Biodiversity and Conservation Biology

Chapter 53

Biodiversity

Biodiversity is the richness of living systems

Genetic variation: the raw material for adaptation, speciation, and evolutionary diversification

Species richness: the number and variety of species within a community – influences its overall characteristics, population interactions, and trophic structure

Ecosystem level: complex interactions bind species in an ecosystem together, and different ecosystems interact within the biosphere

53.1 The Biodiversity Crisis

Biodiversity is declining dramatically, perhaps faster than ever before in Earth’s history

Three broad threats are caused by humans and exacerbated by global climate change:

Clearing of forests (deforestation and desertification)

Commercial overexploitation of marine fish populations

Hydrologic alterations of freshwater ecosystems

Deforestation

Global deforestation occurs at a rate of 13 million hectares per year

More than 90% of occurs in tropical regions

Forests most often cut for grazing livestock

25% of all deforestation in Brazil, which has 27% of the planet’s above-ground woody biomass

Most forests are burned as they are cleared

Adds CO₂ to the atmosphere, enhancing global climate change

Deforestation also reduces the amount of carbon (CO₂) removed from the atmosphere by photosynthesis

Deforestation- Amazon Basin

Tropical Rain Forests

~ 50% have already been cleared globally

Cleared to harvest valuable timber, and to make way for subsistence farming.

Deforestation

Temperate forest of North America

Clearing began with arrival of European settlers.

Cleared to harvest timber & turpentine, and for agriculture.

< 1% of the original temperate forest in southeast US survives.

Desertification

When large tracts of subtropical forest are cleared and overused, the land often undergoes desertification:

Groundwater table recedes to deeper levels

Less surface water is available for plants

Soil accumulates high concentrations of salts (salinization)

Topsoil is eroded by wind and water

Desertification speeds the loss of biodiversity locally, sometimes eliminating entire ecosystems

The Sahel Region of Africa

Overexploitation

Excessive harvesting of an animal or plant species (overexploitation) can cause evolutionary changes and local extinctions

Overexploitation severely threatens marine ecosystems

Yield of fish stocks in Grand Banks has declined to less than 10% of highest historic levels

Cod now mature at a younger age and smaller size

The Grand Banks: Cod

Hydrologic Alterations

Hydrologic alterations (such as dams) are changes to the pathways through which water moves in the hydrologic cycle

Water is used for agricultural, industrial, homes; generate hydroelectric power; mitigate flooding

Hydrologic alterations have practically eliminated some freshwater ecosystems – including the Nile and the Colorado River

More than 30% of the native species in North America are now threatened with extinction

Three Gorges Dam

Water Use and the Florida Everglades

53.2 Specific Threats to Biodiversity

When humans colonize a habitat, they build roads and clear isolated areas for specific uses – reducing areas of intact habitat to small, isolated patches (habitat fragmentation)

The combination of small population size and genetic isolation reduces genetic variability and fosters extinction

Habitat Fragmentation

Habitat Fragmentation

Habitat fragmentation

Reduction to small, isolated patches

Smaller patches, lower carrying capacity

Patches separated by unsuitable habitat

Increasing edge effects reduces remaining habitat quality

Local environmental changes

Songbird reduction from habitat types, brood parasitism and nest predation

Experimental Research:
Predation on Songbird Nests

Habitat Fragmentation

Specific Threats to Biodiversity

Pollutants are materials or energy in forms or quantities that organisms do not usually encounter

Chemical pollutants, the by-products or waste products of agriculture and industry, are released locally – but many spread in water or air on a continental or global scale

Sulfur dioxide (SO₂) from coal-burning power plants and acid precipitation

Acid Precipitation

Patz, 2005. “Impact of regional climate change of human health”

Introduction of Exotic Species

The introduction of nonnative organisms (exotic species) into new habitats poses a serious threat to biodiversity

Exotic species prey upon, parasitize, outcompete native species, leading to their extinction

Many have r-selected life histories – mature and reproduce quickly, and thrive in degraded habitats

In the absence of competitors, predators, and parasites – exotics often experience exponential population growth

Starling Range Expansion

Kudzu

Hemlocks Killed by Woolly Adelgids

Spread of Disease-Causing Organisms

53.3 The Value of Biodiversity

Human activities are causing the current dramatic decline in biodiversity

Arguments for conserving biodiversity fall into three general groups:

Its direct benefit to humans

Its indirect benefit to all living systems

Its intrinsic worth

The Pacific Yew Tree and Teosinte

Indirect Benefits of Biodiversity

Humans and other species derive indirect benefits when ecosystems perform the ecological processes on which all life depends (ecosystem services)

Decomposition of wastes, nutrient recycling, oxygen production, maintenance of fertile topsoil, and purification of air and water

Photosynthetic organisms also mitigate global warming by withdrawing CO₂ from the atmosphere and incorporating it into wood or shells (carbon sequestration)

Intrinsic Value of Biodiversity

Ethicists argue that biodiversity has intrinsic worth as living species, independent of direct or indirect value to humans

Countering this position is the view that human needs should always rank above those of other species and that we should use them to maximize our own welfare

This debate deals more with philosophy and public policy than biology – nevertheless, many people feel that the natural landscape enhances human existence in intangible ways

53.4 Where Biodiversity Is
Most Threatened

Researchers have pinpointed 34 biodiversity hotspots – where biodiversity is concentrated and endangered by human encroachment

A biodiversity hotspot must harbor at least 1,500 endemic plant species (those that are found nowhere else), and it must have already lost at least 70% of its natural vegetation

Locally distributed species account for much of Earth’s biodiversity; and if the local habitats where these species occur are at risk of development, the species are also at risk

Sites Where Extinctions Are Imminent

In 2005, they identified 595 locations in tropical forests, on islands, or mountainous regions where 794 highly endangered species (“trigger species”) are confined to a single site

An endangered species is one that is “in danger of extinction throughout all or a significant portion of its range”

Imminent Danger of Extinctions

53.5 Conservation Biology:
Principles and Theory

Conservation biology is an interdisciplinary science that focuses on the maintenance and preservation of biodiversity

Conservation biologists use theoretical concepts from systematics, population genetics, population ecology, and behavior to develop ways to protect habitats and the endangered species that live within them

Population Genetics

When populations are reduced to a small size, genetic drift reduces their genetic variability and evolutionary potential to adapt

The loss of even a small fraction of a species’ genetic diversity reduces its survival potential

Conservationists try to increase both the population sizes of threatened and endangered species and their genetic variation within and between populations

Whooping Cranes

Population Ecology and Behavior

Conservation programs require data about a target species’ ecology and behavior, including its feeding habits, movement patterns, and rates of reproduction

Sea otters (a keystone predator)

Hunting reduced otters to about 3,000 individuals

Populations of sea urchins (favored prey) exploded

Sea urchins decimated kelp beds, disrupting animal communities in that habitat

To facilitate recovery, conservation biologists reintroduced otters to several regions

Geographical Range of Sea Otters

Population Viability Analysis

Conservation biologists often conduct a population viability analysis (PVA) to determine how large a population must be to ensure its long-term survival

PVAs evaluate habitat suitability, likelihood of catastrophic events, and other factors that may cause fluctuations in demographics, population size, or genetic variability

When conducting a PVA, researchers must decide what level of risk is acceptable for a given survival time – e.g. a 95% probability that the species will survive for 100 years

Population Viability Analysis (cont.)

An increase in either survival probability or survival time requires an increase in the size of the population that must be conserved

The minimum viable population size identifies the smallest population that fits the specifications of the conservation plan

Example: Biologists used PVA in the conservation of an Australian marsupial, the yellow-bellied glider

The Yellow-Bellied Glider

PVA helped biologists determine which remaining forest tracts are large enough to sustain a yellow-bellied glider population

Landscape Ecology

Conservation biologists often use landscape ecology to design the size and geometry of nature reserves and other protected areas

Landscape ecology analyzes how large-scale ecological factors (distribution of vegetation, topography, human activity) influence local populations and communities

Some protected areas consist of one large habitat patch – others consist of several smaller patches

Landscape Ecology (cont.)

Some conservation biologists argue that clusters of physically separate preserves connected by corridors are most effective in maintaining metapopulations of endangered species

Studies suggest that habitat patches connected by corridors retain more native plant species than isolated patches, and that corridors did not promote the entry of exotic species

The Florida Panther

The Florida Panther

53.6 Conservation Biology:
Strategies and Economic Tools

There is little point in trying to preserve natural populations of individual species if their habitats are in jeopardy

Habitat Protection includes:

Preservation enforces strict land use, sometimes precluding human use

Mixed-use conservation protects some areas and controls development in others

Restoration re-establishes vitality of community or ecosystem

Preservation

Mixed-Use Conservation

When complete preservation is impractical, conservationists advocate mixed-use conservation, which combines protection of some land parcels with controlled development of others

Ngorongoro Conservation Area (NCA), Tanzania

Ngorongoro Conservation Area

Restoration

Conservation biologists sometimes create restoration plans to reestablish a previously disrupted community or ecosystem

Restoration requires the removal of contaminants, impediments to the natural flow of water, and barriers to animal movement – as well as restoration of natural processes, such as periodic fires or floods

Most restoration projects also require replanting key plant communities and long-term management after restoration

Brazilian Atlantic Forest: Restoration

Economic Factors

To be successful, a conservation plan must be economically feasible and provide direct benefits to local residents whose lives it will affect

Conservation plans are more successful if they provide local residents with benefits that depend on the existence of a preserve

The Chitwan National Park, Nepal

originally a hunting ground for local royalty

Today, humans are excluded from the park for most of the year

Chitwan National Park, Nepal

Ecotourism

In some preserves, governments enlist local residents in park operations, providing them with a viable livelihood

The most successful approach has been ecotourism in which visitors, often from wealthier countries, pay to visit a nature preserve where local people work

Critics note that increased human (and automobile) traffic degrades protected habitats – and unregulated ecotourism can eventually lead to overdevelopment

Ecotourism in Costa Rica

http://www.costarica-ecotourism.com/

Ecosystem Valuation

In the mid-1990s, conservation biologists and economists developed the concept of ecosystem valuation, in which ecosystem services are assigned an economic value

The global value of ecosystem services has been estimated around $33 trillion – almost twice the value of all goods produced by all humans on the planet

Ecosystem Services & Intrinsic Worth

An Increase in Extinction Rates

Background extinction rates eliminate a few species per year

At least five mass extinctions appear in the fossil record

The greatest mass extinction of all time is occurring now at 1,000 times background rate

What should we do?

Should we act to stop the loss of biodiversity?

If the land mechanism as a whole is good, then every part is good, whether we understand it or not. If the biota, in the course of aeons, has built something we like but do not understand, then who but a fool would discard seemingly useless parts? To keep every cog and wheel is the first precaution of intelligent tinkering.

Aldo Leopold, 1953

Should we act to stop the loss of biodiversity?

How realistic is it to try to save every species?

If we can’t save every species, then what do we save and what do we let go?

Ecosystems

Chapter 52

Energy and chemicals flow through ecosystems

52.2 Energy Flow and
Ecosystem Energetics

Ecosystems receive a steady input of energy from an external source, which in virtually all cases is the sun

Photosynthesis converts less than 1% of the solar energy that arrives at Earth’s surface into chemical energy

Globally, primary producers create more than 150 billion metric tons of new biological material each year

Some of the solar energy that producers convert to chemical energy is transferred to consumers at higher trophic levels

Primary Productivity

The rate at which producers convert solar energy to chemical energy is an ecosystem’s gross primary productivity

After deducting the energy producers use for maintenance, the remaining chemical energy is the ecosystem’s net primary productivity

In most ecosystems, net primary productivity is 50% to 90% of gross primary productivity

Variation in Net Primary Productivity

Standing Crop Biomass
and Net Primary Productivity

Biomass and Net Primary Productivity

Energy Transfer

Net primary productivity ultimately supports all consumers in grazing and detrital food webs

Energy from primary producers that is stored in consumer biomass is secondary productivity

Energy is lost from the ecosystem every time it flows from one trophic level to another:

Some energy is used for maintenance or locomotion

No biochemical reaction is 100% efficient

Energy Flow is Inefficient 5-20%

Types of Pyramids

Pyramids of energy typically have wide bases and narrow tops – energy at each trophic level is reduced about 90%

Pyramids of biomass are proportional to the chemical energy temporarily stored there

In terrestrial ecosystems, the base is generally wide

Aquatic ecosystems sometimes exhibit inverted pyramids of biomass due to high turnover rates of phytoplankton

Pyramid of Energy: Silver Springs

Pyramids of Biomass

Trophic Cascades

• Primary productivity can be indirectly regulated by the consumers above it on the food chain through predator-prey effects called a trophic cascade

• Example: Cordgrass in a salt marsh

• Periwinkle snails preferentially eat cordgrass

• When predators (turtles and crabs) reduce snail populations, cordgrass grows abundantly

• When predators are removed, snail populations grow rapidly and virtually eliminate the cordgrass

Experimental Research:
Trophic Cascade in Salt Marshes

Experimental Research:
Trophic Cascade in Salt Marshes

Trophic Cascade

Consumers can influence primary productivity through food preferences (top-down regulation)

Producers can influence consumers by limiting available biomass (bottom-up regulation)

52.3 Nutrient Cycling in Ecosystems

Producers require carbon, hydrogen, oxygen, nitrogen, phosphorus, and other minerals

Earth is essentially a closed system with respect to matter – it already contains virtually all the nutrients that will ever be available for biological systems

Biogeochemical cycles constantly circulate nutrient ions or molecules between the abiotic environment and living organisms

Biogeochemical Cycling in Ecosystems

Generalized Compartment Model

Nutrients circulate in between living organisms and nonliving reservoirs

Nutrients accumulate in four compartments:

Available organic, available inorganic, unavailable organic, and unavailable inorganic

Nutrients move rapidly between available compartments, slowly in unavailable compartments

The Water Cycle

Volume of Water Reservoirs

The Carbon Cycle

Carbon in Major Reservoirs and Carbon Movement Between Reservoirs

The Nitrogen Cycle (Terrestrial)

Processes that Influence
Nitrogen Cycling

The Phosphorus Cycle

Population Interactions
and Community Ecology

Chapter 51

Ecological Community- all species living in the same place

Ecological Communities

An ecological community is an assemblage of species living in the same place

The presence or absence of species may alter interactions within a community in complex ways

Population interactions and factors such as the kinds of species present and the relative numbers of each species influence a community’s characteristics

51.1 Population Interactions

Most organisms experience complex interactions with many other species

New adaptations that evolve in one species often result in evolution of adaptations in other species

The evolution of genetically based, reciprocal adaptations in two or more interacting species is called coevolution

Population Interactions and Effects

Population Interactions

Coevolution produces reciprocal adaptations in species that interact ecologically

Predation and herbivory define many relationships in ecological communities

Interspecific competition occurs when different species depend on the same limiting resources

In symbiotic associations, the lives of two or more species are closely intertwined

Predation and Herbivory

Predation and herbivory are important in ecological communities

Both predators and herbivores have evolved specialized behaviors and anatomical structures that help them obtain and consume food

Some species (specialists) feed on one or just a few types of food – other species (generalists) eat a wide variety of food

Predators are Many

Evolutionary Adaptations to Predation

Cryptic Coloration

Mimicry and Aposematic Coloration

Spiders eat fireflies

Photuris females are readily eaten by spiders

Photuris females that eat Photinus males are completely rejejected by spiders

Photinus males have 3x antipredatory toxin than Photuris males

Predation

Predators may evolve adaptations to counter prey defenses

Herbivores are Many

Interspecific Competition

Interspecific competition (competition between species) occurs when populations of different species use the same limiting resources

Competing populations may experience increased mortality and decreased reproduction

Interspecific competition reduces the size and population growth rate of one or more of the competing populations

Interspecific Competition

Two or more populations of different species using same resources. Outcomes include:

Competitive exclusion

Competition eventually leads to extinction of one competitor

Character displacement

Evolution of morphological traits that lessen competition

Resource partitioning

“Sharing” of resources reduces negative impacts of competition

Competitive Exclusion Principle

Gause’s competitive exclusion principle states that populations of two or more species that rely on the same limiting resources and exploit them in the same way cannot coexist indefinitely

One species is inevitably more successful, harvesting resources more efficiently and producing more offspring than the other

Gause’s Experiments

Character Displacement

A second way populations reduce competition in nature is by character displacement

Sympatric populations (living in the same place) are morphologically different and use different resources

Character Displacement

Resource Partitioning

One way populations reduce competition in nature is by resource partitioning – the use of different resources, or the use of resources in different ways, by species living in the same place

Example: Weedy plants avoid competition for water and dissolved nutrients in abandoned fields by collecting them from different depths in the soil

Resource Partitioning

The Niche Concept

A population’s ecological niche is defined by the resources it uses and the environmental conditions it requires – including food, shelter, nutrients, light intensity and temperature

A population’s fundamental niche includes all conditions and resources it can possibly use – it’s realized niche is the range of conditions and resources it actually uses

Fundamental versus Realized Niches

Interspecific Competition

Ecological Niche

Symbiotic Associations

Biologists define three types of symbiosis (associations between species) that differ in their effects

Commensalism, one species benefits and the other is unaffected (e.g. cattle and cattle egrets) – rare in nature

Mutualism, both partners benefit (e.g. yucca and yucca moth) – extremely common in nature

Parasitism one species (the parasite) uses another (the host) in a way that is harmful to the host

Population Interactions and Effects: Symbiosis

Commensalism in Plants and Animals

Commensalism

One species benefits and other is unaffected

Mutualism between Plants and Animals

Mutualism in Plants and Animals

Coevolved Mutualism between Plants and Animals

Parasites and Parasitoids

Endoparasites live within a host– generally complete their life cycle in one or two host individuals

Ectoparasites feed on the exterior of a host– most animal ectoparasites have elaborate sensory and behavioral mechanisms

Parasitoids are insects that lays eggs in the larva or pupa of another insect species, and her young consume the tissues of the living host

Endoparasites and Ectoparasites

Parasitoids

51.3 Community Characteristics

Communities have structure and function that varies among ecosystems

Communities differ in species richness and the relative abundance of species they contain

Feeding relationships within a community determine its trophic structure

A Marine Food Web

Species Richness

Communities differ greatly in the number of species that live within them (species richness)

Human activities have disturbed patterns of species richness in natural communities throughout the world

Conservation biologists focus on global patterns of species richness to determine which regions of Earth are most in need of preservation

Relative Abundance

Within every community, populations differ in relative abundance of individuals

Some communities have just one or two dominant species that represent a majority of the individuals present, and some rare species represented by just a few individuals

In other communities, species are represented by more equal numbers of individuals

Species Diversity

Species richness and relative abundance together contribute to species diversity

Example: Forest communities

A forest with more species of trees is more diverse that a forest with fewer species of trees (species richness)

A forest with equal distribution of 10 species of trees is more diverse than a forest with one dominant species and 9 rare species of trees (relative abundance)

Species Diversity: Richness and Relative Abundance

Trophic Structure

The trophic structure of a community is a hierarchy of trophic levels

Primary producers (autotrophs)

Consumers (heterotrophs)

primary consumers

secondary consumers

tertiary consumers

Detritivores (scavengers)

Decomposers (bacteria and fungi)

The Marine Food Web

51.4-51.5 Effects of Population Interactions and Disturbances on Community Characteristics

Community characteristics are constantly challenged by abiotic and biotic influences

Structure and function of communities are often dramatically altered by abiotic and biotic events

Communities are rarely at an equilibrium, but often in constant flux

Effects of Storms on Corals of Great Barrier Reef

51.6 Ecological Succession:
Responses to Disturbance

Ecological succession is a somewhat predictable change in species composition over time

Primary succession begins when organisms first colonize terrestrial habitats without soil, such as those created by erupting volcanoes and retreating glaciers

Secondary succession occurs after existing vegetation is destroyed or disrupted by an environmental disturbance, such as a fire, storm, or human activity

Primary Succession

Species Composition During Succession

Early succession

Species richness rises rapidly, changes quickly

Includes short-lived r-selected species

Late succession

Includes long-lived K-selected species

Some communities eventually achieve a relatively stable state

Succession in Plant and Bird Species

Aquatic Succession in Lakes

Aquatic succession (Eutrophication)

Lakes and Ponds: Fill with nutrients and debris from streams and runoff

51.7 Variations in Species
Richness among Communities

For many groups, species richness follows a latitudinal gradient, with the most species in the tropics and a steady decline in numbers toward the poles

Variations in Species Richness among Communities

Some hypotheses propose explanations for the origin of high species richness in the tropics, including high reproductive rates, low migration, and few environmental disturbances

Other hypotheses propose explanations for the maintenance of high species richness in the tropics, including high availability of energy and other resources

Why It Matters. . .

Someone seems to have disappeared. Investigators thoroughly checked the missing subject's known haunts, but found no trace. They questioned others in the neighborhood, but came up with few leads. The subject was last seen alive in 1978. With so cold a trail to follow, investigators reluctantly marked the case file "Missing and Presumed Extinct."

The subject in this case was Miss Waldron's red colobus monkey, Procolobus badius waldroni (Figure 53.1). Named for a traveling companion of the taxonomist who first described it in 1933, this distinctively colored subspecies lived in large and noisy social groups in a remote forest on the border between Ivory Coast and Ghana in West Africa.

John Oates of the City University of New York led a research team that tried to locate Miss Waldron's red colobus. They used every imaginable method, including visual and auditory censuses, searching for scat (dung) in natural habitats, interviewing local people, and looking in marketplaces where monkey meat is commonly traded. In 2000, more than 20 years after the last confirmed sighting, the researchers concluded that this monkey is probably extinct. A later search by a member of the team, William S. McGraw of Ohio State University, found the skin of one monkey that a hunter had shot six months before. But McGraw searched in vain for a living monkey, and he concluded that even if a few are still alive, the population is so small that continued hunting will surely eliminate it.

Procolobus badius waldroni may be the first primate subspecies to become extinct in more than 100 years—and only the second in the last 500 years. Monkeys and other primates are among the most closely monitored and protected species on Earth. Nonetheless, Oates and his colleagues concluded, these monkeys probably became extinct because they were hunted locally for food by a growing human population and because humans have destroyed their natural habitats.

Miss Waldron's red colobus is just one of many species driven to extinction every year. Current threats to biodiversity—all of which ultimately result from our disruption of natural populations, communities, and ecosystems—are massive. The likely loss of this monkey should warn us that many taxa are at risk, even those that are most rigorously protected.

When ecologists speak of biodiversity, they are referring to the richness of living systems. At the most fundamental level of biological organization, biodiversity encompasses the genetic variation that is raw material for adaptation, speciation, and evolutionary diversification (see Chapters 20 and 21). At a higher level of organization, biodiversity includes species richness within communities (see Section 51-3). The number and variety of species within a community influence its overall characteristics, population interactions, and trophic structure. Finally, biodiversity exists at the ecosystem level. Complex networks of interactions bind species in an ecosystem together, and because different ecosystems interact within the biosphere, damage to one ecosystem can reverberate through others.

In this chapter we first describe how human activities threaten biodiversity and reflect on why we should protect it. We then consider theoretical and practical approaches to conservation biology, the scientific discipline that focuses on preserving Earth's biological resources.

The Biodiversity Crisis on Land, in the Sea, and in River Systems

Biodiversity is declining dramatically, perhaps faster than ever before in Earth's history. In this section, we describe the three broadest threats to biodiversity: the clearing of forests; the commercial overexploitation of marine fish populations; and hydrologic alterations of freshwater ecosystems. Bear in mind that these and other challenges are exacerbated by global climate change, which we have discussed in previous chapters.

Deforestation Disrupts the Carbon Cycle and May Lead to Desertification

Forests are among the habitats that humans most frequently clear and convert. According to the United Nations Forest Resources Assessment released in 2005, global deforestation occurs at a rate of 13 million hectares per year, or 25 hectares per minute. In other words, an area of forest equivalent to 46 football fields is cleared of all trees every minute of every day. An updated Forest Resources Assessment is being conducted in 2010, and experts expect its results to be even more alarming than those published in 2005.

Deforestation does not occur uniformly across the globe. Today, more than 90% of deforestation occurs in tropical regions, where many groups of organisms exhibit their highest diversity (described in Section 51-7). Forests are most often cut to clear land for grazing livestock; as a result, a few species of domesticated animals and the grasses they consume replace what had been a species-rich community. Brazil has experienced the most extensive recent damage, accounting for 25% of all deforestation during the late twentieth century (Figure 53.2). This assessment is particularly troubling because Brazil contains approximately 27% of the planet's total aboveground woody biomass.

Compounding the direct environmental damage, most tropical forests are burned as they are cleared, a process that adds CO₂ to the atmosphere, enhancing the greenhouse effect and increasing the rate of global warming (see Section 52-4). According to the Intergovernmental Panel on Climate Change, a Nobel Prize-winning agency of the United Nations, forest cutting now contributes nearly 20% of all greenhouse gases released into the atmosphere. Ironically, intact forests remove substantial quantities of carbon dioxide from the atmosphere, a capacity that is diminished with every tree felled.

Once a forest is cut, heavy grazing or farming drains nutrients from the soil. To remain productive, even the best agricultural or grazing lands require either the application of fertilizers or long periods during which the land lies fallow, allowing plants to replenish the soil naturally. Unfortunately, the soil where tropical forests grow is often of marginal value (for reasons described in Chapter 49), and it is rapidly degraded; it becomes hard, even more nutrient-poor, unable to retain water, and likely to wash away.

When large tracts of subtropical forest are cleared and overused, the land often undergoes desertification: the groundwater table recedes to deeper levels; less surface water is available for plants; soil accumulates high concentrations of salts (a process called salinization); and topsoil is eroded by wind and water. In other words, the habitat is converted to desert.

Desertification speeds the loss of biodiversity locally, sometimes eliminating entire ecosystems. For example, desertification has decimated habitats in the Sahel region of Africa, just south of the Sahara (Figure 53.3). Excessive grazing of cattle and goats by an ever-expanding human population is the main reason for the Sahara's southward expansion at a rate of 5.5 to 8 km per year. Because the sand dunes of the expanding desert shift constantly, agriculture and grazing are nearly impossible, resulting in frequent famines among the inhabitants.

Desertification and salinization have also begun in the Everglades, a unique, shallow "river of grass" that covers much of southern Florida. The amount of fresh water flowing through South Florida to the Everglades has decreased approximately 70% since 1948, when an extensive network of canals and levees was built to reduce flooding. The rapidly growing human population in South Florida contributes directly to desertification, as groundwater is tapped for domestic use and to irrigate lawns, golf courses, and agricultural fields. Salt water from the Gulf of Mexico now intrudes into the water table, causing salinization of the soil. The Comprehensive Everglades Restoration Plan (CERP), approved by the U.S. Congress in 2000, seeks to restore the natural flow of the Everglades by 2030. This project may halt or reverse the desertification process.

Sadly, deforestation, desertification, and global warming reinforce each other in a positive feedback cycle (see Focus on Applied Research in Chapter 52). If scientists' projections are correct, desertification will lead to an increase in the average global temperature, speeding evaporation and the retreat of forests, which, in turn, will increase rates of desertification. If deforestation and desertification continue, we will soon lose a large proportion of Earth's forests and face a decrease in the area of habitable land.

Overexploitation Can Drive Species to Extinction

Many local extinctions result from overexploitation, the excessive harvesting of an animal or plant species. At a minimum, overexploitation leads to declining population sizes in the harvested species. In the most extreme cases, a species may be wiped out completely. Moreover, overexploitation can foster evolutionary changes in the exploited population, much the way guppies respond to natural predators in the streams of Trinidad (described in Focus on Basic Research in Chapter 50).

Humans have overexploited populations in every habitat we occupy. Today, overexploitation severely threatens marine ecosystems, the only environment from which we routinely harvest predators (such as tuna) as food. The fishery on the Grand Banks off the coast of Newfoundland, Canada, provides a sad example (Figure 53.4). For hundreds of years, fishermen used traditional line and small-net fishing to harvest a large but sustainable catch. During the twentieth century, however, new technology allowed them to locate and exploit schools of fishes more efficiently. As a result, roughly half the fish species harvested there are now overfished. Haddock (Melanogrammus aeglefinus) and yellowtail flounder (Limanda ferruginea) have been essentially eliminated from the Grand Banks; their populations will probably never recover. And because fishers preferentially harvest the oldest and largest individuals, which fetch a higher market price, Atlantic cod (Gadus morhua) now mature at a younger age (three years compared with five or six years) and smaller size.

As a consequence of overfishing, the average yield of the Grand Banks has declined to less than 10% of the highest historic levels. In the mid-1960s, Atlantic cod yielded a minimum of 350,000 tons per year. By the mid-1970s, the catch dropped to 50,000 tons per year. The Canadian government finally closed the fishery in 1993, after the cod catch fell below 20,000 tons for several consecutive years. But the damage had already been done: the most heavily exploited species are less marketable because of their smaller size, fish populations have decreased to dangerously low levels, and the fishing industry is itself imperiled. This sequence of events has been replicated in fisheries around the world. Indeed, in a report published in 2003, Ransom A. Myers and Boris Worm of Dalhousie University in Nova Scotia estimated that modern fishing techniques have reduced the biomass of large predatory fishes by about 90% in marine ecosystems.

Hydrologic Alterations Endanger Freshwater and Wetland Ecosystems

Rivers have always played a key role in the development of human settlements because they provide a source of fresh water, a place to discard wastes, and a means to transport goods. Since ancient times, humans have also dammed rivers to capture reliable supplies of fresh water. When the human population—and the dams they built—were small, these hydrologic alterations (that is, changes to the pathways through which water moves in the hydrologic cycle) had primarily local effects. More recently, we have constructed massive dams that capture vast quantities of fresh water in reservoirs (Figure 53.5). We distribute water from these reservoirs for agricultural, industrial, and domestic uses. The dams also generate hydroelectric power and allow us to control water flow to mitigate flooding of lowlying land. Today, the large scale and ubiquity of these hydrologic alterations have made freshwater ecosystems among the most endangered on Earth.

The damming of rivers and the diversion of their flow wreaks havoc on many interconnected ecosystems. As you learned in Section 49-5, the physical characteristics of rivers change predictably from the headwaters of streams to the estuaries where they empty into the sea. River-dwelling organisms are adapted to specific physical conditions—such as temperature, depth, and flow rate—that are characteristic of each section of the river system. And downstream, the flow of water supports distinct communities of organisms, each adapted to the different environments in floodplains, wetlands, and estuaries.

In 2002, Stuart E. Bunn and Angela H. Arthington of Griffith University in Australia identified four ways in which hydrologic alterations threaten freshwater biodiversity. First, the flow rate and volume of rivers are key determinants of their physical habitats, which have a major impact on the organisms that live there. For example, before it was dammed, the River Otra in Norway experienced low winter water flows, but raging summer floods. These conditions established a regular pattern of disturbance that eliminated many rooted plants from the riverbed. Now dammed, the river has a more regulated flow regime that allows a huge accumulation of plant biomass.

Second, the life histories of aquatic species, which evolved in response to natural flow patterns, are disrupted by changes in river flows. For example, reproduction by many aquatic invertebrates and fishes is triggered by temperature and day-length cues. Because the water released through dams is often drawn from the depths of the reservoirs behind them, it is colder than the natural flow, changing the cues available to organisms. Researchers working in China discovered that the cold water released by dams delayed spawning by as much as 30 days in some fish species.

Third, dams reduce a river system's "connectivity" (that is, the continuity of flow through a river and its streams and tributaries). Reduced connectivity prevents fishes and other animals from migrating freely through a river system. For example, salmon undertake a spawning migration from the sea, swimming upstream into the tributaries and streams where they reproduce. Dams hamper this already difficult upstream journey. In the Pacific Northwest, more than 400 hydroelectric dams on the Columbia River system prevent many salmon from reaching their spawning grounds; they have reduced the breeding habitat for Chinook salmon (Onchorhynchus tshawytscha) by 75%. Similar problems have eliminated migratory fish species from rivers throughout the world.

Finally, dams and reservoirs facilitate the introduction and success of non-native species that thrive in disturbed habitats (discussed further below). For example, several species of large fishes collectively described as "Asian carp" have become established in the Mississippi River and its tributaries. These fishes feed voraciously on plankton. For years ecologists feared that they would spread into the Great Lakes, where they would out-compete native fish species that are the basis of a large fishing industry. In 2002 and 2004, the U.S. Army Corp of Engineers built two electric barriers in a canal that connected a tributary of the river to Lake Michigan, hoping to block the fishes' advance. But in late 2009 and early 2010, scientists detected genetic material from the carp on the far side of the fences. Some politicians have called for the closure of the canal as the only way to prevent the advance of these fishes into Lake Michigan; as of this writing, that proposal is the subject of several lawsuits.

Freshwater ecosystems are now under severe pressure. In the worst cases, human-induced hydrologic alterations have practically eliminated them: the Nile and the Colorado River now rarely discharge much water into the sea. Even in less dramatic cases, the effects of hydrologic alterations on biodiversity have been profound. Freshwater fish species have experienced marked declines in the last few decades. One 2006 estimate suggested that 56% of the freshwater fish species endemic to the Mediterranean region, more than 30% of the native species in North America, and 25% of those found in East Africa are now threatened with extinction. The status of freshwater invertebrates, though not as well documented, is probably comparable. Conservation biologists rank the restoration of natural flow patterns in river systems among their highest priorities.

Study Break 53–1

1. What factors have increased the likelihood of desertification in southern Florida?
2. What are the consequences of the overexploitation of fish populations?
3. How does the construction of a dam disrupt the lives of river-dwelling organisms?

Think Outside the Book

Search the Internet for updates about the spread of Asian carp in North America. Have they successfully invaded the Great Lakes? If so, what impact have they had on the native lake fishes?

Specific Threats to Biodiversity

Although the clearing of tropical forests, overexploitation of marine fisheries, and damming of rivers endanger entire ecosystems, many other human activities imperil natural populations. In this section, we briefly describe some of these threats.

Habitat Fragmentation Threatens Many Populations

When humans first colonize a pristine habitat, they build roads and then clear isolated areas for specific uses. Although this pattern of development initially affects only local populations, the negative effects spread rapidly to a regional scale. The remaining areas of intact habitat are inevitably reduced to small, isolated patches, a phenomenon that ecologists describe as habitat fragmentation.

Habitat fragmentation is a threat to biodiversity because small habitat patches can sustain only small populations of organisms. As you learned in Section 50-4, a habitat's carrying capacity, the maximum population size that it can support, varies with available resources. Populations that occupy small habitat patches inevitably experience low carrying capacities, a problem that is especially acute for species at the higher trophic levels (see Section 52-2). Furthermore, fragmented habitat patches are often separated by unsuitable habitat that organisms may be unable or unwilling to cross. As a result, individuals from one isolated population are unlikely to migrate into another, reducing gene flow between them. The combination of small population size and genetic isolation reduces genetic variability and fosters extinction (see Section 20-3).

In addition to reducing the amount of undisturbed habitat, habitat fragmentation jeopardizes the quality of the habitat that remains. Human activities create noise and pollution that spread into nearby areas. The removal of natural vegetation disrupts the local physical environment, exposing the borders of the remaining habitat to additional sunlight, wind, and rainfall. Increased runoff compacts the soil and makes it waterlogged. These phenomena are collectively described as edge effects.

The effects of habitat fragmentation are often profound. For example, populations of forest-dwelling, migratory songbirds have declined markedly in eastern North America since the late 1940s, largely because of habitat fragmentation in their North American breeding grounds and in their Caribbean and South American wintering grounds.

In 1994, Scott K. Robinson of the Illinois Natural History Survey and David S. Wilcove of the Environmental Defense Fund identified three factors that decrease populations of migratory songbirds in fragmented breeding habitats. First, small forest patches often lack specific habitat types—such as streams, cool ravines, or dense ground cover—that many songbird species require. Second, songbirds breeding in forest patches are more likely to suffer from brood parasitism (described in the opening of Chapter 51) by brown-headed cowbirds (Molothrus ater) than are those breeding in intact forests. Parasitized songbirds rear fewer than half as many young as they might otherwise raise, and their populations decline accordingly.

The third factor that reduces songbird numbers in forest fragments is increased nest predation by blue jays (Cyanocitta cristata), American crows (Corvus brachyrhynchos), common grackles (Quiscalus quiscula), squirrels (genus Sciurus), raccoons (Procyon lotor), and domestic dogs and cats. These predators, which feed on songbird eggs and young, are now superabundant in rural and suburban areas, and they enter adjacent forest fragments in search of an easy meal. Wilcove tested the predation hypothesis experimentally by placing artificial nests with quail eggs in intact forests and in forest fragments. Although he did not observe predation directly, he found that predators discovered only 2% of the nests in the largest intact forest, but they often found 50% or more of the nests placed in small, suburban forest fragments (Figure 53.6).

Many Forms of Pollution Overwhelm Species and Ecosystems

The release of pollutants—materials or energy in forms or quantities that organisms do not usually encounter—poses another major threat to biodiversity.

Although chemical pollutants, the by-products or waste products of agriculture and industry, are released locally, many spread in water or air, sometimes on a continental or global scale. Within North America, for example, winds carry airborne pollutants from coal-burning power plants to the Northeast (Figure 53.7). Sulfur dioxide (SO₂), which dissolves in water vapor and forms sulfuric acid, falls as acid precipitation, acidifying soil and bodies of water. Many lakes in northeastern North America have experienced a precipitous drop in pH from historical readings near 6 to values that are now well below 5—a 10-fold increase in acidity. Although the lakes once harbored lush aquatic vegetation and teemed with fishes, they are now crystal clear and nearly devoid of life.

As residents of major cities and industrial areas know all too well, wastes produced by the combustion of fossil fuels in factories and automobile engines cause terrible local pollution, increasing rates of asthma and other respiratory ailments. Some airborne pollutants, notably CO₂, also join the general atmospheric circulation, where they contribute to the greenhouse effect and global warming.

Like air pollution, water pollution originates locally but has a much broader impact. Oil spills, for example, disrupt local ecosystems, killing most organisms near the spill. Because oil floats on water, it spreads rapidly to nearby areas. An explosion and fire on the Deepwater Horizon oil rig in April 2010 allowed many millions of gallons of oil to spill into the Gulf of Mexico. The uncapped well continued to spew oil for three months, until it was partially plugged in July 2010. The oil spill had a devastating short-term effect on organisms in the Gulf and adjacent wetlands; scientists are still unable to predict its long-term effects on this delicate ecosystem.

Pollution can also have serious effects on terrestrial ecosystems. As a recent disaster in India, Nepal, and Pakistan illustrates, the application of synthetic compounds to agricultural fields or livestock can have dire and far-reaching consequences. For thousands of years, enormous populations of vultures (several Gyps species)—estimated at more than 40 million birds—consumed the abandoned carcasses of farm animals across South Asia. In the early 1990s, however, farmers began to administer diclofenac, a new and inexpensive anti-inflammatory drug, to injured livestock. Within a few years, vultures began to disappear; in 2006, scientists estimated that their populations had declined by more than 97%. Researchers determined that diclofenac, which causes fatal kidney failure in birds, was responsible for the deaths: vultures were ingesting substantial doses of the drug from the livestock carcasses they ate. All vulture species in South Asia are now on the verge of extinction, and although governments in the region have banned the sale of diclofenac, wildlife experts say that the vulture populations are unlikely to recover soon, if ever.

The decline in vulture populations has had a disastrous impact on urban and rural communities in South Asia. Livestock carcasses are now consumed by growing populations of feral dogs, many of which carry rabies. India has the world's highest human death toll from rabies—30,000 per year—and two-thirds of the cases are caused by dog bites. Populations of rats and flies also appear to be increasing. Focus on Applied Research in Chapter 52 describes another example of how pesticides and other chemicals accumulate at lethal concentrations in organisms living at higher trophic levels.

The Introduction of Exotic Species Often Eliminates Native Species

As humans travel from one habitat to another, we inevitably carry other species with us. Seeds cling to our legs, insects accompany us in our food and possessions, and some organisms hitch a ride on boats or cars. The introduction of nonnative organisms, called exotic species, into new habitats poses a serious threat to biodiversity.

Exotic species often prey upon, parasitize, or outcompete native species, leading to their extinction. Many have r-selected life histories; they mature quickly and reproduce prodigiously, and they thrive in the degraded habitats that humans so frequently create. In the absence of natural checks on population growth—such as competitors, predators, and parasites—exotics often experience exponential population growth (see Section 50-5).

The European starling (Sturnus vulgaris) provides an example of the explosive population growth and range expansion of an exotic species. These birds were released in North America in 1890 when a misguided individual, who wanted to introduce all of the bird species mentioned by Shakespeare into North America, imported them into Brooklyn, New York. Within 70 years, they had spread across the continent (Figure 53.8); their population size is now estimated at 200 million. Starlings pose a serious threat to native birds, including several woodpecker species, because they successfully compete with them for nesting sites in natural cavities in trees.

Introduced plants often transform entire ecosystems. One of the best-known examples is kudzu (Pueraria lobata), a fast-growing species from Asia. In the early 1900s, it was widely planted in the southeastern United States as a source of animal feed. Later, a government agency planted it to stabilize soils and decrease erosion on deforested hillsides. But when kudzu has access to abundant nutrients and water, it can grow up to 30 cm per day. It spread quickly across the South, literally overgrowing almost all native plants (Figure 53.9).Exotic insects often become pests of agricultural crops and native plants. The hemlock woolly adelgid (Adelges tsugae) was accidentally introduced into North America from Asia. The adelgid kills eastern hemlocks (Tsuga canadensis) by feeding on their sap. It now threatens the trees from North Carolina to Massachusetts (Figure 53.10). But adelgids endanger far more than these evergreen trees. Hemlocks buffer the physical conditions below them: hemlock stands are cool in summer and warm in winter, sustaining a unique community of organisms that includes ruffed grouse (Bonasa umbellus), turkey (Meleagris gallopavo), white-tailed deer (Odocoileus virginianus), and snowshoe hare (Lepus americanus). Infested stands rarely survive more than a few years, and the communities established under pure stands of eastern hemlock will likely become extinct because of feeding by the adelgid.

The Spread of Disease-Causing Organisms Endangers Many Species

As exotic species become established in new habitats, they may carry disease-causing organisms with them. Because native species had no prior exposure to these pathogenic organisms, they never evolved resistance to them, leading to devastating outbreaks of disease. For example, amphibians have been in a worldwide decline since 1980. About one-third of the roughly 6,300 described species are now in danger of becoming extinct, and populations of nearly half of all amphibian species are declining in numbers. The sudden change in the status of these animals has sparked an enormous research effort aimed at identifying the primary causal factors. Although habitat destruction and pollution have undoubtedly taken a great toll, scientists now attribute many recent amphibian declines and extinctions to infection by the chytrid fungus Batrachochytrium dendrobatidis. The fungus has been found in natural populations of more than 200 amphibian species; more than 1,000 others have been identified as susceptible to it (Figure 53.11).

This pathogenic fungus was first described in 1999 by researchers investigating skin infections in amphibians from North America, Central America, and Australia. Scientists have since learned that the fungus is very strange indeed: it is the only species in its group known to infect vertebrates; it infects only amphibians, feeding on keratin in their skin and in the mouthparts of their tadpoles; and even though it has an aquatic life cycle, it can infect fully terrestrial amphibians that never enter standing water. Researchers believe that the fungus interferes with amphibians' oxygen acquisition and osmoregulation, two important physiological processes that are partly mediated by their skin. We still do not know how the fungus spreads among individuals or populations. It may be carried from place to place by amphibians and other vertebrates.

Where did the pathogen originate? In 2004, researchers found the infection in specimens of African clawed frogs (Xenopus laevis) that had been collected in 1938 and preserved in South African museums. Other researchers have found evidence of the infection in a Canadian population of bullfrogs (Rana clamitans) collected in 1961. Thus, scientists hypothesize that the infection originated in southern Africa and was confined there for decades. How did it spread so rapidly to other continents? Beginning in the 1930s, African clawed frogs were exported from South Africa to many countries for use in biological research and for the pet trade. Sadly, the frog's popularity among scientists and hobbyists has resulted in the spread of an infection likely to cause the extinction of many other amphibian species.

Why did the fungus start to devastate amphibian populations only in the 1980s? Some researchers suggest that even small increases in temperature and related changes in cloud cover and humidity—all the result of global climate warming—have favored the growth of the pathogenic fungus in some habitats with high amphibian diversity. Other researchers argue that climate warming and pollution stress amphibian populations, making them more susceptible to infection by many pathogens.

Biologists are working feverishly to learn more about the fungus and its role in amphibian declines before a majority of amphibian species become extinct. In a broader—and even more frightening—context, ecologists who study the dynamics of disease in natural population are just beginning to grapple with the likely effects of climate change and other consequences of human activity on the spread and success of pathogenic organisms, including those that infect humans.

Human Activities Are Causing a Dramatic Increase in Extinction Rates

As you may remember from Section 22-4, extinction has been common in the history of life; roughly 10% of the species alive at any time in the past became extinct within 1 million years. These background extinction rates eliminated perhaps seven or eight species per year. Paleobiologists have also documented at least five mass extinctions, during which extinction rates increased greatly above the background rate for short periods of geological time (see Figure 22.15).

At present, Earth appears to be experiencing the greatest mass extinction of all time. According to Edward O. Wilson of Harvard University, extinction rates today may be 1,000 times the historical background rate, meaning that thousands of species are being driven to extinction each year. The vast majority of extinctions are a direct result of the destructive human activities discussed previously.

If humans are causing the current mass extinction, why didn't it begin long ago? The answer lies in our increased rate of population growth (see Section 50-6). During the nineteenth and twentieth centuries, improvements in food production, sanitation, and health care increased human life expectancy. Our ever-increasing population consumes resources and produces wastes at an escalating rate. And until we change the way we live in relation to the environment that we share with all other species, our negative impact will grow along with our global population.

Study Break 53–2

1. How has habitat fragmentation affected breeding songbird populations in eastern North America?
2. What environmental factor has caused the demise of vulture populations throughout South Asia?
3. How do extinction rates today compare with the background extinction rate evident in the fossil record?

Think Outside the Book

Search the Internet for updated information about the effects of the Deepwater Horizon oil spill in the Gulf of Mexico. Has the ecological impact of the spill become more evident over time?

The Value of Biodiversity

Given the many ways in which human activities are causing the dramatic decline in biodiversity, we should reflect on the present and future value of what we are destroying and contemplate why we might want to preserve it. Arguments for conserving biodiversity fall into three general groups: its direct benefit to humans, its indirect benefit to all living systems, and its intrinsic worth.

Biodiversity Benefits Humans Directly

Scientists constantly search for natural products that might provide humans with better food, clothing, or medicine. The development of a new medicine often begins when a scientist analyzes a traditional folk remedy or screens naturally occurring compounds for curative properties. Chemists then isolate and purify the active ingredient and devise a way to synthesize it in the laboratory. More than half of the 150 most commonly prescribed drugs were developed from natural products in this manner.

For example, Taxol, a drug treatment for breast and ovarian cancer, was isolated from the narrow strip of vascular cambium beneath the bark of the Pacific yew tree, Taxus brevifolia (Figure 53.12). Unfortunately, a fully grown, 100-year-old tree produces only a tiny amount of Taxol, and six trees must be destroyed to extract enough to treat one patient. Pacific yew trees are not abundant, and they grow slowly. Harvesting them for Taxol extraction could quickly lead to their extinction—and an end to the natural source of this life-saving compound. However, after much research, scientists now synthesize this widely used drug in the laboratory.

Wild plants and animals also serve as sources of genetic traits that may improve agricultural crops and domesticated livestock. For example, corn (Zea mays) is an annual plant. Its cultivation requires yearly planting, a laborious activity that leads to the erosion of topsoil. Farmers would rather grow a perennial strain of corn, one that would produce grain for years after a single planting. In 1978, botanists discovered teosinte (Zea diploperennis) a perennial plant closely related to corn, in the mountains of western Mexico. Researchers crossed the two species, producing a perennial corn. If they can increase the yield of this hybrid, it may prove to be an economically valuable crop.

Today, many agricultural researchers use genetic engineering, the transfer of selected genes from one species into another (see Section 18-2), to alter crop plants more precisely than they can using hybridization. The transferred genes may be chosen to increase resistance to pests or environmental stress, promote faster growth, or increase shelf life after harvesting. However, many scientists and environmentalists fear that genetically modified crops may create environmental hazards that will inadvertently endanger biodiversity. For example, a genetically modified plant or animal that escaped into a natural habitat might compete with naturally occurring species. Or a genetically modified plant might poison harmless animals as well as insect pests.

Ecosystem Services Benefit All Forms of Life

Humans and other species derive indirect benefits when ecosystems perform the ecological processes on which all life depends. These ecosystem services, as they are called, include the decomposition of wastes, nutrient recycling, oxygen production, maintenance of fertile topsoil, and air and water purification.

Some ecosystem services can even mitigate environmental damage caused by humans. As you may recall from Section 52-4, the combustion of fossil fuels produces CO₂ and other waste products that accumulate in the atmosphere, increasing the greenhouse effect and fostering global warming. Photosynthetic organisms use CO₂ for essential metabolic processes; thus, the forests that we clear and, even more importantly, communities of marine phytoplankton, withdraw CO₂ from the atmosphere and incorporate it into living organisms (see Figure 52.13), in a phenomenon called carbon sequestration. Recent research indicates that these organisms are essential for limiting the damage caused by the burning of fossil fuels. In the long run, biodiversity's indirect benefits, provided in the form of ecosystem services, may be even more valuable to humans than the direct benefits.

Biodiversity Has Intrinsic Worth beyond Its Utility to Humans

Some ethicists argue that we should preserve biodiversity because it has intrinsic worth, independent of its direct or indirect value to humans. They note that humans are just one species among millions in the remarkable network of life. Countering this position is the view that our immediate needs should always rank above those of other species and that we should use them to maximize our own welfare. The latter view inevitably leads to the disruption of natural environments and the loss of biodiversity. Framed in this way, the debate lies more within the realms of philosophy and public policy than biology. Nevertheless, many people feel an emotional or spiritual connection to natural landscapes and the plants and animals they harbor. Thus, biodiversity enhances human existence in intangible ways.

Study Break 53–3

1. How does biodiversity serve as a storehouse of genetic information that is potentially useful to humans?
2. What ecosystem services do naturally occurring organisms provide to humans?

3. Where Biodiversity Is Most Threatened

   To slow the current rate of extinction and loss of biodiversity, conservation biologists must first identify where species are likely to become extinct.
   

   If we are to limit the effects of human activities and preserve biodiversity, we must know how biodiversity is distributed. Although species richness within communities generally increases from the poles to the tropics (see Section 51-7), broad latitudinal surveys do not provide enough detail to be useful in this effort.

   In a survey published in 2000, Norman Myers of Oxford University and his colleagues in England and the United States pinpointed 25 biodiversity hotspots, areas where biodiversity is both concentrated and endangered by human encroachment. To qualify as a biodiversity hotspot under Myers' criteria, an area must harbor at least 1,500 endemic plant species (those that are found nowhere else), and it must have already lost at least 70% of its natural vegetation. As human activity in natural habitats has increased, the number of terrestrial biodiversity hotspots has now grown to 34.

   Myers used the number of endemic species as a criterion for identifying hotspots because endemics tend to have highly specific habitat or dietary requirements, low dispersal ability, and restricted geographical distributions. Indeed, locally distributed species account for much of Earth's biodiversity; and if the local habitats where these species occur are at risk of development, the species are also at risk. Although the 25 original hotspots occupy only 1.4% of Earth's land surface, they include the only remaining habitat for approximately 45% of all terrestrial plant species and 35% of all terrestrial vertebrate species.
   

   Researchers Now Pinpoint Sites Where Extinctions Are Imminent

   The identification of biodiversity hotspots tells us where biodiversity is both concentrated and threatened, but most of these areas are large and heavily populated. Conservation biologists need more detailed information to identify specific localities where their efforts will have the greatest impact.

   Building on Myers' pioneering study, Taylor H. Ricketts of the World Wildlife Fund, working with 29 collaborators in the United States, Australia, and the United Kingdom, pinpointed sites where extinctions are imminent. In a paper published in 2005, they identified 585 locations in tropical forests, on islands, or in mountainous regions where 794 trigger species—highly endangered species of mammals, birds, reptiles, amphibians, and coniferous trees—are each confined to a single site (Figure 53.13). As defined by the Endangered Species Act, adopted by the U.S. Congress in 1973, an endangered species is one that is "in danger of extinction throughout all or a significant portion of its range."

   The researchers used strict criteria for including a site in the list. First, it must harbor at least one species that has been officially designated as endangered by the World Conservation Union, an international organization. Second, it must contain at least 95% of the world population of that species. Third, it must have clearly definable boundaries within which habitats are distinct from those outside the boundary; examples of such bounded habitats include lakes, mountaintops, and forest fragments. The boundaries of the site thus define the area to be conserved. Although the 595 sites of imminent extinction are included within the biodiversity hotspots that Myers identified, the new approach has the practical advantage of pinpointing localized sites where conservation biologists can focus their efforts.

   Ricketts and his colleagues noted that 794 trigger species are in danger of imminent extinction, compared with 245 species from the same taxonomic groups that are known to have become extinct in the last 500 years. Thus, the rate of extinction in these groups of organisms is accelerating rapidly. Their analysis also reveals that the proportion of extinctions in mainland habitats (as opposed to islands) is also growing: only 20% of historical extinctions occurred in low-lying mainland areas, whereas more than 60% of trigger species live on the mainland today. Moreover, their study detected a taxonomic shift in extinction: 53% of historical extinctions were of bird species, but 51% of the trigger species are amphibians. Finally, the data reveal a geographic shift in extinctions: whereas only 21% of historical extinctions were in the New World tropics, 50% of the trigger species live in Central America, South America, and on Caribbean islands. These results indicate that species living in the New World tropics, especially in wet forests, are in the greatest danger.

   The data from the new analysis allow conservation biologists to target their efforts, but the task is daunting. Although one-third of the 585 sites lie completely within protected areas, more than 40% lack any protection at all. Most of the sites are small (median size approximately 12,000 hectares), suggesting that they might be easy to protect, but small sites are also the most vulnerable to human encroachment. Adding to the difficulty, the human population density in areas surrounding the sites is nearly triple the average density worldwide. Nevertheless, conservation biologists are optimistic that this new approach to pinpointing the areas most in need of their attention will allow them to develop appropriate strategies for preventing the extinction of the trigger species and others that live within these areas.

   Study Break 53–4

   1. What criteria do conservation biologists use to identify sites where extinctions are imminent?
   2. Why are conservation biologists especially concerned about the rapid rate of deforestation in the New World tropics?

   Conservation Biology: Principles and Theory

   Conservation biology is an interdisciplinary science that focuses on the maintenance and preservation of biodiversity. In this section we describe how conservation biologists use theoretical concepts from systematics, population genetics, behavior, and ecology to develop ways to protect habitats and the endangered species that live within them. We introduce practical applications of conservation theory in the next section.

Systematics Organizes Our Knowledge of the Biological World

To develop a conservation plan for any habitat, scientists must start with an inventory of its species. Their primary tool is systematics, the branch of biology that discovers, describes, and organizes our knowledge of biodiversity (see Chapter 23). Cataloguing the diversity of life may be the most daunting task that biologists face. After more than 200 years of work, systematists have described and named approximately 1.6 million species. However, they realize that this number represents only a fraction of existing species.

In 1982, Terry Erwin of the Smithsonian Institution studied beetle biodiversity at the Tambopata National Reserve in Southern Peru. He sprayed biodegradable insecticide into the canopy of one large tree and collected 15,869 individual beetles, which he sorted into 3,429 species. More than 90% of the individual beetles he collected belonged to species that had not yet been described. Erwin used this astounding result and a complex mathematical model to predict that approximately 30 million species currently exist.

Nigel Stork of the Natural History Museum in London later questioned Erwin's conclusions. Using additional data and a modified set of assumptions, he estimated that the actual number of living species was closer to 100 million. If his figure is correct, more than 98% of species—most of them arthropods, nematodes, bacteria, and archaeans—are still unknown to science. Regardless of whether biodiversity encompasses 30 million species or 100 million, systematists clearly have much work to do.

Recently, conservation biologists and systematists have begun to develop a new technology that will simplify the identification of species in the field, thereby facilitating the creation of a catalog of biodiversity. Insights from the Molecular Revolution describes the effort to develop a "DNA barcode scanner."

Population Genetics Informs Strategies for Species Preservation

When populations are reduced to small size, genetic drift inevitably reduces their genetic variability (see Section 20-3) and the evolutionary potential to adapt to changing environments. Thus, the loss of even a small fraction of a species' genetic diversity reduces its survival potential. To avoid this problem, conservationists strive not only to increase the population sizes of threatened or endangered species but to maintain or increase their genetic variation, both within and between populations.

For example, the whooping crane (Grus americana) was once an abundant bird in wet grassland environments through much of central North America (Figure 53.14). By the early 1940s, excessive hunting and habitat destruction had caused their numbers to decline to just 21 individuals in two isolated populations. This population bottleneck and the resultant loss of genetic variability apparently contributed to developmental deformities of the spine and trachea that had not been seen previously.

During the 1970s, biologists began an aggressive conservation program. In addition to preserving habitats in the crane's summer and winter ranges, they initiated a carefully controlled captive breeding program designed to minimize the effects of inbreeding. Although more than 300 whooping cranes now survive in several wild and captive populations, recent research reveals that they still have a remarkably low level of genetic variability. As expected, the genetic effects of a severe population bottleneck may persist long after a population begins to increase in size.

Studies of Population Ecology and Behavior Are Essential to Conservation Plans

Conservation programs for animals also require data about target species' ecology and behavior, including their feeding habits, movement patterns, and rates of reproduction.

Sea otters (Enhydra lutris) are predatory marine mammals that live along the coastline of the North Pacific Ocean. In the early 1700s, they numbered approximately 300,000 individuals (Figure 53.15), but commercial hunting reduced their numbers to about 3,000 individuals by the start of the twentieth century. Sea otters are keystone predators (see Section 51-4), and the destruction of sea otter populations had profound effects on the communities in which they lived. As the numbers of sea otters plummeted, populations of sea urchins, one of their favored prey, exploded; burgeoning sea urchin populations decimated local kelp beds, disrupting the communities of animals that live among these giant algae.

International treaties ended nearly all hunting of sea otters in 1911, and the populations subsequently recovered to about one-third of their original levels. Conservation biologists facilitated the recovery by reintroducing otters into southeastern Alaska, British Columbia, Washington, and California. Before deciding where otters should be reintroduced, scientists had to assess the resources available at different sites and determine how far individual otters would move, how rapidly they would reproduce, and how quickly their populations would spread. The reintroduction effort was successful at first. However, populations in California have experienced high mortality since the mid-1990s, and nearly half of those dying have been adults in their reproductive prime. Researchers have identified parasitic

infections and heart disease as leading causes of death, suggesting that some coastal environments are so badly degraded that they may no longer support populations of this species.

Using complex mathematical models, conservation biologists often conduct a

population viability analysis (PVA) to determine how large a population must be to ensure its long-term survival. PVAs evaluate phenomena that may influence the longevity of the population or species: habitat suitability, the likelihood of catastrophic events, and other factors that may cause fluctuations in demographics, population size, or genetic variability. When conducting a PVA, researchers must decide what level of risk is acceptable for a given survival time. For example, should a conservation plan attempt to ensure a 95% probability that the species will survive for 100 years, or should it specify a 99% survival probability? An increase in either the survival probability or the survival time requires an increase in the size of the population that must be conserved. The minimum viable population size identifies the smallest population that fits the desired specifications of the conservation plan.

3. Principles of Community Ecology and Landscape Ecology Guide Large-Scale Preservation Projects

   Many conservation efforts focus on the preservation of entire communities or ecosystems. These projects often depend on the work of community and landscape ecologists.

   Species/Area Relationships

   As you know from Chapter 51, community composition is dynamic: some species become extinct and others join the community through immigration. If we view patches of intact habitat as islands in a sea of unsuitable terrain, we can apply the predictions of the theory of island biogeography (see Section 51-7) to the design of protected areas. For example, we might expect that the number of species a patch will support depends on its size and proximity to larger patches.

   Indeed, ecologists recognized long ago that large habitat patches sustain more species than small patches do (Figure 53.16). When plotted on an arithmetic scale, the relationship between species richness and habitat area increases sharply at first and then flattens. In other words, for relatively small habitat patches, even minor increases in area allow a large increase in the number of resident species; but as habitat patches get larger, the number of species present eventually levels off. You encountered an example of this relationship in our discussion of bird species richness on islands of different sizes (see Figure 51.31B).

   As habitats become increasingly fragmented, edge effects exaggerate the species/area relationship in mainland habitat patches (Figure 53.17). Consider two hypothetical patches of habitat: one is 100 m on a side, with a total area of 10,000 m²; the other is 200 m on a side, with a total area of 40,000 m². Now, imagine that edge-effect disturbances penetrate 20 m into each patch from all directions. The small patch contains only 3,600 m² of intact habitat, but the large patch contains 25,600 m² of intact habitat. Although the large patch is only four times larger than the small patch, the large patch contains more than seven times as much intact habitat.

   Landscape Ecology

   Conservation biologists often use landscape ecology to design the size and geometry of nature reserves and other protected areas. Landscape ecology analyzes how large-scale ecological factors—such as the distribution of vegetation, topography, and human activity—influence local populations and communities.

   When conservation biologists first applied concepts from landscape ecology to the design of protected areas, they debated whether nature preserves should comprise one large habitat patch or several smaller patches. Based on the species/area relationship, large patches should harbor more species than small patches; large patches would also experience proportionately smaller edge effects; and large patches would better support populations of large animals that need substantial resources. Nevertheless, some conservation biologists argued that clusters of physically separate preserves are more effective in maintaining meta-populations of endangered species (see Section 50-5), especially if the patches are interconnected by corridors of intact habitat. Individuals could move between preserves, reviving any local populations that experience a decline.

   Some conservation biologists have worried that narrow landscape corridors, which by definition have large edges, might allow exotic species to invade protected areas. Ellen I. Damschen of North Carolina State University and several colleagues conducted an ambitious long-term field experiment to test the effects of landscape

   corridors on plant species richness (Figure 53.18). Their results, published in 2006, suggest that habitat patches connected by corridors retain more native plant species than isolated patches do, and that corridors did not promote the entry of exotic species. Thus, corridors appear to be a useful feature in the design of nature preserves.

   Landscape corridors are a key feature of efforts to prevent the extinction of the Florida panther (Puma concolor coryi, shown in the chapter introduction). This subspecies is critically endangered: only 70 to 100 individuals remain from a population that once ranged throughout the southeastern United States. Other panther subspecies still inhabit the western states. Panthers are large predators, and each female requires nearly 20,000 hectares (more than 75 square miles) for hunting and breeding; males each require more than twice as much space.

   Although the state and federal governments have set aside several panther conservation areas in Florida, 52% of the habitat panthers occupy is privately owned, and most of it is highly fragmented. Panthers frequently cross roads, and most panther deaths in Florida are caused by accidents with motor vehicles. Protected landscape corridors might enable panthers to move more safely between conservation areas. A preliminary study found that panthers already use such corridors, typically along wooded riverbanks, when they are available. The Florida Fish and Wildlife Service proposed the creation of an ambitious 6,100-hectare network of such corridors alongside the Caloosahatchee River to link several significant habitat fragments in neighboring counties.

   Beta-Diversity

   Conservation biologists now often focus their efforts preserving assemblages of organisms rather than individual species. As you know from Section 51-2, communities grade into one another as species composition changes across environmental gradients. That discussion focused on diversity within well-defined communities, a characteristic that ecologists identify as alpha diversity. But conservation biologists are increasingly interested in diversity across communities, which they call beta diversity. Beta diversity reflects the increasing numbers of species present in an area that includes a wide variety of habitats, vegetation types, and small-scale environments. The concept of beta diversity is reflected in the slope of the species/area relationship: as the size of an area increases, so does the number of distinct environmental features it includes; and that environmental diversity supports a larger number of species.

   By basing the design of nature preserves on the conservation of beta diversity, conservations biologists can establish reserves that will protect more species. An ideal reserve system might include several large areas, each including a diversity of small-scale environments suitable for species that do not disperse readily, and many small reserves interconnected by landscape corridors. A reserve system distributed over an important environmental gradient would include species that replace each other across it. And as the global climate continues to warm over the coming decades, reserve systems that include protected areas at different altitudes or latitudes might allow some species to migrate into cooler environments over time.

   Study Break 53–5

   1. How does a population bottleneck increase the likelihood that a species will become extinct?
   2. How does a population viability analysis assist in the development of a conservation plan for a species?
   3. Would a single large nature preserve or several small pres
   erves experience greater edge effects?

3. Principles of Community Ecology and Landscape Ecology Guide Large-Scale Preservation Projects

   Many conservation efforts focus on the preservation of entire communities or ecosystems. These projects often depend on the work of community and landscape ecologists.

   Species/Area Relationships

   As you know from Chapter 51, community composition is dynamic: some species become extinct and others join the community through immigration. If we view patches of intact habitat as islands in a sea of unsuitable terrain, we can apply the predictions of the theory of island biogeography (see Section 51-7) to the design of protected areas. For example, we might expect that the number of species a patch will support depends on its size and proximity to larger patches.

   Indeed, ecologists recognized long ago that large habitat patches sustain more species than small patches do (Figure 53.16). When plotted on an arithmetic scale, the relationship between species richness and habitat area increases sharply at first and then flattens. In other words, for relatively small habitat patches, even minor increases in area allow a large increase in the number of resident species; but as habitat patches get larger, the number of species present eventually levels off. You encountered an example of this relationship in our discussion of bird species richness on islands of different sizes (see Figure 51.31B).

   As habitats become increasingly fragmented, edge effects exaggerate the species/area relationship in mainland habitat patches (Figure 53.17). Consider two hypothetical patches of habitat: one is 100 m on a side, with a total area of 10,000 m²; the other is 200 m on a side, with a total area of 40,000 m². Now, imagine that edge-effect disturbances penetrate 20 m into each patch from all directions. The small patch contains only 3,600 m² of intact habitat, but the large patch contains 25,600 m² of intact habitat. Although the large patch is only four times larger than the small patch, the large patch contains more than seven times as much intact habitat.

   Landscape Ecology

   Conservation biologists often use landscape ecology to design the size and geometry of nature reserves and other protected areas. Landscape ecology analyzes how large-scale ecological factors—such as the distribution of vegetation, topography, and human activity—influence local populations and communities.

   When conservation biologists first applied concepts from landscape ecology to the design of protected areas, they debated whether nature preserves should comprise one large habitat patch or several smaller patches. Based on the species/area relationship, large patches should harbor more species than small patches; large patches would also experience proportionately smaller edge effects; and large patches would better support populations of large animals that need substantial resources. Nevertheless, some conservation biologists argued that clusters of physically separate preserves are more effective in maintaining meta-populations of endangered species (see Section 50-5), especially if the patches are interconnected by corridors of intact habitat. Individuals could move between preserves, reviving any local populations that experience a decline.

   Some conservation biologists have worried that narrow landscape corridors, which by definition have large edges, might allow exotic species to invade protected areas. Ellen I. Damschen of North Carolina State University and several colleagues conducted an ambitious long-term field experiment to test the effects of landscape

   corridors on plant species richness (Figure 53.18). Their results, published in 2006, suggest that habitat patches connected by corridors retain more native plant species than isolated patches do, and that corridors did not promote the entry of exotic species. Thus, corridors appear to be a useful feature in the design of nature preserves.

   Landscape corridors are a key feature of efforts to prevent the extinction of the Florida panther (Puma concolor coryi, shown in the chapter introduction). This subspecies is critically endangered: only 70 to 100 individuals remain from a population that once ranged throughout the southeastern United States. Other panther subspecies still inhabit the western states. Panthers are large predators, and each female requires nearly 20,000 hectares (more than 75 square miles) for hunting and breeding; males each require more than twice as much space.

   Although the state and federal governments have set aside several panther conservation areas in Florida, 52% of the habitat panthers occupy is privately owned, and most of it is highly fragmented. Panthers frequently cross roads, and most panther deaths in Florida are caused by accidents with motor vehicles. Protected landscape corridors might enable panthers to move more safely between conservation areas. A preliminary study found that panthers already use such corridors, typically along wooded riverbanks, when they are available. The Florida Fish and Wildlife Service proposed the creation of an ambitious 6,100-hectare network of such corridors alongside the Caloosahatchee River to link several significant habitat fragments in neighboring counties.

   Beta-Diversity

   Conservation biologists now often focus their efforts preserving assemblages of organisms rather than individual species. As you know from Section 51-2, communities grade into one another as species composition changes across environmental gradients. That discussion focused on diversity within well-defined communities, a characteristic that ecologists identify as alpha diversity. But conservation biologists are increasingly interested in diversity across communities, which they call beta diversity. Beta diversity reflects the increasing numbers of species present in an area that includes a wide variety of habitats, vegetation types, and small-scale environments. The concept of beta diversity is reflected in the slope of the species/area relationship: as the size of an area increases, so does the number of distinct environmental features it includes; and that environmental diversity supports a larger number of species.

   By basing the design of nature preserves on the conservation of beta diversity, conservations biologists can establish reserves that will protect more species. An ideal reserve system might include several large areas, each including a diversity of small-scale environments suitable for species that do not disperse readily, and many small reserves interconnected by landscape corridors. A reserve system distributed over an important environmental gradient would include species that replace each other across it. And as the global climate continues to warm over the coming decades, reserve systems that include protected areas at different altitudes or latitudes might allow some species to migrate into cooler environments over time.

   Study Break 53–5

   1. How does a population bottleneck increase the likelihood that a species will become extinct?
   2. How does a population viability analysis assist in the development of a conservation plan for a species?
   3. Would a single large nature preserve or several small pres
   erves experience greater edge effects?

3. Principles of Community Ecology and Landscape Ecology Guide Large-Scale Preservation Projects

   Many conservation efforts focus on the preservation of entire communities or ecosystems. These projects often depend on the work of community and landscape ecologists.

   Species/Area Relationships

   As you know from Chapter 51, community composition is dynamic: some species become extinct and others join the community through immigration. If we view patches of intact habitat as islands in a sea of unsuitable terrain, we can apply the predictions of the theory of island biogeography (see Section 51-7) to the design of protected areas. For example, we might expect that the number of species a patch will support depends on its size and proximity to larger patches.

   Indeed, ecologists recognized long ago that large habitat patches sustain more species than small patches do (Figure 53.16). When plotted on an arithmetic scale, the relationship between species richness and habitat area increases sharply at first and then flattens. In other words, for relatively small habitat patches, even minor increases in area allow a large increase in the number of resident species; but as habitat patches get larger, the number of species present eventually levels off. You encountered an example of this relationship in our discussion of bird species richness on islands of different sizes (see Figure 51.31B).

   As habitats become increasingly fragmented, edge effects exaggerate the species/area relationship in mainland habitat patches (Figure 53.17). Consider two hypothetical patches of habitat: one is 100 m on a side, with a total area of 10,000 m²; the other is 200 m on a side, with a total area of 40,000 m². Now, imagine that edge-effect disturbances penetrate 20 m into each patch from all directions. The small patch contains only 3,600 m² of intact habitat, but the large patch contains 25,600 m² of intact habitat. Although the large patch is only four times larger than the small patch, the large patch contains more than seven times as much intact habitat.

   Landscape Ecology

   Conservation biologists often use landscape ecology to design the size and geometry of nature reserves and other protected areas. Landscape ecology analyzes how large-scale ecological factors—such as the distribution of vegetation, topography, and human activity—influence local populations and communities.

   When conservation biologists first applied concepts from landscape ecology to the design of protected areas, they debated whether nature preserves should comprise one large habitat patch or several smaller patches. Based on the species/area relationship, large patches should harbor more species than small patches; large patches would also experience proportionately smaller edge effects; and large patches would better support populations of large animals that need substantial resources. Nevertheless, some conservation biologists argued that clusters of physically separate preserves are more effective in maintaining meta-populations of endangered species (see Section 50-5), especially if the patches are interconnected by corridors of intact habitat. Individuals could move between preserves, reviving any local populations that experience a decline.

   Some conservation biologists have worried that narrow landscape corridors, which by definition have large edges, might allow exotic species to invade protected areas. Ellen I. Damschen of North Carolina State University and several colleagues conducted an ambitious long-term field experiment to test the effects of landscape

   corridors on plant species richness (Figure 53.18). Their results, published in 2006, suggest that habitat patches connected by corridors retain more native plant species than isolated patches do, and that corridors did not promote the entry of exotic species. Thus, corridors appear to be a useful feature in the design of nature preserves.

   Landscape corridors are a key feature of efforts to prevent the extinction of the Florida panther (Puma concolor coryi, shown in the chapter introduction). This subspecies is critically endangered: only 70 to 100 individuals remain from a population that once ranged throughout the southeastern United States. Other panther subspecies still inhabit the western states. Panthers are large predators, and each female requires nearly 20,000 hectares (more than 75 square miles) for hunting and breeding; males each require more than twice as much space.

   Although the state and federal governments have set aside several panther conservation areas in Florida, 52% of the habitat panthers occupy is privately owned, and most of it is highly fragmented. Panthers frequently cross roads, and most panther deaths in Florida are caused by accidents with motor vehicles. Protected landscape corridors might enable panthers to move more safely between conservation areas. A preliminary study found that panthers already use such corridors, typically along wooded riverbanks, when they are available. The Florida Fish and Wildlife Service proposed the creation of an ambitious 6,100-hectare network of such corridors alongside the Caloosahatchee River to link several significant habitat fragments in neighboring counties.

   Beta-Diversity

   Conservation biologists now often focus their efforts preserving assemblages of organisms rather than individual species. As you know from Section 51-2, communities grade into one another as species composition changes across environmental gradients. That discussion focused on diversity within well-defined communities, a characteristic that ecologists identify as alpha diversity. But conservation biologists are increasingly interested in diversity across communities, which they call beta diversity. Beta diversity reflects the increasing numbers of species present in an area that includes a wide variety of habitats, vegetation types, and small-scale environments. The concept of beta diversity is reflected in the slope of the species/area relationship: as the size of an area increases, so does the number of distinct environmental features it includes; and that environmental diversity supports a larger number of species.

   By basing the design of nature preserves on the conservation of beta diversity, conservations biologists can establish reserves that will protect more species. An ideal reserve system might include several large areas, each including a diversity of small-scale environments suitable for species that do not disperse readily, and many small reserves interconnected by landscape corridors. A reserve system distributed over an important environmental gradient would include species that replace each other across it. And as the global climate continues to warm over the coming decades, reserve systems that include protected areas at different altitudes or latitudes might allow some species to migrate into cooler environments over time.

   Study Break 53–5

   1. How does a population bottleneck increase the likelihood that a species will become extinct?
   2. How does a population viability analysis assist in the development of a conservation plan for a species?
   3. Would a single large nature preserve or several small pres
   erves experience greater edge effects?

Principles of Community Ecology and Landscape Ecology Guide Large-Scale Preservation Projects Many conservation efforts focus on the preservation of entire communities or ecosystems. These projects often depend on the work of community and landscape ecologists. Species/Area Relationships As you know from Chapter 51, community composition is dynamic: some species become extinct and others join the community through immigration. If we view patches of intact habitat as islands in a sea of unsuitable terrain, we can apply the predictions of the theory of island biogeography (see Section 51-7) to the design of protected areas. For example, we might expect that the number of species a patch will support depends on its size and proximity to larger patches. Indeed, ecologists recognized long ago that large habitat patches sustain more species than small patches do (Figure 53.16). When plotted on an arithmetic scale, the relationship between species richness and habitat area increases sharply at first and then flattens. In other words, for relatively small habitat patches, even minor increases in area allow a large increase in the number of resident species; but as habitat patches get larger, the number of species present eventually levels off. You encountered an example of this relationship in our discussion of bird species richness on islands of different sizes (see Figure 51.31B). As habitats become increasingly fragmented, edge effects exaggerate the species/area relationship in mainland habitat patches (Figure 53.17). Consider two hypothetical patches of habitat: one is 100 m on a side, with a total area of 10,000 m2; the other is 200 m on a side, with a total area of 40,000 m2. Now, imagine that edge-effect disturbances penetrate 20 m into each patch from all directions. The small patch contains only 3,600 m2 of intact habitat, but the large patch contains 25,600 m2 of intact habitat. Although the large patch is only four times larger than the small patch, the large patch contains more than seven times as much intact habitat. Landscape Ecology Conservation biologists often use landscape ecology to design the size and geometry of nature reserves and other protected areas. Landscape ecology analyzes how large-scale ecological factors—such as the distribution of vegetation, topography, and human activity—influence local populations and communities. When conservation biologists first applied concepts from landscape ecology to the design of protected areas, they debated whether nature preserves should comprise one large habitat patch or several smaller patches. Based on the species/area relationship, large patches should harbor more species than small patches; large patches would also experience proportionately smaller edge effects; and large patches would better support populations of large animals that need substantial resources. Nevertheless, some conservation biologists argued that clusters of physically separate preserves are more effective in maintaining meta-populations of endangered species (see Section 50-5), especially if the patches are interconnected by corridors of intact habitat. Individuals could move between preserves, reviving any local populations that experience a decline. Some conservation biologists have worried that narrow landscape corridors, which by definition have large edges, might allow exotic species to invade protected areas. Ellen I. Damschen of North Carolina State University and several colleagues conducted an ambitious long-term field experiment to test the effects of landscape corridors on plant species richness (Figure 53.18). Their results, published in 2006, suggest that habitat patches connected by corridors retain more native plant species than isolated patches do, and that corridors did not promote the entry of exotic species. Thus, corridors appear to be a useful feature in the design of nature preserves. Landscape corridors are a key feature of efforts to prevent the extinction of the Florida panther (Puma concolor coryi, shown in the chapter introduction). This subspecies is critically endangered: only 70 to 100 individuals remain from a population that once ranged throughout the southeastern United States. Other panther subspecies still inhabit the western states. Panthers are large predators, and each female requires nearly 20,000 hectares (more than 75 square miles) for hunting and breeding; males each require more than twice as much space. Although the state and federal governments have set aside several panther conservation areas in Florida, 52% of the habitat panthers occupy is privately owned, and most of it is highly fragmented. Panthers frequently cross roads, and most panther deaths in Florida are caused by accidents with motor vehicles. Protected landscape corridors might enable panthers to move more safely between conservation areas. A preliminary study found that panthers already use such corridors, typically along wooded riverbanks, when they are available. The Florida Fish and Wildlife Service proposed the creation of an ambitious 6,100-hectare network of such corridors alongside the Caloosahatchee River to link several significant habitat fragments in neighboring counties. Beta-Diversity Conservation biologists now often focus their efforts preserving assemblages of organisms rather than individual species. As you know from Section 51-2, communities grade into one another as species composition changes across environmental gradients. That discussion focused on diversity within well-defined communities, a characteristic that ecologists identify as alpha diversity. But conservation biologists are increasingly interested in diversity across communities, which they call beta diversity. Beta diversity reflects the increasing numbers of species present in an area that includes a wide variety of habitats, vegetation types, and small-scale environments. The concept of beta diversity is reflected in the slope of the species/area relationship: as the size of an area increases, so does the number of distinct environmental features it includes; and that environmental diversity supports a larger number of species. By basing the design of nature preserves on the conservation of beta diversity, conservations biologists can establish reserves that will protect more species. An ideal reserve system might include several large areas, each including a diversity of small-scale environments suitable for species that do not disperse readily, and many small reserves interconnected by landscape corridors. A reserve system distributed over an important environmental gradient would include species that replace each other across it. And as the global climate continues to warm over the coming decades, reserve systems that include protected areas at different altitudes or latitudes might allow some species to migrate into cooler environments over time. Study Break 53–5 How does a population bottleneck increase the likelihood that a species will become extinct? How does a population viability analysis assist in the development of a conservation plan for a species? Would a single large nature preserve or several small pres erves experience greater edge effects?