Chapter 52

Why It Matters. . .

Poor Lake Erie, the shallowest of the Great Lakes. Several major industrial cities, including Toledo, Cleveland, Erie, and Buffalo, sprawl along its shoreline. Most of its water comes from the Detroit River, which flows past Detroit; other rivers that flow into Lake Erie carry runoff from agricultural fields in Canada and the United States.

When Europeans first settled along its shores roughly 300 years ago, Lake Erie was a wetland paradise. Fishes and waterfowl reproduced in marshes and bays. Even after steel mills and oil refineries were built nearby in the 1860s and 1870s, the lake supported a busy fishing industry and was famous as a recreation area.

By 1970, wetlands had been filled for building, bays had been dredged for shipping lanes, and the shoreline had been converted to beaches. Worst of all, household sewage, industrial effluent, and agricultural runoff had so polluted the lake that it no longer supported the activities that had made it famous (Figure 52.1). The water was murky with algae and cyanobacteria; dead fishes washed up on the shore; local health departments closed beaches; and the fishing industry collapsed.

How can a vibrant natural resource become a foul-smelling dump? The answer lies in the human activities that disrupt an ecosystem, a biological community and the physical environment with which it interacts. Between the 1930s and the 1970s, Lake Erie's concentration of phosphorus, which had been a limiting nutrient, tripled, largely from household detergents and agricultural fertilizers. High phosphorus concentrations encouraged the growth of photosynthetic algae, changing the phytoplankton community. The density of coliform bacteria, which originate in the human gut and serve as indicators of organic pollution, also skyrocketed as a result of the surge in sewage and nutrients entering the lake.

Increased phytoplankton and bacterial populations depleted oxygen in the lake's waters, contributing to changes elsewhere in the lake. Mayflies (Hexagenia species), whose larvae live in well-oxygenated bottom sediments, had once been so abundant that their aerial breeding swarms were a public nuisance. But they became nearly extinct in the polluted lake, replaced by oligochaete worms, snails, and other invertebrates. Along with overfishing, changes in the bottom fauna shifted the composition of the fish community; the catch of desirable food fishes declined to almost zero by the mid-1960s.

In 1972, Canada and the United States began efforts to restore the lake. They spent billions of dollars to reduce the influx of phosphates and limited fishing of the most vulnerable native species. Nonnative salmon (Onchorhynchus species) and other predatory fishes were introduced in the hope that they could bring the lake back to its original condition. Even the accidental introduction of zebra mussels (Dreissina polymorpha), an aquatic pest, inadvertently helped the effort because they feed on phytoplankton.

But, although somewhat improved, Lake Erie will never return to its former glory. Some native species are now extinct there, and the introduced species that replaced them function differently within the ecosystem. The lake still suffers periods of uncontrolled algal growth, fish kills, and high levels of harmful bacteria.

This story of an ecological disaster and partial recovery introduces ecosystem ecology, the branch of ecology that analyzes the flow of energy and the cycling of materials between an ecosystem's living and nonliving components. These processes make the resident organisms highly dependent on each other and on their physical surroundings. Ultimately, the Lake Erie ecosystem unraveled because human activities disrupted the flow of energy and the cycling of materials on which the organisms depended.

Modeling Ecosystem Processes

All organisms require steady supplies of energy and nutrients for their maintenance, growth, and reproduction. Studies of ecosystems often focus on the inputs and outputs (that is, the gains and losses) of energy and nutrients to the ecosystem as a whole as well as the transfer of energy and nutrients within and between the ecosystem's biotic and abiotic components. Although the movements of energy and nutrients through an ecosystem are sometimes coupled, as when you eat a meal that contains both nutrients and calories, the inputs and outputs of energy and nutrients are fundamentally different (see Section 1-1). In virtually all ecosystems, sunlight constantly renews the supply of available energy, but, as dictated by the laws of thermodynamics (see Section 4-1), most of that energy is lost as heat in cellular respiration. By contrast, virtually all the nutrients that will ever be available for biological systems are already present on Earth, and they are constantly recycled between the abiotic and biotic components of ecosystems in what ecologists describe as biogeochemical cycles.

Researchers use several types of models to describe ecosystem processes. Food webs define the pathways through which energy and nutrients move within the biotic component of an ecosystem. Compartment models describe how nutrients move between living and nonliving nutrient reservoirs. Simulation models allow ecologists to predict how ecosystems will respond to perturbations of ecosystem processes.

Food Webs Illustrate the Transfer of Energy and Nutrients among Organisms

Food webs define the pathways by which energy and nutrients move through an ecosystem's biotic components (see Section 51-3). In most ecosystems, they move simultaneously through a grazing food web and a detrital food web (Figure 52.2). The grazing food web includes the producer, herbivore, and carnivore trophic levels. The detrital food web includes detritivores and decomposers. Because detritivores and decomposers subsist on the remains and waste products of organisms at every trophic level, the two food webs are closely interconnected. Detritivores also contribute to the grazing food web when carnivores eat them.

Compartment Models Track the Movement of Nutrients between Food Webs and Abiotic Reservoirs

Ecologists use a compartment model to describe nutrient cycling (Figure 52.3). Two criteria divide ecosystems into four compartments where nutrients accumulate. First, nutrient molecules and ions are described as either available or unavailable, depending on whether or not they can be assimilated by organisms. Second, nutrients are present either in organic material, the living or dead tissues of organisms, or in inorganic material, such as rocks and soil. For example, minerals in dead leaves on the forest floor are in the available organic compartment because they are in the remains of organisms that detritivores can eat. But calcium ions in limestone rocks are in the unavailable inorganic compartment because they exist in a nonbiological form that producers cannot assimilate.

Nutrients move rapidly within and between the available compartments. Living organisms are in the available organic compartment, and whenever heterotrophs consume food, they recycle nutrients within that reservoir (indicated by the oval arrow in the upper left of Figure 52.3). Producers acquire nutrients from the air, soil, and water of the available inorganic compartment. Consumers also acquire nutrients from the available inorganic compartment when they drink water or absorb mineral ions through the body surface. Several processes routinely transfer nutrients from organisms to the available inorganic compartment. As one example, respiration releases carbon dioxide, moving both carbon and oxygen from the available organic compartment to the available inorganic compartment.

By contrast, the movement of materials into and out of the unavailable compartments is generally slow. Sedimentation, a long-term geological process, converts ions and particles of the available inorganic compartment into rocks of the unavailable inorganic compartment. Materials are gradually returned to the available inorganic compartment when rocks are uplifted and eroded or weathered. Similarly, over millions of years, the remains of organisms in the available organic compartment were converted into coal, oil, and peat of the unavailable organic compartment.

Except for the input of solar energy, we have described energy flow and nutrient cycling as though ecosystems were closed systems. In fact, most ecosystems exchange energy and nutrients with neighboring ecosystems. For example, rainfall carries nutrients into a forest ecosystem, and runoff carries nutrients from a forest into a lake or river. Ecologists have mapped the biogeochemical cycles of important elements, often by using radioactively labeled molecules that they can follow in the environment. As you study the details of the four biogeochemical cycles described in Section 52-3, try to understand them in terms of the generalized compartment model of nutrient cycling.

Simulation Models Predict the Effects of Perturbations on Ecosystem Processes

The compartment model described above is a conceptual model of how ecosystems function. In other words, it ignores the nuts-and-bolts details of exactly how a specific ecosystem functions in favor of a generalized portrait of how all ecosystems function. Although it is a useful tool, a conceptual model doesn't really help us predict what would happen, say, if we harvested 10 million tons of introduced salmon from Lake Erie every year. We could simply harvest the fishes and see what happens. But ecologists prefer less intrusive approaches to study the potential effects of disturbances.

To understand how an ecosystem will respond to specific changes in physical factors, energy flow, or nutrient availability, ecologists turn to simulation modeling. Researchers gather detailed information about a specific ecosystem and then create a series of mathematical equations that define its most important relationships. For example, one set of equations might describe how nutrient availability limits photosynthesis by autotrophs. Another might relate population growth of zooplankton to the abundance of phytoplankton. Other equations would relate the population dynamics of primary carnivores to the availability of their food, and still others would describe how the densities of primary carnivores influence reproduction in populations at both lower and higher trophic levels. Thus, a complete simulation model is a set of interlocking equations that collectively predict how changes in one feature of an ecosystem might influence others.

Creating a simulation model is no easy task, because the relationships within every ecosystem are complex. First, you would identify the important species, estimate their population sizes, and measure the average energy and nutrient content of each. Next, you would describe the food webs in which they participate, measure the quantity of food each species consumes, and estimate the growth and reproduction of individuals in each population. For the sake of completeness, you would also determine the ecosystem's energy and nutrient gains and losses caused by erosion, weathering, precipitation, and runoff. You would repeat these measurements seasonally to identify annual variation in the factors. Finally, you might repeat the measurements over several years to determine the effects of year-to-year variation in climate and chance events.

After collecting these data, you would write equations that quantify the relationships in the ecosystem, including information about how temperature and other abiotic factors influence the ecology of each species. At last, you could begin to predict—possibly in great detail—the effects of harvesting 10 million or even 50 million tons of salmon annually from Lake Erie. Of course, you would have to refine the model whenever new data became available.

As we attempt to understand larger and more complex ecosystems—and as we create larger and more complex environmental problems—modeling becomes an increasingly important tool. If a model is based on well-defined ecological relationships and good empirical data, it can allow us to make accurate predictions about ecosystem changes without the need for costly and environmentally damaging experiments. But like all ideas in science, a model is only as good as its assumptions, and models must constantly be adjusted to incorporate new ideas and recently discovered facts.

Study Break 52–1

1. In the generalized compartment model of biogeochemical cycling, how are the compartments where nutrients accumulate classified?
2. What are the advantages and disadvantages of relying on conceptual models that describe ecosystem function?
3. What data must ecologists collect before constructing a simulation model of an ecosystem?

Energy Flow and Ecosystem Energetics

Ecosystems receive a steady input of energy from an external source, which in almost all cases is the sun. But as energy flows through an ecosystem, much of it is lost as heat without being used by organisms. In this section, we consider the details of energy flow and the efficiency of energy transfer from one trophic level to another.

Sunlight Provides the Energy Input for Practically All Ecosystems

Every minute of every day, Earth's atmosphere intercepts roughly 19 kcal of solar energy per square meter. (Recall from Chapter 2 that 1 kcal = 1,000 calories.) About half that energy is absorbed, scattered, or reflected by gases, dust, water vapor, and clouds without ever reaching the planet's surface (see Chapter 49). Most energy that reaches the surface falls on bodies of water or bare ground, where it is absorbed as heat or reflected back into the atmosphere; reflected energy warms the atmosphere, as we discuss later in this chapter. Only a small percentage contacts primary producers, and most of that energy evaporates water, driving transpiration in plants (see Section 32-3).

Ultimately, photosynthesis converts less than 1% of the solar energy that arrives at Earth's surface into chemical energy. But primary producers capture enough energy to create an average of several kilograms of dry plant material per square meter per year. On a global scale, they produce more than 150 billion metric tons of new biological material annually. Some of the solar energy that producers convert into chemical energy is transferred to consumers at higher trophic levels.

The rate at which producers convert solar energy into chemical energy is an ecosystem's gross primary productivity. But like all other organisms, producers use energy for their own maintenance. After deducting the energy devoted to these functions, which are collectively called cellular respiration (see Section 8-1), whatever chemical energy remains is the ecosystem's net primary productivity. In most ecosystems, net primary productivity is between 50% and 90% of gross primary productivity. In other words, producers use between 10% and 50% of the energy they capture for their own respiration.

Ecologists generally measure primary productivity in units of energy captured (kcal/m²/yr) or in units of biomass created (g/m²/yr). Biomass is the dry weight of biological material per unit area or volume of habitat. (We measure biomass as the dry weight of organisms because their water content, which fluctuates with water uptake or loss, has no energetic or nutritional value.) You should not confuse an ecosystem's productivity with its standing crop biomass, the total dry weight of plants present at a given time. Net primary productivity is the rate at which the standing crop produces new biomass.

The energy captured by plants is stored in biological molecules—mostly carbohydrates, lipids, and proteins. Ecologists can convert units of biomass into units of energy or vice versa as long as they know how much carbohydrate, protein, and lipid a sample of biological material contains (4.2 kcal/g of carbohydrate; nearly 4.1 kcal/g of protein; and 9.5 kcal/g of lipid). Thus, net primary productivity is a measure of the rate at which producers accumulate energy as well as the rate at which new biomass is added to an ecosystem. Because it is far easier to measure biomass than energy content, ecologists usually measure changes in biomass to estimate productivity. New biomass takes several forms: the growth of existing producers; the creation of new producers by reproduction; and the storage of energy as carbohydrates or lipids. Because herbivores eat all three forms of new biomass, net primary productivity also measures how much new energy is available for primary consumers.

Primary Productivity Varies Greatly on Global and Local Scales

The potential rate of photosynthesis in any ecosystem is proportional to the intensity and the duration of sunlight, which vary geographically and seasonally (see Chapter 49). Sunlight is most intense and day length least variable near the equator. By contrast, light intensity is weakest and day length most variable near the poles. Thus, producers at the equator can photosynthesize nearly 12 hours a day, every day of the year. Near the poles, photosynthesis is virtually impossible during the long, dark winter; in summer, however, plants can photosynthesize around the clock.

Sunlight is not the only factor that influences the rate of primary productivity, however; temperature as well as the availability of water and nutrients also have big effects. For example, many of the world's deserts receive plenty of sunshine but have low rates of productivity because water is in short supply and the soil is nutrient-poor. Thus, mean annual net primary productivity varies greatly on a global scale (Figure 52.4), reflecting variations in these environmental factors (see Chapter 49).

On a finer geographical scale, within a particular terrestrial ecosystem, mean annual net productivity often increases with the availability of water (Figure 52.5). In systems with sufficient water, a shortage of mineral nutrients may be limiting. All plants need specific ratios of macronutrients and micronutrients for maintenance and photosynthesis (see Section 33-1). But plants withdraw nutrients from soil, and if nutrient concentration drops below a critical level, photosynthesis may decrease or stop altogether. In every ecosystem, one nutrient inevitably runs out before the supplies of other nutrients are exhausted. The element in short supply is called a limiting nutrient because its absence limits productivity. Productivity in agricultural fields is subject to the same constraints as productivity in natural ecosystems. Farmers increase productivity by irrigating (adding water to) and fertilizing (adding nutrients to) their crops.

In freshwater and marine ecosystems, where water is always readily available, the depth of the water and the combined availability of sunlight and nutrients govern the rate of primary productivity. Productivity is high in near-shore ecosystems where sunlight penetrates shallow, nutrient-rich waters. Kelp beds and coral reefs, for example, which occur along temperate and tropical coastlines respectively, are among the most productive ecosystems on Earth (Table 52.1). By contrast, productivity is low in the open waters of a large lake or ocean: sunlight penetrates only the upper layers, and nutrients sink to the bottom. Thus, the two requirements for photosynthesis, sunlight and nutrients, are available in different places.

Although ecosystems vary in their net primary productivity, the differences are not always proportional to variations in their standing crop biomass (see Table 52.1). For example, biomass in temperate deciduous forests and temperate grasslands differs by a factor of 20, but the difference in their rates of net primary productivity is only twofold. Most biomass in trees is present in nonphotosynthetic tissues such as wood. As a result, their ratio of productivity to biomass is low (1,200 g/m² ÷ 30,000 g/m² = 0.040). By contrast, grasslands don't accumulate much biomass because annual mortality, herbivores, and fires remove plant material as it is produced; and their productivity to biomass ratio is much higher (600 g/m² ÷ 1,600 g/m² = 0.375).

Some ecosystems contribute more than others to overall net primary productivity (Figure 52.6). Ecosystems that cover large areas make substantial contributions, even if their productivity is low. Conversely, geographically restricted ecosystems make large contributions if their productivity is high. For example, the open ocean and tropical rain forests contribute about equally to total global productivity, but for different reasons. Open oceans have low productivity, but they cover nearly two-thirds of Earth's surface. Tropical rain forests cover only a small area, but they are highly productive.

Some Energy Is Always Lost Before It Is Transferred from One Trophic Level to the Next

Net primary productivity ultimately supports all the consumers in grazing and detrital food webs. Consumers in the grazing food web eat some of the biomass at every trophic level except the highest; uneaten biomass eventually dies and passes into detrital food webs. However, consumers assimilate only a portion of the material they ingest, and unassimilated material is passed as feces, which also supports detritivores and decomposers.

As energy is transferred from producers to consumers, some is stored in new consumer biomass, called secondary productivity. Nevertheless, two factors cause energy to be lost from the ecosystem every time it flows from one trophic level to another. First, animals use much of the energy they assimilate for maintenance or locomotion rather than the production of new biomass. Second, as dictated by the second law of thermodynamics, no biochemical reaction is 100% efficient; thus, some of the chemical energy liberated by cellular respiration is always converted to heat, which most organisms do not use.

Ecological efficiency is the ratio of net productivity at one trophic level to net productivity at the trophic level below it. For example, if the plants in an ecosystem have a net primary productivity of 100 g/m²/yr of new tissue and the herbivores that eat those plants produce 10 g/m²/yr, the ecological efficiency of the herbivores is 10%. The efficiencies of three processes—harvesting food, assimilating ingested energy, and producing new biomass—determine the ecological efficiencies of consumers.

Harvesting efficiency is the ratio of the energy content of food consumed to the energy content of food available. Predators harvest food efficiently when prey are abundant and easy to capture (see Section 51-1).

Assimilation efficiency is the ratio of the energy absorbed from consumed food to the food's total energy content. Because animal prey is relatively easy to digest, carnivores absorb between 60% and 90% of the energy in their food; assimilation efficiency is lower for prey with indigestible parts like bones or exoskeletons. Herbivores assimilate only 15% to 80% of the energy they consume because cellulose is not very digestible.

Production efficiency is the ratio of the energy content of new tissue produced to the energy assimilated from food. Production efficiency varies with maintenance costs. For example, endothermic animals often use less than 10% of their assimilated energy for growth and reproduction, because they use energy to generate body heat (see Section 46-8). Ectothermic animals, by contrast, channel more than 50% of their assimilated energy into new biomass.

The overall ecological efficiency of most organisms is between 5% and 20%. As a rule of thumb, only about 10% of the energy accumulated at one trophic level is converted into biomass at the next higher trophic level, as illustrated by energy transfers at Silver Springs, Florida (Figure 52.7). Producers in the Silver Springs ecosystem convert 1.2% of the solar energy they intercept into chemical energy (represented by 20,810 kcal of gross primary productivity). However, they use about two-thirds of this energy for respiration, leaving only one-third to be included in new plant biomass, the net primary productivity. All consumers in the grazing food web (on the right in Figure 52.7) ultimately depend on this energy source, which dwindles with each transfer between trophic levels. Energy is lost to respiration and export (that is, the transport of energy-containing materials out of the ecosystem by flowing water) at each trophic level. In addition, substantial energy flows into the detrital food web (on the left in Figure 52.7) as organic wastes and uneaten biomass. To determine the ecological efficiency of any trophic level, we divide its productivity by the productivity of the level below it. For example, the ecological efficiency of midlevel carnivores at Silver Springs is 111 kcal/yr ÷ 1,103 kcal/yr = 10.06%.

The low ecological efficiencies that characterize most energy transfers illustrate one advantage of eating "lower on the food chain." Even though humans digest and assimilate meat more efficiently than vegetables, we might be able to feed more people if we all ate more vegetables directly instead of first passing these crops through another trophic level, such as cattle or chickens, to produce meat. The production of animal protein is costly because much of the energy fed to livestock is used for their own maintenance rather than the production of new biomass. But despite the economic—not to mention health-related—logic of a more vegetarian diet, a change in our eating habits alone won't eliminate food shortages or the frequency of malnutrition. Many regions of Africa, Australia, North America, and South America support vegetation that is suitable only for grazing by large herbivores. These areas could not produce significant quantities of edible grains and vegetables.

Ecological Pyramids Illustrate the Effects of Energy Losses

All organisms in a trophic level are the same number of energy transfers from the ecosystem's ultimate energy source. Plants are one energy transfer removed from sunlight; herbivores are two transfers away; carnivores feeding on herbivores are three transfers away; and carnivores feeding on other carnivores are four transfers away. As energy works its way up a food web, energy losses are multiplied in successive energy transfers, greatly reducing the energy available to support the highest trophic levels.

Consider a hypothetical example in which ecological efficiency is 10% for all consumers. Assume that the plants in a small field annually produce new tissues containing 100 kcal of energy. Because only 10% of that energy is transferred to new herbivore biomass, the 100 kcal in plants produces only 10 kcal of new herbivorous insects; only 1 kcal of new songbirds, which feed on insects; and only 0.1 kcal of new falcons, which feed on songbirds. Thus, after three energy transfers, only 0.1% of the energy from primary productivity remains at the highest trophic levels.

The inefficiency of energy transfer from one trophic level to the next has profound effects on ecosystem structure. Ecologists illustrate these effects in diagrams called ecological pyramids. Trophic levels are drawn as stacked blocks, with the size of each block proportional to the energy, biomass, or numbers of organisms present; primary producers are illustrated at the bottom of the pyramid and higher-level consumers at the top.

Pyramids of energy typically have wide bases and narrow tops because each trophic level contains on average only about 10% as much energy as the trophic level below it, as illustrated by the pyramid of energy for Silver Springs, Florida (Figure 52.8).

The progressive reduction in productivity at higher trophic levels, as illustrated in Figure 52.7, usually establishes a pyramid of biomass (Figure 52.9). The biomass at each trophic level is proportional to the chemical energy temporarily stored there. Thus, in terrestrial ecosystems, the total mass of producers is generally greater than the total mass of herbivores, which is, in turn, greater than the total mass of predators (see Figure 52.9A). Populations of top predators—animals like mountain lions or alligators—contain too little biomass and energy to support another trophic level; thus, they have no nonhuman predators.

Freshwater and marine ecosystems sometimes exhibit inverted pyramids of biomass (see Figure 52.9B). In the open waters of a lake or ocean, primary consumers (zooplankton) eat the primary producers (phytoplankton) almost as soon as they appear. As a result, the standing crop of primary consumers at any moment in time is actually larger than the standing crop of primary producers. Food webs in these ecosystems are stable, however, because the producers have exceptionally high turnover rates. In other words, the phytoplankton divide and their populations grow so quickly that feeding by zooplankton doesn't endanger their populations or reduce their productivity. And on an annual basis, the cumulative total biomass of primary producers far outweighs that of primary consumers.

The reduction of energy and biomass also affects the population sizes of organisms at the top of a food web. Top predators are often relatively large animals, thus concentrating the limited biomass at the highest trophic levels in relatively few individuals (Figure 52.10). The extremely narrow top of this pyramid of numbers has grave implications for conservation biology. Because each top predator must patrol a large area to find sufficient food, the members of a population are often widely dispersed within their habitats. As a result, they are highly sensitive to hunting, habitat destruction, and random events, which can lead to local extinction (see Chapter 53). Top predators may also suffer from the accumulation of poisonous materials that move through food webs (see Focus on Applied Research). Even predators that feed below the top trophic level often suffer the ill effects of human activities.

Consumers Sometimes Regulate Ecosystem Processes in a Trophic Cascade

As you know from the preceding discussion, numerous abiotic factors—the intensity and duration of sunlight, rainfall, temperature, and the availability of nutrients—have significant effects on primary productivity. Primary productivity, in turn, has profound effects on populations of herbivores and the predators that feed on them. But what effect does feeding by these consumers have on primary productivity?

Recent research suggests that consumers may sometimes influence rates of primary productivity, especially in ecosystems with low species diversity and relatively few trophic levels. For example, food webs in North American salt marshes depend primarily on the productivity of salt marsh cordgrass (Spartina alterniflora), a foundation species that defines the nature of that coastal ecosystem (see Section 51-3). Cordgrass is consumed by herbivores, including the marsh periwinkle snail (Littoraria irrorata). In the southeastern United States, these herbivores are in turn consumed by blue crabs (Callinectes sapidus), mud crabs (Eurytium limosum), and terrapins (Malaclemys terrapin).

For many years, ecologists believed that the productivity of cordgrass was largely determined by the availability of nutrients in the salt marsh. However, research by Brian R. Silliman and Mark D. Bertness of Brown University showed that the productivity of cordgrass is actually regulated by what ecologists call a trophic cascade—predator–prey effects that reverberate through the population interactions at two or more trophic levels (Figure 52.11). Populations of the herbivorous periwinkle snails are controlled by the crabs and turtles that eat them. In the presence of these predators, snail populations are reduced in size, and the cordgrass grows luxuriantly. But when these predators are removed from the system, the snail populations grow rapidly, and feeding by the snails virtually eliminates the cordgrass, converting a highly productive ecosystem into a barren mudflat. Thus, cord grass productivity is indirectly controlled by the abundance of predators that eat the snails that eat the cordgrass.

Study Break 52–2

1. What is the difference between gross primary productivity and net primary productivity?
2. What environmental factors influence rates of primary productivity in terrestrial and aquatic ecosystems?
3. Why is energy lost from an ecosystem at every transfer from one trophic level to the trophic level above it?
4. How can the presence of predators influence an ecosystem's productivity?

   Nutrient Cycling in Ecosystems

   The availability of nutrients is as important to ecosystem function as the input of energy. Photosynthesis—the conversion of solar energy into chemical energy—requires carbon, hydrogen, and oxygen, which producers acquire from water and air. Producers also need nitrogen, phosphorus, and other minerals (see Table 33.1). A deficiency in any of these minerals can reduce primary productivity.

   Earth is essentially a closed system with respect to matter. Thus, unlike energy, for which there is a constant cosmic input, Earth 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. Unlike energy, which flows through ecosystems and is gradually lost as heat, matter is conserved in biogeochemical cycles. Although there may be local shortages of specific nutrients, Earth's overall supplies of these chemical elements are never depleted.

   Nutrients take various forms as they pass through biogeochemical cycles. Some materials, such as carbon, nitrogen, and oxygen, form gases, which move through global atmospheric cycles. Geological processes move other materials, such as phosphorus, through local sedimentary cycles, carrying them between dry land and the seafloor. Rocks, soil, water, and air are the reservoirs where mineral nutrients accumulate, sometimes for many years.
   

   The Hydrologic Cycle Recirculates All the Water on Earth

   Although it is not a mineral nutrient, water is the universal intracellular solvent for biochemical reactions. Nevertheless, only a fraction of 1% of Earth's total water is present in biological systems at any time.

   The cycling of water, called the hydrologic cycle, is global, with water molecules moving from the ocean into the atmosphere, to the land, through freshwater ecosystems, and back to the ocean (Figure 52.12). Solar energy causes water to evaporate from oceans, lakes, rivers, soil, and living organisms, entering the atmosphere as a vapor and remaining aloft as a gas, as droplets in clouds, or as ice crystals. It falls as precipitation, mostly in the form of rain and snow. When precipitation falls on land, water flows across the surface or percolates to great depth in the soil, eventually reentering the ocean reservoir through the flow of streams and rivers.
   

   The hydrologic cycle maintains its global balance because the total amount of water that enters the atmosphere is equal to the amount that falls as precipitation. Most water enters the atmosphere through evaporation from the ocean, which represents the largest reservoir on the planet. A much smaller fraction evaporates from terrestrial ecosystems, and most of that results from transpiration in green plants.

   The constant recirculation provides fresh water to terrestrial organisms and maintains freshwater ecosystems such as lakes and rivers. Water also serves as a transport medium that moves nutrients within and between ecosystems, as demonstrated in a series of classic experiments in the Hubbard Brook Experimental Forest, described in Focus on Basic Research.
   

   The Carbon Cycle Includes a Large Atmospheric Reservoir

   Carbon atoms provide the backbone of most biological molecules, and carbon compounds store the energy captured by photosynthesis (see Section 9-1). Carbon enters food webs when producers convert atmospheric carbon dioxide (CO₂) into carbohydrates. Heterotrophs acquire carbon by eating other organisms or detritus. Although carbon moves somewhat independently in the sea and on land, a common atmospheric pool of CO₂ creates a global carbon cycle (Figure 52.13).
   

   The largest reservoir of carbon is sedimentary rock, such as limestone or marble. Rocks are in the unavailable inorganic compartment, and they exchange carbon with living organisms at an exceedingly slow pace. Most available carbon is present as dissolved bicarbonate ions (HCO₃^–) in the ocean. Soil, the atmosphere, and plant biomass form other significant, but much smaller, reservoirs of available carbon. Atmospheric carbon is mostly in the form of molecular CO₂, a product of aerobic respiration. Volcanic eruptions also release CO₂ into the atmosphere.

   Sometimes carbon atoms leave the organic compartments for long periods of time. Some organisms in marine food webs build shells and other hard parts by incorporating dissolved carbon into calcium carbonate (CaCO₃) and other insoluble salts. When shelled organisms die, they sink to the bottom and are buried in sediments. The insoluble carbon that accumulates as rock in deep sediments may remain buried for millions of years before tectonic uplifting brings it to the surface, where erosion and weathering dissolve sedimentary rocks and return carbon to an available form.

   Carbon atoms were also transferred to the unavailable organic compartment when soft-bodied organisms were buried in habitats where low oxygen concentration prevented decomposition. Under suitable geological conditions, these carbon-rich tissues were slowly converted to gas, petroleum, or coal, which humans now use as fossil fuels.

   The Nitrogen Cycle Depends on the Activity of Diverse Microorganisms

   All organisms require nitrogen to construct nucleic acids, proteins, and other biological molecules. Earth's atmosphere had a high nitrogen concentration long before life originated. Today, a global nitrogen cycle moves this element between the huge atmospheric pool of gaseous molecular nitrogen (N₂) and several much smaller pools of nitrogen-containing compounds in soils, marine and freshwater ecosystems, and living organisms (Figure 52.14). Molecular nitrogen is abundant in the atmosphere, but triple covalent bonds bind its two atoms so tightly that most organisms cannot use it. However, three biochemical processes—nitrogen fixation, ammonification, and nitrification (Table 52.2)—convert nitrogen into nitrogen compounds that primary producers can incorporate into biological molecules such as proteins and nucleic acids. Secondary consumers obtain their nitrogen by consuming primary producers, thereby initiating the movement of nitrogen through the food webs of an ecosystem.
   

   In nitrogen fixation (see Section 33-3), molecular nitrogen (N₂) is converted into ammonia (NH₃) and ammonium ions (NH₄⁺). Certain bacteria, including Azotobacter and Rhizobium, which collect molecular nitrogen from the air between soil particles, are the major nitrogen fixers in terrestrial ecosystems. The cyanobacteria partners in some lichens (see Section 28-3) also fix molecular nitrogen. Other cyanobacteria, such as Anabaena and Nostoc, are important nitrogen fixers in aquatic ecosystems; the water fern (genus Azolla) plays that role in rice paddies. Collectively, these organisms fix an astounding 200 million metric tons of nitrogen each year; nitrogen fixation can also result from lightning and volcanic action. Plants and other primary producers assimilate and use this nitrogen in the biosynthesis of amino acids, proteins, and nucleic acids, which then circulate through food webs.

   Some plants, including legumes (such as beans and clover), alders (Alnus species), and some members of the rose family (Rosaceae), are mutualists with nitrogen-fixing bacteria. These plants acquire nitrogen from soils much more readily than plants that lack such mutualists. Although these plants have the competitive edge in nitrogen-poor soil, nonmutualistic species often displace them in nitrogen-rich soil.

   In addition to nitrogen fixation, several other biochemical processes make large quantities of nitrogen available to producers. Ammonification of detritus by bacteria and fungi converts organic nitrogen into ammonia (NH₃), which dissolves into ammonium ions (NH₄⁺) that plants can assimilate; some ammonia escapes into the atmosphere as a gas. Nitrification by certain bacteria produces nitrites (NO₂^–), which are converted by other bacteria to usable nitrates (NO₃^–). All of these compounds are water-soluble, and water rapidly leaches them from soil into streams, lakes, and oceans.

   Under conditions of low oxygen availability, denitrification by still other bacteria converts nitrites or nitrates into nitrous oxide (N₂O) and then into molecular nitrogen (N₂), which enters the atmosphere, completing the cycle. This action can deplete supplies of soil nitrogen in waterlogged or otherwise poorly aerated environments, such as bogs and swamps. In an interesting twist on the usual predator–prey relationships, several species of flowering plants that live in nitrogen-poor soils, such as Venus' fly trap (Dionaea muscipula), capture and digest small insects as their primary nitrogen source.
   

   The Phosphorus Cycle Includes a Large Sedimentary Reservoir

   Because phosphorus compounds lack a gaseous phase, this element moves between terrestrial and marine ecosystems in a sedimentary cycle (Figure 52.15). Earth's crust is the main reservoir of phosphorus, as it is for other minerals, such as calcium and potassium, that undergo sedimentary cycles.
   

   Phosphorus is present in terrestrial rocks in the form of phosphates (PO₄^3–). In the phosphorus cycle, weathering and erosion carry phosphate ions from rocks to soil and into streams and rivers, which eventually transport them to the ocean. Once there, some phosphorus enters marine food webs, but most of it precipitates out of solution and accumulates for millions of years as insoluble deposits, mainly on continental shelves. When parts of the seafloor are uplifted and exposed, weathering releases the phosphates.

   Plants absorb and assimilate dissolved phosphates directly, and phosphorus moves easily to higher trophic levels. All heterotrophs excrete some phosphorus as a waste product in urine and feces, which are decomposed, and producers readily absorb the phosphate ions that are released. Thus, phosphorus cycles rapidly within terrestrial communities.

   Supplies of available phosphate are generally limited, however, and plants acquire it so efficiently that they reduce soil phosphate concentration to extremely low levels. Thus, like nitrogen, phosphorus is a common ingredient in agricultural fertilizers, and excess phosphates are pollutants of freshwater ecosystems. For many years, phosphate for fertilizers was obtained from guano (the droppings of seabirds that consume phosphorus rich food), which was mined on small islands off the Pacific coast of South America. Most phosphate for fertilizer now comes from phosphate rock mined in Florida and other places with abundant marine deposits.

   Study Break 52–3

   1. How does the global hydrologic cycle maintain its balance?
   2. What processes move large quantities of carbon from an organic compartment to an inorganic compartment?
   3. What microorganisms drive the global nitrogen cycle, and how do they do it?
   4. What is Earth's main reservoir for phosphorus, and why is it recycled at such a slow rate from that reservoir?

      Human Disruption of Ecosystem Processes

      Human activities—industrial processes, agricultural practices, and development—often disrupt biogeochemical cycles by moving materials from one nutrient compartment to another at unnaturally rapid rates. In this section we consider the consequences of human disruption of the carbon and nitrogen cycles.
      

      Our Use of Combustible Energy Sources Has Disrupted the Carbon Cycle

      The combustion of fossil fuels (oil, coal, and peat) and wood is transferring carbon from the reservoirs of organic nutrients to the atmospheric reservoir of inorganic nutrients at an unprecedented rate. Virtually all scientists agree that the resulting change in the worldwide distribution of carbon is having severe consequences for Earth's climate, including a general warming that will cause a significant rise in sea level.

      Concentrations of certain gases in the lower atmosphere have a profound effect on global temperature, which in turn has enormous impact on global climate. Molecules of carbon dioxide (CO₂), water, ozone, methane, nitrous oxide, and other compounds collectively act like a pane of glass in a greenhouse (hence they are described as greenhouse gases). They allow the short wavelengths of visible light to reach Earth's surface; but they impede the escape of longer, infrared wavelengths back into space, trapping much of that energy as heat. In short, greenhouse gases foster the accumulation of heat in the lower atmosphere, a warming action known as the greenhouse effect, which prevents Earth from being a cold and lifeless planet.

      Since the late 1950s, scientists have measured atmospheric concentrations of CO₂ and other greenhouse gases at several remote sampling sites, which are free of local contamination and reflect the average concentrations of these gases in the atmosphere. Results indicate that concentrations of greenhouse gases have increased steadily for as long as they have been monitored (Figure 52.16). The graph of atmospheric CO₂ concentration through time has a regular zigzag pattern that follows the annual cycle of plant growth in the northern hemisphere. Photosynthesis withdraws so much CO₂ from the atmosphere during the northern hemisphere summer that its concentration falls. The concentration is higher during the northern hemisphere winter, when aerobic respiration continues, returning carbon to the atmosphere, and photosynthesis slows. The zigs and the zags in the data for CO₂ represent seasonal highs and lows, but the midpoint of the annual peaks and troughs has increased steadily. Many scientists interpret these data as evidence of a rapid buildup of atmospheric CO₂, which represents a shift in the distribution of carbon in the major reservoirs on Earth. Scientists estimate that the atmospheric CO₂ concentration has increased 35% in the last 150 years and 15% in the last 30 years.
      

      What has caused the increase in the atmospheric concentration of CO₂? Burning of fossil fuels and wood is the largest contributor, because CO₂ is a combustion product of this process. Today, humans burn more wood and fossil fuels than ever before. In a paper published in Nature Geoscience in December 2009, Corinne Le Quéré at the University of East Anglia, UK, and colleagues from other institutions estimated that CO₂ emissions from the combustion of fossil fuels increased 29% between 2000 and 2008. Although slightly more than half the CO₂ emissions are absorbed by forests and marine phytoplankton, vast tracts of tropical forests are being cleared and burned (see Section 53-2), reducing the biosphere's capacity to maintain the carbon cycle as it existed before recent human activities disrupted it.

      Why is an increase in the atmospheric CO₂ concentration so alarming? As Insights from the Molecular Revolution describes, scientists are just beginning to discover many previously unforeseen effects of rising CO₂ levels in the atmosphere and in the oceans. Moreover, simulation models of the global climate suggest that increasing the concentration of any greenhouse gas is likely to intensify the greenhouse effect, contributing to a well-documented trend of global warming. According to the Intergovernmental Panel on Climate Change, an agency of the United Nations, the average global surface temperature increased more than 0.7°C during the twentieth century, with most of the increase occurring after 1980. Scientists have already documented significant changes in the geographical distributions of many organisms in response to that modest level of global warming (see Section 49-2).
      

      Many different climate models now predict that the mean temperature of the lower atmosphere will rise 2.0°–4.5°C during the twenty-first century, enough to increase ocean surface temperatures. Water expands when heated and the global sea level could rise as much as 0.4 m just from this expansion. In addition, because atmospheric temperature is rising fastest near the poles, global warming has already started melting Arctic glaciers and the Antarctic ice sheet, which might raise sea level even more, inundating low coastal regions. Waterfronts in Vancouver, Los Angeles, San Diego, Galveston, New Orleans, Miami, New York, and Boston could be submerged. So might agricultural lands in India, China, and Bangladesh, where much of the world's rice is grown. Global warming could also disturb regional patterns of precipitation and temperature. Areas that now produce much of the world's grains, including parts of Canada and the United States, would become arid scrub or deserts, and the now-forested areas to their north would become dry grasslands.

      Most scientists believe that atmospheric levels of greenhouse gases will continue to increase at least until the middle of the twenty-first century and that global temperature will inevitably rise by several degrees. Although governments have repeatedly promised to stabilize and eventually reduce their CO₂ emissions, they have consistently failed to meet those objectives. Some countries fear that reforming our relationship to the carbon cycle will be too costly, but the economic and social costs of not doing so will be far higher. Stabilizing emissions at current levels will not reverse the damage already done, nor will it stop the trend toward global warming. Many scientists agree that in addition to reducing our emissions of greenhouse gases as soon as possible, we must increase reforestation efforts because large tracts of forest can withdraw significant amounts of CO₂ from the atmosphere. We might also step up agricultural research to develop heat-resistant and drought-resistant crop plants, which may provide crucial food reserves in regions that will be subject to the most drastic climate change.
      

      Human Activities Have Altered the Nitrogen Cycle

      Human activities are also disrupting the nitrogen cycle, primarily through the use of nitrogen-containing fertilizers, the farming and processing of livestock, and the combustion of fossil fuels.

      Of all nutrients required for primary production, nitrogen is often the least abundant. By definition, agriculture depletes soil nitrogen by removing nitrogen-containing plants from fields. Irrigation also fosters soil erosion and leaching, which remove even more. Traditionally, farmers rotated their crops, alternately cultivating nitrogen-fixing legumes and other types of plants in the same fields. In combination with other soil-conservation practices, crop rotation stabilized soils and kept them productive, sometimes for thousands of years.

      In traditional agriculture, nearly all the nitrogen in living systems was made available by nitrogen-fixing microorganisms. Today, however, industrialized agriculture relies on the application of synthetic nitrogen-containing fertilizers. The production of synthetic fertilizers is expensive, and it uses fossil fuels both as a raw material and as an energy source; thus, fertilizer becomes increasingly costly as supplies of fossil fuels dwindle. Furthermore, rain and runoff leach excess fertilizer from agricultural fields and carry it into aquatic ecosystems. Like the phosphorus in Lake Erie, nitrogen has become a major pollutant of freshwater ecosystems, artificially enriching the waters and allowing producers to expand their populations (see Section 49-5).

      Human activities also add an enormous volume of nitrogen compounds to the atmosphere. For example, large-scale livestock production leaves behind mountains of organic wastes, which, through the action of the anaerobic microorganisms that decompose it, releases nitrous oxide (NO₂), a greenhouse gas. Moreover, whenever we burn fossil fuels at high temperatures in combustion engines, molecular nitrogen (N₂), which is abundant in the atmosphere, combines with molecular oxygen (O₂) to form nitric oxide (NO). This molecule readily converts to nitrogen dioxide gas (NO₂) or nitric acid vapors (HNO₃), which are major components of smog and acid rain.

      According to the Millennium Ecosystem Assessment, a report published in March 2005 with the support of the United Nations, human activities more than doubled the amount of nitrogen released from land into other nitrogen reservoirs in the second half of the twentieth century. The report projects that the amount will double again over the next 50 years. These transfers of material contribute significantly to the pollution of aquatic ecosystems, acid rain, and global climate change.

      Study Break 52–4

      1. What is the greenhouse effect, and how does an increase in atmospheric CO₂ concentration affect it?
      2. What human activities release the most CO₂ into the atmosphere?
      3. What agricultural practices contribute to the disruption of the nitrogen cycle?

      Think Outside the Book

      Examine the ingredients listed on the labels of three cleaning products that you use in your household or residence hall. Search the Internet or sources in your library for information about how the disposal of these compounds on land or in sewage might contribute to the disruption of a biogeochemical cycle.