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