Biology II - Community Ecology

Community Concept

• A community is an assemblage of populations interacting with one another within the same environment.– Composition is a listing of various species

in the community.– Diversity includes both species richness

and species diversity.

Community Structure

• Competition for limited resources between two species has a negative effect on the abundance of both species.– Predation and parasitism are expected to

increase the abundance of the predator and parasite at the expense of the abundance of the prey and its host.

Habitat and Ecological Niche

• Habitat is the area an organism lives and reproduces in.

• Ecological niche is the role an organism plays in its community, including its habitat and its interactions with other organisms.– Fundamental niche - All conditions under

which the organism can survive.– Realized niche - Set of conditions under

which it exists in nature.

Feeding Niches for Wading Birds

Competition Between Populations

• Interspecific competition occurs when members of different species try to utilize a resource in limited supply.– Competitive Exclusion Principle - No two

species can occupy the same niche at the same time.

Resource Partitioning decreases competition.

Can lead to character displacement.

Competition Between Barnacle Species

Character Displacement in Galápagos Finches

Predator-Prey Interactions

• Predation occurs when one living organism, the predator, feeds on another, the prey.– Presence of predators can decrease prey

densities, and vice-versa.

Lynx-Snowshoe Hare Interactions

Prey Defenses

• Prey defenses are mechanisms that thwart the possibility of being eaten by a predator.– Spines– Tough Epidermis– Poisonous Chemicals– Camouflage– Bright Coloration– Flocking Behavior

Camouflage in the Anglerfish

Mimicry

• Mimicry occurs when one species resembles another that possesses an overt antipredator defense.– Batesian - Mimic lacks defense of the

organism it resembles.– Müllerian - Mimic shares same protective

defense.

Symbiotic Relationships

• Symbiosis refers to interactions in which there is a close relationship between members of two populations.– Parasitism

Parasite derives nourishment from a host, and may use host as habitat and mode of transmission.

Endoparasites Ectoparasites

Commensalism

• Commensalism is a symbiotic relationship in which one species is benefited and the other is neither benefited nor harmed.– Remoras and Sharks

Many examples may turn out to be mutualism or parasitism.

Amount of harm or benefit two species do to one another is partially determined by the investigator.

Mutualism

• Mutualism is a symbiotic relationship in which both members of the association benefit.– Often help organisms obtain food or avoid

predation.Bacteria in human intestinal tact.

– Need not be equally beneficial to both species.

Cleaning Symbiosis

Mutualism Between Bullhorn Acacia and Ants

Cleaning Symbiosis

Community Development

• Ecological Succession– A change involving a series of species

replacements in a community following a disturbance.

Primary Succession occurs in areas where there is no soil formation.

Secondary Succession begins in areas where soil is present.

Pioneer Species

Secondary Succession in a Forest

Succession Models

• Facilitation Model– Succession in a particular area will always

lead to the same type of community.Climax Community

Each stage facilitates invasion and replacement by organisms of the next stage.

Succession Models

• Inhibition Model– Colonists hold onto their space and inhibit

growth of other plants until the colonists are damaged or die.

• Tolerance Model– Different types of plants can colonize an

area at the same time.Chance determine which seeds arrive

first.

Community Diversity

• Community stability can be recognized in three ways.– Persistence through time.– Resistance to change.– Recovery once a disturbance has

occurred.

Intermediate Disturbance Hypothesis

• Moderate amounts of disturbances at moderate frequency are required for a high degree of community diversity.– If widespread disturbances occur

frequently, diversity will be limited.

Intermediate Disturbance Hypothesis

Predation, Competition, and Biodiversity

• In certain communities, predation by a particular species reduces competition and increases diversity.– Predators that regulate competition and

maintain community diversity are referred to as keystone predators.

• Introduction of exotic species into a new area may lead to unbridled competition and resultant reduction in biodiversity.

Review

• Diversity and Composition Models• Habitat and Ecological Niche• Competition Between Populations• Predator-Prey Interactions• Symbiotic Relationships• Community Development• Community Diversity