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