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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.
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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.
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.
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.
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
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
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
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.
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