Chapter 53 Community Ecology Overview What Is a Community A community is an assemblage of species living close enough together for potential interaction Communities differ in their species richness the number of species they contain and the relative abundance of different species Concept 53 1 There are a number of possible interspecific interactions that link the species of a community Interspecific interactions relationships between species of a community The possible interactions that can link species include competition predation herbivory symbiosis parasitism mutualism and commensalism and disease Interspecific interactions can be symbolized by the positive or negative effects of the interaction on the individual populations 0 indicates that a population is not affected by the interaction The effect of an interaction between two species may change as circumstances change Competition Interspecific competition occurs when species compete for a specific limited resource When two species compete for a resource the result is detrimental to one or both species Strong competition can lead to the local elimination of one of the two competing species a process called competitive exclusion The competitive exclusion principle states that two species with similar needs for the same limiting resources cannot coexist in the same place Ecological Niche The ecological niche is the sum total of a species use of abiotic and biotic resources in the environment In the analogy stated by ecologist Eugene Odum an organism s habitat is its address and the niche is the organism s profession An organism s niche is its ecological role how it fits into an ecosystem Using the niche concept to restate the competitive exclusion principle Two species cannot coexist in a community if their niches are identical However ecologically similar species can coexist in a community if their niches differ in one or more significant ways A species fundamental niche is the niche potentially occupied by that species The fundamental niche may differ from the realized niche the niche a species actually occupies in a particular environment Resource Partitioning When competition between species having identical niches does not lead to the local extinction of either species it is generally because one species niche becomes modified Evolution by natural selection results in modification of the resources used by one of the species Resource partitioning is the differentiation of niches that enables two similar species to coexist in a community Resource partitioning in a community is indirect evidence of earlier interspecific competition resolved by the evolution of niche differentiation Character Displacement Character displacement is the tendency for characteristics to be more divergent in sympatric populations geographically separate of two species than in allopatric populations geographically overlapping of the same two species Predation is a interaction between species in which one species the predator kills and eats Predation the other the prey The term predation elicits images such as a lion attacking and eating an antelope This interaction also includes interactions such as seed predation in which seed eating weevils eat plant seeds Natural selection favors adaptations of predators and prey 1 Predators have many feeding adaptations including acute senses and weaponry such as claws fangs stingers or poison to help catch and subdue prey Predators that pursue prey are generally fast and agile those who lie in ambush are often camouflaged Prey animals have evolved adaptations that help them avoid being eaten Behavioral defenses include fleeing hiding and self defense Alarm calls may summon many individuals of the prey species to mob the predator Animals also display a variety of morphological and physiological defensive adaptations Camouflage or cryptic coloration makes prey difficult to spot against the background Some animals have mechanical or chemical defenses Chemical defenses include odors and toxins Animals with effecting chemical defenses often exhibit bright warning aposematic coloration Predators are cautious in approaching potential prey with bright coloration One prey species may gain protection by mimicking the appearance of another In Batesian mimicry a harmless palatable species mimics a harmful unpalatable model In M llerian mimicry two or more unpalatable species resemble each other Each species gains an additional advantage because predators are more likely to encounter an unpalatable prey and learn to avoid prey with that appearance Predators may also use mimicry Some snapping turtles have tongues resembling wiggling worms to lure small fish Herbivory is a interaction in which an herbivore eats parts of a plant or alga Herbivores include large mammals and small invertebrates Herbivores have specialized adaptations Many herbivorous insects have chemical sensors on their feet to recognize appropriate food Herbivory plants vegetation prevent herbivory Mammalian herbivores have specialized dentition and digestive systems to process Plants may produce chemical toxins which may act in combination with spines and thorns to Parasitism Parasitism is a symbiotic interaction in which a parasite derives its nourishment from a host which is harmed in the process Endoparasites live within the body of the host Ectoparasites live and feed on the external surface of the host Parasitoidism is a special type of parasitism in which an insect usually a wasp lays eggs on or in living hosts The larvae feed on the body of the host eventually killing it Many parasites have complex life cycles involving a number of hosts Some parasites change the behavior of their hosts in ways that increase the probability of the parasite being transferred from one host to another Parasites can have significant direct and indirect effects on the survival reproduction and density of their host populations Pathogens are disease causing agents that have deleterious effects on their hosts Pathogens are typically bacteria viruses or protists Fungi and prions can also be pathogenic Parasites are generally large multicellular organisms while most pathogens are microscopic Many pathogens are lethal Disease Mutualism Mutualism is an interspecific symbiosis in which two species benefit from their interaction 2 Examples of mutualism include nitrogen fixation by bacteria in the root nodules of legumes digestion of cellulose by microorganisms in the guts of