Chapter 53 Community EcologyOverview: 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 ofeither 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- Predation is a +/− interaction between species in which one species, the predator, kills and eats 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 Müllerian 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- 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 plants. Mammalian herbivores have specialized dentition and digestive systems to process vegetation.- Plants may produce chemical toxins, which may act in combination with spines and thorns to prevent herbivory.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 densityof their host populations.Disease- 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