community ecology

CHAPTER 55- COMMUNITY ECOLOGY 55.1 WHAT ARE ECOLOGICAL COMMUNITIES • Species that live and interact in an area constitute an ecological community • Ecological communities are not assemblages of organisms that move together as units when environmental conditions change, rather each species has unique interactions with its biotic and abiotic environments • Organisms in a community can be divided into trophic levels based on their source of energy • Trophic level: consists of the organisms whose energy source has passed through the same # of steps to reach them • Photosynthesizers=primary producers • All nonphotosynthetic organisms (heterotrophs) consumer either directly or indirectly the energy-rich organic molecules produced by primary producers • Primary consumers: eat plants constitute a trophic level called herbivores • Secondary consumers: eat herbivores • Decomposers: detrivores eat dead bodies • Organisms that obtain food from more than one trophic level are omnivores • Sequence of interactions in which a plant is eaten by an herbivore, which is in turn eaten by a secondary consumer-food chain; food web • Most communities have 3-5 trophic levels; biomass: weight of living matter • in most terrestrial ecosystems photosynthetic plants dominate, both in terms of the energy they represent and the biomass they contain • mammals may consume 30-40 percent of the annual aboveground grassland plant biomass; insects may consume an additional 5=15 percent; soil organisms 6-40 percent of the belowground biomass in grasslands • in aquatic ecosystems, dominant photosynthesizers are bacteria and protists ; high rates of cell division • predators restrict habitat and geographic distribution of their prey • austrialian biogeographic region is home to a birds- megapodes (mound- builders) that don’t incubate eggs; lay eggs in a mound of decomposing vegetable material; warmed by decomposition; parent visits regularly to add or remove decaying material for temperature • prey species have evolved a rich variety of adaptations that make them difficult to capture, subdue and eat; toxic hairs, bristles, tough spines, noxious chemicals, camouflage and mimicry of inedible objects or of larger dangerous animals • Batesian mimicry: palatable species may mimic an unpalatable or noxious one-process; works because a predator that captures an individual of an unpalatable or noxious species learns to avoid other prey individuals of similar appearance • Mullerian mimicry: two or more unpalatable or noxious species may converge to resemble one another • for a microparasite to persist in a host population, atleast one new host must become infected with the microparasite before each infected host dies • 3 distinct clases: susceptible, infected and recovered • A microparasite can readily invade a host pop’n dominated by susceptible individuals but as infection spreads, fewer and fewer susceptible remain; a point is met where infected don’t transmit infection to atleast one other; infection dies out • Microparasite can be passed by direct body contact, breath, body fluids waste products • All species share at least part of their diet and use of other resources with others but resource sharing influences abundances and distributions of species only if individuals reduce the ability of others to access resources, either by interfering with their activites- interference competition-or by reducing the availability resources-exploitation competition • Intraspecific competition-among individuals of same species-may result in reduced growth and reproductive rates, may exclude some from habitats and cause death • Interspecific competition-among different species • Competitive exclusion: a superior competitor can prevent all members of another species from using a habitat • Photosynthetic plants typically compete for space, sunlight, water, minerals • Sessile animals may restrict their habitat distribution; Balabus balanoides(bb) and Chthamalus stellatus (ch); adult ch live higher in the intertidal zone than bb, little overlap between areas occupied by two species • Joseph Connell showed that vertical range of adults of each species is greater in absence of other species • Species can restrict range of another species by reducing populations of shared prey to such low levels that the other species cannot persist • Amensalisms are widespread and inevitable interactions • There is no benefit to the mammals in trampling the plants; destruction of the plants is unintentional but inevitable •