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Chapter 55.doc

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BIOL 1020U

Chapter 55 Community Ecology551 What Are Ecological CommunitiesThe species that live and interact in an area constitute an ecological communityCommunities are loose assemblages of speciesEcological communities are not assemblages of organism that move together as units when environmental conditions change Rather each species has unique interactions with its biotic and abiotic environmentThe organisms in a community use diverse sources of energyA trophic level consists of the organisms whose energy source has passed through the same number of steps to reach them Plants and other photosynthetic organisms constitute a trophic level called photosynthesizers or primary producers The produce energy rich organic molecules that nearly all other organisms consumeHeterotrophs consume the energyrich organic molecules produced by primary producersOrganisms that eat plants constitute a trophic level called primary consumersOrganisms that eat herbivores are called secondary consumersOrganisms that eat the dead bodies of organism or their waste products are called detritivores or decomposersOrganisms that obtain their food from more than one trophic level are called omnivores Many species are omnivores trophic levels are often not clearly distinctBiomass the weight of living matterDistributions of energy and biomass for a particular ecosystem usually have similar shapes Variations in their dimensions depend on the nature of the dominant organism at each trophic level and how they allocate their energyIn most terrestrial ecosystems photosynthetic plants dominateRelative to the biomass of plants the biomass of herbivores is larger in grasslands than in forestsIn most aquatic ecosystems the dominant photosynthesizers are bacteria and protists which have such high rates of cell division that a small biomass of photosynthesizers can feed a much larger biomass of herbivores which grow and reproduce more slowly inverted distribution of biomassDetritivores transform detritus into free mineral nutrients that can again be taken up by plants If there were no detritivores nutrients would eventually be tied up in dead bodies where they would be unavailable to plants552 What Processes Influence Community StructureInteractions of organism with one anotheroPredation or parasitism interactions in which one participant is harmed but the other benefitsinteractionsoCompetition interactions in which two organisms use the same resources and those resources are insufficient to supply their combined needsinteractionsoMutualism interactions in which both participants benefitinteractions
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