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Chap 57.doc

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Jon Houseman

Communities are loose assemblages of species • Ecological 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 environment. The organisms in a community use diverse sources of energy • A 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 consume. • Heterotrophs consume the energy-rich organic molecules produced by primary producers. • Organisms that eat plants constitute a trophic level called primary consumers. • Organisms that eat herbivores are called secondary consumers. • Organisms that eat the dead bodies of organism or their waste products are called detritivores or decomposers. • Organisms that obtain their food from more than one trophic level are called omnivores. Many species are omnivores; trophic levels are often not clearly distinct. • Biomass: the weight of living matter. • Distributions 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 energy. • In most terrestrial ecosystems, photosynthetic plants dominate. • Relative to the biomass of plants, the biomass of herbivores is larger in grasslands than in forests. • In 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 biomass). • Detritivores 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 plants. What Processes Influence Community Structure • Interactions of organism with one another: o Predation or parasitism: interactions in which one participant is harmed, but the other benefits (+/- interactions). o Competition: interactions in which two organisms use the same resources and those resources are insufficient to supply their combined needs (-/- interactions). o Mutualism: interactions in which both participants benefit (+/+ interactions). o Commensalism: interactions in which one participant benefits but the other is unaffected (+/0 interactions). o Amensalism: interactions in which one participant is harmed but the
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