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Chapter 18

Bio Chapter 18.doc

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James Thomson

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Chapter 18Community Structure 369376 A 377 B 378387 C 388389 AClements proposed the holistic concept of communities His work focused mainly on vegetation and his theory offered that communities are discrete entities which are very strongly interdependent His theory was held in very high regard for a whileGleason contested this with the individualistic concept defining a community as merely being a fortunate coming together of organisms which are able to coexist and tolerate similar environmental ranges Ultimately Gleasons theory was proven correct over many elements of ClementsCommunity the links between organisms as determined by feeding patterns nutrient cycling energy flow etc It is complex interactions both direct and indirect with define communities To visualize the geographic organization of a community one can use a gradient of environmental conditions and plot the abundances of species across it A set of species with overlapping distribution on the gradient is said to be a closed community Boundaries of these communities which are marked by rapid replacement of one set of species for another are called ecotones Alternately if there seems to be little coincidence between species on the gradient it is an open community Ecotones mark the edge of a species distribution Sharp boundaries are drawn with physical differences eg aquatic vs terrestrial north vs southfacing but often species distribution isnt determined solely by the range of conditions they can tolerate When one species can dominates strongly enough to determine to affect distribution of others they outcompete other species Connells barnacles or change the conditions of a region pine needles increase soil acidity broadleaved and needleleaved forests well defined grass reduces moisture content in surface layer soilgrass and scrublands are welldefined Fire can also play a role in defining ecotones particularly between prairie and forest as some plants can resist fire while others cannot Continuum concept ecotones are less likely to occur along gradients of environmental change eg forests may be welldefined between deciduous and coniferous trees by temperature range but the forests themselves are not homogenousfew species have broadly overlapping geographic ranges meaning it is an open community Species will continually replace each other along gradients of physical conditions Gradient analysis abundances of each species are plotted on a continuous gradient of one of more physical conditions In the work of
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