Lecture 14 - Community Ecology Community definitions and...

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1 Community Ecology Community definitions and concepts Types of interspecific interactions Trophic structure Top-down and Bottom-up control • Diversity • Succession Latitudinal gradients The equilibrium theory of island biogeography Community Ecology A biological community – Is an assemblage of populations of various species living close enough for potential interaction Contrasting views of community structure are the subject of continuing debate Two different views on community structure – Emerged among ecologists in the 1920s and 1930s Community concepts Integrated Hypotheses (Clements) A community is an assemblage of closely linked species, locked into association by mandatory biotic interactions Individualisitc hypothesis (Gleason) – Proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements The integrated hypothesis (Clements) Predicts that the presence or absence of particular species depends on the presence or absence of other species One should see sharp ecotones between distinct communities with little overlap in the species present in alternative communities Population densities of individual species Environmental gradient (such as temperature or moisture) (a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together. Figure 53.29a
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2 The individualistic hypothesis (Gleason) – Predicts that each species is distributed according to its tolerance ranges for abiotic factors Population densities of individual species Environmental gradient (such as temperature or moisture) (b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar abiotic needs. Figure 53.29b In most studies, community composition change continuously, with each species more or less independently distributed. This favors the individualistic view. Number of plants per hectare Wet Moisture gradient Dry (c) Trees in the Santa Catalina Mountains. The distribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities. 0 200 400 600 Figure 53.29c Populations in a community are potentially linked by interspecific interactions – A community’s interactions include competition, predation, herbivory, symbiosis, and disease Interspecific Interactions Table 53.1 Competition Interspecific competition – Occurs when species compete for a particular resource
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