pg - Community Ecology Community definitions and concepts Types of interspecific interactions Trophic structure Top-down and Bottom-up control Diversity
pg - Community Ecology Community definitions and concepts...
1Community 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 biogeographyCommunity Ecology•A biological community– Is an assemblage of populations of variousspecies living close enough for potentialinteraction•Contrasting views of communitystructure are the subject of continuingdebate•Two different views on communitystructure– Emerged among ecologists in the 1920sand 1930sCommunity concepts
2Integrated Hypotheses(Clements)•A community is an assemblage ofclosely linked species, locked intoassociation by mandatory bioticinteractionsIndividualisitc hypothesis(Gleason)– Proposes that communities are looselyorganized associations of independentlydistributed species with the same abioticrequirements•The integrated hypothesis (Clements)–Predicts that the presence or absence of particularspecies depends on the presence or absence ofother species–One should see sharp ecotones between distinctcommunities with little overlap in the speciespresent in alternative communitiesPopulationdensities ofindividualspeciesEnvironmental gradient(such as temperature or moisture) (a) Integrated hypothesis. Communities are discrete groupingsof particular species that are closely interdependent andnearly always occur together.Figure 53.29a
3•The individualistic hypothesis (Gleason)– Predicts that each species is distributedaccording to its tolerance ranges for abioticfactorsPopulationdensities ofindividualspeciesEnvironmental gradient(such as temperature or moisture) (b) Individualistic hypothesis. Species are independentlydistributed along gradients and a community is simply theassemblage of species that occupy the same area because ofsimilar abiotic needs.Figure 53.29b•In most studies, community compositionchange continuously, with each speciesmore or less independently distributed.This favors the individualistic view.Number ofplantsper hectareWetMoisture gradient Dry(c) Trees in the Santa Catalina Mountains. Thedistribution of tree species at oneelevation in the Santa Catalina Mountains of Arizona supports the individualistichypothesis. Each tree species has an independent distribution along the gradient,apparently conforming to its tolerance for moisture, and the species that livetogether at any point along the gradient have similar physical requirements.Because the vegetation changes continuously along the gradient, it is impossible todelimit sharp boundaries for the communities.0200400600Figure 53.29c•Populations in a community are potentiallylinked by interspecific interactions– A community’s interactions include competition,predation, herbivory, symbiosis, and diseaseInterspecific Interactions
4Table 53.1Competition•Interspecific competition– Occurs when species compete for a particularresource•Strong competition can lead to competitiveexclusion