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Biological Sciences
Marc Cadotte

17 SpCommunitiessity in Chapter 18 Recall The competitive exclusion principle : Two species that use a limiting resource in the same way cannot coexist. Joseph Grinnell first used the term ‘niche’ in 1917, as : “no two species of birds or mammals will be found to occupy precisely the same niche” Introduction Paradox of the plankton G.E. Hutchinson Many plankton; few resources Introduction Proposed solutions to the‘paradox of the plankton’ Heterogeneity Fluctuations/ Grazing/ disturbances enemies Community Membership Species richness differs among communities due to variation in regional species pools, abiotic conditions, and species interactions. Figure18.4 Community Membership:A Series of Filters Community Membership 1. The regional species pool provides an upper limit on the number and types of species that can be present in a community. The importance of dispersal can be seen in cases of non-native species invasions. Figure18.5 A Humans Are Vectors for InvasiveSpecies Pg. 390-391 Regional pool Unsaturated Proportional Saturated Regional richness Community Membership 2. A species may be able to reach a community butbephysiologically unable totolerate the abiotic conditions of the environment. Someabiotic constraints are obvious (e.g., fish on land, or more subtle; a lake might not supportorganisms that require fast -flowing water). Community Membership 3.Thefinal cutrequires coexistence with other species. For species that depend on other species for growth, reproduction, or survival, those other species must be present. Species may be excluded from a community by competition, predation, parasitism, ordisease. Biotic resistance Biotic resistance Exotic: Garlic mustard Community Membership There are three schools of thought what controls community diversity: • Equilibrium theory—ecological and evolutionary compromises lead to resource partitioning. • Nonequilibrium theory—fluctuating conditions keep dominant species from monopolizing resources. • Neutral theory—species do not differ and diversity patterns are a product of dispersal, speciation and demographic stochasticity. Equilibrium theories Resource partitioning among the species in a community reduces competition and increases species richness. The competitive exclusion principle : Two species that use a limiting resource in the same way cannot coexist. Equilibrium theories Resource partitioning—competing species are more likely to coexist when they use resources in differentways. Figure 18.7 A Resource Partitioning Figure18.7 B, C, D Resource Partitioning Figure18.8 Resource Partitioningby Warblers MacArthur (1958) looked at resource partitioning in whole communities. Equilibrium theories In further studies, MacArthur and MacArthur (1961) looked at bird communities in 13 different habitats. There was a positive relationship between bird species diversity and foliage height diversity (number of vegetation layers, an indication of habitat complexity). Figure18.9 Bird Species Diversity Is Higher in More Complex Habitats Equilibrium theories Recall Tillman’s experiments with two species of diatoms that competed for silica. Resource Partitioning To explain how diatom species coexist in nature, Tilman proposed theresource ratio hypothesis —species coexist by using resources in different proportions. Resource Partitioning
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