Resources: A Graphical-Mechanistic Approach to Competition and Predation

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Abstract
The growth response of a population to the resources in a particular environment is used to classify pairs of resources as being either essential, hemi-essential, complementary, perfectly substitutable, antagonistic or switching. Nutrition is 1 important factor determining resource type, but the growth response of a population to resources also depends on the interaction between a species'' foraging methods and the spatial distribution of the resources. Two resources which are nutritionally perfectly substitutable may be operationally switching, antagonistic or complementary because of spatial heterogeneity. A graphical, equilibrium theory [model] of resource competition allows prediction of the outcome of interactions between several consumers [animals] for the various classes of resources. The technique requires information on resource type (growth isoclines), resource preference, resource supply processes and mortality rates for all species. For all resource types, the major criterion for stable coexistence is that each species consume relatively more of the 1 resource which more limits its own growth rate. The patterns of species dominance and coexistence in a community depend on the types of resources for which competition occurs. It may be possible for an unlimited number of species competing for essential resources to stably coexist in a spatially heterogeneous environment, but only 2 spp. competing for 2 switching resources may stably coexist in an equally heterogeneous environment. The latter community may be susceptible to invasion by other species which respond to these same resources in a non-switching manner.