Competition and the form of habitat shift.

This study derives simple models of competition and applies them to the description and prediction of habitat shift. Habitat shift is a change from one species population to another in the habitats occupied by most of their individuals. Interference models in which individuals are limited in their time for feeding by other individuals can give rise to competition equations structurally identical to Lotka-Volterra models. These models have linear zero-isoclines. Interference models in which individuals as a group are energy-limited produce nonlinear, concave downward, zero-isoclines. These are asymptotic to the axes. Unlike the linear-isocline case, such models always produce a unique stable equilibrium. Asymptotes may be made nonzero by inclusion of a refugium into the isocline equation. Exploitation models in which individuals of different species feed with finegrained selection on the same resources but gain different energies from these give parallel linear isoclines. They cannot produce a stable point equilibrium, whether consumption per individual is fixed or not. A simple model, in which individuals have flexible requirements and feed with fine-grained selection, has a unique stable equilibrium if each species has some resources not usable by the other. This model has concave downward isoclines. If one species utilizes only resource kinds included within those of another, both can coexist if the first is sufficiently better at obtaining the shared resources and if there are enough such resources. Systems of equations specifying growth of both consumers and resources can sometimes give isoclines that are linear (MacArthur, R. H. 1968, The theory of the niche in “Population Biology and Evolution,” Syracuse Univ. Press) or that are of the same curvilinear form as isoclines for the pure consumer systems. One way to relate competition to habitat shift is to assume the isocline equations hold separately for each of a set of habitats. Then if estimates of carrying capacities are available, models for isoclines can be constructed and competition parameters computed by regression techniques, using data for each habitat as a separate point. Relative carrying capacity in Locality S (with competitors) can be estimated as follows. Measure the relative abundance of the species in its “fundamental” niche in Locality A (without competitors); then weigh it by the ratio of available resources in Locality S to available resources in Locality A. This method provides a possible structure for formalizing how competition transforms the “fundamental” into the “realized” niche. The method is applied to a shift in structural habitat (perch height and diameter) in some Caribbean lizards. Habitat availability is measured as relative surface area, estimated from vegetation using a new sampling technique. Results are: (1) nonlinear isoclines fit better than linear ones; (2) concave downward isoclines fit much better than convex downward isoclines; (3) an exploitative model fits better than all others, including interference and descriptive models; and (4) competitors of different sizes are more likely to fit interference than exploitative models; those of the same size show the opposite; and species best fitting interference models are more often the smaller member of pairs of differently sized competitors.