Habitat Selection and Population Interactions: The Search for Mechanism

Abstract

The study of habitat selection and population interactions has a history about as long as any integrative topic in ecology. Its literature is classically scientific in the sense that it combines pattern discovery, theory, and empirical testing. It is, implicitly, one of the significant demonstrations that general ecological principles exist and can be discovered by hypotheticodeductive methodologies. I outline the development of the topic in the context of two questions, both of which take it as an assumption that habitat selection is an optimal-foraging process. How does habitat selection alter (or reinforce) the dynamics of population interactions? How do those interactions alter the optimal behavior? Competition within a single species forces the use of a wide variety of habitats. This has been shown in many taxa, and Fretwell's theory gives it a sound basis in evolutionary ecology. In other words, we ecologists know that the pattern exists, and we know why it exists. Ecologists are capitalizing on their understanding of single-species habitat selection to design methods for comparing fitnesses (and components of fitnesses) in different habitats. It has been known for 40 yr that a second, competing species can reverse the effects of competition within a species, producing habitat selection where it would not otherwise exist. For the past decade, however, we have been discovering that this effect is really a cluster of effects. Whether competition restores selectivity depends on the kind of competition at work and on the information-gathering abilities of foragers. So does the extent to which it is restored. Generally, the strongest separation of species by habitat selection results from distinct-preference competition between species whose individuals have very limited abilities to assess the density of food in a patch. (This model yields the "ghost of competition past.") But if individuals have much ability to assess food density, there is little or no increase likely in their selectivity compared with the single-species case. Other sorts of competition produce intermediate results. The effects of interspecific competition on habitat selection have been investigated with theory and experiment working closely together. The by-products of the combination have been that new sorts of competitive relationships have been suggested and that it has been possible to compare these relationships with those suggested from other areas of investigation such as plant-herbivore interactions, succession, and plant specializations. Habitat selection may stabilize predation as well as competition. Sih has shown that the conditions required for stabilization closely resemble those predicted by optimal-foraging theory: Victims need to prefer the safer habitat most when their populations are lowest or when their chance of being killed in the riskier habitat is highest. Numerous experiments show that foragers behave appropriately to protect themselves from predation. Multispecies studies emphasize the potential for differential susceptibility and optimal habitat selection to maintain diverse communities. Despite, or perhaps because of, the rich understanding that the study of habitat selection has already engendered, many new and exciting topics are emerging. Perhaps the most challenging is the combination of time scales that allows phenotypes to change morphologically and physiologically as well as behaviorally. It seems probable that investigators interested in these topics will continue to receive substantial payoffs for their efforts.