The Information Costs of Generalism

Abstract

The determinants and consequences of niche width have long been focus of interest for ecologists and evolutionary biologists. From macroevolutionary perspective, specialisation of feeding habits and habitat is thought to increase the rate of species turnover by increasing the rate of both speciation and extinction (e.g. Stanley 1974, 1979, Eldredge 1989). Given that speciation events are thought to be necessary to conserve adaptive change by genetically isolating locally adapted demes (Eldredge 1989), niche width therefore has important consequences for significant evolutionary change. The implications for ecology revolve around niche overlap and competition within trophic level (see Leigh 1990 for review), and density dependent population dynamics between levels (e.g. Fryxell and Lundberg 1994). Organisms that exploit wide range of resources and habitats will experience relatively more interspecific interactions and, depending on the nature of those interactions, should thereby increase interspecific competition within any given trophic level. Generalism may also reduce the dependence of the mortality, or birth, rate of that species' own population, and/or that of their prey, on population density. This is because generalist is likely to experience reduction in both the influence of particular prey type on its own population dynamics, and its influence on the populations of each of its prey species, relative to the appropriate specialists. These factors are thought to have important effects on the stability of the constituent populations and on dynamics as whole (e.g. MacArthur 1955, Connell 1983, Schoener 1983). So the distribution of resource and habitat use patterns between avatars (economically interacting local populations of conspecifics (Damuth 1985)), and the trade-offs underlying these patterns, are implicated in the determination of structure (e.g. Brown 1990, Brown and Pavlovic 1992, Wilson and Yoshimura 1994). A species' potential niche width depends on both the genotypic variation and the reaction norms of those genotypes. The evolution of reaction norms is topic of current interest within evolutionary ecology (e.g. Steams 1989, Houston and McNamara 1992, Via et al. 1995). Furthermore, the factors that determine the realised niche of given phenotype have been focus for the optimality approach to individual behaviour right from its conception (Emlen 1966, MacArthur and Pianka 1966). Theoretical considerations of the dietary aspect of this problem (prey models (Stephens and Krebs 1986)) have focused on individual patterns of habitat use and switching (e.g. Werner and Mittelbach 1981). The general predictions of such models have found broad support in both laboratory and field studies (see Stephens and Krebs (1986) for review). There is widespread agreement on the importance of niche width across range of biological disciplines. Despite this, there is little consensus regarding its general determinants. Futuyma and Moreno (1988), in their review of the current hypotheses on the evolution of specialization and generalization, reasoned that there are many sources of natural selection that may favour one or other strategy. They concluded that it is therefore impossible to assume any common factor(s) priori. Moreover, Fox and Morrow (1981), in an earlier review focusing on herbivorous insects, concluded in favour of rejecting niche width (or specialization, their particular consideration) as species attribute. Better to consider it a flexible attribute of population that is responding to features of its particular community when confronted with vast array of mechanisms leading to dietary specialisation. Rather than dissecting this undoubted complexity, this paper is an attempt to offer coherent conceptual framework for confronting the myriad of mechanisms that determine niche width across the biological spectrum.