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Strictly speaking, a community is composed of all the organisms that live together in a particular habitat. Community structure concerns all the various ways in which the members of such a community relate to and interact with one another, as well as community-level properties that emerge from these interactions, such as trophic structure, energy flow, species diversity, relative abundance, and community stabil ity. In practice, ecologists are usually unable to study entire communities, but instead interest is often focused on some convenient and tractable subset (usually taxonomic) of a particular community or series of communities. Thus one reads about plant communities, fi sh communities, bird communities, and so on. My topic here is the structure of lizard communities in this somewhat loose sense of the word (perhaps assemblage would be a more accurate description); my emphasis is on the niche relationships among such sympatric sets of lizard species, especially as they affect the numbers of species that coexist within lizard communities (species den sity). So defined, the simplest (and perhaps least interesting) lizard communities would be those that contain but a single species, as, for instance, northern populations of Eumeces msciatus. At the other extreme, probably the most complex lizard commu nities are those of the Australian sandridge deserts where as many as 40 different species occur in sympatry (20). Usually species densities of sympatric lizards vary from about 4 or 5 species to perhaps as many as 20. Lizard communities in arid regions are generally richer in species than those in wetter areas; therefore, because almost all ecological studies of entire saurofaunas have been in deserts (l8, 20, 25), this paper emphasizes the structure of desert lizard communities. As such, I review mostly my own work. Other studies on lizard communities in nondesert habitats are, however, cited where appropriate. Historical factors such as degree of isolation and available biotic stocks (particu larly the species pools of potential competitors and predators) have profoundly shaped lizard communities. Thus one reason the Australian deserts support such very rich lizard communities may be that competition with, and perhaps predation pressures from, snakes, birds, and mammals are reduced on that continent (20).

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Desert lizards are typically either widely foraging or sit—and—wait predators, and these foraging modes are correlated with major differences in ecology. Foraging mode is related to the type of prey eaten by lizards. Widely foraging lizards in the Kalahari desert, the Western Australian desert, and the North American desert generally eat more prey that are sedentary, unpredictably distributed, and clumped (e.g., termites) or that are large and inaccessible (inactive scorpions) than do sit—and—wait lizards. In contrast, sit—and—wait lizards eat more prey that are active. Foraging mode also appears to influence the types of predators that in turn eat the lizards. For example, a sit—and—wait snake eats predominately widely foraging lizards. Crossovers in foraging mode thus exist between trophic levels. Widely foraging lizards may also encounter predators more frequently, as suggested by analyses of relative tail lengths; but tail break frequencies are ambiguous. Daily maintenance energetic expenditures of widely foraging lizards appear to be about 1.3—1.5 times greater than those of sit—and—wait lizards in the same habitats, but gross food gains are about 1.3—2.1 times greater. Widely foraging species also have lower relative clutch volumes, apparently in response to enhanced risks of predation. Foraging mode within one species varies with changes in food availability. Physiology, morphology, and risk of predation might generally restrict the flexibility of foraging mode. Because foraging mode constrains numerous important aspects of ecology, any general model of foraging velocity must be complex.

Current theory predicts a distinct upper limit on the permissible degree of niche overlap; moreover

2016-12-23T18:52:10

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