TESTS OF COMMUNITY‐WIDE CHARACTER DISPLACEMENT AGAINST NULL HYPOTHESES

Ecological character displacement is unusual dissimilarity among sympatric species in features such as body size or trophic morphology (Grant, 1972), which allows coexistence by causing species to use the environment in fashions so different that competitive exclusion is avoided (Brown and Wilson, 1956). Character displacement could arise either as a result of divergent natural selection of coexisting species, or by selective survival of immigrating species as a function of distinctness from species already present (Grant, 1969, 1970). But what dissimilarity among species is unusual? What sympatric differences are character displacement? Most evidence for the phenomenon has been taken from species pairs with morphological size ratios that are greater between sympatric than between allopatric populations. The classic example treated a pair of rock nuthatch species (Vaurie, 1951; Brown and Wilson, 1956), populations of which appeared more different in sympatry than in allopatry. However, Grant (1975) has pointed out that bill length varies over geographic clines in both species, and that the zone of sympatry has no discernible influence upon clinal variation of either. A literature review led Grant (1972) to conclude that most examples of ecological character displacement are equivocal. In only two instances do known morphological differences really conform to the pattern, one between two congeneric skinks (Huey and Pianka, 1974; Huey et al., 1974), and another between two species of snails (Fenchel, 1975). The role of competition in these differences is not well established. Community-wide morphological patterns among species have also been taken as evidence of character displacement. The reasoning is that the degree of size difference required to allow coexistence is the same among all contiguous pairs of species within communities. Among species ranked by size, constant ratios of 1.14, 1.2, 1.4, and 2 have been used in this vein (Hutchinson, 1959; Schoener, 1965; Emlen, 1966; Wilson, 1975; Terborgh et al., 1978). However, Hespenheide (1971, 1973) has shown that arbitrary ratios among sympatric species are not good evidence for character displacement, because relationships between the size of trophic structure (or body size) and food size vary substantially within and among taxa. A ratio that provides food separation for one set of species, for one circumstance, will not do so for another. Horn and May (1977) have also commented upon some of the pitfalls of deducing species interaction from constant ratios. Yet, the enthusiasm for character displacement, and for inferring avoidance of interspecific competition from interspecific size ratios, continues. The notion is described as a community principle in recent textbooks of ecology (Odum, 1971; Colinvaux, 1973; Collier et al., 1973; Ricklefs, 1973; Smith, 1974; Pianka, 1978), and is not infrequently given as research motivation or as a theorem in the current primary literature (although under various names [fide Hespenheide, 1973]). Our interest is with the logic of empirical studies of character displacement that involve large fractions of faunas (e.g., Lack, 1947; Hutchinson, 1959; Schoener, 1965; Grant, 1968; Abbott et al., 1977), rather than with studies of particular species pairs. These community-wide studies have uniformly concluded that there is a tendency for coexisting species to be unusually dis-