An Analysis of the Spatial Structure of Mammalian Distribution Patterns in Texas

Abstract

I examined mammalian distribution patterns and species richness in the state of Texas, USA, along two major environmental gradients. I drew a system of square quadrats on a map of the state. I recorded the presence of absence of each native mammalian species and fifteen environmental variables. I analyzed these data using classification and ordination procedures and by statistical regression methods. The purpose of the analysis was to determine if a hierarchical classification explains the spatial structure mammalian distributions well, or if they are best understood as a continuum resulting from the individual responses of species to history and environmental conditions. I used quadratic response surface analysis to relate changes in species richness to environmental gradients and to test ecological hypotheses that richness is explained by climatic stability and primary productivity. Correlation analysis of community ordination axes with a suite of environmental variables indicated that mammalian species distributions differentiate along two major clines: a dominant east—to—west gradient of decreasing precipitation and productivity, and a south—to—north gradient of decreasing mean annual temperature, increasing winter cold, and increasing seasonal variation of temperature. Mammalian community—units, as identified by hierarchical cluster analysis, are essentially continuous with each other. This interpretation is reinforced by the substantial boundary overlap of 95% confidence ellipses about the association centroids. Quadrat ordination scores, contoured on a map of the study area, exhibited rather even patterns of species turnover in both east—to—west and north—to—south directions. This indicates that there are no zones of rapid species replacement followed by areas of faunal homogeneity. The distributions of species that range widely within quadrat sets show little fidelity to the areas to which they are most highly associated. Species appear to be geographically distributed in an individualistic manner according to the vicissitudes of history and their own ecological tolerances. These findings are consistent with the continuum concept community structure for mammals. Quadratic response surface analysis of species richness (bats, rodents, carnivores, and all mammals) with environmental variables estimating productivity and stability of temperature did not support either the productivity or stability hypotheses of species richness. The relationship of these four taxa to a resource gradient of increasing productivity shows that the curvilinear models used by Grime (1979), Huston (1979), and Tilman (1982) for plants apply also to mammals.

Vertebrate species richness at the mesoscale: relative roles of energy and heterogeneity

“…In counter examples, studies of rodents in Israel (Abramsky & Rosenzweig, 1984) and carnivores in Texas (Owen, 1990) demonstrate that mammal richness may follow a unimodal relationship with primary productivity in which richness peaks at intermediate productivity levels*. The present analysis of Wyoming vertebrates found little evidence to support the intermediate productivity hypothesis (Tilman, 1982; Abramsky & Rosenweig, 1984; Huston, 1994) in addition to the monotonic, species‐energy hypothesis (Wright, 1983).…”

“…This theory predicts that levels of energy limit richness: a minimum amount of energy must be available in the environment for rarer species to maintain viable populations (Hutchinson, 1959; Brown, 1981; Wright, 1983). Qualitative observations also suggest that habitat heterogeneity may be an important determinant of diversity (Simpson, 1964; Owen, 1990; Pagel, May, & Collie, 1991; Blackburn & Gaston, 1996). Relatively few studies (Bohning‐Gaese, 1997; Kerr & Packer, 1997), however, have attempted to quantify the large‐ scale influence of habitat variability, probably due in part to the difficulty in measuring macrospatial heterogeneity.…”

Bat and Rodent Diversity in a Fragmented Landscape on the Isthmus of Tehuantepec, Oaxaca, Mexico

“…Several authors have proposed that species richness increases with increasing food diversity [23,[40][41][42][43] and habitat diversity [23,44]. In our study area, species richness is probably related to the diversity of both food resources and habitat types.…”

Spatial structure of avifauna along urban‐rural gradients

“…These results conformed to the gradient paradigm (Whittaker 1976). Owen (1990) has concluded that the mammalian community changes continu-ously in Texas in a manner consistent with the continuum concept of community structure. The general change in biological communities along the environmental gradient in urban areas displays a gradual slope from simple to complex.…”

“…CCA, a direct gradient analysis, is a modification of Correspon-dence Analysis in which the ordination axes are required to be weighted sums of the environmental variables (ter Braak 1986). We computed CCA by means of a personal computer program, CANOCO (ter Braak 1987, 1990.…”