Spatial Organization of a Herpetofauna on an Elevational Gradient Revealed by Null Model Tests

Abstract

Five null model tests were applied to the herpetofaunal assemblage on the western slope of Mount Kupe, Cameroon. Based on the pattern of species range boundaries and abundances along the primary forest elevational gradient, ranging from 900 to 2000 m, the relative importance of interspecific competition and ecotones in structuring the assemblage was assessed. Tests were run for (1) all species, (2) amphibians, (3) reptiles, (4) amphibians dependent on streams for reproduction, and (5) amphibians that do not use streams for reproduction. For three null models, the observed patterns did not differ from random expectations. The results indicated that there are very few species whose gradient distributions may be limited by interspecific competition between congeners. Significant discontinuities in abundance patterns and range boundary dispersion revealed zonations in all subsets analyzed but neither indicated distinct species groups with sharp exclusion boundaries nor a strong response to vegetational ecotones. Physical factors varying in parallel with the gradient and specific habitat components, particularly water bodies suitable as amphibian breeding sites, are suggested to be the dominant factors limiting gradient distributions of amphibians and reptiles on Mount Kupe. The zonations revealed suggest a pattern of three spatially nonexclusive species groups: physical factors separate distinct lowland and montane species limited by physiological constraints and produce faunal discontinuities in the lower submontane forest around 1300 m; this boundary is encompassed by the range of a group of anuran species, whose distributions on the gradient are centered at intermediate elevation and appear to be limited by specific habitat requirements. The response to predominantly abiotic factors suggests a basic difference from endotherms, where biotic factors seem to be of major importance in limiting elevational distributions.