Spatial organization of a herpetofauna on an elevational gradient revealedby null model tests

Five null model tests were applied to the herpetofaunal assemblage on the w estern slope of Mount Kupe, Cameroon. Based on the pattern of species range boundaries and abundances along the primary forest elevational gradient, r anging from 900 to 2000 m, the relative importance of interspecific competi tion and ecotones in structuring the assemblage was assessed. Tests were ru n for (1) all species, (2) amphibians, (3) reptiles, (4) amphibians depende nt 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 exp ectations. The results indicated that there are very few species whose grad ient distributions may be limited by interspecific competition between cong eners. Significant discontinuities in abundance patterns and range boundary dispersion revealed zonations in all subsets analyzed but neither indicate d distinct species groups with sharp exclusion boundaries nor a strong resp onse to vegetational ecotones. Physical factors varying in parallel with th e gradient and specific habitat components, particularly water bodies suita ble as amphibian breeding sites, are suggested to be the dominant factors l imiting gradient distributions of amphibians and reptiles on Mount Kupe. Th e zonations revealed suggest a pattern of three spatially nonexclusive spec ies groups: physical factors separate distinct lowland and montane species limited by physiological constraints and produce faunal discontinuities in the lower sub-montane forest around 1300 m; this boundary is encompassed by the range of a group of anuran species, whose distributions on the gradien t are centered at intermediate elevation and appear to be limited by specif ic habitat requirements. The response to predominantly abiotic factors sugg ests a basic difference from endotherms, where biotic factors seem to be of major importance in limiting elevational distributions.