Identifying ecological gradients at the range edge of a species is an essential step for revealing the underlying mechanisms and constraints that limit the species’ geographic range. Here, we aimed to describe the patterns of variation in chimpanzee density and habitat characteristics perpendicular to the northern range edge and to investigate potential environmental mechanisms for chimpanzee distribution in a savanna-mosaic habitat. We estimated chimpanzee densities at six sites forming a 126km latitudinal gradient at the biogeographical range edge of the western chimpanzee in the savanna-mosaic habitats of southeastern Senegal. To accompany these data, we used systematically placed vegetation plots to characterize the habitats at each site for habitat heterogeneity, tree density and size, floral assemblages, among other variables. We found that both biotic and abiotic factors represent potential determinants of the chimpanzee range limit in this ecoregion. Specifically, chimpanzee-occupied landscapes at the limit had smaller available floral assemblages, less habitat heterogeneity, and contained fewer closed canopy habitats in which chimpanzees could seek refuge from high temperatures than landscapes farther from the range limit. This pattern was accompanied by a decline in chimpanzee density with increasing proximity to the range limit. Our results provide several indications of the potential limits to the chimpanzee niche and the implications of these limits to chimpanzee biogeography, especially in the face of climate change predictions, as well as to species distributional modeling more generally.
Supplementary notes can be added here, including code and math.