Identifying ecological gradients at the range edge of a species is an essential step in revealing the underlying mechanisms and constraints that limit the species’ geographic range. We aimed to describe the patterns of variation in chimpanzee (Pan troglodytes verus) density and habitat characteristics perpendicular to the northern edge of their range and to investigate potential environmental mechanisms underlying chimpanzee distribution in a savanna–mosaic habitat. We estimated chimpanzee densities at six sites forming a 126-km 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, and floral assemblages, among other variables. We found that both biotic and abiotic factors are 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 in 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 of food species diversity, thermal refuge, and water availability to the chimpanzee niche and the implications of these limits for chimpanzee biogeography, especially in the face of climate change predictions, as well as for species distributional modeling more generally.

中文翻译：

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黑猩猩（Pan troglodytes verus）生物地理范围边缘的密度和环境梯度

识别物种范围边缘的生态梯度是揭示限制物种地理范围的潜在机制和约束的重要步骤。我们旨在描述黑猩猩（Pan troglodytes verus）密度的变化模式和垂直于其分布范围北边缘的栖息地特征，并研究热带稀树草原马赛克栖息地中黑猩猩分布的潜在环境机制。我们估计了六个地点的黑猩猩密度，这些地点在塞内加尔东南部的稀树草原 - 马赛克栖息地中西部黑猩猩的生物地理范围边缘形成了一个 126 公里的纬度梯度。为了配合这些数据，我们使用系统放置的植被图来表征每个地点的栖息地，以了解栖息地异质性、树木密度和大小以及花卉组合，在其他变量中。我们发现生物和非生物因素都是该生态区黑猩猩范围限制的潜在决定因素。具体而言，与远离范围限制的景观相比，黑猩猩占据的极限景观具有较小的可用花卉组合，栖息地异质性更小，并且黑猩猩可以在其中寻求避难的封闭树冠栖息地更少。这种模式伴随着黑猩猩密度随着越来越接近范围限制而下降。我们的结果提供了一些关于黑猩猩生态位的食物物种多样性、热避难所和水供应的潜在限制的迹象，以及这些限制对黑猩猩生物地理学的影响，尤其是在面对气候变化预测时，