Intrapopulation isotopic niche structure in mammalian carnivores and herbivores across contrasting habitats

Abstract

Natural communities exhibit ecological gradients that result in a variety of competitive interactions and variations in niche patterns. Changes in resource availability affect the size of a species’ niche: it expands when a variety of resources are abundant and contracts when these resources are scarce. The prevalence of ecologically diverse individuals within populations influences individual responses to inter- and intra-species competition or predation, as well as their fitness and population dynamics. Our aim was to compare the isotopic niche width and structures of two trophic groups residing in a xeric and a more mesic environment. We tested an existing hypothesis, based on differences in absolute intake requirement, that large mammalian carnivores exhibit intrapopulation niche structures more frequently than herbivores, and examined whether this was influenced by habitat conditions. Data were collected for within-population trophic niche variation using stable carbon and nitrogen isotope analyses of faeces collected in the Tswalu Kalahari Reserve, South Africa, and the Pendjari Biosphere Reserve, Benin. By analysing the ellipses in isotopic bi-space and employing community-wide metrics on cross-sectional data, we found that populations of carnivores exhibit greater individual separation compared to herbivores, even when accounting for species-level differences in isotopic niche breadths. This supports the prediction that, despite promoting population fitness through reduced competition effects, niche variation across individuals is only a viable strategy if intake requirement is not limiting.