Carbon isotope differences between grassland and savanna herbivores reveal environmentally driven rather than phylogenetically conserved niches

Stable isotope analyses provide insights into large herbivore diets. Carbon isotopes are particularly useful for analysing diet given their ability to reflect C3 or C4 photosynthetic pathways and, therefore, distinguish between browser and grazer diets. Although isotopes are useful for comparing diets across a range of spatial and temporal scales, most studies have focused on either interspecific differences within habitats or single species averages across their geographic range. In this study, we use both new and existing carbon isotopic data to compare large mammal herbivore diets across habitat types in southern and east Africa. We examined whether species in the grassland biome of central South Africa had the same carbon isotopic signatures as counterparts from savannas in southern and east Africa. Within the grassland biome, isotopic signatures from faecal samples were strongly correlated with signatures from bone and dentine collagen samples, supporting our approach of comparing faecal data from the grasslands with savanna data obtained mainly through analysis of skeletal materials. Regression analysis comparing grasslands and savannas confirmed that species' isotopic niches within communities were conserved across different geographic regions. However, herbivore δ13C values were generally lower in the grassland compared with the savanna biome, highlighted particularly by the lower δ13C values of several grazer species (but not browsers), indicating greater consumption of C3 vegetation in grasslands. This finding is counter‐intuitive and suggests that non‐woody C3 vegetation may be an important component of grazer diets in the grassland biome. We speculate on various environmental factors, related to food abundance that could underline the pattern. We compared the diet niches of large mammal herbivores in grasslands with those in two biogeographically distinct savannas. Species' diets showed negligible temporal heterogeneity, but large differences across species (browsers, grazers and mixed feeders) and, among grazers, across the biomes. In East African savannas, grazers are faithful to their niche, while in southern African savannas they incorporate more browse in their diets and, unexpectedly, consume even more browse in the grassland biome.