Do grazers equal grasslands? Strengthening paleoenvironmental inferences through analysis of present-day African mammals

Abstract

Expanding grassy ecosystems through the late Cenozoic may have driven ecological and evolutionary change in hominins and other African mammals. However, we lack a sufficiently robust quantitative framework to understand how species should respond to changing vegetation structure or the magnitude of change needed to drive range shifts, speciation, extinction, or adaptation. A more detailed understanding of present-day species-environment relationships at scales appropriate to paleoenvironmental records can help build this framework. Here, we use generalized additive models to estimate modern-day herbivore responses to woody cover gradients across 100+ present-day African parks and reserves. We examine these responses for 58 herbivore species (Artiodactyla, Perissodactyla, Proboscidea) and with species aggregated by tribe (for Bovidae) and dietary functional group (browser, grazer, mixed feeder, frugivore). At the spatial scales of our analysis (i.e., parks and reserves measuring ∼500–5000 km2), we find that few species have strong associations with open (<0.3 fraction woody cover) or closed (>0.7 fraction woody cover) environments. Most herbivores demonstrate a peak probability of presence around 0.5 fraction woody cover. The latter result is consistent with the ecological conditions previously shown to maximize herbivore diversity. At landscape-to-ecosystem spatial scales (102–103 km2), which average across finer-scale patches of distinct vegetation types (e.g., grassland, woodland, forest), our results indicate that a taxonomically and ecologically diverse group of species are expected to co-occur. We argue that spatiotemporal scale likely accounts for the ubiquity of mosaic habitat reconstructions in time-averaged fossil assemblages from the late Cenozoic of Africa. In addition, we propose that species' broad tolerances for gradients in woody cover renders the expansion of C4-dominated ecosystems an uncertain driver of macroevolutionary change (i.e., speciation and extinction) in Africa through the Plio-Pleistocene.