Long-Term Population Trends in Savanna Herbivores

Abstract

African savanna herbivore communities sustain the world's most spectacular mammalian biomass concentrations and underpin ecosystems of extraordinary functional complexity, yet their long-term population trajectories have been profoundly reshaped over the past five decades by land-use change, human population growth, climate variability, and altered predator-prey dynamics. This study analysed population time series for 12 large herbivore species -- wildebeest, zebra, African buffalo, African elephant, giraffe, hartebeest, topi, impala, waterbuck, eland, Thomson's gazelle, and Grant's gazelle -- spanning 1970-2023 from four iconic East African savanna systems: the Serengeti-Mara ecosystem (Tanzania-Kenya), Amboseli ecosystem (Kenya), Tsavo ecosystem (Kenya), and Laikipia Plateau (Kenya). State-space population dynamic models incorporating rainfall, land-use, and predator pressure covariates were fitted to aerial survey count data (n = 847 aerial counts across 53 years) using maximum likelihood estimation. Six of the 12 focal species showed significant long-term declining trends (mean annual rate: -2.4% +- 0.8%/yr), four showed stable or slightly increasing trends, and two showed non-monotonic trajectories with recoveries following conservation interventions. Annual rainfall explained 47.3% of inter-annual population variance (beta = +0.61, p < 0.001), while land-use intensification index explained a further 28.4% as a negative driver. Species-specific migration corridor blockage -- quantified from landscape permeability models -- reduced wildebeest and zebra population growth rates by -1.84% and -1.21% annually respectively per unit of corridor disruption. These long-term datasets provide the most comprehensive multi-species population trajectory analysis yet conducted for East African savanna herbivores and identify land-use planning and corridor restoration as the highest-priority conservation interventions.