The Effects of Mega-Herbivore Extinctions on Seed Dispersal and Community Structure in An East African Savanna

Abstract

Herbivory is a prominent top-down force shaping the landscape of semiarid savannas in East Africa. Herbivores participate in seed dispersal both by ingesting and defecating the seeds of endozoochorously-dispersed plants and by translocating the seeds of exozoochorously-dispersed plants. In this study, we sought to determine how the extinction of large mammalian herbivores would affect the abundance of five focal species of plants that use fruiting as their primary dispersal mechanism. We also examined how the absence of large mammalian herbivores affected seed predation by small mammals. We counted the number of individual plants for each of the five species in three short-term (5 year old) exclusion blocks and one long-term (15 year old) exclusion block. We found that total abundance of fleshy-fruited species increased as successive levels of large mammalian herbivores were excluded and as the duration of herbivore exclusion increased. Within species, we found that C. orthacantha and S. campylacanthum increased in the absence of herbivores, while O. stricta decreased in the absence of herbivores. B. aegyptiaca and B. albitrunca did not exhibit differences in abundance between different levels of herbivore exclosure. We found that small mammals demonstrated a preference for B. albitrunca and O. stricta seeds, which were consumed primarily by the fringe-tailed gerbil, Gerbilliscus robustus. Understanding the interactions between herbivores and vegetation is vital to conserving plant communities as well as understanding how they may change in the future.