Patterns of plant defense, diversity, and fitness in an African savanna

Abstract

Direct and indirect interactions between plants and herbivores often differ in their impact on plant growth, survival, and fitness: while direct consumption of plants is almost always deleterious, indirect impacts—including structural changes to plant canopies and the removal of dominant competitors—can be unexpectedly beneficial. In this dissertation, I use a variety of techniques to explore four interrelated questions concerning the community ecology of plants and herbivores in African savannas.

In Chapters One and Two, I use manipulative field experiments to show that, contrary to previous reports, megaherbivores indirectly benefit plant communities in African savannas. First, I use large herbivore exclosures and simulated elephant damage to show that elephants indirectly benefit understory vegetation by creating herbivore-free refuge habitats composed of broken tree branches: elephant damage increases understory abundance and diversity by 55% and 21%, respectively. Next, I show that megaherbivores also indirectly benefit savanna trees by dramatically reducing the abundance of climbing woody vines. In the absence of elephants and giraffes, vines quickly smother tree canopies, reducing the growth, survival, and reproductive output of infested trees.

The outcome of interactions between plants and herbivores is further shaped by plant defenses, which include both intrinsic (e.g., spines) and extrinsic (e.g., associational refuges) strategies. In Chapter Three, I show that understory plants growing in close proximity to well-defended neighbors are able to reduce investment in intrinsic defenses by 39–78%. This consistent reduction in defense investment results from a combination of reduced browsing pressure and a diminished capacity for induced responses to herbivory. In Chapter Four, I use reduced representation bisulfite sequencing to explore the molecular mechanism behind observed patterns of defense investment and find that both biotic and abiotic factors influence epigenetic processes.

African savannas are among the last ecosystems to support near-historic densities and diversities of large mammalian herbivores, and their successful preservation requires a comprehensive understanding of how these species impact vegetation communities. Elucidating the full suite of direct and indirect effects of herbivores on plants, and the defensive strategies that plants employ to reduce browsing damage, is a critical first step towards building such an understanding.