HERBIVORE‑MEDIATED EFFECTS OF ECOSYSTEM HOTSPOTS ON THE DEMOGRAPHY OF A WIDESPREAD EAST AFRICAN SAVANNA TREE

Abstract

In African savannas, ‘ecosystem hotspots’ – nutrient-enriched grazing sites that increase system-wide productivity for many taxa – feature specific patterns of tree population density and age structure. Some hotspots have edge effects, the specific extent and nature of which depend on the spatial distribution of the hotspots. The changes to tree populations in and around ecosystem hotspots contributes to the long-term maintenance of the hotspots themselves, and defines the other ecological impacts of hotspot edges. When hotspots occur close together, their edge effects reduce habitat quality for large herbivores and may further bush encroachment. The mechanisms maintaining distinctive tree populations associated with hotspots are an open question, and it is yet unknown whether hotspot edge effects persist over long time scales. Of the possible mechanisms regulating trees in and around ecosystem hotspots, seed predation remains under-explored. I use seed removal experiments to test whether seed predation is a possible mechanism maintaining variation in tree demography in and around ecosystem hotspots. I find that seed predation is only likely to significantly affect tree demography in hotspot edges. I then use a mathematical model to project the long-term persistence or decay of hotspot edge effects, and the relative contributions of seed predators and large mammalian herbivores to this outcome. I find that edge effects can persist for centuries, and suggest that land managers take steps to avoid creating edges with long-lasting detrimental effects. Edge effect persistence depended on all parts of the system, including both edge-edge interactions and a complete network of plant-herbivore interactions. I suggest that predictions regarding the long-term development of edge effects can be made more accurate by taking into account specific edge effects, including edge-edge interactions, and the basic ecology of the system in question. Seed predation is a potentially quite widely overlooked process regulating savanna vegetation, especially in naturally-occurring contexts where large herbivores are at lower-than-normal density.