Top-down determinants of niche structure and adaptation among African Acacias

The role of top-down factors like herbivory and fire in structuring species' niches, even in disturbance-dependent biomes like savanna, remains poorly understood. Interactions between herbivory and fire may set up a potential tradeoff axis, along which unique adaptations contribute to structuring communities and determining species distributions. We examine the role of herbivory and fire in structuring distributions of Acacia saplings in Hluhluwe iMfolozi Park in South Africa, and the relationship of species' niche structure to traits that help them survive herbivory or fire. Results suggest that (1) fire and herbivory form a single trade-off axis, (2) Acacia sapling distributions are constrained by fire and herbivory, and (3) Acacia saplings have adaptations that are structured by the tradeoff axis. Herbivory-adapted species tend to have 'cage'-like architecture, thicker bark, and less starch storage, while fire-adapted species tend to have 'pole'-like architecture, relatively thinner bark, and more starch storage.