The Role of Top-Down Control by Large Herbivores on the Habitat Specialization of Two Acacia Species in an east African Savanna

Abstract

The range of habitats a plant species is able to occupy is determined by a combination of biotic and abiotic factors. Herbivores have long been predicted to play an important role in species sorting by exaggerating the differences between soil types in edaphically patchy landscapes (Janzen, 1974). Our study attempts to quantify the importance of soil*herbivory interactions on the habitat specialization of Acacia trees in an east African savanna with two distinct edaphic environments: black cotton vertisols (“black soil”) and red friable sandy loams (“red soil”). We planted two species – A. drepanolobium (black-soil specialists) and A. breviscipa (red-soil specialists) – in a controlled, replicated, reciprocal-transplant experiment with 2 levels of resource addition (water and fertilizer or none) and 2 levels of herbivory (herbivores allowed or excluded). Herbivore exclusion increased growth rates of both species. For A. drepanolobium, resource addition and herbivore exclusion significantly increased growth and survival respectively on red soil but not on black. This result suggests that a tradeoff between resource allocation towards growth versus defense prevents A. drepanolobium from establishing on nutrient-poor red soil. Colonization by mutualistic ants also increased the success of A. drepanolobium establishment on its non-preferred soil. We found evidence to suggest that stem diameter and height growth responded to different variables. Stem diameter growth tracked rainfall and nutrient cycles while height growth in drepanolobium responded more strongly to the colonization by ants. To our knowledge, this was the first experiment to explicitly study the interactions between biotic and abiotic factors on the habitat specialization of two dominant Acacia species. This experiment aimed to increase our understanding of how the extinction of large herbivores would shape the future distribution of woody species across African savannas.