Increasing woody cover facilitates competitive exclusion of a savanna specialist

Abstract

Anthropogenically modified systems tend to have homogenous wildlife communities. While this process is well documented, the mechanisms driving the convergence of species in human-modified systems are unclear. There are at least two compelling ecological explanations for the loss of specialized species in these settings; changes to the distribution and quality of resources (i.e. environmental conditions) and changes to competitive interactions. Without a better understanding of the mechanisms behind biotic homogenization, it is difficult to take appropriate conservation and management actions to maintain diverse communities.We investigated the roles of environmental conditions and competition in driving biotic homogenization in one of the world's most imperiled ecosystems; the longleaf pine (Pinus palustris) savanna of the American Southeast. Specifically, we investigated how generalist gray squirrels (Sciurus carolinensis) are replacing the more specialized fox squirrels (S. niger) in shrub and hardwood encroached Southeastern savannas. To test how changes to habitat and competition affect the specialist fox squirrel, we undertook a landscape scale manipulation. We experimentally manipulated competition by removing gray squirrels from closed-canopy patches embedded in longleaf pine savanna.Fox squirrels showed a strong response to gray squirrel removal, increasing their activity by 300% in the interior of closed-canopy patches. Fox squirrels in control patches did not use the interior of closed-canopy patches.Our experimental results support the hypothesis that biotic homogenization occurs when human-induced changes to the environment alter competitive interactions.