Causes and Consequences of Desert Ant Community Assembly

Abstract

Understanding the mechanisms that maintain species coexistence and determine patterns of community assembly are fundamental topics of community ecology. This dissertation examines the drivers of ant community assembly mechanisms along environmental stress gradients in dryland ecosystems. This work positions facultative mutualisms and positive interactions alongside competition as key drivers of community structure. To start, I used experimental resource addition to show that the identity of mutualist ant defenders on silver cholla is a consequence of coexistence processes (here, resource partitioning of individual plants based on EFN resource availability). Investment into EFN resources is worthwhile for plants because ant defense benefits silver cholla; however, this investment is mediated by competition within the ant community. I then examined the importance of the EFN-bearing plant community for ant community assembly at macroecological scales using stacked species distribution models. The EFN-bearing plant community predicted the geographic distribution and diversity of the nectar-feeding ant community. However, interactions between the groups are vulnerable to climate change because of the predicted decrease in geographic overlap between ant-EFN-bearing plant pairs under future climate scenarios. I then examined the relative importance of environmental and biotic assembly mechanisms for ants along a desert stress gradient. The functional structure of the ant community was significantly more diverse than expected which is consistent with the mechanisms of limiting similarity and suggests the importance of competition to ant community structure. The strength of this mechanism decreased with increasing stress, demonstrating that environmental and biotic assembly mechanisms jointly and non-independently structure the ant community. Finally, I asked if environmental filtering on the functional trait composition of the plant and granivorous ant communities scales to shape ant-seed interactions. Ant functional traits responded to both environmental stress and to the functional traits of the plant community. However, seed size-dependent foraging choices arose from ant population size and not the functional structure of either community. This work showed that ant-plant interactions are integral to community assembly for both dryland communities.