Local species diversity typically comprises only a subset of the regional species diversity. At this local scale, a variety of ecological processes—from competition to consumption and mutualism—may interact to stabilize diversity, and mounting empirical evidence shows their ubiquity in communities. However, we understand comparatively little of how local interactions and their concomitant effects on diversity vary in space to produce larger-scale patterns. Here we asked how spatial variation in consumption and apparent competition—the indirect interaction of prey through a shared consumer—affects diversity in a community of seven annual plant species and their seed consumer, a harvester ant species. To do this, we constructed a plant population growth model in which plant fitness is lost to pre-seed dispersal competition and post-dispersal consumption. We parameterized our model using data from field experiments and a map of ant consumption intensity across the landscape, then we projected the impacts of competition and consumption on local- and landscape-level plant diversity.
Results/Conclusions
We found that ant consumption of all plant species’ seeds declined with increasing distance from the ant nest, but the slope and intercept of the relationship varied strongly by plant species. High seed consumption in the immediate vicinity of the ant nest was sufficient to drive the most palatable plant species to extinction, freeing the less palatable species—even those with poor competitive ability—to persist. In areas far from nests, pre-dispersal dynamics alone determined local diversity. We found signals of intra-specific apparent competition in most plant species whereby higher conspecific seed density slightly increased the risk of seed consumption by ants. In contrast, increasing seed densities produced a wide range of effects on heterospecifics. Palatable seeds increased the consumption risk of their heterospecific neighbors (inter-specific apparent competition) whereas less palatable seeds had a protective effect on their heterospecific neighbors (inter-specific apparent facilitation). Incorporating these myriad effects into our dynamical model, we found that spatial variation in plant-ant interactions produced a mosaic of plant diversity reminiscent of observed plant diversity patterns in nature. Our model projections suggested that the interplay of competition and consumption caused plant species simultaneously exclude, are excluded by, and are in stable coexistence with their competitors across the landscape. Our results show that a mechanistic accounting of spatial variation in species interactions provides a promising avenue towards a generalized understanding of the patchy diversity patterns in nature.