Interactions between plants and herbivores do not occur in a vacuum. The presence, identity, and density of neighboring plants can significantly decrease (or increase) herbivore performance and preference, a phenomenon known as associational resistance (or susceptibility). Neighboring plants can also ameliorate harsh abiotic conditions, resulting in positive plant-plant associations that affect not only individual growth and survival, but also landscape-scale vegetation patterns. My work in this area aims to link the effects of plant-plant interactions across scales, from individual plant phenotype to whole ecosystem patterns of plant abundance and diversity. Below, I present a selection of my research in this area:
African elephants commonly damage savanna trees while browsing, and elephant damage has been frequently cited as a major concern by those tasked with the conservation and management of savanna plant communities. Contrary to long-held expectations, we found that elephant damage actually increases understory plant biomass and species richness by indirectly inhibiting non-elephant herbivores and creating canopy-scale refuges for understory grasses and forbs. In this way, elephants enhance an existing associational resistance relationship between spiny trees and their understory neighbors.
Positive interactions between plants--including the inhibition of herbivores by neighbors ("associational resistance")--are particularly common in physically stressful ecosystems like African savannas. Using a series of small-scale experiments, we showed that growing in close proximity to spiny Acacia trees not only reduces browsing damage on understory plants, but also allows them to decrease investment in their own defenses and increase investment in growth and reproduction. In contrast, we found that plants growing in abandoned cattle corrals and experimentally cleared plots, both of which attract herbivores, invest significantly more in defense than conspecifics growing among neighbors. Collectively, these results suggest that plant-plant interactions cause consistent differences in herbivore pressure across the landscape, with parallel patterns of plant defense investment at the same scale.
Plant-plant interactions are invoked as a near-universal mechanism behind vegetation spatial patterns, which are ubiquitous in nature. However, these interactions alone cannot explain the complex patterns observed in many landscapes, particularly those with regularly spaced social insect colonies. Using a combination of field surveys and theoretical models, we showed that both plant-plant interactions and antagonistic interactions between neighboring termite colonies are necessary to create the patterns we observed in Kenyan savannas, as well as those found in the famous fairy circle landscapes of Namibia.