DISSERTATION RESEARCH: Plant defenses in African savannas: testing the effects of induced and associational defenses on plant phenotype, fitness and diversity

Princeton University, Princeton NJ

Investigators

Abstract

Understanding the ways in which plants modulate their vulnerability to herbivores is central to understanding patterns of biodiversity and ecosystem functioning. Many plants have physical defenses such as spines, thorns, or prickles that protect them against large mammalian herbivores (e.g., elephants, giraffes, deer, and elk); however, producing and maintaining these defenses is energetically costly. As a result, plants that produce more spines and thorns are less vulnerable to herbivory, but also have fewer resources to invest in growth and reproduction, two critical components of plant fitness. This tradeoff is particularly acute for plants in dry or infertile African savannas, where resources (e.g., water and nutrients) are scarce and the risk of being eaten by large herbivores is high. As a result, theory predicts the emergence of strategies that maximize the effectiveness of defenses while reducing their cost. Two such strategies are induced defenses (produced rapidly in response to damage by herbivores) and associational defenses (reduced incidence of herbivory derived from growing in close proximity to a well-defended neighbor). The goal of this project is to evaluate the relative effectiveness of, and interactions between, these anti-herbivore strategies and to determine how positive associational interactions among plants of different species alter patterns of resource investment. Plants facing the same evolutionary pressures frequently converge on similar suites of defensive characteristics, known as defense syndromes. In African savannas, individuals of the same species can exhibit strongly contrasting defense syndromes depending on their proximity to well-defended neighbors. Plants growing beneath thorny tree canopies, for example, produce fewer spines and exhibit a diminished induced response to simulated herbivory relative to conspecifics growing in open habitat. The researchers hypothesize that this difference is maintained by a disparity in the environmental cues that govern defensive investment in plants and will test this hypothesis using transplant experiments of focal plant species conducted inside and outside of large-herbivore exclosures in the Mpala Research Centre, Kenya. The researchers will also perform surveys of multiple species to evaluate how common intraspecific variation in defense syndrome is across plant taxa. By investigating the biotic and/or abiotic forces that maintain discrete intraspecific defense syndromes, this research will elucidate the independent and interactive effects of different anti-herbivore strategies on plant fitness and help inform the management of rangelands and the conservation of native vegetation in African savannas.