Date of Award

Winter 12-22-2021

Degree Type


Degree Name

Doctor of Philosophy (PhD)




Ritchie, Mark E.


Defense traits, Grazing, Herbivore impact, Savanna, Serengeti, Theory

Subject Categories

Ecology and Evolutionary Biology | Life Sciences


Although studies indicate that plant growth can be co-limited by multiple resources, particularly water, nitrogen (N), and phosphorus (P), the role of co-limiting resources in driving plant-herbivore interactions remains poorly understood. Evidence indicates that the resources that limit plant and herbivore growth may differ which likely influences trophic interactions. In this dissertation, I explored how multiple resource limitation of plants affects herbivore abundance, herbivore impact on plant biomass, and plant traits, using both theoretical and empirical methods. As none of the existing hypotheses account for the effects of multiple resource limitation, I developed a novel theoretical model to predict the response of herbivore abundance and impact on plant biomass along multiple resource gradients. This work showed that the response of herbivore abundance and herbivore impact varied with the identity of the resource. Additionally, I identified new patterns for the response of herbivore abundance and impact along different resource gradients and evaluated some of the model predictions using existing data on insect and mammalian herbivore abundance. Since changing herbivore abundance likely impacts plant biomass, I analyzed a long-term dataset on herbivore impact on plant biomass in the Serengeti National Park, Tanzania. This analysis found that herbivore impact on plant biomass decreased with rainfall, increased with plant P, and did not change with plant or soil N. To understand plant response to both changing resource supplies and herbivory, I explored the effects of multiple resources and resource-driven herbivory on three plant defense traits in a widely distributed herbaceous plant. Although all the defense traits showed considerable intraspecific variation, only thorn density was positively associated with both herbivory intensity and total soil P. Because defense traits tend to be closely associated with other plant functional traits, I integrated the defense traits with leaf economic spectrum traits for the same herbaceous species. The leaf economic spectrum axis explained considerable variation among the traits in a multivariate space but was unassociated with resource supply gradients. Thorn density was orthogonal to the leaf economic spectrum axis but was positively associated with both herbivory intensity and soil P supply. This analysis suggests that resources that influence leaf economic spectrum strategy and herbivory are likely decoupled in our system and can independently influence allocation to defense traits. An emerging theme from this body of work is that P supply is important in terrestrial plant-herbivore interactions, a relatively underappreciated phenomenon. Additionally, resources differed in their influence on herbivore abundance, herbivore impact on plant biomass, and plant traits thus emphasizing the need to consider resources as unique entities rather than pooling them into a generic "resources" category. Given the projected variation in rainfall and anthropogenic inputs of nutrients globally, it is crucial to understand the interactive effects of multiple resources on plant-herbivore interactions.


Open Access

Available for download on Monday, July 01, 2024