Plants defend themselves against herbivores using a myriad of defenses ranging from physical and chemical defenses to recruiting mutualistic defenders. Research on plant and insect herbivores has greatly advanced our understanding of chemical defenses, though the same is not true for physical or structural defenses which may play a crucial role in deterring in large and/or generalist herbivores. Current hypotheses (Resource Availability Hypothesis, Carbon-Nutrient Balance Hypothesis) fail to explain resource driven variability in physical defense traits, especially for within species variation. Therefore, we developed a theoretical model to understand intra-specific variation in plant defenses and extend the existing framework to include different types of defenses. The model consists of a single resource, a plant population and an herbivore population all linked through consumer- resource interactions such that changes in resources would lead to a numeric response of herbivores via changes in plant populations. Based on the current understanding of plant defenses, we assume that plants suffer a reduction in growth by investing in defense but also benefit from a reduction in herbivory due to defenses. Unlike earlier models, we define fitness over a discrete-time interval during which the plants grow at some potential growth rate and experience herbivory with some probability. Consequently, fitness is a product of growth and survival probability. We maximize fitness to predict optimal allocation towards defense. Finally, we contrast the findings of the Single Resource Defense model with that of the RAH, by studying patterns in defense traits along gradients of rainfall, soil nitrogen and phosphorus in the Serengeti National Park.
Results/Conclusions
The Single Resource Defense model predicts that allocation to defense traits should increase with resource availability in response to higher potential herbivory in the system. However, if herbivore populations or densities do not respond to the resource availability then defenses should not change along resource gradients. These predictions are significantly different from that of the RAH and therefore provide testable alternatives. Our data from the Serengeti on Solanum incanum show that that thorn density increases with both rainfall and soil phosphorus providing support for the Single Resource Defense Model. Thus, our model provides an alternative framework to understand variation in plant defenses in addition to laying a foundation for developing more complex mechanistic models.