One of the most well supported theories regarding the evolution of plant defenses is the resource availability hypothesis (RAH). This theory posits that there is a strong interspecific tradeoff between growth and resistance. Among species, those growing in high-resource habitats are predicted to evolve fast growth rates and allocate relatively little to herbivore resistance traits. In contrast, those species adapted to low-resource habitats are predicted to evolve slow growth rates but have higher herbivore resistance. The predictions of RAH have generally been well supported in broad interspecific comparisons of plant resistance. RAH has also frequently been invoked to predict intraspecific patterns of plant resistance across ecological gradients or to describe plastic responses of plants to fluctuating resources. However, within species, whether resources set the template for growth rate and whether growth and resistance tradeoff is an open question. We review work on intraspecific variation in plant defense (both resistance and tolerance) across resource gradients and evaluate the support for extensions of the RAH that are most likely to apply intraspecifically. Specifically, we evaluate whether high-resource environments compared to low-resource environments: 1) select for faster growing plants, and 2) select for reduced resistance, increased tolerance, and/or higher induced resistance.
Results/Conclusions
We found little congruence between theory developed to describe how resource availability affects interspecific patterns of resistance and intraspecific patterns. Within species, we found that growth rate is often positively associated with resource availability, as predicted by RAH. However, there is no strong evidence to support a tradeoff between growth and resistance; both often covary positively with resource availability. We develop an intraspecific framework that predicts that plants from high-resource environments have fast growth rates, high resistance, high tolerance, low inducibility and experience high herbivore pressure. Plants from low-resource environments have slow growth rates, low resistance, low tolerance, high inducibility, and experience low and variable herbivory rates. However, we further suggest that the position of a species on the resource-resistance continuum may shed further light on predicting growth-resistance relationships. For instance, species that evolve in low-resource environments may be constrained by growth and exhibit resource-resistance tradeoffs whereas species that evolve in high-resource environments may have more capability for allocating to both growth and resistance across resource gradients. Our testable framework should help advance growing interest in the drivers of intraspecific variation in plant defense and provides an important step towards reconciling inter- vs. intraspecific strategies of growth and defense.