Effects of resource availability and insect herbivore pressure on plant defense

Plants have evolved a diverse array of defenses to protect against insect damage. Defending against herbivores is thought to come at a cost to growth, resulting in negative correlations between growth and defense. While growth-defense trade-offs are commonly documented across plant species, there is less support for trade-offs within species. Theory predicts two primary mechanisms should drive defense levels and tradeoffs within species: 1) resource availability should favor growth over defense and 2) herbivore pressure should increase defense. Importantly, these mechanisms could evolve, be plastic, or both (i.e., G × E). Few studies, however, have evaluated how resource availability and herbivore pressure may interact to shape plant defense tradeoffs within species. We collected seeds from populations originating from low-resource (arid) grasslands in Montana and from high-resource (mesic) tallgrass prairies in Wisconsin. We grew plants in common gardens in each region in a reciprocal transplant experiment and measured plant growth, herbivore damage, and plant defensive traits. We found greater insect herbivore abundance and damage in the high-resource garden, supporting the hypothesis that herbivore pressure increases with resource availability. Additionally, within both gardens, we found that plants that originated from the high resource region received more damage than plants originating from the low resource region, suggesting defense levels have evolved differently between the resource regions. These patterns were consistent across several tested plant species. We will discuss the roles of putative defensive traits driving these patterns as well as implications for understanding drivers of variation in plant defense.