Effects of resource availability and insect herbivore pressure on plant defense

Background/Question/Methods

Plants have evolved a diverse array of defenses to protect themselves against herbivory. Defending against herbivores is thought to come at a cost to growth, resulting in negative correlations between growth and defense. Theory predicts two primary mechanisms should drive defense levels and tradeoffs: 1) resource availability should favor growth over defense and 2) herbivore pressure should increase defense. While growth-defense trade-offs are commonly documented across plant species, there is less support for trade-offs within species. Here we test the hypothesis that growth-defense should be negatively correlated across species but positively correlated within species. We collected seeds from species originating from low-resource (arid) grasslands in Montana and from high-resource (mesic) tallgrass prairies in Wisconsin. For a subset of species, we collected seed from populations that occur in each region. We grew plants in common gardens in each region in a reciprocal transplant experiment and measured plant growth, herbivore damage, and plant defensive traits.

Results/Conclusions

We found greater insect herbivore abundance and damage in the high- compared to low-resource region, supporting the hypothesis that herbivore pressure increases with resource availability. Across species, we found tradeoffs both across regions and across species within each region. The strength of the tradeoff was more strongly negative across species within the low-resource region, suggesting stressful environments may more strongly constrain growth-defense tradeoffs. Growth-defense correlations within species were mixed, with most intraspecific patterns exhibiting neutral to positive correlations. We will discuss the roles of putative defensive traits driving these patterns as well as implications for understanding drivers of variation in plant defense.