Predictable influence of environmental conditions and conspecific density on herbivore pressure and plant fitness

Insect herbivores can be highly damaging to plants, but the extent to which damage translates into reductions of plant fitness is highly context dependent. At large spatial scales, climate or resource availability may be important predictors of herbivore pressure (i.e., how strongly herbivores impact plant fitness). At local scales, conspecific density can be an important predictor of herbivore pressure. Using Monarda fistulosa as a focal plant species, we sampled populations in different resource regions: a low-resource region in Montana with cooler temperatures and low summer precipitation, and a high-resource region in Wisconsin with warmer temperatures and 3-fold greater precipitation. Within each population, we estimated density of conspecifics, herbivory by estimating seed loss to pre-dispersal seed predators, and seed production. At large scales, we found that seed loss was approximately 2-fold greater and seed weight was 2-fold lower in the high-resource region compared to the low-resource region. At local scales, conspecific density was correlated with greater seed loss and seed weight, suggesting a role of negative density dependence. A supplementary seed addition experiment showed that seed limitation was also greater in the high resource region, suggesting that pre-dispersal seed predation might limit population size in the high resource populations. Our study suggests a predictable influence of environmental context (large scale) and conspecific density (local scale) on herbivore pressure with greater negative impacts in high-resource regions. These results improve understanding of context-dependent herbivory and the negative conspecific density dependence process.