Burn regimes influence the functional diversity of insect herbivores

Background/Question/Methods

Fire is an important disturbance known to affect species composition in many ecosystems. For herbivorous insects, fire can have direct effects through disturbance and subsequent recolonization, or indirect effects by altering plant community composition. Evaluating the response of functional traits to disturbance can elucidate the mechanisms underlying community assembly and ecosystem functioning. While the concept of functional traits has been widely embraced by plant ecologists, there has been less research investigating the trait-based ecology of terrestrial arthropods. Here we evaluate how insect herbivore traits change across a prescribed burn regime. Specifically, we predict that if herbivores respond indirectly to fire through changes in the plant community, functional traits related to their feeding niche should correlate with time since fire. However, if herbivore communities respond directly to fire through disturbance and recolonization, functional traits related to dispersal should correlate with time since fire. To test these hypotheses we used sweep nets to sample the grasshopper communities in the Florida longleaf pine savannah at the Ordway-Swisher Biological Station. We sampled 15 sites (three samples per site) that experienced different prescribed fire treatments at two time points across one season. Grasshoppers were IDed to species and traits were measured on individuals.

Results/Conclusions

The community weighted means of the trait values of the grasshopper communities changed based on the burn frequency of the site being sampled. We found that the effect of fire disturbance on grasshopper trait values is indirect, mediated by changes in the plant communities of sites experiencing different burn intensities. This suggests that insect herbivore assemblages form based on the composition of the plant community. Because the trait values of the herbivores change from site to site, it is likely that herbivore functional traits and plant functional traits are linked. Understanding how and what traits are linked can uncover the mechanisms governing how communities assemble and allow us to make informed predictions about community assembly in other systems.