Anthropogenic legacies mediate contemporary plant-herbivore interactions

Background/Question/Methods

Recent work suggests that herbivore impacts may hinge on characteristics of the plant community, such as composition or diversity. Therefore, anthropogenic disturbances that alter plant communities may influence the impact that herbivores have on these communities. Using a large-scale field study in the longleaf pine savannahs in the southeastern USA, we tested the hypothesis that herbivore damage will be reduced in diverse communities, but that this may depend on the composition of the plant community. Our study sites (50 X 50m separated by >500m within each location, n = 17 sites X 2 locations: NC and SC, USA) had either a history of agricultural disturbance (>50 years since abandonment) or were undisturbed remnant savannas, stratified across a gradient of plant diversity. The vegetation compositions of these two land use types are distinct, allowing us to test how the legacies of anthropogenic disturbances mediate current ecological processes. We quantified grasshopper abundance at each site using walking transects. Herbivore damage was measured on Solidago odora—a widespread plant present in consistent, but relatively low, abundances across sites—by visually estimating the percent area of leaf tissue removed by chewing insects.

Results/Conclusions

Historical agricultural disturbance reduced current plant diversity and marginally reduced grasshopper abundance. Although herbivore damage on Solidago odora increased with increasing grasshopper abundance, it was not significantly predicted by grasshopper abundance. Rather, herbivore damage depended on the interaction between plant diversity and the land use history of the site. In remnant sites, increased levels of plant diversity reduced the amount of herbivore damage on the focal plant species. This protection against herbivores did not occur in disturbed sites, suggesting greater vulnerability of palatable plants in depauperate sites. These findings provide key insights into plant coexistence mechanisms by indicating that herbivore foraging may be diluted across species, and thereby maintaining plant diversity, where plant diversity is high. In contrast, herbivore foraging may be concentrated on palatable species thereby maintaining low levels of plant diversity in disturbed sites. Broadly, these results show that anthropogenic legacies can alter current ecological processes and reinforce the need for preservation and restoration of biodiversity, particularly on the ever increasing area of the earth that has been altered by anthropogenic activities.