Habitat quality and herbivory interact to have species-specific effects on plant growth

Background/Question/Methods

Uncovering the forces reducing biodiversity likely depends on understanding the interplay between habitat context and biotic interactions.  For example, past and present land-use may alter plant dynamics, as well as the nature of plant-herbivore interactions.  However, this remains largely unexamined, particularly at spatial scales relevant to management.  We evaluated the role of land-use history in affecting herbivory by using a landscape experiment that manipulates canopy closure (thinned and unthinned) in remnant (never cultivated) and post-agricultural (50+ years since abandonment) longleaf pine woodlands (n=64 one-hectare patches).  In each patch, we transplanted seedlings of four species of Asteraceae (n=2,112 total seedlings) that vary in their growth rates, and manipulated access to insect herbivores (open or excluded).  We hypothesized 1) that habitat quality would mediate herbivory rates, with higher herbivore abundance leading to higher herbivory rates in thinned patches and 2) that plant species with slower growth rates would be more vulnerable to herbivores during the initial stages of establishment.

Results/Conclusions

We found that thinned patches had greater grasshopper abundance, sunlight availability, and total standing herbaceous biomass.  Despite lower grasshopper abundance, herbivore damage was greater on plants in unthinned patches, resulting in a stronger herbivore effect on plant growth.  Higher herbivory rates on focal plants in unthinned patches likely resulted from lower overall food availability in these patches (i.e., lower standing herbaceous biomass).  Furthermore, plant growth appeared to suffer from both herbivory and low-light conditions in unthinned patches.  However, these herbivore effects were species-specific.  One plant species was not affected by herbivores; two species experienced reduced growth when exposed to herbivores, but only in unthinned (i.e., low light) patches; and the slowest growing species experienced reduced growth when exposed to herbivores in both thinned and unthinned patches.  These results indicate that herbivory and light availability interact to regulate plant community dynamics in contemporary landscapes with disproportionate deleterious effects on slower growing species.