Agricultural land use history and fire suppression strengthen herbivore impacts

Background/Question/Methods

Anthropogenic disturbances can cause significant changes to plant communities that persist for decades.  However, empirical studies examining the mechanisms driving anthropogenic changes focus largely on abiotic factors and competition, whereas the role of trophic interactions in the context of anthropogenic disturbance has received less attention.  Because herbivore impacts can be dependent on characteristics of neighboring plants, such as relative palatability, human activities that alter whole plant communities may change the strength of herbivory.  To evaluate if anthropogenic disturbances alter the strength of herbivory, we conducted a large-scale field study manipulating herbivore access to experimental plants (caged from insects or open) in longleaf pine woodlands with different land use histories (post-agricultural or remnant woodlands) and different degrees of fire frequency (high or low).  After six weeks, we measured herbivore damage and growth on four species of herbaceous plants that are common in longleaf woodlands.

Results/Conclusions

Our experimental results illustrate that agricultural landuse and prescribed fire regime both affect the strength of herbivory.  Herbivore impacts, measured as 1) the proportion of leaves damaged by chewing insects and 2) plant stem growth, differed among the four plant species tested but were generally greater on herbaceous forbs in human altered systems.  Herbivore exclosures significantly reduced damage and increased growth of Solidago odora in post-agricultural sites and fire suppressed remnant woodlands.  Herbivore damage to S. odora decreased as herbaceous plant cover at the site level increased, suggesting that herbaceous plants act to dilute herbivory in this system.  Pityopsis graminifolia growth increased inside herbivore exclosures in fire suppressed sites, regardless of land use history.  No significant herbivore effects were documented on the grass Schizachyrium scoparium or the legume Tephrosia virginiana.  Overall, these results suggest that the ecological effects of anthropogenic activities can strengthen the effects of herbivory, potentially affecting plant establishment in the substantial portions of the terrestrial biosphere that are affected by human agricultural disturbance.