Insect responses to grassland restoration using fire and grazing: Short and longer-term perspectives

Background/Question/Methods

Although grassland insects evolved in the context of both grazing and fire, the appropriate use of grazing and fire in restoration and management of tallgrass prairie ecosystems has been debated among those concerned with protecting insect communities. We established an experiment to test insect responses to three grassland management treatments and to assess community recovery within degraded grasslands over time.  The treatments included 1) patch-burn graze (burning of spatially distinct patches and free access by cattle), 2) graze-and-burn (burning of entire tract with free access by cattle), and 3) burn-only.  We also evaluated effects of time since fire, grazing history, remnant history and pre-treatment vegetation characteristics, which were assumed to be a legacy of prior land-use.  Butterflies (Lepidoptera), ants (Hymenoptera: Formicidae), and leaf beetles (Coleoptera: Chrysomelidae) were surveyed to compare their short-term (1-3 yr.) responses to these treatments as measured by abundance, richness and species diversity.  Each of these taxa is relatively diverse and was expected to have the potential to have strong negative responses to grazing and burning, but we predicted more positive responses to patch-burn grazing.  We were also able to evaluate both short (1-3 yr.) and longer-term (5 yr.) responses from the perspective of the butterfly community.

Results/Conclusions

Within the short-term, land-use legacies affected insect abundance, richness and diversity, but treatments did not.  Ant abundance was lower in tracts with a history of heavy grazing. Ant species richness was positively associated with pre-treatment time since fire and vegetation height and negatively associated with pre-treatment proportion native plant cover. Butterfly abundance was positively associated with pre-treatment litter cover. Leaf beetle diversity was positively associated with pre-treatment native plant cover, and leaf beetle abundance was negatively associated with time since fire.  However, over the longer term, we quantified temporal trajectories in the butterfly community that showed an increasing similarity between recovering grasslands and the higher quality remnant sites we used as benchmarks.  Our results indicate that 1) in the short-term, land-use legacies can exert significant influence on grassland insect community composition and thus should garner more attention in grassland management, 2) conservation of grassland insect communities could be improved by taxon-specific analysis of land-use legacy variables, and 3) the influence of landscape legacies may be reduced with time, but long-term monitoring may be essential to tease out these responses.