Variable impact of late summer fire and post-fire grazing intensity on grasshopper populations in a northern mixed-grass prairie

Grasshoppers are highly responsive to altered habitat structure, with both burning and livestock grazing shown to influence grasshopper population density and community composition. Responses of grasshoppers to late summer fire and post-fire grazing intensity were examined in a replicated field experiment in eastern Montana. The four treatments included fire with 0%, 17% and 50% vegetation utilization, as well as no fire with 0% utilization. The experiment was replicated at adjacent sites burned in August 2003 and 2004. Grazed plots were stocked with ewes in the summer following burning. Grasshopper population density and species composition were assessed pre-fire and for two years post-fire. Post-fire grazing intensity did not affect grasshoppers following the 2003 fire. After grazing was initiated in the plots burned in 2004, grasshopper population densities were significantly lower in 50% utilization plots compared to ungrazed plots. Following the 2003 fire, grasshopper abundance was 74% lower in burned plots in the 1st year post-fire and 51% lower in the 2nd year post-fire. Densities of the two most abundant grasshopper species were reduced over 80% with burning. In 2004 when fuel biomass was low due to drought conditions, burning did not significantly reduce post-fire grasshopper population densities or affect post-fire species composition. Importantly, fuel biomass was low due to drought conditions in 2004. The effects of burning on grasshopper populations were influenced by drought effects on fuel load at the time of burning. Burning when biomass is abundant appears to reduce population densities of some, but not all, grasshopper species.