Effects of patch burning season and livestock grazing on grasshopper populations in northern mixed prairie

Background/Question/Methods

Rangeland management practices such as burning or livestock grazing have the potential to manipulate the habitat of grasshoppers and/or their predators and thus impact population densities and community composition. Few studies have examined the effects of fire and grazing on grasshopper population dynamics in the northern Great Plains, especially during population increases. Interest in fire stems from its use as a management tool and unplanned events; however, there is increased interest in using fire to shift grazing animal distribution and to increase the structural heterogeneity of rangelands. As part of a study examining patch burning effects on plant community dynamics and grazing distribution in northern mixed prairie in Montana, we examined how patch burning and livestock grazing affects grasshopper population dynamics. Homogenous and heterogeneous management treatments were assigned to six pastures, with heterogeneity created by burning 25% of each treatment pasture in a given year. Patch burns occured during spring and fall of 2009. Total grasshopper density was estimated by counting the number of grasshoppers within a series of 60, 0.1m² aluminum wire rings in each plot, with sweep net samples used to establish grasshopper community composition.

Results/Conclusions

Patch burning effects on grasshopper populations differed significantly between the spring and fall patch burn, likely due to differences in the timing of fire. There was no indication of strong effects from a spring patch burn on grasshoppers, when the most abundant grasshopper species would have been in the egg stage. Although a few grasshopper species overwinter as nymphs, these species were not dominant in these pastures. By contrast, fall patch burning negatively affected grasshopper population densities but there was little evident impact of patch grazing. The examination of species diversity metrics was constrained by the low replication in the study, but the effects of cattle grazing on grasshopper populations appeared smaller than patch burning.