Differential effects of livestock grazing intensity on invertebrates in the Pacific Northwest Bunchgrass Prairie: Results of a large-scale manipulation

Background/Question/Methods

Developing sustainable grazing practices for public and private lands depends on a comprehensive understanding of how livestock grazing intensity affects biodiversity and ecological function of many taxa, including invertebrates. Although invertebrates are the most diverse animal group and play significant roles in many ecological processes, including pollination, decomposition, and food provisioning, relatively little is known about their responses to livestock grazing intensity. The goal of our study was to examine how a gradient of grazing intensity in a native bunchgrass prairie affected the abundance of several common and ecologically important invertebrate groups, including spiders, grasshoppers, beetles, butterflies, and true bugs. To address this question, we conducted a large scale manipulative experiment at the Zumwalt Prairie in northeastern Oregon, one of the largest remnants of the Pacific Northwest Bunchgrass Prairie. For two summers, we exposed 16 40 ha pastures to four levels of cattle stocking rates (no cattle, and low, medium, and high stocking rates). Invertebrates were sampled in each pasture using pitfall traps, sweep net sampling, and visual observations along transects in early and late summer of each year. We also measured responses in soil and vegetation characteristics that we hypothesized would influence invertebrate responses.

Results/Conclusions

Taxa showed differential responses to grazing intensity. The abundance of several taxa significantly declined with increased grazing intensity, including some common predators (e.g., spiders), herbivores (true bugs), and pollinators (butterflies). However, other ecologically significant taxa appeared relatively insensitive to grazing intensity, including ground and carrion beetles, short-horned grasshoppers, and leafhoppers. Both soil and plant characteristics showed significant responses to livestock grazing intensity. Soil compaction and the amount of bare ground increased with increased grazing intensity, while soil stability, the amount of litter, vegetation structure, and the availability of floral resources decreased. This study suggests that responses of invertebrates to grazing intensity are not explained simply by functional group or trophic level, but are a complex combination of interacting factors that include diet specificity, microhabitat requirements, and other aspects of life history.