Cascading effects of grazing intensity on predatory arthropod and parasitoid densities

Abstract An important goal in arthropod conservation is to understand how anthropogenic activities affect arthropods. Livestock graze 29% of land area in the United States, which can result in both top‐down and bottom‐up effects on ecosystems that are grazed. Grazing intensity can either increase or decrease species richness and abundance of arthropods, depending on context and taxa, making generalisation difficult. The impacts of grazing intensity on different taxa may also be indirect, depending on trophic interactions with other members of the community. We propose that considering trophic relationships will help clarify the effects of grazing intensity on arthropods. We studied the effects of grazing intensity by ungulates on pompilid wasps ( Hemipepsis and Pepsis hawk wasps) and their hosts, tarantulas ( Aphonopelma hentzi ). We did this by comparing the cover of forbs (flowering plants that provide nectar to hawk wasps) and the densities of hawk wasps and tarantula burrows in areas grazed by cattle to areas with light grazing by wild ungulates in the short‐grass prairie. Grazed areas had lower cover of nectar‐producing plants, fewer parasitoids and more tarantulas, while lightly grazed areas had higher cover of nectar‐producing plants, more parasitoids and fewer tarantulas. We propose that hawk wasp abundance may track the abundance of nectar‐producing plants, enabling parasitoids to exert strong top‐down pressure on tarantulas in lightly grazed areas. Thus, grazing may benefit tarantulas by reducing the abundance of their parasitoids. This study provides a simplified system in which trophic cascades can be observed, demonstrating how anthropogenic land use can drive predator–prey dynamics in invertebrate communities. Additionally, it offers evidence for the impact of human‐modified landscapes on invertebrate biodiversity, highlighting the role of grazing in shaping ecological interactions in the short‐grass prairie.