Question: Which factors influence the effectiveness of biotic refuges for harbouring grazing-sensitive species in pastures with a long history of grazing by large herbivores? Previous research showed that spiny clumps of the cactus Opuntia polyacantha provided refuges from cattle grazing for plants and for inflorescence production on short-grass steppe. In this paper, seven factors that may have a potential positive influence on the refuge effects of cactus at a landscape scale were assessed.

Location: Short-grass steppe of the Great Plains of North America.

Methods: The study was conducted in eight long-term grazed pastures and their respective ungrazed controls that were established 60 years ago.

Results: Heavy grazing intensities were necessary for some positive effects of cactus to manifest, and some refuge effects changed to negative effects under lower grazing pressure. Refuge effects increased with plant community productivity due to greater abundances of grazing-sensitive species, and greater grazing intensities in the more productive areas. Cover of cactus cladodes (spine-covered pads) inside clumps appeared to be the main limiting factor for refuge effects, probably by limiting available space for grazing-sensitive species in the clumps. Other factors such as size and density of cactus clumps, and the presence of large refuges in the proximity of clumps had minor influence on the effectiveness of cactus refuges.

Conclusions: The effects of biotic refuges largely varied with ecological conditions and structural characteristics of the refuge. Refuge effects were mainly influenced by grazing intensity, plant community productivity, and structural characteristics of the biotic refuges. A conceptual model of factors influencing refuge effects at a local landscape scale in plant communities grazed by large herbivores is presented.