Milchunas and Noy-Meir developed a conceptual framework of external plant avoidances to herbivory, based on foraging selection impedances (associational avoidances), behavioral impedances (indirect avoidance), and physical impedances (refuges), organized along axes of efficiency, degree of protection, and necessity of tolerance characteristics. Associational avoidances are uncommon for terrestrial mammalian herbivores compared to plant-insect or marine situations. Indirect avoidances mediated through herbivore territoriality, predator avoidance, and other behaviors independent of foraging decisions are probably common in nature, but few have been formally documented. Biotic and geologic refuges providing a physical impedance are the only avoidances shown to have implications for plant biodiversity. In a survey, 86% of small refuge studies reported positive effects on plant diversity compared to 50% for large refuges. Geologic refuges in more productive environments were more important in protecting diversity than refuges in less productive, semiarid environments, and the effects of protection were greater in communities with short compared to long evolutionary histories of grazing. The authors conclude that is important to note that their evaluation of geologic refuges was based on richness and the harboring of rare or threatened plant species. Grazing animals have small to large effects on species composition that can be independent of effects on richness, and biomass removal by grazers always influences community physiognomy. Therefore, refuges will always have some influence on structural diversity at the larger landscape level.
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