Herbivores are generally thought to enhance plant diversity by their direct consumption of competitively dominant plant species and indirect effects on plants competition. Consequently, management of herbivores has become a crucial component in efforts to restore or maintain biodiversity, particularly in grasslands. However, other studies suggest that herbivores sometimes have weak or even negative effects on plant diversity. Olff and Ritchie reviewed recent work and concluded that herbivores appear to affect plant diversity through their impact on dominant plant species, plant regeneration opportunities and propagule transport. The strength of their effects range from weak to strong across a continuum from small to large herbivores. Furthermore, these effects depend on regional variation in major habitat characteristics, such as soil fertility and water availability, which determine herbivore abundance and the number of plant species that have evolved herbivore avoidance or tolerance strategies. These generalizations may lead to more understanding of why similar herbivores positively influence diversity in some environments but not others. They may also help to understand more of the magnitude and direction of the effects of different types of herbivores and the spatial and temporal scales at which they operate. The authors suggest that placing the results of individual studies in this framework should help to define contexts for biodiversity management and help resolve management controversies that arise from comparing conflicting individual studies.
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