Grazing and "Degradation"

Abstract

It is hard to see, given previous analysis (1), the point of redefining “nonequilibrium” systems as “all systems that are not at equilibrium,” as L. Gillson and M. T. Hoffman do in their Perspective “Rangeland ecology in a changing world” (Perspectives, 5 Jan., p. 53). This would apply almost everywhere and would be inconsistent with many publications. Few ecologists use “equilibrium” to mean that species track their resources precisely; they emphasize constrained variation in abundance, despite stochasticity (2). Correlations between regional and temporal variation in rainfall and herbivore abundance confirm equilibrial consumer-resource dependence when viewed at sufficient spatial and temporal scales (3–5). The real challenge is to distinguish the subset of resources in the system that limit large herbivore numbers from those that do not, and to understand how species of plant and animal in each subset respond to herbivory (6, 7). Crucially, a nonequilibrium area for one species may be a key resource for another.