Because of the complexity of the vegetation-grazer system and the long timescales of vegetation change, especially in semiarid rangelands, grazing models are an integral part of rangeland research. Conceptual models to guide management have reflected the concurrent concepts of theoretical ecology. In fact, the development of grazing models mirrored developments in theoretical ecology closely.
Early models to conceptualize grazing systems were based on equilibrium models, most notably models of ecological succession developed around 1940 (i.e., the Clementsian theory of ecological succession, Grassland Models) and predator-prey-type models developed in the 1960s. However, since the 1970s, the equilibrium paradigm of ecology (see Stability) was increasingly challenged. In the late 1980s, ecological theory became dichotomized into equilibrium and nonequilibrium camps. The first proposed that homoestatic biotic interactions (feedbacks) would cause a system to return to equilibrium after perturbation, the latter argued that environmental fluctuations and disturbances are stronger than any homoestatic biotic forces, leading to a highly stochastic system rarely at equilibrium. This dichotomy is still mirrored in the ongoing equilibrium versus none-quilibrium debate in rangeland ecology. Integral part of this debate is also the issue of spatial scale, the role of spatial heterogeneity in the plant-herbivore system.
Grazing models are as varied as the purposes for which they have been constructed, and cover various spatial and temporal scales and various degrees of detail. The objective of this article is to describe the historical development outlined above based on characteristic and influential examples of grazing models. We use the term 'grazing' in the sense of herbivory by large mammalian herbivores; this includes both grazing of herbaceous plants and browsing of woody vegetation. Reflecting the literature, we emphasize models of livestock grazing (in contrast to native game).
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