Rangeland models

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The range succession model. The range succession model based on the Clementsian theory of ecological succession formed the conceptual framework for most grazing management up to the 1980s. It is supposed that, in the absence of grazing, a rangeland has a single persistent state (the climax), whereas grazing causes continuous and reversible transitions of the grassland state along a single, monotonic gradient between an undisturbed climax and an overgrazed subclimax vegetation state. Therefore, the grazing pressure can be made equal and opposite to the successional tendency, producing an equilibrium in the vegetation at a set stocking rate (Figure 3 a). A sustainable yield of livestock products can be harvested from such an equilibrium. The model recognizes that vegetation is affected when rainfall varies from year to year and supposes that grazing and inter-annual variation in rainfall cause equivalent changes in the vegetation. Therefore, management should respond to drought by reducing grazing.

State-and-transition models. Over the years, however, substantial empirical evidence accumulated of cases where the assumptions of the range succession model were not met, especially in arid and semiarid environments. To deal with the complex dynamics of semiarid and arid ecosystems, scientists such as M. Westoby, B. Walker, and I. Noy-Meir suggested (by the end of the 1980s) that these ecosystems could be described in terms of discrete states and inter-state transitions (Figure 3 c). Transitions could be triggered by natural events (e.g., rainfall, drought, and fire) or by management actions (e.g., removal of herbivores, altered intensity or timing of herbivory, and burning). Changes in range condition are not unidirectional, but multiple pathways of system transitions to alternative states may exist, depending on the particular sequence of driving events (Figure 3 c).

Grazing

Early succesional

Ungrazed climax

Rainfall

Early succesional

Ungrazed climax

Palatable' ^Breon. (incr perennials (incr. II)

Poor condition

Grazing gradient

Moderate

Good

Palatable' ^Breon. (incr perennials (incr. II)

Poor condition

Grazing gradient

Figure 3 Illustration of different conceptual rangeland models. (a) Range succession model. Vegetation changes are reversible and unidirectional in response to grazing and rainfall. (b) Degradation gradient model for South African grasslands. The system responds to smaller perturbations (grazing and rainfall) unidirectionally and reversibly as predicted by the range succession. Larger perturbations (drought, overgrazing) may cause the system to cross an irreversible threshold where changes in soil conditions (related to reduced basal cover) and species composition may hinder the system to return. The pathway of degradation is unidirectional. (c) State and transition model. Multiple equilibria and multidirectional pathways of degradation in response to different driving events. S1, S2: good condition domains; S3, S4: moderate condition domains; and S5, S6: poor condition domain.

Moderate

Good

Such state-and-transition models are valuable tools for describing the structure of the ecosystem to identify irreversible transitions and alternate stable states; however, they provide little information applicable to forecasting and prediction. Additional models are needed to quantify the temporal scales of the transitions, to identify rare events that drive semiarid and arid ecosystems, and to improve the user's understanding of ecosystem dynamics over a long temporal scale.

Degradation gradient models. The degradation gradient model combines in some sense the concepts of Clementsian succession and state-and-transition models (Figure 3b). It is based on the idea that vegetation compositional changes along a grazing gradient are indicative for the ecological condition. This statistical method was developed in the 1990s by O.J. H. Bosch and H. G. Gauch for the semiarid South African grasslands. Key element of the approach is a classification of the species according to their response to grazing as increaser and decreaser species (Figure 3b). This classification is derived by means of multivariate statistics. Species composition data are collected from grasslands in various stages of degradation. Based on a series of statistical analyses of these data, the degradation gradient is constructed. The model recognizes that irreversible transitions may exist which are caused by soil loss or major changes in floristic composition. This method is used for range assessment where species composition data of a sample is compared to the gradient.

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