The Community Approach

The second viewpoint of alternative stable states in the ecological literature has had a different conceptual approach to the question of how communities move from one state to another. The origins of this viewpoint are found in the population and community ecology literature, so we refer to it as the 'community approach'. Once again, the ball and its movement represent the current state and dynamics of the community. Unlike the former view, here the surface remains static with multiple basins of attraction situated in fixed positions. Since the surface remains static, the ball can only move between basins by being actively 'pushed' out, and this can only arise through direct perturbations to the state variables (usually the actual densities of the populations in the community). The ball is less likely to persist in some basins of attraction longer than others because some are shallower and thus smaller and more frequent perturbations permit the ball to escape (Figure 1b).

On a static surface, as envisioned by the community perspective, there can be no direct analog of hysteresis as described in the 'ecosystem approach'. A closely related phenomenon can arise, however, when there are asymmetries in the configurations of basins of attraction. For example, in a shallow basin, perturbations to population densities might easily move the ball up and over a barrier and into a deeper basin (Figure 1b). In this case, a return to the shallow basin is less likely because a more intense perturbation would be required to move the ball up the steeper slope and back into the original basin. Thus, due to topographical asymmetry, equal and opposite perturbations to state variables can have quite different results depending on the nature of a community's state when the original perturbation is applied (Figure 1b).

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