Grazing land managers use knowledge of how grassland plant communities change over time to influence these changes. Accumulated knowledge of plant community change in both moist and arid grassland environments have identified two primary ecological models for this change. These are known as the succession model and the state-and-transition model (Clements 1936; see Sidebar).
In the successional model, biodiversity is considered to be highest at the climax community although earlier stages of succession may support a different suite of species. Since disturbance is thought to knock communities into earlier successional stages the occurrence of some moderate level of disturbance, patchily distributed throughout the habitat will encourage the persistence of a variety of successional stages thereby increasing the landscape level heterogeneity. So the greatest landscape level biodiversity is found in the successional model with a moderate level of disturbance.
In contrast, a state and transition model holds that not all disturbance moves the community in the same direction (not all roads lead to Rome) and not all disturbed communities will eventually proceed to the same climax community (time does not heal all wounds). Instead, a state and transitional model must be developed with local understanding of where landscapes tend towards, where they get stuck, what potential high biodiversity states are possible, and what events seem to kick landscapes from one state towards another.
Land managers can use these concepts to anticipate how management actions will change plant communities. A key component to this is an understanding of your local grazing lands and how they change under various management or disturbance regimes. Resource specialists can facilitate this local knowledge by developing models of potential habitat states, including transition pathways between them.
Sidebar: Succession and State-and-Transition Theories to Plant Community Change
The succession theory (Clements 1936) describes plant community change as rather steady and predictable change from fast-growing, weedy pioneer plants to more complex and stable communities of desirable climax plant species. For practical purposes for management, change in the community is continuous (one condition leads to the next), reversible, and linear. Decrease in disturbance or increase in precipitation is thought to support higher climax communities. Exceptions to this theory have been found in disturbed arid grasslands (Westoby 1989). To address this Westoby (1989) proposed a state-and-transition model to describes plant community change as less predictable and non-linear with more than one potential trajectory. Change between states is abrupt, difficult or impossible to reverse, and often dramatic, leading to multiple different possible steady states after a transition. For practical purposes, the state-and-transition model recognizes that degraded or altered states may only be changed with intensive intervention.