Shrubs v.s. Grasses on Savannas

From RAWorkbook

Savanna ecosystems are found around the world in areas of low to moderate rainfall. They are characterized by sparse tree cover and a rich, diverse ground cover of grasses and forbs.

For thousands of years, humans have used savanna systems for livestock production, primarily cattle or sheep. In many cases humans can control the amount of grazing that occurs, ranging from a few animals per unit area to many. The level of grazing (or grazing pressure) can lead to dramatic changes in the dominant type of vegetation (e.g. shrubs and trees versus grasses).

The savanna system can exist in different configurations - measured by the relative amount of grasses and shrubs. If there are more grasses, the system is considered to be in a grass-dominated state (top image). This state may persist for long periods of time with recurring fires, even with grazing. Hence it is a stable (or quasi-stable) state. Other states, such as a shrub-dominated state can also exist. These states and the transition between them are controlled primarily by interactions between grazing pressure and fires (bottom image).

In the grassy state, the balance of grass and shrubs is maintained by frequent fires and a relatively low grazing pressure. As more animals are added to the pasture, more of the grasses which fuel the fires are removed. The removal of fuel and changes in its spatial pattern limits the system’s capacity to carry a fire. In the absence of regular fire events, more woody shrubs become established. The shrubs inhibit grass growth and do not carry a fire. Thus the shrub-dominated state of this system can persist for decades and may only return to a grass regime through direct manipulation of woody vegetation. Note that the woody state of savanna systems no longer provides the grazing benefits of the grass-dominated state. Hence the woody state is not as desirable for continued livestock production.

Alternate states are identified by a shift in dominant organisms or system structure and a change in the processes that reinforce a particular state. In particular, they are identified by a change in the nature or intensity of feedback in the system. For the savanna example the change in feedback has to do with the feedback from the amount of grass to the intensity of fire. The alternate regimes are characterized by dominance of grasses, or shrubs. Alternate states do not only exist in the ecological communities, as shown in the savanna example. In chapter 1, an exercise was undertaken to identify historical eras or periods. These different periods represent alternative configurations or regimes (or states) in the social or ecological components of these managed systems. In some systems transformation to an alternate regime may be desired.