ABSTRACT
Disturbance has been a popular theme in ecological studies over the last two decades (e.g. Barrett et al. 1976). A working definition of disturbance is any event that disrupts the ecosphere or an ecospheric system. In the present context, it is any event that disrupts a geoecosystem. It would include environmental fluctuations and destructive events (White and Pickett 1985:6). In the ‘brash’ formula, disturbance is a driving variable, even though it may originate from within the geoecosphere. Furthermore, disturbing agencies may be physical or biological. Grazing, for example, may be defined as a biotic driving variable of vegetation communities, although it belongs to the same geoecosystem as the plants that it disturbs. Plainly, disturbance may result from external or internal geoecosystem processes. It is useful, however, to think of disturbance as a continuum between purely endogenous processes and purely exogenous processes (White and Pickett 1985:8). At the exogenous end of the continuum is classical ‘disturbance’. This is demonstrably the outcome of exogenous processes acting at a particular time, creating sharp patch boundaries, and increasing resource availability. At the endogenous end of the continuum is internal dynamics of communities, exemplified by Watt’s (1947) cycles of vegetation change.
