ABSTRACT
Ecosystems exist in a constantly changing world. Change results from processes in the abiotic or physical environment, from changes in the biotic environment, the living ecosystem community, and by human actions. We might consider the last of these to be a kind of biotic environmental process, but because of the extent of the human impact on the biosphere, and the human perspective of knowledge, it is more appropriate to consider human impacts on ecosystems as a separate category of disturbance. Therefore disturbance is both a natural and normal part of the environment, and in the cases of human impacts, increased in scale and accelerated in effect. Disturbance is defined as any influence on an ecosystem which increases the probability of destruction of biomass of the organisms present (see Box 6.I). In disturbed ecosystems these influences may be either biotic or abiotic. For example, grazing is a strong disturbance pressure on plants in a prairie ecosystem, but so are abiotic disturbances, e.g. caused by lightning-induced grass fires.
