ABSTRACT
Communities change. The nature of this change is debatable. The classic view saw climatic climaxes and balanced ecosystems resulting from unidirectional succession. Succession is a complex process and is explained by at least three models-the facilitation model, the tolerance model, and the inhibition model. It may also be driven by factors external to the community (allogenic factors). Primary succession is the colonization of land or submarine surfaces that have never existed before. Secondary succession is the invasion of newly created surfaces resulting from removal of pre-existing vegetation. A modern view of community change stresses the disequilibrium behaviour of communities. It sees succession going in many possible directions, and sees communities as temporary collections of species that assemble and disassemble as the environment changes. Much community change has been caused by land cover transformation over the past two hundred years. Two important aspects of this transformation are habitat fragmentation, and it attendant effects on wildlife, and the loss of wetlands. Global warming during the twenty-first century is likely to put peripheral, geographically restricted, highly specialized, poorly dispersive, and climatically sensitive species under intense pressure. It will also cause significant changes in many communities. Wetlands, tundra, and alpine meadows are especially vulnerable.
