Even when a habitat or ecosystem is not directly destroyed it can be degraded or disturbed as a result of human activities. Our increasing influence over the environment is such that few corners of the earth or depths of the oceans remain that can truly be classified as undisturbed. Disturbance takes many forms; we concentrate here on chemical pollution, introduction of exotic species and diseases, and most recently, genes.

By reading this chapter students will gain an understanding of the range of agents that cause disturbance to natural systems, the mechanisms of disturbance, and be introduced to examples illustrating the variety of impacts disturbance has on species and communities.

Introduction

Ecosystems and the communities they contain can be disturbed by human activity in a variety of ways. The term disturbance is used here to mean the alteration of the natural dynamics of systems. It is distinct from destruction, in that the system remains recognisable, although the level of degradation may be severe. Agents of disturbance include chemical pollutants, introduced species, diseases and genes, each of which are considered below. Disturbance is also manifest on very different scales. Some air pollutants are distributed globally, whereas some heavy metals cause much more localised pollution events. Some species including pathogens have colonised virtually the whole of the land surface along with humans, whilst others have been specifically introduced to small islands with disastrous consequences. Most recently we have the prospect of genetically modified organisms being released into the environment with, as yet, undetermined consequences.