In Chapter 3 the many influences at work which determine the properties and characteristics of soil types were outlined. In a similar manner, each biotic community represents the response of a set of species to a set of ecological factors. An ecological factor is defined as one having an influence, maybe decisive, on the presence, absence or performance of a species. This broad definition reflects the fact that there may be almost countless influences at work on a community: each varying in time and space and acting reciprocally with the other factors. Although some progress has been made in assessing the significance of certain ecological factors on some individual species or commu­ nities , we still know little about how every environmental factor operates on a single species, let alone a simple community.