Introduction

Ecology deals with relationships between plants and animals and between them and the places where they live. Consequently, many questions of interest to ecologists call for the investigation of relationships between variables of two distinct but associated kinds. Such relationships may involve those, for example, between the plant and animal constituents of a biotic community. They might also involve, as in plant ecology, connections between plant communities and their component species, on the one hand, and characteristics of their physical environment on the other. As another example, comparative relationships among a number of affiliated species or populations with respect to a particular treatment regime in a designed experiment might be studied. In more general terms, the question which arises calls for the exploration of relationships between any two or more sets of variables of ecological interest.