TOWARD AN ECOLOGICAL DEFINITION OF AN ISLAND - A NORTHWEST EUROPEAN PERSPECTIVE

The equilibrium theory of island biogeography of MacArthur and Wilson aimed at providing a unifying framework for study insular ecology. However, the theory does not elaborate the concept of an 'island' in an ecological framework. I suggest that different types of islands be ecologically defined on the basis of processes relative to which some ecologically interesting entities on those 'islands' actually are isolated from their surroundings. Four main island scales can be discerned, namely, the scales of (1) individuals, (2) population dynamics, (3) population differentiation, and (4) evolutionary divergence. I illustrate these different island types and their interfaces with examples mainly from northwestern Europe. The equilibrium hypothesis seems relevant primarily on the population dynamics scale. A research programme following such differentiation would include two phases: first, assessment of factors which make the environment studied 'insular', and second, construction of specific models for analysing ecological processes in those particular insular conditions.