The self-identity of ecological units

Ascertaining whether an ecosystem, a community or any other ecological unit has completely changed, whether it has been destroyed or whether it remains 'the same' is a common task for ecologists. At the heart of this task is the question of what constitutes the identity of a unit through time, which we call self-identity. The determination of self-identity requires that we know what the 'essence' of an ecological unit is. This is highly dependent on the definition and specification of the ecological unit of interest, which in turn, depends upon the question(s) being asked. We develop a framework to provide unambiguous definitions of ecological units. The components of this framework are different criteria describing the spatial or process-based boundaries of a unit, the degree of required internal relationship and the level of abstraction at which an ecological unit is specified. These criteria can be represented in a three-dimensional graphical model which locates the definitions of ecological units within a conceptual volume. We describe the application of the model for the assessment of the self-identity of ecological units through time. The specification of ecological units and the possibility of assessing the self-identity of ecological objects are also dependent upon the specific ecological theories applied (and upon issues that reside outside the realm of pure ecological science). The universe of meaningful questions and of ecological units is constrained by both societal and scientific interests. We argue that a precise specification of ecological units is necessary for advancing both ecological understanding and its application.