The sequences of events that comprise ecological processes occur across differing expanses of space and are resolved over different periods of time. This realization has led to the incorporation of the idea of 'scale' as a tool for understanding these spatiotemporal complexities. Although the concept that ecological processes 'occur on different scales' has been discussed widely in the ecological literature, it is often used in different contexts to mean different things. Underlying this confusion is the lack of a well-
developed measurement theory applicable to ecological systems.
Scale, in its most precise sense, refers to the units of space and time used to measure an ecological system. Because units of measurement are arbitrary, most ecolo-gists use the standard International System of Units (SI) to measure ecological processes. Six of the seven base SI units are of particular interest to ecologists. Of these, length (m), mass (kg), time (s), amount of a substance (mol), and temperature (K) are used most frequently. Given these base units of measure, all ecological processes and state variables can be expressed in terms of derived units, which incorporate some combination of the base units.
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