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The complexity of ecological systems requires investigators to deal with aggregates: individuals aggregated into populations or populations aggregated into communities or trophic levels. A number of theoretical studies have indicated that dangers of dealing with the dynamics of aggregated systems. These studies have focused on mathematical models so the results have been of marginal interest to the field ecologist. The present paper develops practical criteria which can be applied when the underlying model is unknown or of little immediate interest. Our approach has been to examine 40 alternative models and to state conclusions that are valid, no matter what the underlying dynamics of the system. Theoretical studies indicate that no error is introduced when two components with identical turnover times are aggregated. This study shows that, for certain problems, the components can be aggregated with <10% error, even if the turnover rates differ by a factor of three. Criteria can be stated even more practically in terms of the ratio between the standing cropos of the components. When measurement errors are considered explicitly, the criteria depend upon the accuracy with which aggregated and disaggregated measures can be estimated.

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