This chapter analyzes the benefits and shortcomings of inferring a temporal trend from a study of different aged sites. This technique, called space-for-time substi- tution, assumes that spatial and temporal variation are equivalent. Although this assumption has been challenged, studies continue to rely on space-for-time sub- stitution due to necessity or convenience. To evaluate the utility and soundness of space-for-time substitution, I review a variety of studies. Studies of vegetation succession illustrate the generalizing power of space-for-time substitution, but point out the power of direct long-term studies for exposing mechanisms. Space-for-time substitution was initially justi- fied by a deterministic theory of succession, but long-term studies have helped replace that theory. Other systems show a different balance between long-term studies and space- for-time substitution. The utility of space-for-time substitution to generate hypotheses about pattern and mechanism is shown in marine upwelling systems. In a chronosequence of strip-mined sites, space-for-time substitution erroneously suggested a succession; historical factors explained much of the temporal pat- tern. Certain forecasts of acidification and recovery of surface waters are subject to the limitations of space-for-time substitution, as is the quantification of other regimes of stress and disturbance. Much of the problem in evaluating space-for-time substitution lies in using time as a surrogate for the past environment and prior system status. Improved determination of the history of sites constituting a chronosequence can help assess the validity of space-for-time substitution. However, certain transient effects can only be determined via long-term studies. Determination of history can also improve understanding of systems not usually considered to have a significant time dimension. For example, debris dams and organic storage in streams introduce a past to certain aquatic systems. The impact of temporal differences in fish recruitment can cascade through trophic webs in lakes and affect productivity over the long term. In summary, space-for-time substitution has been successful where general or qualitative trends are sought, or when hypotheses are to be generated. It has failed where unrecognized effects in the past of a system were oflarge magnitude. Space-for-time substitution has appeared to be irrelevant where the past has had unsuspected effects. Analysis of the influence of past environments or prior system status is needed to justify space-for-time substitution, link short- and long-term studies, and frame and interpret direct long-term studies. The two techniques are not strict alternatives.
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