On Equilibrium in Ecological history

The slow response of vegetation to climate change has interesting implications. If climate changes continuously, as it appears to, the vegetation may never succeed in catching with it. In the words of Margret Davis, plant (and also animal) communities are "in disequilibrium, continually adjusting to climate and continually lagging behind and failing to adjust to equilibrium before the onset of a new climactic trend."

This opinion is not universal. The opposing point of view has been advanced by H. E. Wright, Jr., another leader in the field of Quarternary paleoecology. He assumes that vegetation and climate are at present in equilibrium and describes ancient communities that had what appear (to us) to be mismatched mixtures of species as "disharmonious." The implication is that modern mixtures are harmonious. The argument in favor of this view is that climate changes in stepwise fashion and the last step was taken a long time ago; therefore, because the climate has not changed appreciably for a long time, vegetation has by now had time to come into equilibrium with it.

There is a wealth of evidence, however, showing that climactic change is never ending. Even if major climactic "steps" are comparatively quick, it is almost certain that the climate in the intervals between steps undergoes continual lesser changes. In the light of present knowledge, therefore, Davis's view, that disequilibrium in ecological communities is much commoner than equilibrium, is the more acceptable.

It should lead, in time, to a much needed change in popular thought. The notion espoused by so many nonprofessional ecologists-that the living world is "marvelously" and "delicately" attuned to its environment-is not so much a scientifically reasonable theory as a mystically satisfying dogma. Its abandonment might lead to a useful fresh start in environmental politics.

From After the Ice Age: The Return of Life to Glaciated North America. By E. C. Pielou (University of Chicago, 1991) pp. 100-101