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Father of a successful idea: Henry Cowles. Credit: AMERICAN ENVIRONMENTAL PHOTOS/UNIV. CHICAGO

Henry Chandler Cowles first developed the idea of succession in his studies around the shores of Lake Michigan, where the gradual shrinking of the lake since the last glaciation had progressively exposed new terrestrial habitats. The developing dune ridges represented a series of stages in the colonization of the new land. Vegetation changed from initial specialist grasses to poplars (cottonwoods), then to pines and finally the oak forest that forms the region's 'climax' vegetation. Cowles, very reasonably, took the spatial arrangement of these communities of plants to reflect their progressive development through time. And herein lies one of the great problems in the study of succession, for the timescale of this type of vegetation development (centuries, perhaps running into millennia) is far beyond the lifespan of humans, so the process must be followed by proxy rather than by direct observation.

An important feature of succession is its predictability. Ecosystems pass through a series of stages and eventually stabilize at a predetermined end-point, the climax. This point is essentially controlled by climate, although other factors such as soil conditions may lead to local variation. But the stages may simply be artefacts of the time dimension, mere snapshots in a continuous process. Similarly, the impression of a series of communities may be a mental imposition determined by our temporal limitations: species of plants, animals and microbes may actually arrive and depart in isolation rather than in concert.

The apparently deterministic nature of succession could be due to our own need to seek pattern. Indeed, studies of wetland successions, which leave fossil records of their courses in stratified lake and peat sediments, show that various pathways of development are possible. Stochastic elements, including the availability of certain immigrant organisms, may affect the line that is taken. The existence of an end-point, an ultimate stable equilibrium that is self-sustaining, is increasingly unacceptable as we come to understand the pattern and pace of environmental changes, from catastrophes to climatic shifts. On a scale of centuries or millennia, the Earth will have changed beneath an ecosystem's roots and a new set of conditions will place fresh demands on the turbulent vegetation.

A kind of anthropomorphism has also attended the development of the concept of succession, simultaneously stimulating and blighting it. Frederic E. Clements, at the close of the nineteenth century, and Sir Arthur Tansley, at the beginning of the twentieth, encouraged an organismal view of ecosystem development. They saw succession as a type of embryology leading ultimately to the fully developed ecosystem in its final maturity.

This extreme form of determinism has failed the test of time. The species of plants and animals that are involved in successions arrive when conditions are appropriate and when their populations are geographically available. They depart when conditions are less suitable or when competition for resources proves too severe. Species, as Henry Gleason pointed out in the 1920s, act in an individualistic manner; communities are simply the assemblages of the moment. Palaeoecological studies of plant and animal movements during the past 14,000 years have supported the idea that it is individual species, rather than communities, that move in response to climate change.

In 1969, Eugene P. Odum set out an energetic model for succession by concentrating on the general features of the process. Energy balance in the ecosystem progressively changes, with ecosystem respiration lagging behind production. When the two eventually coincide, equilibrium — climax — is attained. Biomass is generally greatest at this equilibrium stage, nutrient imports to the ecosystem are equalled by exports, and species richness and general complexity are at their peaks. The model has many attractive and useful features, although the final state of equilibrium must still be regarded as a dubious ideal.

What mechanisms underlie the observed sequence of species? Each species may modify the environment in some way, rendering it more suitable for the invasion of others — as in the provision of soil organic matter, soil stability, nitrogen fixation, shade, and so on — processes that have been called facilitation. Or perhaps competitive factors predominate, early colonists exploiting a situation of low competition, only to be displaced as more aggressive but slower invaders arrive. It is likely that both processes are present and interact together.

As in so many areas of science, a lack of full understanding of a process does not necessarily interfere with its exploitation. As we manage ecosystems, we often set succession into reverse gear. We create open water in wetlands, open glades in forests, extract timber, plough land and take harvests from annual plants such as wheat. All these activities are manipulations of the process of succession. Whether seeking enhanced biodiversity or agricultural productivity, we often need to keep succession in check.