Do successions come to an end? It is clear that a stable equilibrium will occur if individuals that die are replaced on a one-to-one basis by young of the same species. At a slightly more complex level, Markov models (see Section 16.5) tell us that a stationary species composition should, in theory, occur whenever the replacement probabilities (of one species by itself or by any one of several others) remain constant through time.
The concept of the climax has a long history. One of the earliest students of succession, Frederic Clements (1916), is associated with the idea that a single climax will dominate in any given climatic region, being the endpoint of all successions, whether they happened to start from a sand dune, an abandoned old field or even a pond filling in and progressing towards a terrestrial climax.
This monoclimax view was challenged by many ecologists, amongst whom Tansley (1939) was prominent. The polyclimax school of thought recognized that a local climax may be governed by one factor or a combination of factors: climate, soil conditions, topography, fire and so on. Thus, a single climatic area could easily contain a number of specific climax types. Later still, Whittaker (1953) proposed his climax pattern hypothesis. This conceives a continuity of climax types, varying gradually along environmental gradients and not necessarily separable into discrete climaxes. (This is an extension of Whittaker's approach to gradient analysis of vegetation, discussed in Section 16.3.1.)
In fact, it is very difficult to identify a stable climax community in the field.
climaxes may be approached rapidly - or, so slowly that they are rarely ever reached
Usually, we can do no more than point out that the rate of change of succession slows down to the point where any change is imperceptible to us. In this context, the subtidal rockface succession illustrated in Figure 16.12 is unusual in that convergence to a climax took only a few years. Old-field successions might take 100-500 years to reach a 'climax', but in that time the probabilities of further fires or hurricanes are so high that a process of succession may rarely go to completion. If we bear in mind that forest communities in northern temperate regions, and probably also in the tropics, are still recovering from the last glaciation (see Chapter 1), it is questionable whether the idealized climax vegetation is often reached in nature.
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