When looking at forest change, it is the implicit assumption that the forest will recover after a disturbance. The exact pathway by which this happens may be different each time but it will return to something similar to what was there before the disturbance. This can be defined as stability, the ability of a forest, or indeed any other ecosystem, to return to equilibrium following a disturbance. To put it another way, stability is the ability to resist permanent change. (There is some debate about precisely how stability should be defined; Johnson et al. (1996) and McCann (2000) give good reviews.)
The role that diversity plays in stability is one of the most controversial concepts in ecology. Beginning with a seminal paper by MacArthur in 1955, a school of thought developed that believed that a more diverse community was more stable. This can be thought of as a spider's web, with the threads representing trophic or feeding links. A web with more links should be more robust and less liable to collapse if some of the links are broken. In the same way, a forest with a higher diversity of species should be less affected with the loss of a species from the food web than a forest with fewer species.
Building on this, it has been claimed by some that communities have functional redundancy; some species can be lost with no effect on overall efficiency because their function in the food web is taken over by other species. In other words, a community is remarkably robust when species are lost. But care is needed, as pointed out by Schulze and Mooney (1993). They liken an ecosystem to a moving car. We can see that many components are essential for the car to keep moving (such as the spark plugs, dynamo and the radiator), while others can be lost without major apparent effect (such as the ash tray). But caution is needed since some components, such as the bumpers/fenders and air-bags, may appear dispensable until there is a low- or high-speed collision, respectively; i.e. some parts function only occasionally. In the same way, some species in a forest may appear to be redundant but in a time of disturbance, for example, may play a crucial role in the early stages of recovery. It can be argued, with justification, that in the real forest, all species play a crucial function at some time in some way.
What practical evidence is there for the idea that diversity leads to greater stability? The relationship between diversity and stability is still unclear and it is difficult to generalize. In Ecotron (artificially enclosed ecosystem)
experiments by Naeem et al. (1994) it was shown that high-diversity communities fixed more carbon and had higher cover. Moreover, there have been a number of studies that have shown less year-to-year variation in biomass production in species-rich grasslands compared with species-poor ones (e.g. Dodd et al., 1994). But this may be specific to these types of communities and the evidence for forests is less clear.
When this whole idea was first raised, it was considered that the high stability of tropical forests (at least those that have remained largely unchanged for thousands of years) was due to their high diversity. However, the true picture is probably exactly the opposite way round: it is the stability of the tropical environment that has led to the high species diversity by allowing each species to occupy a narrower niche safely, as Wright (2002) argues. A bird, for example, can feed on one species of flower in the tropics but in a northern conifer forest where the climate is less predictable, and certain plants may flower poorly in some years, or the forest may burn, each species must have a wide niche in order to be able to survive. There is therefore less room for a large number of species in the conifer forest.
The discussion on the link between diversity and stability continues!
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