It is possible to conceive of a world with just one species of plant (or herbivore) with supreme performance over an enormous range of tolerance. In this scenario the most competitive species (the one that is most efficient at converting limited resources into descendants) would be expected to drive all less competitive species to extinction. The species richness we witness in real communities is a clear demonstration of the failure of evolution to produce such supreme species. An extension of this competitive argument holds that diversity can be explained through a partitioning of resources amongst competing species whose requirements do not overlap completely, as discussed in detail in Section 19.2. However, this argument rests on two assumptions that are not necessarily always valid.
The first assumption is that the organisms are actually competing, which in turn implies that resources are limiting. But there are many situations where physical disturbances, such as storms on a rocky shore or frequent fires, may hold down the densities of populations so that resources are not limiting and individuals do not compete for them. The role of physical disturbances, and the associated patch dynamics view of communities, were discussed in Chapter 16. In an exactly analogous manner, the action of predators or parasites is often a disturbance in the 'normal' course of a competitive interaction; the resulting mortality may open up a gap for colonization in a way that is sometimes indistinguishable from that of battering by waves on a rocky shore or a hurricane in a forest.
The second assumption is that when competition is operating and resources are in limited supply, one species will inevitably exclude another. But in the real world, when no year is exactly like another, and no square centimeter of ground exactly the same as the next, the process of competitive exclusion may not proceed to its monotonous end. Any force that continually changes direction at least delays, and may prevent, an equilibrium or a stable conclusion being reached. Any force that simply interrupts the process of competitive exclusion may prevent extinction and enhance species richness.
A basic distinction can thus be made between equilibrium and nonequilibrium theories. An equilibrium theory, like the one concerned with niche differentiation, helps us to focus attention on the properties of a system at an equilibrium point - time and variation are not the central concern. A nonequilibrium theory, on the other hand, is concerned with the transient behavior of a system away from an equilibrium point, and specifically focuses our attention on time and variation. Of course, it would be naive to think that any real community has a precisely definable equilibrium point, and it is wrong to ascribe this view to researchers who are associated with equilibrium theories. The truth is that investigators who focus attention on equilibrium points have in mind that these are merely states towards which systems tend to be attracted, but about which there may be greater or lesser fluctuation. In one sense, therefore, the contrast between equilibrium and nonequilibrium theories is a matter of degree. However, this difference of focus is instructive in unraveling the important role of temporal heterogeneity in communities.
Thus, predators and parasites, like physical disturbances, can interrupt the process of competitive exclusion, influence profoundly the outcome of competitive processes, and impose their own order on community organization. Predation and parasitism can also affect community structure through the process of 'apparent competition' (see Section 8.6), where one or more prey or host species suffers from the actions of predators or parasites that are sustained by the presence of other species of prey or hosts. We turn to predation and parasitism in the next two sections.
equilibrium and nonequilibrium theories
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