The Developmental Process

A nonlinear change in the number of species in a community or any other ecological system is a necessary component of the intermediate disturbance hypothesis. This change is usually seen as unimodal or quadratic with a peak in diversity at some intermediate point in time. This is in contrast with two other potential patterns, a continuous increase in species number over time and an asymptotic increase with species number reaching and stabilizing at some maximum (Figure 4). Assuming that there is some finite pool of available species, then stabilization at this number of species represents the upper limit to diversity. It follows that if over time diversity increases or increases to an asymptote without any decrease, then no frequency or magnitude of disturbance can result in higher diversity and the intermediate disturbance hypothesis would be rejected.

Time

Figure 4 Alternate patterns of temporal changes in diversity. If diversity is asymptotic or increasing intermediate disturbance would not maximize diversity.

Time

Figure 4 Alternate patterns of temporal changes in diversity. If diversity is asymptotic or increasing intermediate disturbance would not maximize diversity.

Beyond a pattern of increasing and then decreasing diversity over time, the process by which a community develops is unimportant. If development proceeds in an orderly successional sequence of species or through random colonization, the aspect critical to the intermediate disturbance hypothesis is that diversity peaks at some intermediate stage of the process. On the other hand, the characteristics of the system, especially the taxa or species included, the variability in their life histories, and their trophic relationships, have important consequences to the application and testing of the hypothesis.

Connell was careful to restrict the intermediate disturbance hypothesis to particular types of species when he applied it to the structural tree and coral species in forest and reef systems. It is logical to assume that within these taxa there will be a limited range of life histories such that high to low rates of disturbance can be defined. However, if we included all species from bacteria to vertebrates that might be considered to be part of tropical reef or forest systems, then imposing a single meaningful scale of disturbance is difficult. Disturbance is relative to the taxa being examined and their life histories or generation time. An intermediate rate of disturbance for a bacterial community may be on the order ofhours to days while it may be decades or more for corals or trees. A similar argument can be made for the size of a disturbance with large disturbances to a bacterial community being too small to be disturbances to trees. In such complex systems it may be necessary to classify disturbances in terms of frequency, magnitude, and spatial extent simultaneously and identify a combination that maximizes overall diversity. For example, if an intermediate disturbance frequency for trees is on the order of decades, smaller-magnitude daily intermediate disturbances for bacteria will occur multiple times within the period of highest tree diversity. This would produce a set of multiple intermediate disturbances of specific frequencies and magnitudes that result in several diversity maxima.

Including multiple trophic levels within the system being considered produces a similar set of problems. Modeling has demonstrated that the degree to which disturbances affect each trophic level can influence the overall impact on diversity. If a disturbance affects all levels equally then an intermediate level producing the highest diversity will exist. Alternatively, ifthe impact of a disturbance is disproportional there may not be an intermediate level producing the highest diversity. For example, if a disturbance principally causes mortality in a lower trophic level this would open resources at this level and some intermediate disturbance rate should lead to higher diversity at this trophic level. At the same time herbivores or predators in the next trophic level would experience reduced resources because of the lowered overall abundances of prey. This should lead to greater competition, lower diversity in the higher trophic level, and an unknown change in overall diversity. If species at higher trophic levels are more selective in their prey then the changes in their resources with disturbance and the effect on overall diversity are even more difficult to predict.

In a similar manner, the degree or number of positive symbiotic interactions within any system may affect directly the relationship between disturbance and diversity. At its core the intermediate disturbance hypothesis depends on resources becoming limiting and resulting negative effects on abundances and the survival of species. If a significant number of interspecific interactions within a system are positive, then disturbances may actually reduce available resources or the ability of species to utilize them. For such a system no level of disturbance may increase diversity.

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