The attractiveness of the intermediate disturbance hypothesis is its deceptive simplicity of applying some form of disruption to the observable process of community development. Clearly as a barren patch of habitat is colonized, species richness should increase regardless of whether one envisions the community developing as a defined succession or as a random accumulation of species. Eventually resources should become scarce with competition causing some decline in diversity. It follows that if this process of change can be halted at the right time diversity will be maintained at the highest level. Reality, however, is not so tractable. Even within a homogeneous habitat, community development will vary as a consequence of random recruitment as well as differences in competitive ability or susceptibility to predators. Recruitment, itself, may vary with season, distance from source, interspecific differences in productivity, etc., and not be random. Together these can result in delaying any competition for resources and a decline in diversity. Likewise, the simultaneous variability in the magnitude, frequency, and spatial scale ofdisturbances, whether they operate catastrophically or noncatastrophically, whether they are seen as operating within or among patches, or whether one or more types occur, all create a complexity that makes testing the hypothesis difficult.
This has resulted in both empirical and theoretical challenges to the hypothesis. For most of these, it is intermediate disturbance as a causal mechanism that is challenged not the pattern of highest diversity at some intermediate state in community development. Two types of alternatives based on empirical studies are the importance of considering interacting processes and the control ofthe recolonization ofdisturbed sites by external rather than internal processes. The importance of environmental gradients is an example of the former and recruitment limitation of the latter.
Although physical disturbance may produce light gaps in forest or overturn rocks and boulders in the rocky intertidal, these habitats, as well as others can be subject to gradients in environmental variables that produce a similar unimodal or quadratic pattern in diversity. Within the rocky intertidal, desiccation and high temperatures in the high intertidal and predation and herbivory in the low intertidal can result in different communities at the two extremes with an intermediate zone that mixes both sets of species and has a higher diversity. Disturbances from rock turnover resulting from wave action can also produce a similar gradient of diversity based on rock size in the same location. Separating the impact of these processes based on pattern alone is difficult, if not impossible. Even patterns produced by disturbance may not be a consequence of some intermediate level of disturbance. For example, the colonization of an abandoned old field next to a forest can result in an ecotone community ofhighest diversity between the field and forest. The field may have been created by disturbance, but the pattern results from a gradient ofenvironmental conditions between two extremes rather than ongoing differences in disturbance rates or magnitude.
Recruitment limitation, unlike intermediate disturbance does not rely on limiting resources within a disturbed patch to control diversity. Differential recruitment from the species surrounding or closest to the patch controls the development of the community within the patch, making colonization by species more distant or excluded from that part of the landscape much less likely. Colonization is not a lottery and disturbances do not create sites or resources for early-succession species that would be outcompeted over time. Species diversity is maintained in the system by the overall patchiness ofthe system and the inability of superior competitors to reach and colonize distant disturbed sites with poorer competitors. It is not clear how recruitment limitation operates over long time periods in which rare events of superior competitors colonizing distant sites might eventually occur.
Theoretical and modeling studies have led to variations and modifications of the intermediate disturbance hypothesis, including variations in the community or ecological system, variations in disturbance characteristics, and linking or contrasting other environmental parameters with disturbance. For example, when the complexity of the community is increased by the inclusion of multiple trophic levels or groups of species with very different life histories, intermediate disturbance becomes harder to define. A single frequency or magnitude of disturbance is unlikely to affect species of vastly different sizes and generation times in the same way or necessarily be intermediate for all. Likewise, disturbances that maintain higher diversity and presumably lower population levels of prey may have the opposite effect on predators. Characteristics of disturbance such as the timing or phasing of disturbances among patches can also modify the overall effect of disturbance. Even at the same magnitude and frequency, whether a disturbance affects one or all patches at the same or different times will determine its impact and what level of disturbance produces the highest diversity. Other parameters such as recruitment or productivity can also have a nonlinear impact on diversity and change the effect of disturbance. Low and high levels of recruitment may minimize the effects of disturbance. At low recruitment resources may not be limiting and disturbance would have little effect while at high recruitment levels any effect of disturbance would be quickly overwhelmed. Only at intermediate levels would differences in disturbance affect diversity.
In summary, the intermediate disturbance hypothesis remains a compelling concept that provides a nonequili-brium view of how communities can gain or maintain a richness in species without an intricacy of adaptations. Strong and weak competitors, good and poor colonizers, can all coexist in a system where some process such as disturbance prevents the monopolization of resources at different times and places. Intermediate disturbance remains a hypothesis that is far from proven or universally applicable to all communities. It needs to be tested in each community or habitat with an open mind as to what constitutes a community, a disturbance, and an intermediate frequency, magnitude, or spatial scale of the latter.
See also: Colonization; Community. Further Reading
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