Introduction

The intermediate disturbance hypothesis postulates that for any ecological system the diversity of species will be highest when, or where, disturbance is at some intermediate level. The hypothesis simply overlays environmental disturbances that negatively impact one or more species onto the temporal process of community development or succession. Starting with a new, unoccupied patch of habitat the number of species in it will increase as it is colonized. As population densities and species diversity continue to increase, resources will eventually become limiting and the species will compete for these resources. With increased competition, inferior competitors will be displaced and eventually lost. At some point species losses should be greater than gains and the number of species will decline and, in the extreme, end with a single dominant species remaining. Thus, the temporal pattern of diversity can be represented by a unimodal or quadratic curve in which diversity initially increases and then declines over time (Figure 1). Any disruption or disturbance of this process will likely reset the community to

Colonization Maximum Competition species

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High Intermediate Low

Community age

Figure 1 General concept of the intermediate disturbance hypothesis which overlays disturbance at different times on the diversity changes as a community develops from early stages of colonization with few species to maximum diversity prior to the reduction in diversity through competition after resources become limiting.

some earlier state. High rates of disturbance will keep the community in an early stage with few species and low rates of disturbance will allow competitive exclusion to reduce diversity. Therefore, some rate or magnitude of disturbance should exist that maintains the community in an intermediate developmental state of highest diversity.

At its core the hypothesis makes four principal assumptions: (1) populations and the communities that they form are constantly changing; (2) these changes can result in the gain or loss of species; (3) the process of community change can be disturbed or disrupted by any number of physical and biological events that remove individuals, populations, species, or the whole community; and (4) the change in the number or diversity of species is nonlinear as a consequence of the interaction between species recruitment and the limits placed on continued increases by finite available resources. The first three of the assumptions are straightforward. First, normal processes of individual birth, growth, and mortality will produce change. These changes may be limited to replacement of individuals in stable populations with little measurable change at the community level. It is more likely that populations will fluctuate in size and produce changes in species' relative abundances including complete loss of some species. Second, for any community, there is always some probability of new species immigrating or existing species emigrating or going extinct locally. Third, disturbances that produce losses certainly can occur in any system, but their effects will vary based on their magnitude, frequency, and spatial extent. The fourth assumption is probably the most contested. The increase in species as a habitat is colonized clearly must occur and the rate of increase will decline as both available resources and the pool of available new species decrease with the addition of each new species. Thus the critical aspect is whether after some period of time competition for limiting resources or some other process results in sufficient species losses to cause a decline in diversity.

Although the intermediate disturbance hypothesis is most often tied to changes in species associated with succession, it is not really dependent on any particular process of community development. Whether one perceives the species composition of a community changing as a fairly systematic successional sequence of species or as a process of random immigrations and extinctions, the process can be interrupted or perturbed by weather, fire, flood, drought, herbivores, predators, disease, tree falls, waves, etc. These disturbances can affect any or all species, randomly or selectively. If diversity increases and then decreases as the community develops through time, then some level of disturbance is likely to maintain the community at a state of maximum diversity.

Because of its apparent generality the intermediate disturbance hypothesis has been studied and tested in a vast array of empirical studies of most types of natural communities and habitats, in experimental communities in small laboratory chambers, and in larger ecosystems and landscapes that can include multiple habitats and communities. There have also been a substantial number of theoretical and modeling studies that have tried to develop and test constraints on the hypothesis and its applicability. As a consequence, there is a diversity of opinions about when, where, and to what systems the hypothesis should be applied. Before limiting the hypothesis to one extreme of catastrophic disturbances that remove all species in fragments or patches within a landscape or to the other of noncatastrophic disturbances across homogeneous habitat that maintain high diversity by reducing populations to noncompetitive levels, it is important to examine the historical context of the hypothesis and how it has evolved.

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