Hypothesis History and Range

The idea that an intermediate level of disturbance can lead to the highest diversity within a system has been proposed independently by numerous ecologists based on their observations of a variety of communities. Although observations of the phenomenon can be found in studies from at least the early 1900s, J. H. Connell is usually credited with giving the first formal definition and description of this idea as the intermediate disturbance hypothesis in his 1978 Science paper. Connell found the intermediate disturbance hypothesis to be the best of six explanations for the high diversity of species found in the tropical coral reefs and rainforests that he was studying. Alternate explanations included 'equal chance' in which all species are equal and diversity is a function of the environment and species available in a region, 'gradual change' in which environmental changes prevent competitive exclusion; 'niche diversification' or the finer division of resources among competing species; 'circular networks' in which competition is not hierarchical; and 'compensatory mortality' in which noncompetitive mortality is greatest for dominant species. Connell was careful to limit the scope of the intermediate disturbance hypothesis to the main structural species of these two systems, corals and trees. He saw them as representative of communities of sessile, often long-lived, species with dispersal largely limited to the larval and seed life stages produced through reproduction. He specifically excluded more motile species which have the potential to avoid or quickly adjust to the impacts of disturbance.

In this context, Connell described these diverse systems as dynamic, nonequilibrium communities that did not approach maximal diversity by the coexistence of species which have divided available resources on finer and finer scales. Rather than a balance or equilibrium among coexisting species each with their unique way of using some portion of the available resources, species contested limiting resources with those superior competitors ultimately pushing out the inferior. Nevertheless, within long-lived coral reef and forest communities competitive exclusion is not instantaneous and may take decades to centuries. In these long time periods any number of events can disrupt the competitive process, remove or reduce the abundance of dominants, and make resources more available. Although one can imagine how disturbances such as disease, wind, or waves might exclusively or disproportionately affect large dominant coral or tree species, such types of disturbance are not necessary for disturbance to contribute to higher diversity. Disturbances that randomly remove individuals or parts of the habitat will create areas available for colonization with available resources and reduced competitor abundance. New or previously lost species have the opportunity to colonize these areas with the potential of increasing local diversity.

At its core the intermediate disturbance hypothesis is recognition that environmental change can alter ecological processes and patterns, particularly when the change is of sufficient magnitude, frequency, or spatial extent to cause mortality and the local loss of populations, species, or whole communities. How disturbance disrupts processes, the nature of the processes being disrupted, and ultimately, the consequences for diversity have had their own diversity of interpretations. The most dominant view has been that of physical processes operating at variable frequencies causing interruptions (e.g., tree falls, rock turnovers) of a successional sequence that progresses from good colonizers to competitive dominants. Even with these constraints intermediate disturbance can be viewed differently. At one end of the spectrum intermediate disturbance can be seen as some frequency of catastrophic or complete removal of the community in patches within a fragmented landscape. If this produces patches of different ages, disturbed at different times then some intermediate frequency of disturbance will maximize the cumulative diversity of all patches

Figure 2 Distributions of patch ages at different frequencies of disturbance showing the proportion of patches at maximum diversity assuming this is attained in communities 5-8 months old.

Figure 2 Distributions of patch ages at different frequencies of disturbance showing the proportion of patches at maximum diversity assuming this is attained in communities 5-8 months old.

(Figure 2). At the other end of the spectrum intermediate disturbance can be seen as part of secondary succession in which noncatastrophic disturbances operating in a homogeneous system keep abundances sufficiently low and resources sufficiently available to prevent competitive exclusion from occurring. In this context intermediate disturbances provide the environmental variability and temporal diversity in resource availability that can produce opportunities for a greater diversity of species life histories or adaptations.

Alternately, the link between high diversity and intermediate levels of diversity can be maintained independent of any of the specific interspecific relationships that are part of succession. For example, island biogeographic theory as proposed by R. H. MacArthur and E. O. Wilson presents an alternate scenario for the colonization of an island or patch of habitat that is neutral to species identities. The number of species present will be a function of the number of species immigrating minus the number of species lost or going extinct locally. Immigration rate will be a declining function of the number of species present, reaching zero when all species in the available pool of species are present. With all species having some probability of being lost, the cumulative extinction rate will increase as the number of species present increases. Therefore, some balance or equilibrium in species number should exist when immigration and extinction rates are equal. Species extinction rates should also be higher when they begin interacting or when population sizes are limited by available resources. Thus there should be a higher noninteractive equilibrium species number that might be expected to precede a lower interactive equilibrium species number (Figure 3). In patchy environments, the catastrophic random disturbance of individual patches will produce a distribution of patches with a mean age equal to the mean disturbance rate. At some intermediate rate of disturbance, the

Figure 3 Diversity (S) changes during colonization as a function of species gains and losses (immigration and extinction) (based on island biogeographic theory). Inset: When species interact or compete for resources, the extinction rate increases resulting in a lower number of species. Thus, disturbance at intermediate frequencies can result in highest diversity.

Time

Figure 3 Diversity (S) changes during colonization as a function of species gains and losses (immigration and extinction) (based on island biogeographic theory). Inset: When species interact or compete for resources, the extinction rate increases resulting in a lower number of species. Thus, disturbance at intermediate frequencies can result in highest diversity.

majority of patches will be at an age when they are at or near the noninteractive equilibrium number of species creating the overall highest diversity for the system (Figure 2). Likewise, noncatastrophic disturbance can lower extinction rate by opening resources and keep individual patches at the highest diversity. This approach is not limited to specific types of species, makes no assumptions about competitive dominance or the relative strengths or directions of any interactions. It relies only on the system reaching some dynamic balance between species losses and gains that will change once resources become limiting. Regardless of whether the probabilities of both immigration and extinction are equal or variable among species, some intermediate level of disturbance should produce the highest number of species.

An important question arises as to whether the intermediate disturbance hypothesis is a logical tautology that is unfalsifiable because intermediate disturbance cannot be defined independently of diversity. First, the hypothesis can be falsified if it can be shown that no decrease in diversity occurs as a community develops over time. In this case no intermediate frequency or magnitude of disturbance can increase diversity. Second, disturbance can be defined relative to the life histories or generation times of the species within a community for disturbance frequency or the degree of mortality for disturbance magnitude. In this sense the extremes of high and low disturbance can be defined with an array of intermediate disturbance rates or magnitudes in between. If diversity is not higher at a reasonable suite of the definable intermediate rates or magnitudes, then the hypothesis is falsified. If however, the extremes cannot be defined, then intermediate cannot be defined and falsification is not possible. This may be a problem for slow-developing communities of long-lived organisms for which it may be difficult to test the hypothesis over an adequate range of measurable intermediate disturbances within one's life span. Nevertheless, if maximum diversity is hypothesized to develop before resources become limiting or interactions increase mortality rates, then the intermediate rate or magnitude of disturbance may be defined as that which limits the use of available resources to some measurable level such as less than 90% of the space that can be occupied.

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