The Intermediate Disturbance Hypothesis (IDH) states that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem.[1] At high levels of disturbance, due to frequent forest fires or human impacts like deforestation, all species are at risk of going extinct. According to IDH theory, at intermediate levels of disturbance, diversity is thus maximized because both competitive K-selected and opportunistic r-selected species can coexist.
This coexistence is a result of the differing life history strategies of species, which dictate a preference for high or low disturbance. K-selected species tend to be more competitive, because they invest a larger proportion of resources into growth and competition and thus generally dominate stable ecosystems over long time periods. In contrast, r-selected species, which colonize open areas quickly, can dominate landscapes recently cleared by disturbance. Therefore, in areas where disturbance occurs occasionally, both species can take advantage of the same region. This effect is observed for the most part in sessile species.
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The notion that disturbance can increase biodiversity opposes the older idea that diversity is highest in undisturbed ecosystems. It was first proposed by J. Philip Grime in 1973.[2] It was then proposed again by Henry S. Horn in 1975,[3] and then by Joseph Connell in 1978.[4] That Connell's later paper is frequently given priority over Grime's earlier paper has attracted comment in the scientific literature.[5]
This hypothesis caused concern among the marine science community because of the discrepancy with the 1976 Competition/Predation/Disturbance model proposed by Menge and Sutherland.[6] In this model, low disturbance influences high predation and high disturbance creates low predation, causing competitive exclusion to take place. Menge & Sutherland formulated a new model, one that incorporated Connell's ideas in a two part graph published in the American Naturalist (1987). This model proposes that predation, competition, and disturbance are all responsible for shaping the diversity of a community under certain circumstances.
Research regarding the effects of intermediate disturbance is ongoing. In one study, dry and tropical forest regions were compared to determine how the effects of IDH change due to varying climate.[7]
Debate over the validity of the IDH is ongoing as the theory is tested in various ecological communities. Other evidence exists for[8][9] and against[10][11] the hypothesis.
One alternative proposed in 1985 by Denslow[12] states that the species diversity in a disturbance-mediated coexistence between species is maximized by the presence of a disturbance regime resembling the historic processes. This is because species generally adapt to the level of disturbance in their ecosystem through evolution (whether disturbance is of high, intermediate or low level). Many species (e.g. ruderal plants, fire-adapted species) even depend on disturbance in ecosystems where it often occurs.
However, the IDH theory has gained traction in the field of ecology and remains a useful framework for understanding the influence of disturbance on biodiversity within communities. Due to the complicated interactions of ecosystems, the IDH, like all theories, represents a simplification of competition interactions. Thus, some have come to understand the theory as a series of similar phenomena dictated by the unique aspects of each community.[13]