Intermediate disturbance hypothesis
[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 species that thrive at both early and late successional stages can coexist.
In contrast, r-selected species colonize open areas quickly and can dominate landscapes that have been recently cleared by disturbance.
[2] In this paper, he explains that the idea of disturbance relating to species richness can be traced back to the 1940s in Eggeling 1947,[7] Watt 1947,[8] and Tansley 1949.
[2] This graph appeared first in Grime's 'Competitive exclusion in herbaceous vegetation'[10] where it was used to show the relationship between species density and both environmental stress and intensity of management.
The graph appears again in Horn's 'Markovian properties of forest succession'[11] and Connell's 'The influence of interspecific competition and other factors on the distribution of the barnacle'.
The intermediate disturbance hypothesis has been supported by several studies involving marine habitats such as coral reefs and macroalgal communities.
In shallow coastal waters off of south-west Western Australia, a study was conducted to determine whether or not the extremely high diversity observed in macroalgal communities was due to disturbance from waves.
[13] The study provided evidence that biodiversity in microalgal reef communities possess some relationship with their proximity to the outer edge of lagoon systems typical of the Western Australian coast.
Research using an individual-based, eco-evolutionary system demonstrates disturbance on small spatial scales increases species richness.
[19] Additionally, a study done in the Virgin Islands National Park found that species diversity, in some locations, of shallow coral reefs increased after infrequent hurricane disturbance.
[14] In 1980, Hurricane Allen increased species diversity in shallow zones of the Discovery Bay Reef in Jamaica because the more dominant corals were reduced; giving the other types a chance to propagate following the disturbance.
Moreover, a study was conducted in Fynbos, South Africa to test the intermediate disturbance hypothesis over different spatial scales ranging from 1 m^2 to 0.1 hectares.
[21] In this experiment, Bongers, Poorter, Hawthorne, and Sheil evaluate the IDH on a larger scale and compare different tropical forest types in Ghana.
[21] Their results generally supported the IDH as an explanation of why diversity varies across sites, but concluded that disturbance is less important for species richness patterns in wet tropical rain forests than previously thought.
[15][22][23][24] The rationales behind these discrepancies have been leveled at the simplicity of IDH and its inability to grasp the complexity found within the spatial and intensity aspects of disturbance relationships.
