The intermediate disturbance hypothesis (IDH) suggests 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. 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. IDH is a nonequilibrium model used to describe the relationship between disturbance and species diversity. IDH is based on the following premises: First, ecological disturbances have major effects on species richness within the area of disturbance. Second, interspecific competition results in one species driving a competitor to extinction and becoming dominant in the ecosystem. "Gause's Law", also known as competitive exclusion, explains how species that compete for the same resources cannot coexist in the same niche. proposed that relatively low disturbance leads to decreased diversity and high disturbance causes an increase in species movement. These proposed relationships lead to the hypothesis that intermediate disturbance levels would be the optimal amount of disorder within an ecosystem. Once K-selected and r-selected species can live in the same region, species richness can reach its maximum. The main difference between both types of species is their growth and reproduction rate. These characteristics attribute to the species that thrive in habitats with higher and lower amounts of disturbance. K-selected species generally demonstrate more competitive traits. Their primary investment of resources is directed towards growth, causing them to dominate stable ecosystems over a long period of time; an example of K-selected species the African elephant, which is prone to extinction because of their long generation times and low reproductive rates. In contrast, r-selected species colonize open areas quickly and can dominate landscapes that have been recently cleared by disturbance. Watt 1947, and Tansley 1949. Though studies supporting the hypothesis began in the 1960s, the first concrete statements of the intermediate disturbance hypothesis didn't occur until the 1970s. 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' and Connell's 'The influence of interspecific competition and other factors on the distribution of the barnacle'. 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). and protist microcosms.
Support and critiques
Debates over the validity of the IDH are ongoing within the discipline of tropical ecology as the theory is tested in various ecological communities. Other evidence exists for and against the hypothesis. 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.
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. Fynbos is a place where fire is one of the most prevailing disturbance. However, the intermediate fire frequency regime had lower species richness than did the frequently burned sites. It was also found that community heterogeneity was highest at the least frequently burned sites and lowest at the sites that experienced an intermediate fire frequency. In this experiment, Bongers, Poorter, Hawthorne, and Sheil evaluate the IDH on a larger scale and compare different tropical forest types in Ghana. Their dataset consisted of 2504 one-hectare plots with a total of 331,567 trees. These plots were divided classified into three forest types: wet (446 plots), moist (1322 plots), and dry forest (736 plots). Criticisms have focused on the increasing amount of empirical data that disagrees with IDH. This can be found within approximately 80% of over 100 reviewed studies that are examining the predicted peak of diversity in intermediate disturbance levels. 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. In addition, many IDH proven circumstances have been suggested to be a reflection of skewed research methods based on researchers looking for humped diversity-disturbance relations only in systems where they believed it has occurred.
Several alternative hypotheses have been proposed. One example is by Denslow, who 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 and fire-adapted species) even depend on disturbance in ecosystems where it often occurs.
thumb|(Disturbed vegetation due to [[milpa farming, Contreras Valley, Cayo District, Belize [Macrae 2010].]]
[[File:IDH example3.jpg|thumb|Disturbance due to tree fall, Gainesville, Florida [Daniel 2012].]]
See also
- Competition-colonization trade-off
- Ecological succession
- Habitat fragmentation
- Patch dynamics
- Wood-pasture hypothesis