Intermediate Disturbance Hypothesis - Support and Critiques

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 . Using a numerical wave model to estimate the forces caused by waves, researchers were able to determine that there was a significant relationship between species diversity and disturbance index; this is consistent with the Intermediate Disturbance Hypothesis . Furthermore, diversity was lower at exposed offshore sites where disturbance from waves was highest, and at extremely sheltered site where disturbance from waves was minimized . 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 . While this study may have been localized to the Western Australian coast, it still provides some evidence to support the validity of the IDH.

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 . In 1982, reefs in Kona, Hawaii were reported to have an increase in diversity after a moderate storm, although the effects of the storm varied with the reef zones . 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 . Similar findings have been reported in shallow reefs in which dominant species of coral have suffered more damage than the less common species . While more long-term studies are required to completely support the Intermediate Disturbance Hypothesis, the studies that have been conducted thus far have proven that IDH does have some validity while attempting to describe the relationship between diversity and the rate of occurrences of disturbance in an area.

Even though the IDH was designed for species-rich environments, like tropical rainforests, "most studies that evaluate the IDH are based on limited data with: few species, a limited range of disturbance and/or only a small geographic area, compared with the scale of interest” . In this experiment, Rogers, 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) . They found that diversity does peak at intermediate level of disturbance but little variation is explained outside dry forests. Therefore, disturbance is less important for species diversity patterns in wet tropical rain forests than previously thought. The number of species was about the same for each forest type, and wet forests had only slightly fewer pioneer species, slightly more shade-tolerant and an equal number of pioneer light-demanders compared with the moist and dry forests . 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 .

IDH has been subjected criticism since its inception but not to the degree that other specie density hypotheses have been. Recently there has been a call for a critical reassessment IDH 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 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 . Other criticisms are suggesting several subtle theoretical issues with IDH. First, while disturbances weaken competition by reducing species’ densities and per-capita growth rates it also reduces the strength of competition needed to push per capita growth into a negative territory and reduce density to zero see . Second, intermediate disturbances slow competitive exclusion by increasing the long-term average mortality rate, thereby reducing the differences in the average growth rates of competing species. The difference in the growth rates between competitively superior and inferior species that determine the rates of competitive exclusion; therefore intermediate disturbances are affecting species’ abundance but not coexistence . Third, intermediate disturbances temporarily affect relative specie fitness. However, no matter what the rate of disturbance is, the species with favored fitness will out-compete the rest of the species .

Several alternatives hypotheses have been proposed. One example is by Denslow, he 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. 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.