Model proposing regional biodiversity is increased by a moderate level of ecological disturbance
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.[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. 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.[2][3][4] Second, interspecific competition results in one species driving a competitor to extinction and becoming dominant in the ecosystem.[2][3][4] Third, moderate ecological scale disturbances prevent interspecific competition.[2][3][4]
The hypothesis is ambiguous with its definitions of the terms "intermediate" and "disturbance". Whether a given disturbance can be defined as "intermediate" inherently depends on the previous history of disturbances within a given system, as well as the component of disturbance that is evaluated (i.e. the frequency, extent, intensity, or duration of the disturbances).
Graph shows principles of intermediate disturbance hypothesis: I. at low levels of ecological disturbance species richness decreases as competitive exclusion increases, II. at intermediate levels of disturbance, diversity is maximized because species that thrive at both early and late successional stages can coexist, III. at high levels of disturbance species richness is decreased due to an increase in species movement.
Disturbances act to disrupt stable ecosystems and clear species' habitat. As a result, disturbances lead to species movement into the newly cleared area.[2] Once an area is cleared there is a progressive increase in species richness and competition takes place again. Once disturbance is removed, species richness decreases as competitive exclusion increases.[5] "Gause's Law", also known as competitive exclusion, explains how species that compete for the same resources cannot coexist in the same niche.[3] Each species handles change from a disturbance differently; therefore, IDH can be described as both "broad in description and rich in detail".[2] The broad IDH model can be broken down into smaller divisions which include spatial within-patch scales, spatial between-patch scales, and purely temporal models.[5] Each subdivision within this theory generates similar explanations for the coexistence of species with habitat disturbance. Joseph H. Connell[6] 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.[4] An ideal examples of r-selected groups are algae. Based on the contradictory characteristics of both of these examples, areas of occasional disturbance allow both r and K species to benefit by residing in the same area. The ecological effect on species relationships is therefore supported by the intermediate disturbance hypothesis.
Disturbed vegetation due to milpa farming. Cayo District, Belize. [Macrae 2008].
^Dial, R.; Roughgarden, J. (1988). "Theory of marine communities: the intermediate disturbance hypothesis". Ecology. 79 (4): 1412–1424. doi:10.1890/0012-9658(1998)079[1412:TOMCTI]2.0.CO;2.
^ abcdeWilkinson, David M. (1999). "The Disturbing History of Intermediate Disturbance". Oikos. 84 (1): 145–7. doi:10.2307/3546874. JSTOR 3546874.
^ abcdKricher, John C. (2011). Tropical Ecology. New Jersey, Princeton: Princeton University Press.[page needed]
^ abcdCatford, Jane A.; Daehler, Curtis C.; Murphy, Helen T.; Sheppard, Andy W.; Hardesty, Britta D.; Westcott, David A.; Rejmánek, Marcel; Bellingham, Peter J.; et al. (2012). "The intermediate disturbance hypothesis and plant invasions: Implications for species richness and management". Perspectives in Plant Ecology, Evolution and Systematics. 14 (3): 231–41. doi:10.1016/j.ppees.2011.12.002.
^ abVandermeer, John; Boucher, Douglas; Perfecto, Ivette; de la Cerda, Inigo Granzow (1996). "A Theory of Disturbance and Species Diversity: Evidence from Nicaragua After Hurricane Joan". Biotropica. 28 (4): 600–13. doi:10.2307/2389100. JSTOR 2389100.
^Connell, J. H. (1978). "Diversity in Tropical Rain Forests and Coral Reefs". Science. 199 (4335): 1302–10. Bibcode:1978Sci...199.1302C. doi:10.1126/science.199.4335.1302. PMID 17840770.
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