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Intermediate Disturbance Hypothesis - WikiVisually
Finally, the higher diversity found in the soils less enzymatically active and relatively poorer in nutrients, may be explained to an extent by niche-based theories such as the resource heterogeneity hypothesis and Connell's intermediate disturbance hypothesis. ID: 330404
IDH has been subject to criticism since its inception but not to thedegree that other species density hypotheses have been. Recently therehas been a call for a critical reassessment of IDH. Criticisms havefocused on the increasing amount of empirical data that disagrees withIDH. This can be found within approximately 80% of over 100 reviewedstudies that are examining the predicted peak of diversity inintermediate disturbance levels. The rationales behind thesediscrepancies have been leveled at the simplicity of IDH and itsinability to grasp the complexity found within the spatial andintensity aspects of disturbance relationships. In addition, many IDHproven circumstances have been suggested to be a reflection of skewedresearch methods based on researchers looking for humpeddiversity-disturbance relations only in systems where they believed ithas occurred. Other criticisms are suggesting several subtletheoretical issues with IDH. First, while disturbances weakencompetition by reducing species densities and per-capita growth rates,it also reduces the strength of competition needed to push per capitagrowth into a negative territory and reduce density to zero. Second,intermediate disturbances slow competitive exclusion by increasing thelong-term average mortality rate, and thereby reducing the differencesin the average growth rates of competing species. The difference inthe growth rates between competitively superior and inferior speciesdetermines the rates of competitive exclusion; therefore intermediatedisturbances are affecting species’ abundance but not coexistence. Third, intermediate disturbances temporarily affect relative speciesfitness. However, no matter what the rate of disturbance is, thespecies with favored fitness will out-compete the rest of the species.
Graph shows principles of Intermediate Disturbance Hypothesis: I
The INTERMEDIATE DISTURBANCE HYPOTHESIS (IDH) suggests that localspecies diversity is maximized when ecological disturbance is neithertoo rare nor too frequent. At high levels of disturbance, due tofrequent forest fires or human impacts like deforestation , allspecies are at risk of going extinct. According to IDH theory, atintermediate levels of disturbance, diversity is thus maximizedbecause species that thrive at both early and late successional stagescan coexist. IDH is a nonequilibrium model used to describe therelationship between disturbance and species diversity. IDH is basedon the following premises: First, ecological disturbances have majoreffects on species richness within the area of disturbance. Second,interspecific competition results from one species driving acompetitor to extinction and becoming dominant in the ecosystem. Third, moderate ecological scale disturbances prevent interspecificcompetition. Graph shows principles of IntermediateDisturbance Hypothesis: I. at low levels of ecological disturbancespecies richness decreases as competitive exclusion increases, II. atintermediate levels of disturbance, diversity is maximized becausespecies that thrive at both early and late successional stages cancoexist, III. at high levels of disturbance species richness isdecreased due an increase in species movement.
David Wilkinson gives a thorough history of the hypothesis in hispaper titled, "The disturbing history of the intermediatedisturbance". In this paper, he explains that the idea of disturbancerelating to species richness can be traced back to the 1940s inEggeling 1947, Watt 1947, and Tansley 1949. Though studiessupporting the hypothesis began in the 1960s, the first concretestatements of the didn't occuruntil the 1970s. The hypothesis was initially illustrated using whathas been referred to as a "hump-backed model", which graphed theproposed relationship between diversity and disturbance. This graphappeared first in Grime 's 'Competitive exclusion in herbaceousvegetation' where it was used to show the relationship betweenspecies density and both environmental stress and intensity ofmanagement. The graph appears again in Horn's 'Markovian properties offorest succession' and Connell's 'The influence of interspecificcompetition and other factors on the distribution of the barnacle'. Though Grime was the first to provide a model for the relationship andHorn was the first to explicitly state the hypothesis, Connell isgenerally cited in text books and journals as the founder of thehypothesis.
Intermediate Disturbance Hypothesis in Phytoplankton Ecology
Additionally, a study done in the Virgin Islands National Park foundthat species diversity, in some locations, of shallow coral reefsincreased after infrequent hurricane disturbance. In 1982, reefs inKona, Hawaii were reported to have an increase in diversity after amoderate storm, although the effects of the storm varied with the reefzones. In 1980, Hurricane Allen increased species diversity inshallow zones of the Discovery Bay Reef in Jamaica because the moredominant corals were reduced; giving the other types a chance topropagate following the disturbance. Similar findings have beenreported in shallow reefs in which dominant species of coral havesuffered more damage than the less common species. While morelong-term studies are required to completely support the IntermediateDisturbance Hypothesis, the studies that have been conducted thus farhave proven that IDH does have some validity while attempting todescribe the relationship between diversity and the rate ofoccurrences of disturbance in an area.
The hypothesis caused concern among the marine science communitybecause of the discrepancy with the 1976Competition/Predation/Disturbance model proposed by Menge andSutherland In this model, low disturbance influences high predationand high disturbance creates low predation, causing competitiveexclusion to take place. Menge this is consistent with theIntermediate Disturbance Hypothesis. Furthermore, diversity was lowerat exposed offshore sites where disturbance from waves was highest,and at extremely sheltered site where disturbance from waves wasminimized. The study provided evidence that biodiversity inmicroalgal reef communities possess some relationship with theirproximity to the outer edge of lagoon systems typical of the WesternAustralian coast. While this study may have been localized to theWestern Australian coast, it still provides some evidence to supportthe validity of the IDH.
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Joseph Connell discovered that Balanus barnacles were excluded ..
From the sites with different degree of fire impact, we found higher diversity in intermediately disturbed sites than in severely disturbed or non-disturbed site, supporting the intermediate disturbance hypothesis.
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