# 18.2: Effects of Disturbance

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Changes in community structure and composition over time are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state. In primary succession, newly exposed or newly formed land is colonized by living things; in secondary succession, part of an ecosystem is disturbed and remnants of the previous community remain. Thus, disturbance can initiate successional change.

Species that are well adapted for exploiting disturbance sites are referred to as pioneers or early successional species. These shade-intolerant species are able to photosynthesize at high rates, produce a lot of offspring, and grow and mature quickly. Their fast growth is usually balanced by short life spans. Furthermore, although these species often dominate immediately following a disturbance, they are unable to compete with shade-tolerant species later on and are replaced by these species through succession. However these shifts may not reflect the progressive entry to the community of the taller long-lived forms, but instead, the gradual emergence and dominance of species that may have been present, but inconspicuous directly after the disturbance.1 Disturbances have also been shown to be important facilitators of non-native plant invasions.2

While plants must deal directly with disturbances because of their lack of mobility, many animals are mobile and thus are not as immediately affected by disturbance. For example, some animals could successfully evade the initial destruction of a forest fire, but can later return to the burned area and thrive on new growth on the forest floor. Disturbed communities (such as a forest after a fire) often support a wider variety of plants compared to pre-disturbance vegetation. The plants in turn support a variety of wildlife, temporarily increasing biological diversity in the forest.3

## Intermediate Disturbance Hypothesis

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.4 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 intermediate disturbance hypothesis theory, at intermediate levels of disturbance, diversity is thus maximized because species that thrive at both early and late successional stages can coexist. Intermediate disturbance hypothesisis a nonequilibrium model used to describe the relationship between disturbance and species diversity.

The intermediate disturbance hypothesis is based on the following premises:

1. ecological disturbances have major effects on species richness within the area of disturbance,
2. interspecific competition results from one species driving a competitor to extinction and becoming dominant in the ecosystem, and
3. moderate ecological scale disturbances prevent interspecific competition.5,6,7

Figure $$\PageIndex{1}$$: I describes how, at low levels of ecological disturbance species richness decreases as competitive exclusion increases; II shows that at intermediate levels of disturbance, diversity is maximized because species that thrive at both early and late successional stages can coexist; III shows that at high levels of disturbance species richness is decreased due to an increase in species movement. "Intermediate Disturbance Hypothesis Graph" by Sciencerelatedusername is licensed under CC BY-SA 4.0.

Disturbances act to disrupt stable ecosystems and clear species' habitat. As a result, disturbances lead to species movement into the newly cleared area (secondary succession).5 Once an area is cleared there is a progressive increase in species richness and competition between species takes place. Once the conditions that create a disturbance are gone, and competition between species in the formerly disturbed area increases, species richness decreases as competitive exclusion increases.8

"Gause's Law", also known as competitive exclusion, explains how species that compete for the same resources cannot coexist in the same niche.6 Each species handles change from a disturbance differently; therefore, intermediate disturbance hypothesis can be described as both "broad in description and rich in detail".5 The broad intermediate disturbance hypothesis model can be broken down into smaller divisions which include spatial within-patch scales, spatial between-patch scales, and purely temporal models. Each subdivision within this theory generates similar explanations for the coexistence of species with habitat disturbance. Joseph H. Connell9 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.

Another way of thinking about the intermediate disturbance hypothesis requires that we consider the types of organisms that could specialize in areas with different levels 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. In contrast, r-selected species colonize open areas quickly and can dominate landscapes that have been recently cleared by disturbance.7 These characteristics attribute to the species that thrive in habitats with higher and lower amounts of disturbance. Based on the contradictory characteristics of both of these examples, areas of occasional disturbance allow both r and K species to flourish in the same area. If K-selected and r-selected species can live in the same region, species richness can reach its maximum.

Several alternative hypotheses to the intermediate disturbance hypothesis have been proposed10. One alternative hypothesis 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). In addition, many species (e.g. ruderal plants and fire-adapted species) even depend on a specific disturbance in ecosystems where it often occurs.

## References

1. Nobel, I. The use of vital attributes to predict successional changes in plant communities subject to recurrent disturbances.
2. Lembrechts, J.J., Pauchard, A., Lenoir, J., Nuñez, M.A., Geron, C., Ven, A., Bravo-Monasterio, P., Teneb, E., Nijs, I., & Milbau, A. (2016). Disturbance is the key to plant invasions in cold environmentsProceedings of the National Academy of Sciences, 113(49), pp. 14061–14066. doi:10.1073/pnas.1608980113ISSN 0027-8424PMC 5150417PMID 27872292.
3. Pringle, L. (1979). Natural fire: Its ecology in forests. William Morrow and Company, New York, pp. 27-29.
4. Dial, R., & Roughgarden, J. (1988). Theory of marine communities: The intermediate disturbance hypothesis. Ecology, 79(4), pp. 1412–1424. doi:10.1890/0012-9658(1998)079[1412:TOMCTI]2.0.CO;2
5. Wilkinson, D.M. (1999). The disturbing history of intermediate disturbance. Oikos, 84(1), pp. 145–7.
6. Kricher, J.C. (2011). Tropical Ecology. New Jersey, Princeton: Princeton University Press.
7. Catford, J.A., Daehler, C.C., Murphy, H.T., Sheppard, A.W., Hardesty, B.D., Westcott, D.A., Rejmánek, M., Bellingham, P.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), pp. 231–41. doi:10.1016/j.ppees.2011.12.002
8. Vandermeer, J., Boucher, D., Perfecto, I., & de la Cerda, I.G. (1996). A theory of disturbance and species diversity: Evidence from Nicaragua after Hurricane Joan. Biotropica, 28(4), pp. 600–13. doi:10.2307/2389100JSTOR 2389100
9. Connell, J.H. (1978). Diversity in tropical rain forests and coral reefs. Science, 199(4335), pp. 1302–10. Bibcode:1978Sci...199.1302C
10. Hall, A.R., Miller, A.D., Leggett, H.C., Roxburgh, S.H., Buckling, A., & Shea, K. (2012). Diversity-disturbance relationships: Frequency and intensity interactBiology Letters, 8(5), pp. 768–71. doi:10.1098/rsbl.2012.0282