Intermediate Disturbance Hypothesis

The Intermediate Disturbance Hypothesis (IDH) states that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. 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 both competitive K-selected and opportunistic r-selected species 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 from one species driving a competitor to extinction and becoming dominant in the ecosystem . Third, moderate ecological scale disturbances prevent interspecific competition

Disturbances act to disrupt stable ecosystems and clear species’ habitat. As a result, disturbances lead to species movement into the newly cleared area . 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 . “Gause’s Law”, also known as competitive exclusion, explains how species that compete for the same resources cannot coexist in the same niche . Each species handles change from a disturbance differently; therefore, IDH can be described as both “broad in description and rich in detail” . 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 . Each subdivision within this theory generates similar explanations for the coexistence of species with habitat disturbance. Connell 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 is elephant herds, which are 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 . An ideal example of R-selected groups is algae. Based off the contradictory characteristics of both species, areas of occasional disturbance allow both groups to benefit off of residing in the same area. The ecological effect on species relationships is therefore supported by the Intermediate Disturbance Hypothesis.