Intermediate Disturbance Hypothesis Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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The intermediate disturbance hypothesis (IDH) predicts that the highest levels of biotic diversity are to be found at intermediate levels of disturbance and at intermediate time spans following the disturbance. Other influential ideas about species diversity include island biogeography theory, the time hypothesis, niche partitioning/diversification, and productivity-stability hypothesis.

Disturbances to ecological systems can emanate from diverse sources that may be natural or anthropogenic, including fire, wind, grazing and predation, human management regimes, land cover change, chemical, and thermal contamination, tides, floods, tectonic activity and other forces. According to IDH, relatively low numbers of species may be expected to prevail in ecological systems that are undisturbed or usually subject to very low levels of disturbance, as well as in ecosystems that suffer highly frequent or intense disturbances. Ecosystems characterized by intermediate levels of disturbance, by comparison, are predicted by the IDH to exhibit the highest species richness and diversity (species richness and abundance). A graphical representation of the diversity-disturbance relationship, according to the IDH, would approximate an inverted U-shape when diversity is mapped along the Y/vertical axis, and disturbance intensity/frequency along the X/horizontal axis.

Nonequilibrium Theory

The theories explaining the IDH and its inverted U-shaped relationship between disturbance and diversity are linked to concepts derived from community ecology and ecological succession, and the IDH may be seen as one among several nonequilibrium theories of biodiversity. In the IDH view, the prevalence of disturbance in ecological communities prevents them from reaching equilibrium states. Disturbance in a system results in the creation of gaps and spatial heterogeneity, and opens up spaces for various species to colonize. The gaps and the ecological community then begin a trajectory along a new successional sequence, or revert to a sequence similar to the pre-disturbance succession.

At very high levels of disturbance, the ecological community and its gaps do not progress beyond the pioneer stage of the successional sequence. The species composition is dominated by a few early successional, pioneer species often referred to as “r-strategists” for their life history strategies geared toward high population reproductive rates (r) rather than adaptations geared toward competitive advantage. Such dominance leads to low species diversity.

At very low levels of disturbance, on the other hand, successional pathways are quickly followed to the final climax/equilibrium stage. This leads to the dominance of later successional species, often referred to as “K-strategists” for their prevalence at or near the population carrying capacity (K). Such species are adapted to compete successfully for limited resources, and thus exclude other species, leading also to low species diversity overall in the ecological community.

At intermediate levels of disturbance, neither pioneer nor late successional species manage to dominate, and the species mix reflects a higher diversity than expected under lower or higher levels of disturbance.

An early formulation of the IDH was presented by J.H. Connell (1978), who studied species diversity patterns within local areas rather than across large-scale geographic gradients such as temperate to tropical ecosystems. Specifically, Connell focused on tropical forests and coral reef ecosystems, hypothesizing that such systems were characterized by disturbances that maintained them in nonequilibrium states, which helped maintain and explain their high levels of biotic diversity. He also sounded a cautionary note about human disturbance, stating in his article that although the IDH proposed diversity benefits of disturbance regimes, many anthropogenic disturbances-such as mass-scale tropical deforestation or chemical pollution-were in fact qualitatively different from many natural disturbance regimes to which organisms had the opportunity to adapt over long periods of time. Therefore, he warned, such human-caused disturbances had the capability to cause species extinctions, particularly in highly diverse tropical ecosystems with low species populations.

Since Connell’s article, several studies from marine and freshwater ecology as well as terrestrial ecology have produced evidence corroborating IDH, while others have presented empirical data and/or simulation models contradicting the hypothesis. For instance, disturbance in the form of selective logging often leads to a loss, rather than an increase in the number of understory deciduous plant species. Another important factor is the effect of changing spatial scale. In certain tallgrass prairie ecosystems, for instance, intermediate levels of disturbance (annually burned versus unburned or burned once every few years) are linked to lower within-site diversity of vegetation; but the relationship reverses when larger spatial areas are considered.

Other experiments in tallgrass prairies have garnered support for the lesser-explored component of IDH, temporal scale (time elapsed since disturbance), while contradicting the disturbance-diversity predictions of IDH. Scale dependence is also seen in coral reef systems. Additionally, coral diversity may often be high in deeper zones or under greater coral cover despite the fact that deeper reefs and greater cover are usually subject to or reflect lower disturbance.

In most cases, a complex of several factors may interact and collectively influence diversity patterns: disturbance, soil fertility and/or nutrient versus limitation, resource partitioning, native vs. invasive/exotic species assemblages (and their evolutionary/life history characteristics), climate change and seasonality.

Bibliography:

H. Connell, “Diversity in Tropical Rainforests and Coral Reefs,” Science (v.199, 1978);
R. Pianka, “On r and K Selection,” American Naturalist (v.104, 1970);
E. Ricklefs and G.L. Miller, Ecology, 4th ed. (W.H. Freeman and Company, 2000).