Intermediate disturbance hypothesis

Intermediate disturbance hypothesis, or IDH, is a theory in ecology. It proposes that the variations and diversity in the plant and animal life in an area reaches its greatest heights when there are neither too few nor too many significant change events, or disturbances. These events include such occurrences as fires, floods, lava flows, or human actions such as logging or mining.

These disturbances result in varying levels of destruction of the life forms in the area. Some will be almost completely destroyed, others will experience medium levels of destruction, and still others will be minimally affected. As those that survived repopulate, the area will have plant and animal life at various stages of development. This is known as ecological succession. According to IDH, when there are moderate amounts of disturbances, there will be a greater diversity of the types and stages of life in the area. This, the theory proposes, works to the mutual benefit of all the species living in that location.

Background

The intermediate disturbance hypothesis was proposed by American ecologist Joseph H. Connell. His professional career began in the early 1960s with highly acclaimed research related to the competition between two species of barnacles. The idea of competition between organisms and the structure of the communities between different species of life forms became a cornerstone of Connell’s professional work.

In the 1970s, Connell focused much of his research on coral reefs and rainforests. He became particularly interested in how damage or destruction within these ecosystems affected the plant and animal life. Connell studied how the flora and fauna recovered in various circumstances, including various levels of damage from disturbances and different rates of repetition of the disturbance. In 1979, he published the hypothesis of intermediate disturbance.

Connell’s work became one of the theories related to ecological succession. This is an important concept in the scientific field of ecology that addresses how plant and animal species respond after an environmental change that reduces the ecosystem to a simpler form. It was first proposed in the nineteenth century, when one of its earliest proponents was writer and naturalist Henry David Thoreau.

There are two forms of ecological succession, primary and secondary. Primary succession occurs when none of the previous life survives the disturbance. This might happen after a catastrophic landslide, severe fire, or volcanic eruption with significant lava flows or ash cover. New plant and animal life must start from nothing to repopulate the area.

Secondary succession is the result of a less-severe disturbance. While there is significant disruption to some or all of the life in the area, some life persists. Some species may have been affected more than others, leaving the plant and animal life in different stages of growth and community organization. This might happen after a less-severe forest fire, flood, or human activity such as logging.

Ecologists have formed various theories to understand and explain ecological succession and how various ecosystems respond to disturbances. At their simplest, these theories address how one type of plant or animal life responds after a disturbance from the event to its full recovery, or climax. Others address various factors that might interrupt that progress, such as the introduction of a new type of plant or animal life. Theories allow for life forms that might not recover fully, or become a subclimax community, and those that overcome the interruption and fully recover to a climax state.

Many of these theories follow the progress of one affected species at a time. By the beginning of the twentieth century, ecologists became interested in how the interactions between the different types of plant and animal life in an area affected its ultimate recovery from a disturbance. Some, such as ecologist Henry Chandler Cowles, established a new focus on how different aspects of the environment affected the life in that environment.

As a result, ecologists began studying factors such as distance from water sources, the age of the area at the time of the disturbance, and the types and ages of the non-living portions of the environment such as dirt and rock. Eventually, this led to the type of study done by mid-century ecologists such as Connell, which looked at the area as a whole and how the various living parts of the system interacted with and affected each other in forming a new community or ecosystem. They also began investigating the concept that a certain amount of change or disturbance in an ecosystem is necessary for the area to reach its fullest potential in terms of diversity and viability of plant and animal life.

Overview

The Intermediate Disturbance Hypothesis refers to the idea that all ecosystems experience disturbances and that the ecosystem of a given area needs a certain amount of disturbance in order to reach maximum potential in diversity and growth. Rather than being seen as a wholly disastrous and destructive force, events that alter an ecosystem can sometimes play an essential part in how an area grows and develops. However, this depends on the nature of the disturbance. Disturbances often alter the type and variety of plant and animal life in an area, changing its diversity and biological richness. They can also lead to conditions that allow one species to dominate at the expense of others.

This led to Cowell’s theory that the amount of disturbance is important. Too little disturbance could lead to some species dominating and never allowing others to fully develop. Too much disturbance could mean that much of the life in an area is in an infancy stage of development, or the plant and animal life has to start over too many times after repeated disturbances. This prevents any of the species from reaching its full potential. The hypothesis proposes that an area needs an intermediate, or moderate, amount of disturbance to reach maximum growth and development potential.

Disturbances, whether from natural sources such as disasters or from human-influenced changes, rework the ecosystems in place in an area. Some life forms may be wiped out or severely diminished. The removal of an entire rainforest of trees in a logging operation would be an example of this type of change. In other cases, some of the life in an area might be affected, such as some trees being burned in a fire while others are damaged or escape entirely. Sometimes, the species survives but loses something it needs for survival. For example, when water temperatures change because of climate alterations, some aquatic plants and animals might be directly affected by the temperature change while others might lose a source of food as those life forms die out.

All these changes alter the environment in an area, clearing the way for a new environment to form. This revised environment will include species that survived the disturbance with little effect, some that survived but need to reestablish themselves, and new species that take advantage of the change to expand their habitat. It will not include some plants and animals that did not survive the disturbance. As a result, a new and more varied environment will form, including species at various stages of growth and with different needs for the resources available in the environment. This reduces competition for the same resources and enhances the biological richness of the area.

Not all ecologists agree with the hypothesis, however. It is more widely accepted in some branches of ecological study, such as those involved with coral reefs and other marine life. In other branches, however, less correlation has been found between the frequency or type of disturbance and the biodiversity and richness of an area. Some ecologists have criticized the theory as having been developed with too few studies involving too few species in an area, inadvertently slanting the data to show a greater correlation than really exists between disturbance frequency and diversity.

Some subsequent studies have shown weaknesses in the hypothesis’s conclusions related to how the disturbances affect competition between species. Others note that it does not fully account for factors such as inherent differences in growth rates between species that could have some forms of plant or animal life reaching maturity faster than others. This alters the hypothesis’s findings that use the profusion of a particular species as a measure of the effects of a disturbance. Critics also say the theory does not adequately account for the fact that some plant and animal species are simply more resilient than others and will always recover better than others, which also affects the rates of diversity in an area.

As a result, some scientists have moved to alternate ways of addressing how an area will recover after a disturbance. These tend to rely on mathematical formulas for determining probability. While they consider such factors as interdependence and competition between species, they are more focused on the mathematical likelihood of a particular species surviving or recovering from a given disturbance.

Bibliography

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