28.12 : Ecological Disturbance

In many ecological communities, the diversity and composition of species are in flux. Ecological disturbances⁠—events that influence the makeup of an ecosystem⁠—contribute to these changes by adding or removing organisms or altering the level of available resources.

Natural physical factors, such as drought, may cause ecological disturbances. Biological factors, such as disease, also disturb an ecosystem. Human activity, such as deforestation, causes the largest disturbances today.

The intensity of a disturbance affects its impact on an ecosystem. For example, a low-intensity storm may damage a few trees.  A high-intensity flood may eliminate many plants and animals.  

The geographic extent of the disturbance also affects its impact on an ecosystem. For instance, while a single tree falling causes only local damage, a fire may cause damage spanning hundreds of miles.  

A third major factor in the ecological impact of a disturbance is the frequency of the disturbance. While a single fire can rejuvenate a forest by promoting regrowth, biodiversity and renew soil nutrients, repeated fires can negatively affect the ecosystem, reducing time for native plants to recover and limiting soil fertility. 

Ecological disturbances can influence species diversity in a community. According to the intermediate disturbance hypothesis, moderate levels of disturbance promote more diversity than high or low levels. 

High levels of disturbance can lower diversity by exceeding the tolerance and recovery times of species that grow or colonize slowly, allowing faster growing species to fill the gaps. 

Low levels of disturbance can also reduce diversity by allowing the most competitive species to monopolize resources, which prevents less dominant species from becoming established. 

Whether their impact is low, moderate, or high, ecological disturbances have a broad range of causes and effects that continually shape ecosystems across the globe.