Disturbance (ecology)

In the study of ecology, a disturbance is a change in environmental conditions that causes a pronounced change in an ecosystem.

These events can alter the physical structure or the arrangement of biotic and abiotic elements. Biotic elements are the living parts, like plants and animals, while abiotic elements are the non-living parts, such as soil or sunlight. Disturbances can happen very quickly or occur over a long period of time. They can also change the population size or the species richness of a community. Species richness refers to the number of different species found in a specific area.

Disturbances function by changing the availability of space and resources. Immediately after an event, there is often a pulse of recruitment, which is the process of new individuals growing into a population. This regrowth happens because there is very little competition for space or nutrients. However, once a plant is established, it becomes very difficult to displace. The spatial scale of a disturbance also modulates its effect on a community. For example, seed dispersal and herbivory might decrease as you move further from the edge of a burn. This means the interior of a large fire responds differently than a small one.

Disturbances can be classified by their source and their pattern. Many natural disturbances follow a cyclical or periodic pattern. These include wildfires, floods, droughts, landslides, and insect outbreaks. For instance, longleaf pine ecosystems in the southeastern United States often experience natural fire disturbances due to high lightning rates. Other natural events include volcanic eruptions, earthquakes, and impact events. In contrast, anthropogenic disturbances are caused by humans. These include logging, mining, dredging, and the conversion of land for agriculture.

Human activity can also influence ecosystems in complex ways. Some human actions, like the controlled burns used by Native Americans, help maintain stability and biodiversity. These burns support fire-dependent ecosystems and enhance overall ecosystem health. However, other human actions can create novel ecosystems called agroecosystems. These are environments created by agricultural practices that are colonized by species adapted to disturbance, often called weeds. Furthermore, anthropogenic climate change is considered a major source of change for future successional trajectories. This refers to the predictable path an ecosystem takes as it changes over time.

When an ecosystem recovers, it undergoes a process called ecological succession. This is the change in the abundance of different species over time. After a disturbance, ephemeral herbaceous life-forms, which are short-lived plants, often appear first. Over time, these are overtopped by taller perennials, shrubs, and trees. Without further disturbance, many ecosystems trend back toward their pre-disturbance conditions. In this process, long-lived species eventually become dominant. Succession often leads to conditions that make the ecosystem ready for a new disturbance.

A clear example of this cycle involves the mountain pine beetle, or *Dendroctonus ponderosae*. In 2004, these beetles affected more than 90,000 square kilometres in western North America. The beetles exist in both endemic and epidemic phases. During an epidemic phase, swarms of beetles kill large numbers of old pines. This mortality creates openings in the forest canopy. In these openings, spruce, fir, and younger pines thrive.

As the pines grow back and reach the canopy, they eventually become more susceptible to infestation again, creating a temporal mosaic of different tree ages.

Some organisms are specifically adapted to exploit these disturbed sites. These are known as pioneer species or early successional species. These species are often shade-intolerant, meaning they require high levels of sunlight to grow. They can photosynthesize at high rates, allowing for rapid growth, though they usually have short life spans. For example, the Jack pine in boreal forests has specialized serotinous cones. These cones only open and disperse seeds when they are exposed to the heat of a fire.

Disturbance is essential for maintaining biological diversity. In ponderosa pine forests in the western United States, frequent surface fires thin out existing vegetation. This thinning allows for new growth and prevents a single species from taking over. If fire is suppressed, shade-tolerant species like Douglas fir may replace the pines. Because Douglas firs have dense crowns, they limit the sunlight reaching the forest floor. This lack of light can severely limit new growth and cause a decline in the diversity of forest flora. Therefore, natural disturbances ensure that many different types of life can coexist.