Lesson 6 Text

1 Disturbance Ecology

Lesson 6: Disturbance Ecology
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Disturbances are ecosystem processes which affect the composition, structure and function with in an ecosystem. Their effects include changes to parent material, soil, hydrology and the destruction of biota all of which than lead to alterations in species composition, structure and ecosystem function. Disturbances also promote changes in the mutulaistic and competitive nature between organisms which than influences the rate of vegetation change and therefore succession. Natural disturbances such as fire, insects and diseases and wind have shaped the landscapes and vegetation found in all ecosystems.


USDA Forest Service - Northeastern Area Archives, USDA Forest Service, www.forestryimages.org

In addition to the natural disturbance factors which shape our ecosystems, we also must consider the disturbances caused by humans. We will discuss the different types of disturbances as they relate to secondary secession later in this lesson.

LESSON 6
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2 Disturbance as a Ecosystem Process

Lesson 6: Disturbance Ecology

The term disturbance will be used here to represent any discrete event in time that disrupts ecosystem composition, structure or function (Barnes et al. 1998). Due to the nature of the disturbance process we often associate it with death and destruction; however we must consider the idea that disturbances are in fact natural events in all ecosystems.


Dave Powell, USDA Forest Service, www.forestryimages.org

When discussing disturbances we must consider the concept of scale. At large scales such as the western U.S. macroclimate and physiography will strongly affect the kinds of disturbances found. As we move from a macro scale to a local scale we must consider specific ecosystem characteristics as well as the relationship of one ecosystem to other ecosystems. These factors will help us determine the kind of disturbances that an ecosystem may encounter as well as their frequency and severity.

As we have mentioned disturbances not only effect individual plants and communities but have effects on the physical site as well (Barnes et al. 1998). Disturbances such as glaciation and volcanism have major influences on site development by creating new landforms and rearranging parent material especially at large scales. On smaller scales wind and fire can change the physical site as well. The point to this is that not only do disturbances have direct effects on plants and plant communities but they also have many indirect effects.

LESSON 6

3 Types of Disturbance

Lesson 6: Disturbance Ecology

Disturbances are often times ecosystem specific. So to best understand the different types of disturbances we must always think in terms of specific ecosystems with which they occur. When we view disturbances in this way we see that the frequency and severity of a disturbance is often closely related to the specific site conditions.


USDA Forest Service - Rocky Mountain Region Archives, USDA Forest Service, www.forestryimages.org

There are many disturbance agents throughout the world which interact with each other as well as with the environment and the biota to create disturbance regimes. Although we will discuss several specific types of disturbances you should always remember that there are at any one time several different types of disturbances interacting in various ways.

LESSON 6

4 Land Movement

Lesson 6: Disturbance Ecology

Events such as earthquakes, landslides, and volcanoes are all major disturbance agents which can create new ecosystems with extremely different biota. In many parts of the world landslides have been linked to drastic changes in vegetation. Just like landslides volcanic eruptions are also present in many parts of the world and may be responsible for large scale vegetation changes. Recent events such as the eruption of Mt. St. Helens are one example of how volcanic activity has shaped many ecosystems around the world.


Cascades Volcano Observatory, USGS, www.usgs.gov
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5 Fire

Lesson 6: Disturbance Ecology

Fire may be one of the most dominate disturbance agents world wide. A large number of forested and non forested ecosystems for example have been burned over at more or less frequent intervals for the last several thousand years. In many of these ecosystems fire plays an important role in reproduction and regeneration, reduction of competition, nutrient cycling and forest succession.


Dale Wade, Rx Fire Doctor, www.forestryimages.org
LESSON 6

6 Wind

Lesson 6: Disturbance Ecology

The effects of wind on ecosystems can be found at regional and local scales, from hurricanes which may affect a large portion of the east coast to down bursts which may only affect a few trees in a forested stand. In either case the effects of wind as a disturbance agent can not be ignored.


Ronald F. Billings, Texas Forest Service, www.forestryimages.org
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7 Insects and Disease

Lesson 6: Disturbance Ecology

Insects and pathogens are major regulators of forest in many ecosystems, each year there are millions of hectors affected by insects and pathogens throughout North America. Although the visual effect of insects and pathogens is less dramatic than that of fire or mass land movements their ability to kill either individual trees or small groups of trees in a forest has a major influence in controlling forest succession. However recent outbreaks of bark beetles and the introduction of invasive pathogens have greatly influenced forest succession on large scales. There ability to have dramatic influences on forest succession is even more pronounced due to the fact that many insects and pathogens attack only one species. Insects and diseases like all other disturbance agents tend to work together this is particularly true with respect to fire, although little information is available about this relationship.


James Solomon, USDA Forest Service, www.forestryimages.org
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8 Human Disturbances

Lesson 6: Disturbance Ecology
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Human disturbances have probably had the most drastic effects on succession in both forested and non-forested lands. Activities ranging from over grazing, to land clearing and logging have dramatically changed the composition, structure and function of ecosystems around the world. In the western United States fire prevention is one of the most ecologically damaging practices humans have introduced. Other issues such as fragmentation may have greatly disturbed large scale ecosystem processes.

Other human caused disturbances include the introduction of non-native species, the elimination of species and climate change. The effects of these and other human caused disturbances has had dramatic effects on ecosystems on both local and landscape scales, and as with global warming the effects on vegetation and secession will be seen across the world.


Reto Stöckli, NASA Goddard Space Flight Center, www.nasa.gov


Dave Powell, USDA Forest Service, www.forestryimages.org

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