Ecological Succession - New Castle Community School ...



Ecological Succession

Raging fires transformed thousands of acres of wilderness in Yellowstone National Park in 1988. Mt. St. Helen's violent volcanic eruption in 1980 leveled thousands of hectares of forest and killed millions of animals. Hurricanes that bear down on the Caribbean topple trees and disturb coral reef communities. These are all examples of disturbances that happen naturally to communities. But human activities can also disturb communities. In this section you will read about the long-term effects of disturbances on communities.

Disturbances to Communities

Communities tend to be in a continual state of change. Natural disturbances such as fires, volcanic eruptions, floods, storms, and droughts affect communities by destroying organisms and changing the availability of resources such as shelter and water. The effects of the disturbance are not necessarily negative. Despite the deaths caused by the disturbance, some organisms thrive in the wake of the destruction. For example, seeds of some plant species require exposure to the extremely high temperature of a forest fire before they can sprout. Disturbances can also be caused by human activities, such as clearing a forest to grow crops or build homes. Like natural disturbances, certain disturbances caused by human activities can have some positive effects. For example, when a ship sinks in the ocean, it disturbs the ocean floor. However, the wreckage also provides new nooks and crannies in which fish can hide. Barnacles, mussels, seaweed, and other marine species can attach to its surfaces.

Communities may change drastically as a result of a disturbance. A variety of species may colonize the disturbed area. These species may be replaced as yet other species move into the area later. This process of community change is called ecological succession.

Primary Succession When a community arises in a lifeless area that has no soil, the change is called primary succession. Examples of such areas are new islands created by erupting volcanoes, or the bare rock left behind a retreating glacier. Autotrophic microorganisms are generally the first organisms to appear. Then lichens and mosses, which grow from windblown spores, colonize the barren ground. Pioneer species are the first organisms to inhabit an environment. Usually adapted to low nutrient needs. (Bacteria, algae, fungus, lichen and weeds) Soil develops gradually from the action of these early colonizers and from their decomposed remains. Once soil is present, the lichens and mosses may be overgrown by grasses, shrubs. These grasses and shrubs are called the Seral community. The seral community is an intermediate community that alters the physical factors of the area in a way that makes it impossible for the community to regenerate itself. These plants' seeds may have blown in from other areas or been carried in by animals. Finally, these grasses and shrubs are in turn replaced by trees. The trees are the Climax community which is a relatively stable, almost permanent community that creates conditions in which its young can regenerate the community Primary succession from barren ground to a community such as a deciduous forest can take hundreds or even thousands of years.

Secondary Succession When a disturbance damages an existing community but leaves the soil intact, the change that follows is called secondary succession. An example is a forested area that has been cleared for farming and then abandoned. Grasses that grow from seeds carried by animals or the wind may be the first plants to grow in abandoned fields. They may be replaced by shrubs, and eventually by trees similar to those in the original forest.

Human Activities and Species Diversity

Of all species, humans have had the greatest impact on communities worldwide. Currently 60 percent of Earth's land is used by humans, mostly as cropland or rangeland. Unfortunately, human disturbances usually have a negative effect on species diversity in those communities.

Clearing the Land Many areas of forest have been cut down for lumber or to provide land for farming and building. Similarly, much of the natural North American prairie has been converted to farmland. In some cases the diverse forest or grassland community is replaced with crops of a single plant species, such as corn. Other areas may be paved over or eventually recolonized by weeds and shrubs, as in abandoned city lots. Usually the species diversity of these environments is reduced by the disturbances.

Introduced Species Sometimes called exotic species, introduced species are organisms that humans move from the species' native locations to new geographic areas, either intentionally or accidentally. One intentional introduction is kudzu, a Japanese plant that was planted widely in the American South in the 1930s to help control erosion, especially along irrigation canals. But kudzu soon grew out of control, taking over vast expanses of landscape.

Many introduced species are ill-suited to their new environment and don't survive to reproduce. However, some introduced species gain a foothold and may disrupt their new community. Some introduced species prey on native species. Introduced species may also outcompete native species that have similar niches.

1. Provide an example of an ecological disturbance and describe its effects on a community.

2. What do we call the gradual change in a community over time?

3. What is the difference between a pioneer community, a seral community and a climax community?

4. What is the main difference between primary and secondary succession?

5. Give an example of an introduced species and describe its effect on species diversity.

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