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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.