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Fact sheet.........73
Ecological Succession
What is ecological succession?
The ecosystem is made up of living and non-living components which
interact with each other to form a stable, self-sustaining system in nature.
In certain habitats, however, communities tend to succeed one another in
a relatively definite sequence even when there are no changes in climatic
or topographic conditions.
This sequence of ecological changes is known
as ecological succession, and is usually the result of activities of existing
organisms.
How does ecological succession occur?
Due to the activities of existing species, the environment is gradually
changed such that it becomes more favourable to other species.
These
new species will compete with and eventually replace the existing species
which have created the modified conditions.
The new species will
dominate the habitat for a period and form a closed community.
In the
course of time, conditions become further modified by the actions of the
existing species so that they can no longer compete successfully with new
invaders.
In ecological succession, one community continues to follow
another until, in many situations, a type of community is reached that
cannot be replaced under the prevailing conditions.
The community that
can maintain itself indefinitely in a habitat is known as the climax
community.
When a climax community is established, it tends to remain
in possession of the area because it does not change the environment so as
to favour the growth of other species.
The climax community will not be
replaced unless the environment is dramatically changed.
Types of ecological succession
There are two types of ecological succession: primary succession and
secondary succession.
Primary succession is the colonisation of an
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area which has not supported life before, e.g. volcanoes, sand dunes, and
glaciated surfaces. It usually occurs from the starting point of bare rock.
Secondary succession is ecological succession in an area which previously
supported an ecosystem has been denuded of vegetation by factors such
as fire or grazing.
It may occur after a forest fire or clearance of
agricultural land.
Examples illustrating the change underlying ecological succession
1.
Establishment of a tropical rain forest
In August 1883, two volcanoes exploded in Krakatoa, an island
between Java and Sumatra. The lava and volcanic ash covered the
whole island and all life forms disappeared.
After nine months, some
blue-green algae appeared on the rock surface of the shore.
After
three years, ferns and grasses were found over most part of the island.
Seven years later, there were young coconut trees, lizards, snakes and
a large number of insects and spiders.
Finally, a dense tropical rain
forest becomes established on the island again and the community is
in a state of balance.
Krakatoa has reached a climax community with
eight hundred different species being recorded in 1983 one hundred
years after the clearance of the island.
The recovery of the forest on Krakatoa illustrates how progressive
changes in a community can occur over a long period of time.
At first,
the bare rocks could hardly support any organisms except the
primitive algae and lichens.
These first organisms to move in are
called pioneer species which can penetrate the rock surface and trap
organic materials to begin the formation of humus.
Weathering of the
rock and action of algae and lichens on the rock formed a thin layer of
soil which favoured the survival of mosses. The mosses outgrew the
algae and lichens.
They trapped more dust and formed more soil
when they died.
The accumulating soil in turn favoured the
establishment of grasses and ferns, and they in turn broke the rocks
further.
Finally the soil was deep enough for the establishment of the
trees which became the dominant species.
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2.
Changes in mangrove community
In coastal areas of tropical regions, the mangroves grow into the sea
by producing special seedlings called droppers that will root in shallow
water.
The spreading prop roots of the mangroves trap soil particles
and dead organic matter between them thus raise up the sea bottom.
As the soil becomes drier, this change leads to the gradual elimination
of the mangroves and allows the establishment of the more terrestrial
shrubs.
3.
Changes in a deserted land
A deserted field or construction site is usually first invaded by weeds.
As more soil is formed by the activities of the weeds, the weeds are
succeeded by shrubs which in turn add more soil to the land.
Finally,
trees can establish on the deeper soil and replace the shrubs.
4.
Changes of a forest after a fire
When a forest is cleared by fire, if conditions become suitable once
again, the bare land will begin to change back into a forest.
happens very slowly and gradually.
This
Spores, seeds and organs of
vegetation may remain viable in the soil and thus influence the
succession. There may also be an influx of plants and animals through
dispersal and migration from the surrounding area.
In these
circumstances the succession will not begin with pioneer species but
with organisms from subsequent successional stages.
A forest cleared by a fire