AQA A2 Biology Unit 4 3.4.7 Succession

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3.4.7 Succession
SUCCESSION is the process in which a community alters its environment and is replaced by a new community over a period of
Succession takes place in a series of stages, where each species present changes the environment, making it suitable for
another species to colonise. The new species then usually out-competes the previous species for resources such as
nutrients, space or light.
In the early stages of succession, ABIOTIC factors are the most important in determining which organisms colonize a habitat.
After the pioneer species have become established, BIOTIC FACTORS become increasingly important in the habitat. As
succession continues, the species diversity of the community increases, and the food web of the ecosystem becomes more
There are two types of succession that can occur:
PRIMARY SUCCESSION is the change in species composition over time in a habitat that has not previously been
SECONDARY SUCCESSION is the change in species composition over time in a habitat that has previously been
Primary succession
Primary succession may start from newly exposed bare rock, volcanic islands, glacial deposits of rock, new ponds or lakes, or
from sand dunes deposited on a shore.
Only those organisms that can tolerate the hostile abiotic environment can establish themselves. These are called PIONEER
SPECIES. These species must have features that allow them to colonise such an inhospitable environment.
1. The pioneer species are often lichens. These slow-growing organisms produces organic acids that break down the
rock surface to provide essential mineral ions.
2. The increase in mineral ions allows more plants to colonise
3. As these plants die, the ground becomes richer in humus and soil begins to form.
4. As the soil and humus content increases, more water can be retained and the increased depth available allows more
plants to grow.
5. The progression of plants is usually mosses following lichens, then ferns, grasses, shrubs and trees.
Ecological succession leads to more complex communities with a high species diversity that is in equilibrium with its
environment. This has been called the CLIMAX COMMUNITY. In the UK, the final climax community formed is usually
deciduous oak woodland.
It is generally assumed that an increase in species diversity makes an ecosystem more stable because of the large number
of alternative links between members of the community.
Several species might be carrying out the same function, such as decomposition or primary production. If one species
became extinct, the others would still be available to carry out that function.
Example: Primary Succession on Sand Dunes
Sand dunes are deposited by wind and water, in coastal regions throughout the world. They are not permanent. The
environment is very severe, with high salt concentrations in the water at high tide and dehydration at low tide. Only a few

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As the distance from the shore increases, the sand becomes more stable and
succession towards the climax community begins.
Grasses are a common pioneer plant on a sand dune. As the grasses extend over the surface of the dune, their roots help to
stabilise the dune. It is now possible for mat-forming shrubs to colonise the dunes, and the stability of the dune increases.
Much later, fast-growing trees such as birch replace the shrubs, which in turn are replaced by oaks.…read more


Laura Tuppen

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