AQA A2 Biology Unit 3: Succession

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  • Created on: 22-08-13 17:21
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Succession is the process by which an ecosystem changes over time.
The biotic conditions change as the abiotic conditions change.
There are two types of succession:
Primary Succession Secondary Succession
Happens on land that's been newly formed This happens on land that's been cleared of
or exposed. There's no soil or organic all the plants, but where the soil remains.
material to start with, just bare rock.
Where a volcano has erupted to form a new After a forest fire or where a forest has been
rock surface or where sea level has dropped cut down by humans.
exposing a new area of land.
Primary Succession
Succession occurs in stages called seral stages
The first stage of primary succession is the colonisation of an inhospitable environment by
organisms called pioneer species.
Pioneers species often have features that suit them to colonisation, including:
Production of vast quantities of wind-dispersed seeds or spores so they can easily
reach isolated situations
Rapid germination of seeds on arrival as they do not require a period of dormancy
The ability to photosynthesise, as light is normally available b ut other `food' is not
The ability to fix nitrogen from the atmosphere because, even if there is soil, it has
few or no nutrients
Tolerance to extreme conditions
Succession takes place in a series of stages. At each stage, certain species can be identified
which change the environment, especially the soil, so that it becomes suitable for other
species. These other species then out-compete the species in the existing community and so
a new community is formed.
As succession goes on, the ecosystem becomes more complex. New species move in
alongside existing species which means the species diversity increases.
The final stage is called the climax community ­ the ecosystem is supporting the largest and
most complex community of plants and animals it can. It won't change much ­ it's in a
steady state. It is the abiotic factors such as climate that determine the dominant species of
the community.

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Secondary Succession
Secondary succession happens in the same way, but because there's already a soil layer
succession starts at a later seral stage ­ the pioneer species in secondary succession are
larger plants.
The process by which the ecosystem returns to its climax community is the same as
described above, except that it normally occurs more rapidly.…read more

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Different ecosystems have different climax communities.
Which species make up the climax community depends on what the climate's like in an
ecosystem. The climax community for a particular community is called its climatic climax.
For example:
Temperate Climate Polar Climate
There's plenty of available water, mild There's not much available water,
temperatures and not much change temperatures are low and there are massive
between the seasons. changes between the seasons.…read more

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Conservation is the management of the Earth's natural resources in such a way that
maximum use of them can be made in the future.
This involves active intervention by humans to maintain ecosystems and biodiversity.
The main reasons for conservation are:
Reason Explanation
Ethical Other species have occupied the Earth far longer than we have
and should be allowed to coexist with us.
Respect for living things is preferable to disregard for them.…read more

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Ways to Conserve Species and Habitats
Different species and habitats need to be conserved in different ways.
Conserving habitats by Managing Succession
Conservation sometimes involves preventing succession in order to preserve an ecosystem
in its current seral stage.
One example of this is the moorland that exists over much of the higher ground in the UK.
The burning of heather and grazing by sheep has prevented this land from reaching its
climax community.…read more

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Animals can be conserved using Captive Breeding Programmes
Captive breeding programmes involve breeding animals in controlled environments.
Species that are endangered, or already extinct in the wild, can be bred in captivity to help
increase their numbers.
However, some animals can have trouble breeding outside their natural habitat, which can
be hard to recreate in a zoo.…read more

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Conflicting Interests
One challenging conservation issue in the UK is the conflict between the conservation of hen
harriers and the commercial hunting of red grouse.
One scientific survey investigated the effect of predation by hen harriers on the breeding
success of red grouse on managed moorland in Scotland.…read more


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