AQA A2 BIOLOGY UNIT 4: Ecological Succession

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  • Created on: 18-04-14 14:15
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Ecosystems are not fixed, but constantly change with time.
This change is called succession.
Different species of plants naturally colonise a habitat in a predictable order, until finally a stable
community is reached, called the climax community.
Each plant species in turn changes its environment (e.g. by creating deeper soil, or providing shade),
making the environment more suitable for new species to colonise.
These new species are usually bigger plants with a larger photosynthetic area, so they outcompete and
replace the older species.
So each plant effectively causes its own demise.
The plants colonising early in succession (the pioneer species) tend to be small and fast growing, with
shallow roots and wind-dispersed seeds.
The plants colonising late in succession tend to be tall and slow growing, with deep roots and animal-
dispersed seeds.
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
· Rapid germination of seeds on arrival as they do not require a period of dormancy
· The ability to photosynthesise, as light is normally available but other `food' is not
· The ability to fix nitrogen from the atmosphere because, even if there is soil, it has few or no
· Tolerance to extreme conditions
The successive stages are called seral stages, or seral communities, and the whole succession is called
a sere.
It will usually take a few hundred years to reach a stable climax community.
The climax community is usually a forest, though this varies depending on the climate and the
underlying rock.
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.
During any succession there are a number of common features that emerge:
The non-living environment becomes Soil forms, more nutrients available and
hostile more plentiful
A greater number and variety of habitats Increase biodiversity
Increased biodiversity As different species occupy these habitats
More complex food webs Lead to increased biomass
Increased biomass Especially during mid-succession

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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 global gene pool is a resource for learning more about life on Earth, and some genes may be able
to provide us with useful products for medicine and biotechnology.…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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