Download Preview OCR A2 Geography Student Book sample pages 54-55

plants. Gradually, taller and more
diverse communities colonise the
clearing. Birch saplings, which like
sunlight, become established and
underneath these oak seedlings
begin to germinate. Oak requires
shade to begin to grow, but
eventually the trees overtop and
shade out the birch. In some areas,
beech eventually replaces oak to
become the climatic woodland.
A2 Geography for OCR
54
1 Why are legumes such as alder important in the
succession?
2 Why might species diversity decline under the climax
forest?
3 Suggest naturally occurring events which might
prevent the succession reaching its climax.
4 How might the soil pH change as the succession
changes?
Discussion point
Figure 2.11 Lowland heathland
Key terms
Eutrophication: the nutrient enrichment of water
bodies causing algae to proliferate.
Plagioclimax community: plant community
permanently arrested by human activity.
Secondary succession: plant succession which
develops on land that has been vegetated before.
U3
2
Look at the plant succession in Figure 2.10.
Other human activities causing change
Human activity
Effects
Drainage of wetlands
and coastal salt-marsh
This lowers water-tables and alters habitats.
Trampling
Light trampling increases species diversity, but heavy pressure damages plants. Some species are
more resistant to trampling than others, which is why tolerant plants such as plantain are often
found along footpaths.
Fertiliser application by
farmers
Nitrate and phosphate fertilisers increase crop yields, but excess fertilisers not taken up by the
plant often find their way into ditches and lakes causing nutrient enrichment or eutrophication.
Algae thrive in the nutrient-rich water and shade out waterweed. On death, the algae sink to
the floor of the lake where they are decomposed by bacteria which consume oxygen and this
de‑oxygenates the water.
Species removal
In the 1920s the wolf was deliberately removed from Yellowstone Park in the USA because it
was considered a threat to elk and buffalo herds. Elk numbers subsequently rose and consumed
young aspen, willow and cottonwood trees. Wolves have recently been reintroduced into
Yellowstone to control the elk numbers and restore the vegetation.
Introduction of alien
plants and animals
(accidential and
deliberate)
This brings about change, particularly on islands where many species are endemic and have no
fear of predators. On Hawaii, for example, the introduction of the mongoose and the rat has
posed a major threat to flightless, ground-nesting native birds. Fireweed, a plant introduced from
the Canary Islands, has also displaced native species on volcanic soils.
The burning of fossil
fuels
This creates acid rain which adversely affects lakes and coniferous forests. Acid rain which falls on
already acid soils developed under coniferous woodland, leaches nutrients such as potassium and
magnesium from the soil. Acidity also increases the solubility of normally insoluble minerals such as
iron and aluminium, which make the soil toxic. Pine needles turn yellow and die and trees lose their
vigour and become vulnerable to insects. Acidity also alters plant compositions often causing a
decline in species diversity. Heavy metals, such as mercury, kill sensitive species such as trout in lakes.
Should lowland heathlands be managed?
Theory into practice
Look out for local examples of colonisation and
succession, such as plants developing on paving
stones and walls, or on derelict land, or on farmland
which has been taken out of production (set-aside).
Figure 2.12 Salt-marsh backed by sand dunes
tides (MHWS). Marshes form in sheltered estuaries and
on the landward margins of spits and bars (Figure 2.12),
where the water is shallow and the coastal gradient is
low. Creeks cross the marsh surface which fill and drain
with each incoming (flood) and outgoing (ebb) tide
(Figure 2.13). The marsh is broadly divided into a lower
area which is covered and exposed twice daily by tides,
and an upper zone which is submerged by spring tides
which occur only a few times each month.
Key term
U3
Salt-marsh: coastal vegetated mudflat.
2.2 What factors give
the chosen ecosystem
or environment its
unique characteristics?
A salt-marsh is a vegetated mudflat found on the
coast. It develops between the mean low water of
neap tides (MLWN) and the mean high water of spring
Stores and flows within a
salt‑marsh ecosystem
The salt-marsh ecosystem contains a variety of different
organisms. Rooted plants such as glasswort grow on
the marsh surface, while Enteromorpha, a green alga,
lives on the mudflats. Bacteria live on and within the
mud and decompose the algae and decaying plant
matter. Crustaceans and molluscs live within or on the
surface of creeks and mudflats
which are periodically covered by
the tides. Brent geese graze the
marshes in winter, while wading
birds, such as redshank, breed
on the marshes in summer and
feed on crustaceans, worms
and molluscs on the mudflats
and within creeks. Fish, such as
the goby, live in the creeks and
crabs scavenge for food on the
mudflats.
2
Ecosystems and environments under threat
Humans can also initiate secondary
successions by clearing and
later abandoning land. Where a
succession is more permanently
prevented from developing, a
plagioclimax community is created. One example
seen in southern Britain is lowland heathland which
is maintained by continuous grazing and periodic
burning (Figure 2.11). Left to itself, it would be invaded
by bracken, gorse and birch and eventually be
replaced by oak woodland.
Activities
Figure 2.13 A salt-marsh creek
55