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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