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Stage 5: Review

Stage 5: Conclusions | Evaluation

Conclusions

Link your results to the process of succession. Moving inland you might expect to find

• greater vegetation cover
• deeper soil
• higher soil moisture content
• lower soil pH
• a change in species composition and species diversity

These can all be linked to the concept of facilitation - the death and decay of plants from an earlier seral stage facilitates the invasion of plants in the next seral stage.

Evaluation

Why might the results not be as you expect?

1. Erosion of the foredunes

Sand grains are continually being transported between the beach and the foredunes. If the amount of erosion is equal to the amount of deposition, the dunes are stationary.

If there is more erosion of sand in the foredunes than deposition, the dunes are described as eroding. Over time, the sea will appear to advance inland. The photo (right) shows the eroding east-facing side of dunes at Dawlish Warren in Devon.

If there is more deposition of sand in the foredunes than erosion, the dunes are described as accreting. Over time, the dune system will advance out to sea as new dune ridges build up the level of the old sea bed.

In a 2006 survey of the sand dunes of England and Wales, 35% of the dune frontage was recorded as stationary, 30% was accreting and 35% was eroding or protected by hard engineering sea defences (DEFRA, 2007).

2. Grazing

Grazing by livestock, like sheep and cattle, is an arresting factor which can halt succession. The main effects of grazing are listed below.

• Vegetation height - higher in ungrazed areas. Animals bite off the top of plant material ,with sheep able to bite off plant material closer to the ground than cattle.
• Bare ground - heavy grazing and trampling by hooves can result in removal of parts of the turf and the creation of bare ground. These may be enlarged by wind to form blow outs. Alternatively, bare patches of sand can be colonised by less vigorous species which are not good competitors.
• Species composition - the relative abundance of different species. Species which are more resitant to trampling will increase in relative abundance.
• Growth form of plants - rosette and trailing growth forms are relatively more frequent in grazed areas. Grazing animals damage plants by removing growing tips, so plants with a growing point close to the ground are more likely to survive.

3. Trampling

Trampling by people is another arresting factor which can halt succession. The main effects of trampling are listed below.

• Physical damage - the crushing and flattening of leaves and stems. Over time this leads to more bare ground and a reduction in mean plant height.
• Species composition - the relative abundance of different species. Species which are more resitant to trampling will increase in relative abundance.
• Species richness - the number of different species present.
• Growth form of plants rosette and trailing growth forms are relatively more frequent in trampled areas. Grazing animals damage plants by removing growing tips, so plants with a growing point close to the ground are more likely to survive.
• Soil compaction - heavy footpath use can lead to the formation of blowouts. It has been estimated that as few as 80 pedestrian passes per month can cause bare ground to appear