Ecology of the Pembrokeshire Islands: II. Skokholm, Environment and Vegetation

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Abstract
Skokholm, a small rather flat, rocky island off the coast of Pembrokeshire, has a relatively uniform light soil and a mild climate which is counteracted to a large extent by strong winds which retard spring growth and flowering. Although it is one of the windiest places in the British Isles in Winter, it is not nearly so exposed as many in Summer, and for this reason, it has been possible to grow crops successfully in the past. Violent storms deposit sea spray over the land surface, and this tends to increase the pH of the soil nearer the coast and favor the growth of many halophilous spp., although it may scorch other spp. The extent to which winds affect the vegetation depends on their direction, the sheltering influence of local topography and the turbulence caused by irregularities in the vegetation surface. The prevailing winds of the region around Skokholm are more likely to blow equally from the north-west and west, and not equally from the south-west and west as usually shown. The major plant communities which are Armerietum maritimae, Caricetum arenariae, Grassland (mixtures of Holcus lanatus, Agrostis tenuis and Festuca rubra). Callunetum vulgare, Pteridietum aquilini and Wet Heath are described and classified in accordance with exposure to such winds. Skokholm supports large breeding populations of Orvctolagus cuniculus and in summer, sea birds. The windiness and animal activity account for the treeless nature of the island, which is covered with a sub-maritime grass heath in which zonation of plant communities depends in general upon exposure to prevailing winds and spray. Minor communities dominated respectively by Erodium maritimum, Plantago coronopus, Silene maritima, Cochlearia spp., Rumex spp., Senecio jacobaea and freshwater communities are described. The effects of animal activity are summarized. It is possible that Skokholm was once wooded, the trees being destroyed at some time after the 9th century A.D. Many spp. both introduced and native to Britain appear to have been introduced on Skokholm as weeds of cultivation. On the decline of agriculture the ploughed land reverted to grass heath once more, and many introduced weeds have found suitable sites in the ground disturbed by animal activities. It is possible to suggest that intensive animal occupation would probably destroy certain heavily grazed communities which would on cessation of animal activity show very marked changes.

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