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

G. T. Goodman and M. E. Gillham
Journal of Ecology
Vol. 42, No. 2 (Jul., 1954), pp. 296-327
DOI: 10.2307/2256864
Stable URL: http://www.jstor.org/stable/2256864
Page Count: 32
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Ecology of the Pembrokeshire Islands: II. Skokholm, Environment and Vegetation
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Abstract

The purpose of this paper is to introduce and discuss the importance of the main environmental factors which influence Skokholm as a plant habitat and to describe briefly the principal types of plant communities which are present. 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 favour the growth of many halophilous species, although it may scorch other species. 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 shown by Tansley (1939) after Bilham (1938). The major plant communities are classified in accordance to exposure to such winds. Skokholm supports a large breeding population of rabbits and, in summer, sea birds. The windiness and animal activity account for the treeless nature of the island, which is covered with a sort of maritime grass-heath in which zonation of the constituent plant communities depends in general upon exposure to prevailing winds and spray. The major and minor communities comprising the heathland are described and the effects of animal activity are summarized. It is possible that Skokholm was once wooded, the trees being destroyed at some time after the ninth century A.D. Many species, 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 submaritime grass-heath once more, and many of the introduced weeds have found suitable sites in the ground disturbed by animal activities. From general observations recorded throughout the text and from a few earlier records on the influence of animals upon the various communities 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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