Sheep Population and Environment in Snowdonia (North Wales)

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Abstract
There is a north - south cline of sheep number per unit area which appear to be partly determined by climate and partly by difference in soil and vegetation which are, in turn, influenced by climate. In the north, where mean annual rainfall is less than 100 inches, the mean sheep number per acre in ewe units is 1.94. Rainfalls exceed 100 inches in the south and the average sheep population amounts to 1.27 ewe units per acre. Where mean annual rainfall is less than 100 inches, geology, soil and related vegetation play a paramount part in determining sheep population densities. The basic igneous rocks, which give rise to brown earth soils and bear an Agrostis-Festuca grassland, carry the greater sheep number per acre (2.8 ewe units per acre as compared with 1.7 on acid igneous soils). Such differences are not so evident at high rainfall, though recent detailed work does show that differences do exist in the sheep population borne by acid and basic igneous soils, even where rainfall exceeds 150 inches. Recent work by R. I. Davies on the cobalt and copper content of these soils suggests that the high concentration of these elements in basic igneous soils may be an important factor in determining the high sheep population densities which they bear. In turn, the resulting heavy grazing must profoundly affect the nature of the vegetation. As an essential background to these studies a general account is given of the influence of variation in local climatic conditions and geology on soil and vegetation. Emphasis is placed on the need in plant ecological studies of giving a quantitative estimate of seasonal variation in grazing intensities to which specific communities are submitted.