The Pattern of Species Density of Old Stone Walls in Western Ireland

Abstract

The number of plant species per unit of area, 'species density', is known to vary, and in an apparently predictable fashion, with latitude (Stehli 1968) and time (Stehli, Douglas & Newell 1969), and along environmental gradients (Whittaker 1965). Presumably, such variation is vegetation's response to changes in soil nutrient and water availability, and ambient temperature. This paper describes a search for patterns of species density in an area of strong climatic gradients. The experimental design required a single and distinctive site type for plant growth, of common occurrence in an extensive study area. After the rejection of such site types as boulders in till fields (Pettersson 1929a), pit heaps (Richardson 1957), tree stumps (Pettersson 1929b), and the walls of old buildings (Raistrick & Gilbert 1963; Rishbeth 1948; Woodell & Rossiter 1959), it was decided to investigate the vascular plant flora of field boundaries, 'stone walls', in parts of counties Galway and Mayo, western Ireland. The approach is quasi-experimental in that every attempt was made to limit the number of independent variables to aspect of the regional slope on which each wall is located, and degree of exposure of the site to maritime air masses. In this, it compares with the work of Suominen (1969) in which the effect of aspect on plant distributions was studied in the flora of railway embankments in Finland. Exposure is clearly of considerable importance with respect to the representation of species in mural communities (Segal 1969).