Effects of social behaviour on the spatial distribution of sheep grazing a complex vegetation mosaic

Abstract

In complex environments, the spatial distribution of preferred food types will be a major factor influencing the distribution of foraging animals. However, in highly social animals, such as sheep, social interactions may modify foraging behaviour and hence influence both where animals feed and their impacts on the food resource. This process was investigated in a replicated experiment with six groups of Scottish Blackface sheep ( n = 6), each grazing a separate 1 ha plot containing a natural vegetation mosaic, consisting of preferred (grass) and less preferred (heather, Calluna vulgaris) species. Grass covered 18% of the total area and was distributed across the plots as a complex network of patches, ranging in size from 1 to 690 m 2. In each plot, the sheep showed a preference for grazing on large patches, and were seen together in groups of 4 or more animals more often than would be expected by chance. Irrespective of patch size, mean nearest neighbour (NN) distances for pairs of sheep grazing together on the same patch (4.9 m) were much shorter than those for nearest neighbours grazing different patches (13.4 m) or heather (9.6 m), and similar to values for sheep grazing on larger grass swards. It was concluded that the ability to graze in groups at their preferred spacing was an important factor influencing the preference of sheep for large patches in this mosaic. There were differences in individual mean NN distances, measured over the whole mosaic, which ranged from 6.2 to19.3 m. However, there was no correlation between mean NN distance and the overall percentage of time spent on grass. The results suggest that although social behaviour influenced the choice of grass patches, the scale of heterogeneity in this particular mosaic was such that dietary preferences were not compromised.