Impacts of sheep grazing a complex vegetation mosaic: Relating behaviour to vegetation change

The impact of grazing animals on vegetation is usually considered as a single process, despite the knowledge that behaviours such as defoliation and trampling are likely to have different effects. A 3-year study was carried out to assess the different impacts of sheep on a natural dwarf-shrub and grass mosaic, by relating animal behaviour to vegetation change, using a spatially explicit design. Sheep grazed at average stocking rates of 2, 3 or 4 animals ha−1 in six 1 ha plots. Spatial patterns of defoliation were obtained from field measurements of grazing damage, other aspects of behaviour from direct observation and vegetation change from aerial photographs.Over the 3 years, there was a net change of 907 m2 (1.5% of the total study area) from ‘shrub’ (heather Calluna vulgaris and blaeberry Vaccinium myrtillus) to ‘mixed’ (grass and blaeberry) vegetation and degraded heather, but no overall net change in grass cover. Over the whole study area there were significant linear relationships between heather defoliation rate and the percentage change in shrub and mixed vegetation at defoliation sites, where relatively small, local changes in the vegetation were seen. However, differences between individual plots showed no relationship with sheep stocking rate. Larger, spatially aggregated changes were also seen, but these were not associated with grazing; they tended to be concentrated in areas used by the sheep for resting. Grass and mixed vegetation cover increased by 351 m2 at these sites, equivalent to 27% of the total area initially identified as resting sites. Since the impacts of the sheep differed in both intensity and spatial pattern for the processes of defoliation and resting, we suggest that the effects of these component behaviours should be considered separately when attempting to predict vegetation change in complex environments.