Grazing across grass:shrub boundaries: Can spatial heterogeneity increase resistance?

In a three-year grazing experiment, three sheep stocking rate treatments were applied to six plots, each containing a natural dwarf-shrub:grass mosaic. Defoliation of the dwarf-shrub, heather ( Calluna vulgaris ), was recorded using a spatially-explicit sampling design providing information across multiple spatial scales. Heather defoliation by sheep across the dwarf-shrub:grass mosaics was not homogeneous, but was strongly influenced by the pattern of the preferred vegetation type (grass). While previous studies found that heather defoliation declines with increasing distance from the preferred forage, grass, this experiment showed that the spatial gradient of heather defoliation away from the edge is also affected by the local grazing pressure which is strongly influenced by the local vegetation heterogeneity. Results indicate that the greater the habitat heterogeneity, the more inappropriate a simple vegetation-based stocking rate will be in achieving a desired vegetation condition. The findings support the hypothesis that increased heterogeneity of grass:dwarf-shrub mosaics leads to increased resistance to herbivory, as herbivore impacts are dispersed across the greater total area of ‘impact zone’. The results also highlight the importance of understanding the contrasting spatial patterns of trampling and defoliation impacts, differences which can be further enhanced with increasing heterogeneity of the vegetation.