Spatial and temporal patterns of heather use by sheep and red deer within natural heather/grass mosaics.
Abstract
Fragmentation of heather and conversion to grass under heavy grazing is a widespread phenomenon. In the British uplands it is attributed in particular to the effects of sheep and red deer [Cervus elaphus], but little is known about the processes by which fragmentation occurs under grazing by these different herbivores. This has hindered the prediction of herbivore impacts and thus the design of appropriate management schemes for the protection of heather-dominated vegetation. The results are given of an experiment examining the patterns of impact by sheep and farmed red deer grazing separately or together within an area of fragmenting heather in north-east Scotland during 1991-95. Heather utilization by both herbivores decreased rapidly with distance from grass, with both herbivores grazing similar amounts of heather, except in autumn where heather utilization by deer was greater than that by sheep. Differential patterns of grass patch use by sheep and red deer resulted in greater heather utilization around larger grass patches by red deer, but no consistent differences in edge-heather utilization by sheep. Both species grazed less edge-heather downhill of grass patches than uphill or across the slope. It is hypothesized that on sloping ground grazing impacts will be greatest uphill from grass for both herbivores, but their patterns of impact around different grass patches will diverge when a range of grass patch sizes is present. Patterns of impact as a result of trampling damage differed from those due to grazing, with damage concentrated at downhill edges of patches and paths, with corresponding increases in cover of species such as Vaccinium myrtillus where the heather had declined. It is suggested that on sloping ground at lower herbivore densities, trampling may have more important impacts on the heather than grazing, but as herbivore densities increase the relative importance of grazing will increase, thus changing the patterns of impact on the vegetation. The implications of these differences in herbivore impacts are discussed in relation to the management of heather-dominated vegetation under grazing by sheep or red deer and the limitations of approaches based on stocking rates alone.