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1. Heather moorland is an internationally important resource; it is valued as food and habitat for a range of herbivores as well as for landscape, conservation and recreation. In many parts of its range, grazing by large herbivores has impacted greatly on its current status and distribution. The interactions between two widespread herbivores, sheep and red deer, and the vegetation were examined within a naturally fragmented heather/grass mosaic in north-east Scotland from 1991 to 1996. This paper reports their foraging behaviour and inter-species interactions, and discusses the implications for grazing management within this vegetation type.
2. Both species foraged widely in this highly fragmented mosaic (red deer more widely than sheep) and consumed similar proportions of grass and heather. Neither species showed any major dietary shift as grass availability declined during experimental periods.
3. Grass patch size affected foraging behaviour differentially. Sheep spent more time grazing smaller grass patches (1–6 m2), whereas red deer showed no consistent grass patch size preferences (range: 1 m2 to over 200 m2). When lying down, red deer lay within heather almost exclusively, whereas sheep spent equal amounts of time within heather and small grass patches.
4. Faecal distribution also differed in a similar way, with concomitant implications for the spatial distribution of both physical and chemical impacts on the vegetation.
5. Contrary to previous hypotheses, the overall patterns of foraging behaviour by sheep and deer were little affected by the presence or absence of the other species. This suggests that range overlap between these species may have much less of an impact on their vegetation use than previously assumed.
6. Heather moorland is frequently found on sloping ground, which was shown to have a major effect on foraging, with a strong preference by both species for grazing facing either uphill or across the slope. This, and other slope-related impacts, resulted in spatially different patterns of heather use around the edges of grass patches.
7. The primary conclusions are that, although the overall use of grass and heather by sheep and red deer was similar within this fragmented mosaic, vegetation pattern clearly affected their foraging behaviour differentially. Sheep appeared to be much more affected by the scale of heather fragmentation than were deer and their habitat use was more closely focused on paths and grass patches, which contrasts with the more even use of the mosaic by deer. The implications of these behavioural differences for the management of upland grazings under one or both of these herbivore species are discussed.
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Large herbivores have a major influence on the function and dynamics of most terrestrial biomes (Hodgson & Illius 1996). Through their grazing, trampling, defecation and urination they affect nutrient flows, vegetation community dynamics and the responses of associated fauna. In turn, ecosystem characteristics such as resource composition, productivity and distribution determine the individual and population nutrition of the herbivore (Clutton-Brock & Albon 1989; Langvatn et al. 1996). The spatial distributions of different plant communities can thus have a strong influence on the foraging behaviour of free-ranging herbivores (Senft et al. 1987; Cougenhour 1991; Gordon & Illius 1992; Gross et al. 1995; Owens et al. 1995). Although such differences are known to be important, for many herbivores and vegetation types the interrelationships between vegetation pattern, foraging behaviour and plant community use are still poorly understood, leading to poorly developed management strategies for agricultural or other land use objectives (Bailey, Dumont & Wallis de Vries 1998). Many studies have focused primarily on the preferred vegetation ‘patch’ (Illius, Clarke & Hodgson 1992; Jiang & Hudson 1993a; Langvatn & Hanley 1993), yet the more fragmented or widely distributed that preferred vegetation is, the greater the impacts on the surrounding vegetation are likely to be, which has important implications for management. In semi-natural vegetation, food is generally distributed in variably spaced patches of different size and quality (reflected by nutrient status or biomass, for example) which can also change seasonally. Thus, the management of semi-natural vegetation by grazing animals depends fundamentally on an adequate understanding of the interrelationships between the animal and the complexities of the vegetation (Hanley 1997; Milne et al. 1998). Furthermore, the greater the impact of vegetation pattern on the foraging behaviour of a particular species, the greater may be the divergence in impact of that species on the vegetation between areas which have different distributions of the same vegetation types (Cougenhour 1991; Clarke, Welch & Gordon 1995b; Bailey, Dumont & Wallis de Vries 1998).
The system selected for study here represents a fragmented example of an internationally important vegetation type, heather moorland (Gimingham 1972), grazed by two widespread herbivore species, Scottish Blackface sheep and red deer Cervus elaphus L. Heather moorland (dominated by Calluna vulgaris L. Hull) is found in varying amounts throughout north-western Europe (de Smidt 1995), yet there has been considerable fragmentation and contraction of its range in many areas, partly as a result of grazing impacts (Anderson & Yalden 1981; Froment 1981; Bokdam 1995). Northern Britain represents a major stronghold of this community, with heather-dominated vegetation still covering about 30% of the land area of Scotland, for example (Gimingham 1972; Gauld et al. 1991; Thompson, Hester & Usher 1995). However, even here, there has been widespread grazing-related fragmentation, loss and conversion to grass-dominated vegetation (Sydes & Miller 1991; National Countryside Monitoring Scheme 1993; Staines, Balharry & Welch 1995; Tudor & Mackay 1995).
Domestic sheep and wild red deer range freely in large numbers over extensive areas of heather-dominated vegetation in northern Britain, and are also found within this vegetation type in other parts of Europe (Ahlen 1965; Schmidt 1983; Kottman, Schwöppe & Willers 1985; Staines, Balharry & Welch 1995). Both these herbivores are mixed strategy feeders, with preferences for grass species such as Agrostis/Festuca over woody species such as heather (Mitchell, Staines & Welch 1977; Hofmann 1989). Grass patches within heather moorland will therefore act as attractants, leading to increased pressure on the heather immediately adjacent to the areas of grass (Grant et al. 1978; Clarke, Welch & Gordon 1995a,b), thus affecting the rates and patterns of any subsequent heather decline. In many such areas the impacts of these herbivores on the vegetation have been marked, and the need for more appropriate grazing management has long been recognized (e.g. Anderson & Yalden 1981; Sydes & Miller 1991; Milne et al. 1998). Within the UK, for example, heather management through reduced grazing forms one of the requirements of management within the government's Environmentally Designated Areas Scheme (Henderson et al. 1994; Nolan et al. 1994). However, monitoring of the success of this scheme has already revealed some of the inadequacies of generalized herbivore stocking rate prescriptions that do not specifically take vegetation pattern into account (e.g. Nolan et al. 1994). There is a clear need to improve our understanding of how these herbivores interact with vegetation pattern in heather-dominated systems to enable robust predictions to be made about their likely impacts, and to guide current and future herbivore management prescriptions.
The main objective of this research, therefore, was to test how foraging behaviour and grass patch selection by sheep and red deer was affected by the size and distribution of natural grass patches within an area of fragmenting heather moorland. Naturally fragmenting moorland contains a variety of sizes, shapes and distributions of patches of grass, which we hypothesized would differentially affect the foraging behaviour of sheep and red deer. This experiment was set up to complement the work of Clarke, Welch & Gordon (1995a,b), who artificially created large, uniformly distributed squares of grass within heather and compared the foraging behaviour of sheep and red deer when presented with one of three different patch sizes (one size of patch per plot) within a heather mosaic.
As sheep and red deer commonly graze within the same upland areas (Clutton-Brock & Albon 1989), and both species seem to occupy similar foraging niches in these areas (Hunter 1962; Clutton-Brock, Guinness & Albon 1982; Osborne 1984), the second objective of this research (not tested by Clarke, Welch & Gordon 1995a) was to quantify the extent of interaction and feeding overlap between sheep and red deer when grazing together under controlled experimental conditions. This would enable us to test current hypotheses about differences in their observed feeding strategies (Osborne 1984; Clutton-Brock, Iason & Guinness 1987; Illius & Gordon 1987). Circumstantial evidence from field observations (Osborne 1984) suggested, for example, that red deer may feed more on heather in the presence of sheep than when alone. This may have been an indirect effect caused by sheep grazing the grass to a lower height than that required to sustain red deer; alternatively direct behavioural factors, such as avoidance of the other herbivore species, may have been important in this particular study. If such interactions occur between sheep and red deer, their impacts on heather/grass mosaics could differ greatly according to whether both sheep and red deer or just one of the species are present. Any such differences would clearly affect the design of appropriate management strategies for this vegetation type, depending on the herbivore species using the area.