Red deer (Cervus elaphus) grazing on vegetation mosaics grazing patterns and implications for conservation management

Abstract

Grazing is widely used as a tool in conservation management. Many plant communities of conservation importance are dependent on grazing for their existence, maintenance of species diversity and other valued characteristics. Plant community response to grazing depends on many factors, including site productivity and dominant plant species; setting appropriate grazing levels can therefore be challenging. The problems are magnified when more than one species or plant community is the target of conservation goals as they may need different levels of grazing. Where multiple plant communities are present in a mosaic, grazing pressure on the higher productivity community (usually the more attractive to herbivores) can affect the utilisation of the lower productivity communities: grazing on the less productive community is elevated in close proximity (a few metres) to the productive community. This increases the possibility of conflict in managing grazing for the conservation of both communities as low productivity communities can sustain only low levels of grazing. Less well studied are the effect of community layout at larger spatial scales (100s – 1000s of metres) and the effect of vegetation pattern on grazing on the productive community. It is also not well known how the spatial pattern of grazing is affected by changes in herbivore density. I investigated the consequences of the spatial pattern of plant communities and changing herbivore density for grazing patterns on a complex multi-community mosaic and assessed the probable consequence for conservation of these plant communities. The plant mosaic comprised a mixture of species-rich grassland and several less productive communities, primarily heaths and bogs; the main grazers were red deer (Cervus elaphus). The grassland needs higher grazing levels than the others to meet management goals. I used small scale experiments to investigate the effects of reducing grazing on grassland and how the effects varied within the grassland community. Elimination of grazing caused a rapid switch from short, herb-rich grassland towards a graminoid dominated, less diverse sward, as expected. The degree of change in diversity and herb cover was dependent on productivity. Experimental reduction in grazing had mixed consequences for grassland in relation to conservation goals due to pre-existing variation in intensity of grazing on the grassland. The condition of areas of initially heavily grazed and short vegetation improved, whilst taller grasslands deteriorated. Analysis of large-scale datasets was used to investigate the influence of spatial pattern of community types and differences in large scale deer density on the distribution of grazing. There was increased grazing pressure on less productive plant communities where grassland was abundant within 1km and this was fairly consistent across communities and across different grazing indicators. There was an effect on grazing levels on grassland, but the explanatory power was generally lower and the effect less consistently present across indicators of grazing. Sward height and litter depth measures from one dataset indicated heavier grazing with more grassland present nearby (250m); however, lower grazing pressure was indicated by sward height and a combined grazing index when there was more grassland in a more distant zone (500-1000m). Deer density had limited power to explain large scale variation in impacts, probably due to the coarse scale of the information available and correlation with other variables. This limited the ability to thoroughly test the consequences of changes in deer density on the spatial pattern of impacts or investigate whether there was an interaction between deer density and spatial pattern. The inherent conflict in conservation management of grazed communities of different productivities is increased by the influence of the spatial distribution of plant communities on the distribution of grazing; conservation management goals need to account for this and identify a suitable trade-off.