Red deer management and vegetation on the Isle of Rum.
Understanding the effects of red deer Cervus elaphus grazing on a range of plant communities is important for formulating guidelines for the management of forestry plantations, nature reserves and deer hunting enterprises. The effects of red deer on the species composition of plant communities varying in productivity were examined on the Isle of Rum, Inner Hebrides, north-west Scotland. The study compared vegetation inside and outside fences erected to exclude deer for more than 20 years, and between areas of the island where different deer culling policies were instigated in 1991. The effect of the exclusion of deer was directly proportional to vegetation type. Long-term deer exclusion (20-40 years) had negligible effects on the species composition of the most unproductive plant communities, but caused a marked decline in the species richness of productive grassland. The decline was due to the loss of prostrate herb species inside exclosures where Festuca rubra had assumed a high dominance. Changes in the size and sex ratio of the deer populations were not associated with major changes in plant species richness or relative abundance. The most notable change occurred in productive grassland. Here, Festuca rubra had greater cover and several low-growing herbs such as Polygala serpyllifolia and Thymus polytrichus had lower cover in areas where hind numbers had been reduced than in areas where there was no culling. High deer densities within an unculled food-limited population were associated with negligible effects on plants in Molinia caerulea flush and Calluna-Molinia wet heath vegetation. Tree regeneration inside the exclosures or in areas where deer were culled was minor; only two solitary rowan Sorbus aucuparia saplings were found. Flowerhead densities of herbaceous plant species and grasses showed trends that could be associated with cattle grazing. It is suggested that this may lead to changed recruitment, although seed sowing experiments are required to test this. The results suggest that red deer grazing sustains the plant species diversity of productive grasslands, with reduced deer grazing leading to the loss of plant species in these communities. In contrast, the effects of reduced deer densities and altered culling policies on unproductive vegetation types are negligible.