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Individual study: Red deer Cervus elaphus grazing sustains plant species diversity of productive grasslands on the Isle of Rum, Western Isles, Scotland

Published source details

Virtanen R., Edwards G.R. & Crawley M.J. (2002) Red deer management and vegetation on the Isle of Rum. Journal of Applied Ecology, 39, 572-583

Summary

Grazing is often important in the maintenance of herb-rich grasslands. The effects of red deer Cervus elaphus grazing on species composition of grassland and other plant communities was examined on the Isle of Rum (a 10,684 ha nature reserve), north-west Scotland. The study compared vegetation inside and outside fences erected to exclude deer (over 20 years old), and between areas of the island where different deer culling policies were instigated in 1991 to reduce deer densities.

The effect of grazing on the abundance of vascular plants and bryophytes was examined in several vegetation types including: heather Calluna vulgaris-purple moor-grass Mollinia caerula heath/moorland, Agrostis-Festuca dominated grassland and marram grass Ammophila arenaria dunes. These were chosen to represent the dominant vegetation types on the island and to include a range of plant productivity levels.

Vegetation was monitored inside and outside of fenced areas (excluding deer) in May and late July 1997. Percentage cover of each bryophyte and vascular plant species was recorded in 2 × 2 m quadrats by visual estimates of leaves (both dead and live). The height of the dominant species (maximum height of canopy measured at the quadrat centre) and thickness of the moss carpet were recorded for each vegetation type.

Long-term red deer exclusion (20-40 years) or reduced deer densities had negligible effects on plant species composition of the less productive (heath/moor and dune) plant communities. In contrast, exclusion caused a marked decline in species richness of productive grassland. This decline was mostly due to the loss of prostrate herbs inside the exclosures where red fescue Festuca rubra had become dominant.

Where hind numbers had been reduced compared to areas where there was no culling (as within exclosures), the most notable change occurred in productive grassland. F.rubra had greater cover and several low-growing herbs e.g. heath milkwort Polygala serpyllifolia and wild thyme Thymus polytrichus had lower cover in these less intensively deer-grazed areas. Tree regeneration throughout was negligible; the only species regenerating was rowan Sorbus aucuparia with only two saplings found (inside exclosures/where deer culled).

The results suggest that red deer grazing on Rum sustains plant species diversity of productive grasslands by allowing persistence of low-growing herbs that would otherwise tend to be out-competed by grasses. Reduced grazing intensity (or deer exclosure) in grasslands led to loss of some species in these communities with a rise to dominance of one or two grass species.


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