Impacts of woodland deer on small mammal ecology

The possible impacts of increased deer populations on small mammal ecology in British lowland woodlands are reviewed. These impacts occur mainly through two pathways. First, by the modification or removal of habitat which influences food supply, cover and the balance of competition between the species. Secondly, through direct competition for resources, particularly food supplies. Losses of woodland ground vegetation may provoke major changes in small mammal community structure which, in the extreme, may decrease from five to eight species, or more, to just wood mice (Apodemus sylvaticus). Reduction of understorey species such as hazel (Corylus avellana) or loss of shrub diversity, would be expected to lead to declines in yellow-necked mice (Apodemus flavicollis) and common dormice (Muscardinus avellanarius). Reduction of early succession mixed scrub and grassland may reduce colonization by shrews, harvest mice (Micromys minutus) and field voles (Microtus agrestis), although in woodland rides, field voles may benefit from preferential grazing of grasses by deer. Impacts on regeneration which may delay or inhibit canopy/understorey closure may encourage the ground and shrub vegetation and so favour species with a preference for ground cover such as the bank vole (Cletbrionomys glareolus). Reductions in the availability of foods (e.g. mast crops) may reduce the potential for high population densities, particularly of bank voles, wood mice and yellow-necked mice, decrease the lengths of breeding seasons, and further affect community structure. If rodent densities are lowered, avian and terrestrial predators are likely to suffer reduced breeding success and tawny owls (Strix aluco) may prey more heavily on bank voles if their favoured ground cover is reduced. Studies at Wytham Woods, near Oxford, from 1949 to 1999, indicate that a significant reduction in bank vole, but not wood mouse, numbers has occurred; this is suggested to be the result of observed changes in the woodland vegetation, including a significant reduction in the ground cover of bramble (Rubus fruticosus agg.). Deer, as well as increased canopy shading, are probably the major factors causing this change in habitat and small mammal density.