15
MAMMAL CONFERENCE
TOO MANY DEER IN THE WOODS? IMPLICATIONS FOR BIRDS AND OTHER WILDLIFE – A SYNOPSIS ROB FULLER Numbers of all species of free-living deer have increased very strongly in many parts of Britain in recent decades and East Anglia is one of the regions where these increases have been most pronounced. The mammal monitoring undertaken by the BTO as part of the BTO/JNCC/RSPB Breeding Bird Survey indicates especially large national increases for Reeves’ muntjac Muntiacus reevesi (114%) and roe deer (see Cover) Capreolus capreolus (73%) since 1995. Muntjac have shown a striking expansion into East Anglia during this period. The reasons for these rapid increases are various. They include the widespread use of winter cereals (providing overwinter food), a benign climate (allowing greater winter survival), expansion of tree cover (more habitat for deer) and fewer people working in the countryside (less disturbance). The implications for other wildlife are complex. Deer are keystone species because they can completely modify the structure of woodland by removing a large portion of the understorey with escalating consequences for the composition of plant and animal communities. Additionally, deer are selective in the tree and other plant species they eat, causing gradual reduction in tree and shrub diversity and the loss of palatable herbs but increases in unpalatable plants notably coarse grasses, sedges and ferns. These multi-trophic effects on woodland biodiversity have been most clearly unravelled in North America. Several studies of both black-tailed deer Odocoileus hemionus (a mainly western species) and white-tailed deer Odocoileus virginianus (a mainly eastern species) show that intense and prolonged deer browsing can reduce both local and regional populations of some species including understorey-dependent birds and many herbs. White-tailed deer in particular have massively increased from a very low population level in the early 20th century following decades of overexploitation. This has triggered cascading ecological impacts in the forests of Michigan, Wisconsin, Pennsylvania, West Virginia and other eastern states. Here in Britain, there have been similar patterns of change in woodland induced by the rising deer populations, though the studies are not generally as comprehensive or rigorous as those in the USA. Woodland plant species thought to be susceptible to grazing include bluebell Hyacinthoides non-scripta, various orchids, dog’s mercury Mercurialis perennis, oxlip Primula elatior, wood anemone Anemone nemorosa, bramble Rubus fruticosus, ivy Hedera helix, sallows Salix spp. and ash Fraxinus excelsior. Species that are resistant to deer, and potentially expand as a consequence, include pendulous sedge Carex pendula , ground ivy Glechoma hederacea, several grasses and alder Alnus glutinosa. There is now strong evidence that deer can reduce local populations of some woodland bird species in Britain. Much of this evidence comes from a long-term exclosure experiment in Bradfield Woods, Suffolk. In the late 1990s a ‘split plot experiment’ was started in which one sub-plot was fully protected from deer and the matched sub-plot was effectively unprotected. This involved erecting a steel deerproof fence around one half of a freshly cut coppice compartment with the other half Trans. Suffolk Nat. Soc. 51 (2015)