Too many deer in the woods? implications for birds and other wildlife - a synopsis

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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)

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only temporarily protected by a brushwood fence, which the deer can usually penetrate within two years. Eight such split plots were established. After a few years of coppice growth, the complexity and density of low vegetation was typically considerably higher in the fully protected sub-plots than in the unprotected sub-plots. As expected, breeding densities of several species of birds that require complex low vegetation were found to be higher within the fully protected sub-plots than in the matched sub-plots. These species included blackcap Sylvia atricapilla, garden warbler Sylvia borin, willow warbler Phylloscopus trochilus, dunnock Prunella modularis and nightingale Luscinia megarhynchos. The overall territory density of migrant birds was consistently higher in the sub-plots plots fully protected by deer fences. Numbers of breeding long-distance migrant birds in Bradfield Woods have declined substantially since the late 1980s; the experiment suggests that this may be partly a consequence of intensified deer browsing. Unfortunately, there have been no nightingale territories in Bradfield Woods since the late 2000s. As the population dwindled, so the birds became increasingly associated with coppice plots that were fully protected from deer – this strongly suggests that deer browsing reduced habitat quality for the species, at least in coppiced woodland. Analysis of national data by the BTO indicates that nightingales have declined most heavily in areas where deer numbers have increased the most. It seems probable that increasing deer pressure is one factor contributing to the widespread decline of nightingales in the country.

Loss of understorey vegetation also occurs through increased shading, for example as a consequence of reduced management. Therefore, in wooded areas where there is high canopy closure and large deer populations, species that require open habitats or dense low vegetation will find little suitable habitat. However, such woodland structures can potentially benefit a different range of species, including ones that prefer an open understorey and others that utilise decaying wood which may develop through death of trees and limbs as a consequence of competition for light.

Moderate levels of deer browsing may actually be beneficial in terms of biodiversity because this can help create a greater patchiness in vegetation and microhabitats. In the case of species that depend on very open sunny habitats, such as some butterflies, browsing by deer may actually prolong the period when these habitat patches remain open and suitable. If pressure from deer is too heavy, this advantage will be lost because important food plants are likely to be eaten.

Under some circumstances it may be possible to undertake woodland management in ways that help to reduce the impacts of deer on tree regeneration and biodiversity. Maintaining open canopy areas may stimulate understorey regeneration provided that the density of deer is not too high. Woodland managers

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are frequently faced with difficult decisions about the extent to which deer should be excluded by fencing or controlled directly through the use of deer managers. Effective fencing solutions may involve the use of various kinds of temporary moveable fencing. In many cases, however, it is unavoidable that numbers of deer have to be maintained at levels that are compatible with the objectives of woodland management, whether those are concerned with production, biodiversity or both. An emerging issue is how deer will interact with the growing problem of ash dieback. Ash is a principal food of woodland deer but that does not necessarily mean that the disease will result in fewer deer or reduced impacts. Large scale death of ash may simply promote an increase in other favoured browse species such as sallow. Deer are likely to remain a significant issue for woodland managers for many years to come.

Acknowledgements Pete Fordham and the Suffolk Wildlife Trust have been immensely supportive of the work conducted at Bradfield Woods. Various friends and colleagues have contributed to the work outlined in this article and I would especially like to acknowledge Chas Holt, Paul Dolman and Robin Gill.

Further sources of information Dolman, P., Fuller, R., Gill, R., Hooton, D. & Tabor, R. (2010). Escalating ecological impacts of deer in lowland woodland. British Wildlife 21: 242–254.

Frerker, K., Sabo, A. & Waller, D. (2014). Long-term regional shifts in plant community composition are largely explained by local deer impact experiments. PLoS ONE Doi:10.1371/journal.pone.0115843 (open access online) Fuller, R. J. & Gill, R. M. A. (2001). Ecological impacts of increasing numbers of deer in British woodland. Forestry 74: 193–199.

Holt, C. A., Fuller, R. J. & Dolman, P. M. (2010). Experimental evidence that deer browsing reduces habitat suitability for breeding Common Nightingales Luscinia megarhynchos. Ibis 152: 335–346. (based on work in Bradfield Woods) Holt, C. A., Fuller, R. J. & Dolman, P. M. (2011). Breeding and post-breeding responses of woodland birds to modification of habitat structure by deer. Biological Conservation 144: 2151–2162. (based on work in Bradfield Woods) Holt, C. A., Fuller, R. J. & Dolman, P. M. (2013). Exclusion of deer affects responses of birds to woodland regeneration in winter and summer. Ibis 156: 116–131. (based on work in Bradfield Woods) Martin, J.-L., Stockton, S. A., Allombert, S. & Gaston, A. J. (2010). Top-down and bottom-up consequences of unchecked ungulate browsing on plant and animal diversity in temperate forests: lessons from a deer introduction. Biological Invasions 12: 353–371.

Newson, S. E., Johnston, A., Renwick, A. R., Baillie, S. R. & Fuller, R. J. (2012).

Modelling large-scale relationships between changes in woodland deer and bird populations. Journal of Applied Ecology 49: 278–286.

Rob Fuller British Trust for Ornithology (BTO), The Nunnery, Thetford, Norfolk IP24 2PU rob.fuller@bto.org Trans. Suffolk Nat. Soc. 51 (2015)