Woods, especially ancient woodland, exist over much of the country as small patches surrounded by a totally different habitat, whether lowland farmland or upland moor. During the 1970s and 1980s it became common to compare such patches to real islands surrounded by sea and to apply island biogeography ideas to terrestrial conservation questions. The continuing influence of this and other landscape ecological concepts can be seen in the major report on the state of England’s protected sites produced for the Department of Environment, Food and Rural Affairs (Defra) (Lawton et al. 2010).
A woodland island amongst moorland on Hoy; a summary of conservation actions needed from the Lawton Report (figure reproduced by permission of Natural England): (a) improving the quality of habitat patches; (b) making existing sites bigger, which can include creating ecotones (c ); (d) enhancing connectivity through a continuous corridor or (e ) through a series of stepping stones; (f) creating new sites; reducing pressures on sites through (g) establishing buffer zones, or (h) enhancing the wider environment.
Certainly the analogy between habitat patches and islands can work as a first approximation – various studies have shown strong correlations between size of patch and number of specialist woodland plant species; other studies show that these species colonise new woodland more rapidly if it is next to an existing ancient woodland than if it is remote from such a seed source.
Oxlips, Dog’s Mercury and Bluebell have colonised the new woodland next to Hayley Wood in Cambridgeshire, but are less commonly found in more isolated farm woodland patches.
Studies of large blocks of forest that are undergoing fragmentation have emphasised that this also involves the creation of new woodland edges, where conditions differ from that in the interior. The ratio of edge to interior habitat has at times been suggested as important in assessing the conservation value of different patches – so small patches or long thin patches which have a high proportion of edge might be deemed less valuable than a similar extent which was part of a larger, more compact block.
Cutting a wood in two, as with this bypass, involves not just direct loss of habitat along the carriageway, but the creation of two new edge zones exposed to increased nitrogen pollution from the cars.
No analogy is perfect however and some of the assumptions underlying ideas about ‘woods as islands’ or the undesirability of high ‘edge to interior’ ratios don’t always hold in practice. Conditions are not uniform across a single wood or across woods in a landscape, so protecting a series of small woods across a landscape spread over several soil types for example might allow the conservation of more plant species than going for a single big uniform wood.
Most of the large mammals that might have required large blocks of woodland to survive went extinct long ago: woodland plants often appear able to survive for years as viable populations in quite small patches if conditions of light and nutrients remain fairly stable. It is possible that the Herb Paris in Cotting Wood, Morpeth is a direct descendent of the plant described from there by William Turner in the 16th Century. New edges exposed to more pollution may be bad, but some woodland specialist plants are more likely to be found at old edges of woods because they are not deep-shade species or else prefer the slightly dryer conditions associated with the old earth banks that may mark the woodland boundary. The grass Wood Melick is one that I tend to look for in such places.
Herb paris – a long term survivor in a small wood; Wood Melick – a grass often found near wood-edges.
Another complication with treating woods as ‘islands’ is that the surrounding landscape is seldom as hostile to woodland species as an ocean is to real island plants and animals. Woodland plants, such as ‘wood’ anemone may be found in moorland or unimproved meadows; and even in intensively farmed countryside there may be Bluebells in the hedges or Dog’s Mercury picking out where a hedge may once have been. The wood-island populations of plants may not be as isolated as a glance at a map may suggest.
Bluebells in a hedge; Dog’s Mercury hanging on even though the hedge has gone.
Despite these caveats, isolation and small size do contribute to scarcity of woodland specialists in newly created woods but rather than just accepting this, perhaps we should consider this as an opportunity for creative conservation. We humans could take on the role, perhaps once played by the wild ox, boar and moose when they roamed the landscape, of spreading seed and plants into new woods. If they can’t survive there, they won’t survive, but we could be speeding up a colonisation process that might otherwise take many decades.
Lawton, JH, Brotherton, PNM, Brown, VK, Elphick, C, Fitter, AH, Forshaw, J, Haddow, RW, Hilborne, S, Leafe, RN, Mace, GM, Southgate, MP, Sutherland, WJ, Tew, TE, Varley, J,Wynne, GR (2010) Making space for nature:a review of England’s wildlife sites and ecological network. Defra, London
The Old Man of Wytham 