One of the few, reasonably consistent, rules in conservation, is that big sites tend to hold more species than small ones. If species number is plotted against the logarithm of wood area for example there is usually a good straight-line fit. Does that mean we should always give priority to the largest sites? Can conservation be based on the extent and disposition of the green areas on Ordnance Survey maps?
Would that it was that simple. An implicit assumption behind the species-area relationship is that the patches are similar in all other respects than area and that is almost never true. These other differences between the woods may be sufficient to completely override the size effect.
Mike Sykes and Dave Horrill (1979) from the ITE (now Centre for Ecology and Hydrology) recorded thoroughly a series of woods as part of a study of survey methods. To no-one’s surprise, Colt Park, a 25 ha wood on limestone pavement had far more plant species than the 160 ha of Kirkconnell Flow, a birch-pine woodland on peat. George Peterken and Meg Game (1984) showed that, under the relatively uniform conditions of central Lincolnshire, the larger ancient woods had more species than recently-formed woods of a similar extent. Woods with a mixture of vegetation types tend to be richer than a wood of the same size that is all one type.
Variations in composition and structure made Roudsea the richest of the woods surveyed by Sykes and Horrill.
The same tends to be true for woodland structure: a wood where stands of different ages present, from recently cut-over areas through to mature and veteran stands, will usually hold more plant species than a wood that is even-aged. One explanation for the decline in the local plant richness in woods since the Second World War is that many of them have been left largely alone for the last 40 year and are now in the relatively dark, species-poor stage of the woodland cycle. A small worked coppice wood for example may have more species than a much larger area of high forest managed by group-felling on 100-yr rotation.
The relationship between species richness and area can be further complicated if we consider other types of wildlife. There are very few species for which the whole wood is a homogenous habitat patch. Some fritillary butterflies may be using only the bits within the wood that are currently open; some dead wood beetles may need large hollow trees; pied flycatchers may focus on the stands with a relatively open understorey. There may or may not be any link between the extent of the wood as indicated on the map and the extent of the habitat that is actually used by any given species. Nor is the area of habitat for one species necessarily linked to the effective habitat area for another.
Wytham Woods cover about 400 ha, but only about 10 ha has mature beech habitat and less than 5 ha is open stage coppice.
More mobile animals such as the larger birds and mammals may not even regard the wood as a discrete patch at all, merely as one part of a daily, seasonal or annual range that may include several other woods. Looking at groups of woods raises other questions: on the one hand it might be sensible to conserve groups of sites that are close together, because they act as one ‘site’ for a mobile species, or show occasional interchange of genes and individuals between populations in the different patches, that is they function as a metapopulation. The alternative strategy is to pick patches that are more dispersed, since these are often collectively contain more species than a single patch of the same total extent, because the dispersed patches cover more of the available environmental variation.
Whether this landscape is seen as one wood or many depends on the species concerned.
Is there a minimum size of woodland that we should aim for in protected sites? The species-area relationship as usually plotted would suggest that we should keep going for ever bigger sites, until the maximum possible number of species has been included. However, because this is a logarithmic relationship, the additional species you get for each additional hectare, for each additional pound spent, declines as the sites get bigger. In practice, there is likely to come a point, just considering species numbers, where it is not cost-effective to go for a larger site. Larger woods may still be desirable because they allow for larger population sizes and hence species may be less vulnerable to local extinction, but evidence for what size wood is needed for this is very limited.
Larger woods may also suffer less from negative edge effects (drying out of edges, potential spray-drift or eutrophication from adjacent fields), although conversely species that use or are commonly found on wood-edges will have less available habitat.
A high edge-to-core ratio may be a good or bad thing for conservation, depending on what the wood is edging on to.
Woodland extent is important, but not the universal guide to conservation value that it at first appears, as Rackham (2003) also argued. We need to consider the particular species or assemblages we wish to conserve and the landscape context of each particular case.
PETERKEN, G. & GAME, M. 1984. Historical factors affecting the number and distribution of vascular plant species in the woodlands of central Lincolnshire. Journal of Ecology, 72, 155-182.
RACKHAM, O. 2003. Ancient woodland: its history, vegetation and uses in England (revised edition), Dalbeattie, Scotland, Castlepoint Press.
SYKES, J. M. & HORRILL, A. D. 1979. Survey methods within woodlands. ITE Contract Research Report. Peterborough: Nature Conservancy Council.