Fragmentation and connectivity

The Earth's surface now supports a relatively minute proportion of truly natural landscape. As we have seen in the previous chapter, this is particularly true of woodlands and forests. One of the most insidious effects of these changes is the fragmentation of forests into smaller parcels. Even large countries with relatively unpopulated areas such as the USA, where forest covers 31.4% of the land, are suffering from extensive forest fragmentation. Riitters et al. (2002) determined that 44% of the forest area within the contiguous US states is within 90 m of a forest edge and 62% is within 150 m. People can drive to within a kilometre of 82% of all land in the contiguous US states (Riitters and Wickham, 2003). Nevertheless the forest still tends to be in fairly large areas such that at least 73% of all forest is in landscapes that are at least 60% forested. Consequently, around half of the forest edge found by Riitters et al. is created by small perforations in forest of less than 7.29 ha. Thus, the forest is large with holes cut within it. In contrast, the British landscape is just the opposite. England has no virgin forests and until very recently it seemed that even its last reserves of ancient woodland (land continuously wooded since AD 1600) faced a very uncertain future. Only 11.8% of the UK is covered by forest and much of this is introduced conifers. Not only is the amount of ancient woodland very low; it is also highly fragmented, being scattered in very many relatively small sites which are often far apart. In contrast to the USA, the agricultural landscape makes up the broad matrix (the most extensive and connected landscape element), perforated by small areas of woodland. In England and Wales, 83% of ancient woodlands (amounting to 31% of ancient woodland total area) is less than 20 ha in size, and more than 40% is smaller than 5 ha (Thomas et al., 1997).

The contrast between England, with its high human population density and fragmentary ancient forest, and New Zealand, with its very large areas of well-protected and largely natural forest, is extreme. Before the relatively recent arrival of Polynesians and then Europeans, the only mammals present were either marine or bats. It is suggested that globally 4% of the boreal forest has been removed or fragmented compared to >50% of temperate broadleaf forest and nearly 25% of tropical rain forest (Wade et al., 2003).

Fragmentation can have a large effect on the living biota in a forest. As noted above under metapopulations, the resulting lack of connectivity between forest areas makes it much more difficult for native species to migrate and expand within this important habitat. In recent years, however, vigorous action has been taken in many parts of the world (see Box 10.1) and especially the UK. Thomas et al. (1997) present an encouraging picture of firm action being taken in the UK to prevent further loss of ancient semi-natural woodland and to de-coniferize and reclaim areas where woods long dominated by broadleaved trees, especially the English oaks, had been planted over. New

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