Habitat changes in British lowland landscapes and their
implications for bird populations

by Rob Fuller

2. Habitat loss, habitat deterioration and habitat fragmentation

A distinction needs to be made between impacts that result in the loss or destruction of habitat, and impacts that leave the habitat apparently intact, but in some way reduced in quality. The destruction of habitat can be clear-cut, for example the destruction of a wood and its conversion to agriculture or the building of a housing estate on a heath. There have been virtually no studies of what happens to individual birds displaced by such extreme habitat loss but presumably many die, especially when alternative habitat is scarce. Habitat loss should be regarded as the most powerful threat to the persistence of populations at all scales.

Habitat fragmentation has become widely regarded as a critical issue in conservation science (Hansson et al. 1995, Wiens 1995, Villard 2002). Habitat fragmentation is concerned with spatial processes, such as negative edge effects (e.g. predation pressure) and dispersal problems that become increasingly severe as habitat is divided into smaller units. Several studies have indicated that fragmentation effects are likely to become relevant once the total amount of habitat has dropped below a certain critical threshold. When patches become small they are more vulnerable to processes that will lead to habitat deterioration. In practice it has proved extremely difficult to separate effects of habitat loss from those of habitat fragmentation. This is partly because habitat fragmentation can, by definition, only occur when the extent of habitat is reduced.

Two points are worth bearing in mind with respect to the effects of habitat fragmentation on birds in lowland Britain. Firstly, many habitats in lowland Britain have been fragmented into relatively small patches for many hundreds of years so that species have had a long period to adapt. Secondly, compared with most taxa, birds have relatively strong dispersal ability and, with a few exceptions, do not have great difficulty moving between habitat patches. In summary, habitat fragmentation is increasingly thought of as a secondary issue to that of habitat loss. This is not to say that fragmentation of blocks of extant habitat, for example by roads, is a trivial issue but the main thrust of conservation effort should be on maintaining or restoring large areas of good quality habitat.