Chalk grassland habitat audit

Chalk grasslands develop on shallow lime-rich soils, notably on the downland of south-east England. The habitat supports a wealth of wildflowers and a wide array of butterflies, grasshoppers and other invertebrates, many of which are restricted to chalk soils.

In London, chalk grassland is largely restricted to the southern edge of the metropolitan boundary. Here parts of the North Downs lie within the Boroughs of Sutton, Croydon and Bromley. Another area of chalk lies on the extreme north-western edge, in the Borough of Hillingdon, where outliers of the Chiltern Hills are just within the Greater London boundary. Further small patches of grassland containing species typical of the chalk can be found scattered throughout London growing on artificial calcareous substrates such as railway ballast and fly ash.

Greater London's chalk grassland supports a number of nationally rare species. Many of these are continental in distribution and occur in Britain only on the downland of the Southeast, where climatic conditions are comparable to those of mainland Europe.

Traditionally, sheep grazing maintained a short sward and prevented scrub invasion, but with intensification of farming this traditional management practice has largely been abandoned in London. The decline in sheep pasturing and rabbit grazing (following myxomatosis) has resulted in many chalk grasslands succumbing to scrub invasion and natural succession to woodland. Other remaining chalk grassland sites have been modified by applications of fertiliser, partial reseeding and frequent mowing. The continued sprawl of urban London has led to large losses of habitat and conversion to arable has been a problem in the past.

All these factors have led to a reduction in the extent and distribution of this habitat and continue to threaten remaining chalk grassland. The fragmented, isolated nature of the remaining sites makes further decline in their nature conservation interest more likely, particularly the loss of small populations of vulnerable animal species.

Efforts to reverse this trend have been made on a number of sites with some success, particularly through the removal of invasive scrub and restoration of grazing. Where former chalk grassland has been lost to previous arable conversion, there is the potential for reversion to grassland which can become quite species rich. Arable reversion can provide an opportunity for linking together isolated chalk grasslands by providing stepping stones, habitat corridors or extensions to existing habitat.