Like oases in asphalt deserts, city parks and gardens shine as green beacons for wildlife. Yet increasingly urban biodiversity hotspots are coming in shades of brown and grey.

Brownfield sites - underused or abandoned and reclaimed by nature - are seen as ripe for redevelopment: indeed some environmentalists urge developers to concentrate on them and leave the countryside unspoiled. Cluttered though they may be by industrial remnants and haphazard foliage, they are often important reserves for wildlife.

Fortuitously free from human intervention, unattended brownfields often provide niche habitats for flora and fauna declining elsewhere. Many contain complex ecosystems replete with wildflowers, ferns, goldfinches, skylarks, butterflies, bats, bumblebees and beetles. Some boast a startling array of biodiversity that rivals even famous botanical gardens, public parks and waterways. One in every four wildlife sites in London (home to Kew Gardens, Hyde Park and the River Thames) are partly or wholly brownfield.

Not all brownfields are equally valuable. Some are indeed ideal for redevelopment in cities pressed for building space. But in other cases, rehabilitating these undisturbed, derelict areas - even into green spaces meant to restore biodiversity - may be missing the forest for the trees.