Urban Habitats — Rare Invertebrates Colonizing Green Roofs in London

Urban Habitats -- Rare Invertebrates Colonizing Green Roofs in London

Introduction

Even our most industrial, built-up cities need not be completely devoid of green space and wildlife. While parks and gardens come to mind as obvious refuges for nature, plants and animals are often more adventurous with regard to the places they colonize and use. Not many people associate rooftops with wildlife habitats, but if suitable niches are available or provided, plants and animals will rapidly move in and establish communities. In some cases, green roofs offer the only valuable wildlife sanctuaries in our cities and towns. Of particular importance is the fact that these rooftops already exist, so no additional space has to be sacrificed. The potential to provide habitat for wildlife on green roofs is tremendous. In London, for instance, 26,000 hectares of available roof space could be greened with little effort, and this would create 28 times the green space of Great Richmond Park (Grant, Engleback Nicholson, 2003 ).

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Figure 1: Retail Sedum Roof, Canary Wharf, London.

The term "green roof" describes both intensive, ornamental roof gardens and extensive roofs with more naturalistic plantings or self-established vegetation. Intensive green roofs are like parks and gardens at roof level and require deep soil and regular maintenance. Extensive roofs have more naturalistic plantings and shallower natural substrates and are either sown with (local) wildflower mixes or Sedum matting or left to colonize naturally. Extensive green roofs require little or no maintenance and are relatively inexpensive to establish.

The environmental benefits provided by green roofs are well documented (Grant, Engleback Nicholson, 2003 ; Getter Rowe, 2006 ). What green roofs can achieve in terms of biodiversity, however, is less well known. They may provide new habitats in areas that currently lack suitable wildlife space, act as green corridors linking existing habitats, facilitate wildlife movement and dispersal, and serve as refuges for declining and rare species. One of the most pressing issues in the U.K. is the role that green roofs might play in terms of habitat mitigation for the lost biodiversity of redeveloped brownfield sites. (In the U.K. "brownfield" land is land that has had a previous industrial use but can be built on; it is not necessarily contaminated.)

Brownfield sites include some of the most species-diverse habitats left in the U.K. They are sometimes referred to as "English rainforests" ("A Bleak Corner of Essex," 2003 ), because some of them harbor the same number of rare invertebrates that can be found in ancient woodlands (Gibson, 1998 ). The best sites may contain up to half of an entire county’s invertebrate fauna (Gibson, 1998 ; www.buglife.org.uk ). With the intensification of modern farming methods in rural areas, these sites, which have largely escaped improvement, have become wildlife refugiahabitat "islands" in a "sea" of industrial agriculture (Angold et al. 2006 ; www.buglife.org.uk ).

Urban Habitats -- Rare Invertebrates Colonizing Green Roofs in London

So what is the problem? There is increasing pressure to redevelop the brownfield sites. In London, for example, according to the latest estimates, 24,000 new homes are expected to be built each year (DETR, 2000 ). The general government strategy is to build 60% of these homes on brownfield sites (DEFRA, 2003 ). Huge swathes of industrial brownfield along the Thames estuary are slated for redevelopment, and this will have an immense impact on wildlife.

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Figure 2: Laban Dance Centre (brown/biodiverse) roof.

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