Brown roofs for invertebrates
scrambler biking, cycling, and even horse riding. These activities introduce disturbance, which
helps maintain bare ground and structural diversity without destroying the continuity of the all
important forage and stem resources. This is not so different to priority semi-natural habitats such
as heathland and ancient woodlands, where footpaths, disturbance and coppicing are important
elements in the maintenance of a proportion of bare and open ground within the overall habitat.
This same disturbance, created by military activities, is also crucial in providing valuable bare
ground habitat, herb-rich grassland and structural mosaic in the nationally important Salisbury
Plain (Toynton & Ash 2002).
Brown roofs as replacement habitat for invertebrates
Many rare or scarce species that might benefit from brown roofs include various species associated
with open, dry and sunny habitats containing sparsely vegetated and bare ground, such as heathland,
dry grassland and dune species. Other species such as those that nest inside dead bramble stems are
associated with a drought-stressed scrub component.
As yet there is insufficient research in Britain to adequately assess the long-term value of brown
roofs to invertebrates, and they have not been specifically designed with invertebrates in mind.
However there seems to be no reason why, if suitably designed, they should not be able to make a
substantial contribution to the invertebrate biodiversity of urban and city areas. It seems likely that
brown roofs, like brown field land, will take many years to develop substantia] diversity and an
important associated fauna. Unfortunately research in Britain has been limited to green or brown
roofs that are relatively young, and not surprisingly many invertebrates recorded have been pioneer
or ubiquitous species. Nevertheless there have been surprises, for example the dry heathland spider
Steatoda phalerata found in the very centre of London, near Kings Cross station, on the roof of the
temporary home of the Almeida theatre. This green roof was covered mostly with Sedum plants,
but also had a wide range of herbs that had established as a result of minimal management on the
roof. Consequently this habitat had a very varied flora and fauna. Unfortunately th is roof has since
been removed, as it was only a temporary home (Kadas 2002).
In general, green/brown roofs are likely to be especially valuable for invertebrates if they can
provide friable (e.g. sandy) substrates with a topographical variety, plenty of scattered rocks, rubble
and dead wood and logs, and with an open and diverse vegetation cover and plenty of bare ground.
Plants that provide nectar and pollen resources are especially important, and in many cases specific
plant species support specific invertebrate species. Unmanaged or sporadically managed vegetation
provides a continuity and large resource of dead herbaceous stems, fruit heads and seed heads for
stem nesters and seed feeding invertebrates. Plants stressed by mineral deficiency and high water
deficit appear to provide an especially valuable resource.
The aim should be to provide or encourage the development of plant diversity, forage continuity,
structural diversity and topographical variation. Drought and mineral deficiency stress should be
viewed as positive. The name of the game is therefore 'variety, bare ground and stress'.
The main issues that are likely to affect the importance of a brown roof to invertebrates are;
• The type of substrate material used. This should as a general principle make use of nutrient
poor, friable material such as crushed concrete, sands, gravels and rubble. It may be possible to
use material already present at a site before development. The particle composition should be
varied over different parts of the roof. Dune species will be helped by areas of loose sandy
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