Brown roofs for invertebrates
infertile mineral deficient substrates, combined with the unique climate of the East Thames Corridor,
especially in south Essex near the Thames, provide an abundance of sparsely vegetated ground that
takes decades to gradually develop towards scrub and woodland. This bare and sparsely vegetated
ground warms up quickly and provides important habitat for many rare species at the edge of their
range in Britain, as well as providing essential exposures of friable substrate for ground nesting
bees and wasps. Key (2000) emphasizes the many benefits of bare ground for the conservation of
invertebrates. Infertile substrates and drought promote the development of a diverse flower-rich
vegetation, with an associated invertebrate diversity.
The requirement of many invertebrate species for a suitable foraging area where nectar and pollen
may be gathered is paramount to their survival. Whilst many species may forage on a range of
flower species, others are highly plant-specific. Long tongued bumblebee species such as the Brown-
banded Carder Bee Bombus humilis depend on a continuity of suitable forage resources throughout
the whole season. In addition to forage areas, predatory species require suitable hunting areas of
open flower-rich vegetation, grassland or scrub that provide the specific prey species and it seems
probable that fairly extensive areas are needed by some species.
There is a parallel need for suitable larval development or nesting habitat and, in the case of parasitic
species, there is a requirement for the presence of a viable host population. Larval development
may take place on or in roots, stems, leaves, buds and seeds of plants or nesting may take place in
the ground. For stem nesting species and their cleptoparasites it is critical that these resources
remain in situ from one generation to the next for the survival of these species and their parasites.
For stem nesting species it is vital that these resources remaining in situ from one generation to the
next for the survival of these species and their cleptoparasites. Drought-stressed vegetation seems
to be especially important for many plant dependent invertebrate species, and the Blue Carpenter
Bee Ceratina cyanea for example is frequently found nesting only in stems of severely drought-
stressed bramble.
The invertebrate importance of the open flower-rich vegetation is also largely dependent on its
relatively unmanaged nature enabling the survival of species which live inside leaves, stems, flower
heads and seeds and which overwinter inside or at the bases of dead herbaceous stems and in
ground litter. The lack of regular management, often combined with sporadic disturbance from
various activities such as scrambler biking or occasional small-scale industrial activities, is crucial
to the development and survival of the large populations of rare and scarce species currently found
in the region. Management such as mowing or cutting that removes herbaceous stems on a regular
basis is a very harmful practice for these species, and amenity grasslands have virtually no value at
all. Tree and scrub planting, a fashionable and frequently carried out operation on 'restored'' land
and roadside verges, is also highly detrimental, shading out the valuable open habitat and replacing
it with a relatively poor diversity of widespread species with low nature conservation value.
The age of sites is important. There is often an assumption that brownfield sites are important
because they consist of early successional and ruderal habitat. Contrary to these perceptions, it is
the author's experience that brownfield sites do not develop their nature conservation importance
over short timescales. Older sites are almost always the most important, and it seems that the order
of 10 years abandonment, often much longer, is needed before they develop important invertebrate
assemblages. Early successional habitat is important, but only as a component of the whole habitat
mosaic, together with bare ground, herbaceous diversity, extensive flower-rich areas and an element
of scrub. After being abandoned, the fences that prevent public access to many brownfield sites
gradually get damaged and children and adults use the sites for various activities such as walking,
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Essex Naturalist (New Series) 21 (2004)