Green Roofs & Brownfield Biodiversity
The research in Basel has lead to modifications in the design and topography of the green roofs and informed green roofing policies at a regional level. The benefits to biodiversity of green roofs in urban situations are probably far reaching and their potential is not fully understood or realised in this country. However, two studies of invertebrates on green roofs are being undertaken in London, one funded by English Nature. The results will be included on this site when available.
For further information on green roofs may we suggest you visit our new sister site www.livingroofs.org which has been developed specifically as an information resource on this subject.
Birds
Green roofs are liable to benefit a number of bird species. In Urban situation 2 species which are likely to benefit are House Sparrow (Passer domesticus) and Linnet (Carduelis cannabina). The Swiss research noted that along with black redstarts, house sparrows were the most active birds encountered on roofs. The House Sparrow is a PRIORITY species for the London Biodiversity Partnership and there is widespread concern for its conservation due to substantial decline in its population. The Linnet is a UK PRIORITY species and there is a national action plan foor this species. In London Linnets are encountered regularly on brownfield land and wastelands. The author has observed this species feeding on moss and sedums on spontaneous green roofs and redundant gutters on numerous occasions. A complexity of structural diversity of plants should provide this species with ample foraging habitat and provide a positive benefit for its conservation in nationally in urban areas.
Invertebrates
Many rare or scarce species that might benefit from green roofs include various species associated with open, dry and sunny habitats containing sparsely vegetated and bare ground. Examples are many, but include Ponera coarctata the Indolent Ant, a nationally scarce species referred to in the Lee Valley Biodiversity Action Plan, the Red Data Book bee Ceratina cyanea which nests in bramble stems and has a number of localities in the East Thames region and Colchester in open unmanaged or sporadically managed grasslands and waste ground, the nationally scarce stem nesting bees Hylaeus cornutus and H. signatus, which are especially associated with Reseda and Daucus in open grassland and waste ground in southern England.
Two long-tongued bumblebees on English Nature’s Species Recovery Program might benefit from suitably designed and managed green roofs. Both have nationally important populations in the East Thames region and south Wales. Green roofs supplying suitable forage habitat could help the survival of the regional metapopulations. Both bumblebee species require abundant forage plants especially Fabaceae, Scrophulariaceae, and to a much lesser extent Asteraceae, Lamiaceae. Workers particularly favour plants such as Lotus, Odontites, Ballota, Trifolium pratense and queens will use species such as Fodder vetch Vicia villosa and Broad-leaved Everlasting pea Lathyrus latifolius, which could usefully be seeded or planted on green roofs.
One Red Data Book solitary wasp Cerceris quinquefasciata, included in English Nature’s Species Recovery Program, might benefit from suitably designed green roofs. The wasp is currently widespread in Britain only in the East Thames region, but most sites are threatened by development. It excavates its nests in hot sandy conditions and provisions them with common weevil species. The species is typically associated with open unmanaged or sporadically managed flower-rich grasslands, waste ground and heath edge.
Gymnosoma nitens is a Red Data Book tachinid fly parasitic on the nationally scarce shieldbug Sciocoris cursitans, a species of dry sandy and calcareous places. Most records of G. nitens are recent, from waste ground and open unmanaged or sporadically managed grasslands in the East Thames region. Both species might benefit from suitably designed and managed green roofs.
Lasioglossum pauperatum is a Red Data Book mining bee which nests in sandy ground and forages on yellow composites. It is currently widespread in the East Thames region on open unmanaged or sporadically managed grasslands and waste ground, and would probably make use of green roof habitat.
A number of bees and wasps usually associated with dune habitats are found in open sandy waste ground habitats in the East Thames region, such as the nationally scarce bees Dasypoda altercator and Megachile leachella and the sphecid wasps Astata pinguis, Gorytes tumidis and its nationally scarce cleptoparasite Nysson dimidiatus. These might be accommodated by green roofs which provided areas of loose sandy substrate.
Spider hunting wasps, or pompilids, include various species that might benefit from green roofs, especially where these include sandy substrate and open sporadically managed vegetation. Many scarce beetles associated with open dry grasslands and waste ground are widespread in the East Thames region, including a number of Red Data Book tumbling flower beetles Mordellistena species that develop inside the herbaceous stems of plants such as Wormwood Artemesia. Scarce spiders such as the ant eating Zodarion italicum and the jumping spider Bianor aurocinctus are especially widespread in the East Thames region in open dry grasslands and waste ground.
The Adonis Ladybird (Hippodarnia variegata} is another species which could, especially if plants such as artemesia and mayweed are present. This species main prey is aphids that may be associated with these plants.
Ecology and design for invertebrates
The design of green roofs has been mentioned earlier it is worth noting the ecological principles that should be applied to the design of green roofs from an invertebrate point of view.
Although the availability of specific plants and the resources they provide is a key factor for many invertebrate species, the structural characteristics of habitats are also very important. A varied sward and mosaic of vegetation heights, a continuity throughout the season of various leaf, stem, bud, seed, nectar and pollen resources and including the availability of over-wintering dead stems and seed heads are all essential for many species.
Open bare areas are an important and often undervalued resource. Bare soil offers a number of benefits. In particular it warms up rapidly in sunshine, it is used by burrowing and ground nesting species and for egg laying and it provides a clear visual field for predators (Key, 2000).
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. 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 it is critical that these resources remain in situ from one generation to the next for the survival of these species and their parasites.
Hence the invertebrate importance of waste ground is largely dependent on the unmanaged nature of the vegetation enabling the survival of species which live inside leaves, stems, flower heads and seeds and which over winter inside or at the bases of dead herbaceous stems and in ground litter. For stem nesting species it is critical that these resources remaining in situ from one generation to the next for the survival of these species and their cleptoparasites. Management that removes dead wood or herbaceous stems on a regular basis is therefore a very harmful practice for these species.
Many invertebrates that are found on brownfield sites in the UK are likely to benefit from the provision of green roofs in development schemes. A list of the species likely to benefit is provided.
In general, green 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 openopen 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 main issues that will affect the importance of a green roof to invertebrates are:
- the type of substrate material used. This will vary from site to site but should as a general principle make use of the existent material to recreate the original conditions. The substrate should be as nutrient poor as possible, and the friability and particle composition should be varied over different parts of the roof. Vegetation stressed by drought and low nutrient levelsnutrient levels is (vegetation is surely singular(areis especially valuable.
- the topography of the green roof. This should be varied, with small scalesmall-scale hollows, mounds and banks to increase the physical diversity and consequent associated biodiversity.
- Management. Management should aim to provide continuity through the summer and winter of forage, stems, fruits and seeds, to enable a wide diversity of species to feed and over winter successfully. The majority of invertebrates on brownfield sites, especially the rare and scarce species, depend on a variety and continuity of varied plant resources. Any form of regular management that removes these resources will be very harmful to the associated invertebrate fauna. Cutting will be destructive unless it is done on small areas on a long rotational basis. Occasional disturbance to re-establish bare ground will be beneficial.
Flora
Green roofs will also provide a great deal of botanical interest. This could come through natural colonisation or seeding. Specific certain plants are particularly important for invertebrates and birds:
Bird’s foot trefoils Lotus corniculatus & L. glaber
Black Horehound Ballota nigra
Common Stork’s-bill Erodium cicutarium
Hardhead Centauria nigra
mayweeds Tripleurospermum, Matricaria
Mignonette Reseda lutea
mugwort, wormwood Artemesia spp.
Ragwort Senecio jacobeia
Red Bartsia Odontites verna
Thistles Cirsium spp.
Weld Reseda luteola
White Bryony Bryonia dioica
Wild Carrot Daucus carota
Yellow Asteraceae e.g. Hieracium , Picris spp.
