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december 2003

The London Biodiversity Partnership

Wild Roofs:  Current research into green roofs and biodiversity in London
By Dusty Gedge

 www.blackredstarts.org.uk
Photo: Craig Churchill

It was some years ago when a few gallant nature conservationists sat in a pub in London trying to deal with the fate of the black redstart, Phoenicurus ochruros, in the context of whole scale development (and brownfields). This wonderful bird is found in London on old vacant lots and has a particular liking for rubble - certainly not everyone's idea of pristine wilderness - yet in many ways such sites are more interesting for biodiversity than more pristine areas of the English countryside. Therefore, from supping beer and pondering the dilemma of bird and rubble verses swanking new apartments, an idea began: Why not transfer the rubble to the roofs?

The Gallant Few

At that time the gallant few were kind of aware of green roofs but didn't really know anymore than that they were "grassed" areas in the sky. However, the efforts of the gallant few succeeded in ensuring that a whole neighbourhood in Southeast London would have a large amount of 'rubble' roofs to ensure that the black redstart would maintain its fragile population in that corner of our city: Now, green roofs are being used for black redstarts and other nature conservation issues throughout London - in fact, to date the London Biodiversity Partnership's Black Redstart Action Plan is aware of over 100,000 square metres of green roofs planned or already built in London - and that is only the number we are aware of; it is likely to be much greater.

The question that was often raised a number of years ago and is still occasionally encountered today is, "Will they work for nature?" Back in 2000 I was fortunate to encounter the work of Stephan Brenneisen in Basel, Switzerland and have now visited that city a number of times. Stephan's work was the only published work on biodiversity and green roofs at the time, and it demonstrated that many rare species of spiders and beetles had indeed found refuge on the green roofs in his city. This was the kind of evidence we needed in London, especially as Stephan had also found that black redstarts were one of the UK main bird species encountered on roofs.

Goddard & Brenneisen in Basel, Switzerland

The Partnership of Jill Goddard, UK and Stephan Brenneisen, CH
on greenroof in Basel, Switzerland

Many professionals wanted concrete biodiversity material evidence, yet also, at the same time the UK government had highlighted the need to build on brownfield land as a priority. As ecologists started to look at these sites, they found that many of them were incredibly rich in biodiversity. In fact, in 2003, English Nature - the UK Government's advisory body on biodiversity - heralded one such site, Shellhaven, as England's equivalent of a rainforest. It was determined that with such pressure on rich biodiverse sites there well may be a need to ensure that there is habitat mitigation for the loss of biodiversity through the use of green roof systems. However, what kind of green roof systems would best benefit the biodiversity in question? These questions have lead to a number of research projects over the last few years.

Tecticolous Invertebrates

In 2002 English Nature commissioned a survey of invertebrates on a number of roofs in the London area. Richard Jones, a well-known UK entomologist, undertook this effort and the ensuing report was published in early 2003. He used the term tecticolous, first used in study of roof plants by Ron Payne, to describe the type of invertebrates that could be found on green roofs in the London area.

The study aimed to establish what kind of invertebrates could be found on green roof systems and to establish what were the important factors affecting tecticolous invertebrates on such systems. As the majority of the green roofs were of a monocultural or very low variety of sedum mats, they had low plant diversity and lack of species in plant architecture, and also very thin substrates. This, he concluded, limited the number of invertebrate species to be found on green roofs. Not withstanding, he did find a number of very unusual and uncommon species which had not been recorded in the London area before, most notably Tachys parvulus, often referred to by entomologists as "the patio beetle." This species is normally associated with gravel pits and exposed shingle and was a very surprising discovery found atop roofs.

Spider Research

G. Kadas sampling on the Almeida Theatre, London;
Courtesy Dusty Gedge

Running concurrently with the English Nature invertebrate study, a more intense study was undertaken by Ms. Gyongyver Kadas, a M.SC student at the University of London, focusing on spider populations on green roofs.

Ms. Kadas' conclusions were very similar to those of Richard Jones: that plant architecture and substrate depth were the most important factors affecting the population and the variety of species of invertebrates found on green roofs. Ms. Kadas also compared the populations on green roofs with those on a number of brownfield sites and concluded that on the whole, sedum matted roofs did not have the variety of substrate depths and plant architecture that brownfield sites had, and therefore were not mitigating or encouraging such invertebrate species associated with brownfield habitats in the London area in their current design.

Erigone aletris, courtesy Dusty Gedge

Erigone aletris

However, Ms. Kadas did find some very interesting species on green roofs. All in all, she identified 59 different species, which represents 9% of the total UK spider fauna and 26% of the total London spider fauna. Of these species, two were classified as "nationally rare"; six were new to London; and one species, Erigone aletris, was the first recorded specimen for Southern England, and is interestingly a recently colonizer from North America.

Ms. Kadas also noted that on one particular roof, due to its aspect in a sun shadow, it was home to a number of spider species associated with wetland habitats.

Current Factors

Brenneisen, Jones, and Kadas' work highlight that the most important factors affecting rare invertebrate populations on green roofs are substrate type and depth. If the depth of substrate is varied, then the roof provides both arid and verdant areas increasing the species composition. In fact, many of the very rare species in both Switzerland and London are those that prefer shallow bare surfaces, which are not found on matted green roofs.

Further Research

The London Biodiversity Partnership and Royal Holloway University of London have managed to raise over £55,000 to build on the research undertaken in 2002. Ms. G. Kadas started a three-year PhD this year, and her study will focus on spiders, bees and beetles. In the first phase, three green roofs within the Canary Wharf Estate (2 sedum matted, 1 sedum on 7cm of crushed brick) and two so-called brown roofs (rubble based roofs) are being studied along with three terrestrial rubblescapes.

The full analysis of the first year's sampling is not as yet complete. However, it appears that a quick comparison of the samples taken from the roofs at Canary Wharf show that there were more invertebrate species and individuals on the roof with 7cm of crushed brick than were found on the roofs with just sedum mats - concluding that a greater substrate depth will encourage greater invertebrate biodiversity.

Canary Wharf Retail

Left: Retail, Canary Wharf Sedum Mat only study roof; Right: Waitrose, Canary Wharf Sedum mat
laid on 7cm of crushed brick; Photos Courtesy Dusty Gedge

Future Canary Wharf Green Roof Laboratory Site,
HQ4, which houses Northern Trust;
Courtesy Gyongyver Kadas

The London Biodiversity Partnership and Royal Holloway University of London project will soon have established a number of green roof laboratories in London. These laboratories will try out different substrate and planting regimes in order to establish the important or essential design factors to provide mitigation for brownfield invertebrates on green roofs. Expected by March, the future site will be located on Canary Wharf HQ4 and the system is just crushed brick with various depth and planting regimes. This system has not yet been used in the Canary Wharf area and is essentially a ZinCo system without any plug planting.

The project is grateful to a number of sponsors including the People's Trust for Endangered Species (www.ptes.org.uk), Canary Wharf Management Limited (www.canarywharf.com), British Waterways and the London Development Agency. The laboratories have been provided by Alumasc-Exteriors who supply the ZinCo roofing system to the UK market.

The Future

Black Redstart

www.blackredstarts.org.uk;
Photo by Craig Churchill

The efforts of the gallant few have now generated a growing interest in green roofs in London and rest of England. The establishment of a thorough scientific investigation into how they can used to protect rare species will hopefully inform design and lead to an expansion in the application of green roofs in the London area.

The black redstart first raised its red tail above the roof parapets some years ago, but it has now become the flagship species helping the conservation of, and until recently, the little understood and cared for, invertebrate population. The London Biodiversity Partnership and Royal Holloway University of London are interested in hearing from anyone researching biodiversity and green roofs and are more than willing to offer advice and help to those seeking to undertake such research.

Thanks

I would like to thank a number of people who include the gallant few: Jill Goddard, Nick Bertrand, Mike Paice, Richard Jones and Chris Gittner. And since the Deptford Project, Pete Massini, Jenny Scholfield, Adam Ingleby, Trudi Thompson, Alun Rhys Tarr, Peter Allnutt, and the many individuals at Canary Wharf.  Particular thanks goes to Ms. G. Kadas for her belief and diligence and Mathew Frith as my colleague 'in crime' on the subject of green roofs and biodiversity.

Dusty Gedge writes on behalf of the London Biodiversity Partnership and Royal Holloway University of London, and is a leading figure in green roof circles in the UK. He is a consultant ecologist specializing in black redstart conservation and is the principal behind www.blackredstarts.org.uk
He advises a number of government bodies, NGOs, architects, landscape architects and the green roof industry.

Dusty Gedge is assistant supervisor for the Green Roof and Biodiversity PhD at Royal Holloway University of London, and may be contacted at: dustygedge@yahoo.co.uk; 7 Dartmouth Grove, London SE10 8AR; phone: +442086922109.


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