Biodiversity and Wildlife

Green roofs are intrinsically of greater benefit to biodiversity than more traditional roofing methods. Many green roof manufacturers promote green roofs as benefiting wildlife, but with little evidence to demonstrate this. Of course ‘off the shelf’ green roof systems do provide benefits for wildlife compared to non-green roofs, but research in Switzerland and in London shows that green roofs need to be designed to meet specific local biodiversity conservation objectives.

Swiss research

Detailed research into biodiversity and green roofs has been undertaken since 1997. This research was specifically driven by concern over the impacts of new developments on brownfield land in the city. Such land has been recognised as important for a number of national scarce beetles and rare spiders. These species were originally associated with Rhineland alluvial gravel habitat, little of which remains. These species had found refuge on brownfields in Northern Switzerland. Could green roofs be better designed to provide refuge for such species as new developments encroached on their habitats?

A number of design principles were arrived at:

1. The use of local substrates as growing mediums on green roofs helped replicate the conditions at ground level.
   
   
2. Varying the depth of the substrate provided microhabitats for rare spiders and beetles associated with brownfields in the city.
   
   
3. Planting with a local seed mix.
   
   
4. The placing of objects associated with natural habitats such as dead wood and old branches increased the biodiversity of the roofs.

Around the same time the term ‘brown’ roof was coined to ensure that where green roofs are to be placed in developments as mitigation for brownfield biodiversity issues, these roofs would not use ‘off the shelf’ solutions but be designed specifically for the biodiversity that is to be mitigated for.

Biodiverse Green Roof in Switzerland

The design of such roofs is based on the research undertaken by Dr.Brenneisen in Basel and continuing investigations in to roofs in London. Much of this has arisen out of the London Biodiversity Partnership’s black redstart action plan. It is estimated by the plan that an area greater than 100,000m² of roofs is to be built as mitigation for this species. In many cases ‘off the shelf’ solutions are to be used. However, a number of roofs are being designed specifically to meet the needs of this species.

Research in London since 2002 has shown that some green roofs in London are providing refuge for a number of nationally rare and scarce species. However, ‘off the shelf’ systems are not, as yet, providing the ecological requirements to mitigate for brownfield habitats and other well-drained low nutrient habitats.

Interestingly, research from 2003 has shown that a green roof with a sedum mat on 75mm crushed brick is more species diverse than roofs with just sedum mats, and that the species diversity is closer to that of a nearby brownfield habitat.

Research in the UK on biodiversity is continuing and a more detailed assessment of how different green roof systems perform in terms of biodiversity should be available in the next few years. This has demonstrated the need for many more bespoke roofs to be designed in order to meet specific biodiversity benefits. In addition, there is a growing body of research that shows these roofs may also provide better environmental benefits for some situations.

It is likely that green roofs can be designed to meet a number of national, regional and local biodiversity action plan targets. Skylark (Alauda arvensis), a UK Biodiversity Action Plan [BAP] species, are known to breed on green roofs in Germany along with other notable species such as oystercatcher (Haematopus ostralegus), ringed plover (Charadrius hiaticula), little ringed plover (Charadrius dubius) and common tern (Sterna hirundo).