Evaluating The Landscape-scale Benefits Of Sustainable Urban Drainage Systems For Freshwater Biodiversity

More than half of the world's population now lives in urban areas and the ongoing rapid expansion of urban areas provides challenges and opportunities for the conservation of biodiversity. Urbanisation is commonly associated with loss of rare or pollution sensitive species, overall change in communities and the spread and dominance of invasive plants and animals. The development of urban green infrastructure (GI) has the potential to provide a counter-balance to such issues. Urban GI encompasses a wide range of ecosystem features including woodland, grasslands and meadows, and freshwater features. The development of effective GI has the potential to provide enhanced ecosystem services, biodiversity gain and societal benefits such as recreation.

Sustainable Urban Drainage Systems (SUDS) are a common 'blue' component of GI. These are artificial waterbodies principally designed to intercept surface drainage in urban catchments, improving water quality by allowing settlement of sediment and absorption/breakdown of pollutants. Such SUDS, mostly consisting of small ponds and other artificial wetlands, also present an opportunity for enhancing urban freshwater biodiversity; the four 'SUDS pillars' emphasise their role in relation to water quality, quantity, biodiversity and amenity.
The role of SUDS in relation to local biodiversity has been assessed through a number of studies. There's already evidence that SUDS can be important habitats for rare and important to conservation species. What has not been well established is their broader influence on diversity at the landscape or catchment scale, in terms of species richness, population connectivity and genetic diversity within and between populations. Many freshwater taxa are highly dispersive and SUDS can potentially act as important stepping-stone habitats both from other natural ponds and other types of freshwater ecosystems including rivers.Due to this potential connectivity, SUDS might allow otherwise isolated populations to persist as stable metapopulations within an ecosystem network that includes both natural and urban habitats.
In extensive urban areas, where natural freshwaters are often scarce, SUDS may provide links between natural features and other sites such as garden ponds and therefore increase landscape permeability both in terms of population movement and gene flow. Habitat fragmentation associated with urbanisation is known to influence gene flow, with reduction in genetic diversity potentially impacting on the ability to adapt to changing environments. Given significant northwards shifts of many species ranges with changes in climate, such permeability may also be critical in allowing species to bypass urban barriers. Enhanced permeability can have a downside however; the value of SUDS for enhancing biodiversity must be weighed against the value for harbouring and encouraging spread of disease vector and pest organisms, with the planning of SUDS undertaken with consideration of this risk-reward ratio.

Scottish Planning Policy identifies GI as an essential part of our long-term environmental performance and climate resilience. Significant investment is currently taking place in GI in Scotland through initiatives such as the Green Infrastructure Strategic Intervention (GISI). The GISI is part of the current European Regional Development Fund programme in Scotland and funds multifunctional GI in deprived areas of urban Scotland. As well as creating benefits for people and nature in the local areas, the sites will act as demonstrator sites, showing the value of GI and how it can deliver multiple benefits.

With this ongoing investment it is vital to evaluate the outcomes and determine how such developments can be implemented to maximise benefits for native biodiversity whilst performing other functions such as urban water management.