Urbanisation worldwide (UN 2007; UNFPA 2007) is leading to loss of biodiversity due to habitat fragmentation and destruction (Miller and Hobbs 2002; McDonald et al. 2008) and to negative effects on people’s quality of life, including stress and a range of other health issues (Ulrich 1979; Jackson 2003). However, the presence of green areas in cities can counteract the effects of urbanisation. First, they provide important habitats for many species that would otherwise not be able to exist in cities and provide essential nodes in the overall landscape-scale habitat network (Angold et al. 2006; McKinney 2006). In some cases urban areas may even provide habitats for species decreasing elsewhere. For example, in Great Britain an increase in the abundance of the common frog (Rana temporaria) has been observed in cities, while at the same time the species declined in rural areas (Carrier and Beebe 2003). Second, the presence of green areas can have positive effects on people’s mental and physical health (Kaplan 1973; Frumkin 2001; Hill 2002), as illustrated by the faster recovery from surgery of patients with a window view of greenery instead of a wall (Ulrich 1984), or less domestic violence in neighbourhoods with more trees (Sullivan and Kuo 1996).