Such services are regulating (purifying air and water or mitigating floods), provisioning (including supplying food, water, or medicines) and cultural (covering aesthetic, spiritual, recreational, and intellectual benefits). Indeed it is more accurate to affirm that cities are ecosystems than that cities have ecosystems. It is a key tool for promoting a city’s biodiversity objectives, relying on urban green areas as entry ways for flora and fauna from beyond the city system. Each can be divided further into specific biotopes - living spaces that provide suitable conditions for the development of certain living organisms - or localized elements, such as trees (in the green system), sidewalks (grey) and ponds (blue). All three are of equal importance.