While cities only occupy two per cent of the Earth's land surface, they draw on their hinterlands for goods and services. Waste and emissions produced by urban dwellers, moreover have an impact on ecosystems elsewhere, even at the global level. For this reasons, cities depend on so called ecosystem services - benefits that nature provides us for free.
Those ecosystem services have provisioning (such as food and water), regulating (as flood control), cultural (such as for recreation) and supportive characters (like nutrient cycles). Healthy ecosystems and a rich biodiversity, i.e. the multiplicity of living organisms and ecosystems, are vital for cities to function properly. Not only do ecosystem services provide urban residents with food and clean water, they also play an important role for the quality of life in a city: ecosystem services regulate a city's climate, filter emissions and can protect a city against flooding. Despite the importance of a healthy ecosystem, little is done to mitigate the negative impact that by factors including over-exploitation, climate change, land use change, urban sprawl, traffic, air pollution and invasive species have on it.