This figure presents the impact of increasing urban densities on urban ecosystem services (ESS list from Costanza et al. , 1997), as compared to leaving land in natural state. The arrows indicate whether the ability of urban ecosystems to provide the ecosystem service increases, decreases or alters in a way that is neither an increase nor a decrease. The question marks indicate unknowns. For example, the loss of permeable surfaces decreases the ability of urban ecosystems to regulate disturbances such as urban floods; however, while loss of permeable surfaces has an impact on the hydrology of water systems, it does not necessarily impact water regulation, which is defined as provisioning of water for agriculture and industry. It may be noted that, while some scholars argue that services such as food production and raw materials are not relevant within the city, these become relevant in contexts such as urban farming and are therefore included here.The figure is the outcome of an extensive literature review that identified attributes generic to high-density urbanisation and resulting pressures on land, water and air. The investigation reinforced the claim that cities draw on ecosystem services far beyond their boundaries, such as water purification and food, and ecosystems within city boundaries are highly modified by humans (Rees, 1999; Pickett et al., 2001; Gómez-Baggethun and Barton, 2012). It also demonstrates that cities and their inhabitants rely on certain locally generated ecosystem services such as air purification and noise alleviation and ecosystems therefore need to be a part of the city fabric.BibliographyCostanza, R. et al. (1997) ‘The value of the world’s ecosystem services and natural capital’, Nature, 387, pp. 253–260.Gómez-Baggethun, E. and Barton, D. N. (2012) ‘Classifying and valuing ecosystem services for urban planning’, Ecological Economics. Elsevier B.V. doi: 10.1016/j.ecolecon.2012.08.019.Pickett, S. T. A. et al. (2001) ‘Urban Ecological Systems: Linking Terrestrial Ecological, Physical, and Socioeconomic Components of Metropolitan Areas’, Annual Review of Ecology and Systematics, 32(1), pp. 127–157. doi: 10.1146/annurev.ecolsys.32.081501.114012.Rees, W. E. (1999) ‘The built environment and the ecosphere: a global perspective’, Building Research & Information, 27(4–5), pp. 206–220. doi: 10.1080/096132199369336.