The problems facing our cities call for a holistic approach, not just for ecological solutions (Nature 468, 173; 2010). We also need to consider the resilience of the changes we make to the urban landscape in the name of sustainability (see http://www.urban-futures.org) and strike a balance between the benefits and disadvantages of these strategies.

Take street trees planted to improve biodiversity. They reduce air pollution by increasing particle deposition and replenishing oxygen, yet may also exacerbate it by reducing ventilation. They provide shade but limit passive solar heating. Their amenity value may be undermined by high costs for repairing infrastructure damaged by ground shrinking and swelling. Although they help to mitigate light pollution, trees are likely to increase lighting requirements, and although they store water they may need irrigating — and so on.

A continuing positive outcome will depend on thoughtful assessment of the competing and shifting aspects of sustainability.