In the twenty-first century, urban growth will witness the most rapid expansion in human history. In the cities of Asia and Africa, where urbanisation is advancing at the fastest pace, the population will increase by several billion. For this reason, it will be necessary to plan a massive expansion of urban infrastructure — roads, pipe networks, electricity lines and schools. In addressing this challenge, ecologists, urban planners, economists and landscape architects are increasingly required to consider the relationship between cities and nature, and in particular the role of natural infrastructure: that is, habitats or natural spaces recreated by human beings from which city dwellers benefit.

Cities need nature

Whether it is forests maintaining water quality in reservoirs, trees with their canopies keeping cities cool during heatwaves, or parks contributing to a city's success and improving quality of life, natural infrastructure is very much in vogue today. Yet despite the enthusiasm this type of infrastructure generates, it is often difficult to find reference points that explain how to plan and implement such projects.

Moreover, environmentalists who care deeply about nature often regard cities as enemies to be fought. Urban expansion has, in effect, caused the loss of biodiversity.

Nature needs cities

Cities offer many economies of scale, reducing the per capita use of certain resources. Many environmentalists look upon the current urbanisation of our species with a sense of melancholy, arguing that it marks "the end of nature" — to use the words of Bill McKibben, activist of 350.org. He argues that we have reached a point at which every square metre of land and every ecosystem has been altered by human beings, and that therefore nature in the true sense no longer exists. In his view, cities represent the end, the death of nature, because they are spaces entirely created and designed by human beings for themselves.

While wild nature is increasingly rare, natural processes remain vitally important to cities. Rather than viewing the twenty-first century as the end of something, we must begin to see it as a beginning. We are creating a new world, an urban world. This urban world could become a dystopia, a negative utopia, but only if we allow it to — because there is another way to contemplate our relationship with the landscape. Rather than subjugating nature to our will, we must subjugate our will in order to live in harmony with it. In this sense, the science of ecosystem services — defined as the benefits that ecosystems provide to humankind — offers us essential tools for following these alternative paths.

We have become an urban species regardless of whether we are ready for this change. If we choose to ignore the ways of nature, if we treat the landscape as though it were a disposable object, then the dystopia will come to pass. If, on the other hand, we choose to reflect on how to urbanise, then we will have a more beautiful and humane world. In a sense, it is we who choose the urban world we build, and we will get the urban world we deserve.