High density "compact cities" are the favoured model for sustainable living in the 21st Century. But there are drawbacks, says Richard Fuller, because losing urban green spaces will reduce people's quality of life and drive out wildlife that have also made their homes in cities.
We are fast becoming an urban species. More than half of the world's people now live in cities, and in the UK around 90% of us are urbanites.
Evidence from the UK tells us that green space is one of the first casualties of high density urban development
As human populations increase, demand for new housing seems insatiable.
Over the past few decades, this has resulted in the rapid development of urban sprawl - vast expanses of low density suburban housing that have progressively eaten into the countryside around our cities.
Suburban sprawl means that many of us now live miles from "local" shops, our workplaces, friends' houses, and also miles from the nearest park or piece of countryside.
This can result in car-dependence, a lack of exercise, and exhausting, environmentally damaging daily journeys to and from scattered locations across our cities.
Proponents of the compact city have a new vision, which has captured the attention of governments.
They envisage smaller, high density cities that reduce the amount of countryside that needs to be swallowed up by urbanisation; places where people can live closer to work, more journeys can be made on foot or by bicycle, with less air and noise pollution, and a reduced collective carbon footprint to boot.
With such reductions in land-take in mind, the UK government now recommends that all new housing is built at 30-50 dwellings per hectare, more-or-less double the current density. That's an average of more than one household in an area the size of a tennis court.
Scientists have shown that green spaces promote community togetherness, reduce crime, improve our physical health and enhance our psychological well-being
This will pack a lot more people into the same space than we currently do. It is perhaps the single most important piece of housing legislation for decades, yet it is not well known and the potential consequences of it have not been widely debated.
There are significant downsides to the alluring vision of the compact city. Evidence from the UK tells us that green space is one of the first casualties of high density urban development.
Green spaces, including our own domestic gardens, are important even to the most hardened city slickers among us. They are places to sit and contemplate, meet with friends, walk the dog, go for a run, feed the ducks, for children to play.
Scientists have shown that green spaces promote community togetherness, reduce crime, improve our physical health and enhance our psychological well-being. They promote inward investment into cities, and even increase house prices.
But more fundamentally than this, urban green spaces are one of the few places where we can experience nature in our increasingly urbanised world.
Even small scraps of green space can be vitally important - I spent most of my youth in so-called "waste ground" near home collecting bugs, watching bees pollinate the flowers, poking sticks in ant-hills and being captivated by the antics of garden birds.
This access to nature was important in steering me towards biology as a career.
How close is the nearest patch of "waste ground" to today's generation of young explorers? How many of us live near shiny new blocks of executive apartments built on a "brownfield site"?
Bye bye biodiversity
Green spaces are also important for biodiversity, which is declining in the UK at an alarming rate. Several species whose populations have crashed nationally, such as house sparrows, starlings and song thrushes, are now more common within cities than outside them.
This is partly because they can thrive where there is plenty of green space and gardens within our sprawling towns. Leafier suburbs support more biodiversity.
Work at the University of Sheffield has recently shown that building at the kinds of densities required by the UK Government will likely reduce the populations of even those birds that are well adapted to city living.
Finally, green spaces perform important functions such as storing carbon in the vegetation, helping to reduce high temperatures caused by the sheer expanse of asphalt and lack of shade, and helping to prevent floods by soaking up storm-water.
Although these kinds of benefits may be less tangible to us, they all contribute to making city living that little bit more bearable.
So, it's time to rationally debate these issues, and this is an issue that affects at least the nine out of 10 of us that live in cities. It is vitally important that we go into this new, high density era with our eyes open to the potential consequences.
Yes it has clear benefits as we build assertive cities for the 21st century, but by also making them compact cities, we must recognise the risk of isolating ourselves and our children still further from an experience of nature, as well as causing biodiversity around the places where we live to decline precipitously.
The sprawling version of surburbia often supports a great variety of plants and animals. On top of this, there is a real danger that the quality of life of us all will suffer.
So when the next new housing development goes up near you, think about the multitude of ways that this is gradually changing our cities.
Dr Richard Fuller is a post-doctoral researcher at the University of Sheffield, examining the ecological sustainability of urban spaces
If you want to find out more about urban ecology, visit BBC Radio 4's The Material World programme website
The Green Room is a series of opinion pieces on environmental topics running weekly on the BBC News website