Cities are living organisms. There is constant movement. They breathe, or choke. They take in energy, water, food, building materials, and excrete waste.
They have systems for reproduction, in the form of education and training, construction, planning and development.
But are they healthy or natural?
More than half the world's population now live in cities, and that's projected to rise to two thirds by 2050.
So can urban environments be eco-friendly and, if so, how do we make them that way?
It's a question urbanists, planners and architects are wrestling with.
Ludo Campbell Reid, the general manager of Auckland Council's design office, says while Auckland doesn't say it wants to be an eco-city, it wants to be one of the world's most livable cities.
"One of the tenets of being one of the greatest cities on earth will need to be our eco-credentials, so we have a whole range of initiatives from rebuilding our transport network through to zero-waste policies, reducing carbon emissions, investing in an electric fleet, encouraging walking, rebuilding the waterfront.
"Everything we do is about sustainability in its wider sense..." he says.
Campbell-Reid says the council has developed strong criteria around the performance of new buildings, and it is looking at converting road space into green space so the central city streets become about people rather than rush-hour traffic.
"Cities are competing to be the greenest city on earth. That goes back to a range of things including the creative industries. Creative, innovative, entrepreneurial people are drawn to different cities but they can work and live anywhere in the world. One of their major credentials is the green credential of that particular business, what it stands for and what its environmental rating is."
Campbell-Reid says New Zealand has the potential to become a world leader in these issues.
"The rebuild of Christchurch is an opportunity for it to really stamp its mark on its green credentials and eco-city credentials."
That's the perspective from Auckland, but on the ground in the shaken city Lincoln University urban design lecturer Andreas Wesener fears it's an opportunity going begging.
While there may be aspirations to include sustainability and ecological features in the redesign of the city, there is little to see on the ground.
"There are a few flagship projects, a few private developments, but none of these suggest Christchurch will go a different way than before the earthquake," he says.
Some of that may be caused by an insurance regime that insists it is paying to restore what was there before, not use the opportunity to make something better.
"There is also tension between two planning regimes. CERA (the Christchurch Earthquake Recovery Authority) has made its own rules. Christchurch City Council, the government people have voted for, is disempowered and left with only a few thing that it can do to suburbs - it can't do the city centre and the most prestige projects," Wesener says.
He says the greening of the city projects that emerged as people waited for the rebuilb to start were an interesting pointer.
"These were bottom-up projects as opposed to the CERA approach where no one knew what was going on, it was behind closed doors, there was no public participation and people got really angry because there was all this engagement at beginning, then it all shut off and [an attitude of] 'we are doing it how we want to do it'."
Wesener believes major issues like water management still need to be addressed.
"We have a project with students looking at places affected by water on both sides. The projected rise in sea levels of 1.2m to 3m over the next few decades is enormous for a city like Christchurch. A large part of the eastern side will regularly be under water. They are wicked problems with no easy solutions," he says.
What allows a city and its residents to breathe is a balance between residential density and green spaces.
As someone who grew up in apartments in Germany, Wesener sees higher density as an opportunity.
"If you have higher density, you need to come up with higher quality buildings," he says.
New Zealand has a tendency towards suburban sprawl, with a lot of land that would be valuable for other purposes like growing food or recreation being covered with buildings.
Allied with that is urban transport, which is currently dominated by the car. That can change, as can be seen in cities like Amsterdam and Berlin where fewer than 40 per cent of people now rely on their cars.
Since Christchurch's roads need to be rebuilt anyway, it should be easy to add in cycle ways and promote more active modes of transport.
Wesener says New Zealand has a qualitative problem with its planning system, which was developed from a rural rather than an urban perspective. "The Resource Management Act looks at the environmental impact of things but not necessarily at the urban qualities of a development, or the social qualities."
He sees room for more urban agriculture projects as part of the literal greening of cities.
"The interesting thing in the last 20 years in the urban environment is they have proved really beneficial - not just for a green perspective for food resilience - but also a social perspective because these are places people met, social capital is constructed there, there is a lot of research showing it has been beneficial to communities.
"People who get involved with community gardens have healthier lifestyles, less obesity problems, they are socially active.
"If you have high density, these spaces are important."
A positive note on the horizon is national science challenge 11, to build better homes, towns and cities.
Wesener's department is developing proposals so it can bid for some of the research money available.
Back in Auckland, the council's chief sustainability officer John Mauro says the city benefits from the sheer luck of geography with its beaches and bush and natural surroundings.
But its popularity is a challenge, as growing numbers of inhabitants strain its systems.
"I am optimistic if I look at who is coming to Auckland and who is born here. There is a powerful dual wave of young people and their requests for a liveable city, as well as older people who are thinking less of moving to lifestyle blocks outside the city and more about a city being a place to walk and be among people like themselves.
"If the city is going to be a desirable place to live, we need to not only have housing that's affordable and easy transport connections, we need to have green space.
"The research says that's what makes people happy and that's what gives ecological value in a city."
The highest population density in the city is along the ridge of Hobson and Nelson Sts, but it's not a welcoming area to walk or drive.
Mauro says instead of treating the roads as high volume, high speed arterials, there is a case for slowing it down, maybe making the roads two-way again, widening footpaths, activating the streetscape with cafes and shopping so it becomes a liveable front yard for the people there.
"We have the tools but lack the courage and vision to take bold steps. Other cities are saying they are willing to try things that are different and radical."
Mauro recently moved from Seattle, where cycling is promoted as a transport strategy. He's an advocate of cycle lanes, which he says create more jobs than car infrastructure.
"People on bikes or on foot can linger by shops longer, so if you are worried about retail, put a bike lane in front of your business."
Change can be made. He says Copenhagen used to be worse than Auckland for car dependence, but over decades it has made small investments each year to become friendlier to cyclists.
"Over the next three years we will spend $123 million on walking and cycling in Auckland."
Mauro says Auckland Council and its subsidiaries can lead by example, as is showing with Auckland Transport's pivot towards public transport and cycling.
There is also procurement, with a big investment in LED street lights that will not only save an estimated $36 million in power and parts over the next 20 years, but should be a large enough purchase to bring prices down for other firms.
"If we don't act and look at smart green clean tech and great ideas, we will be left behind. There is an opportunity for Auckland to diversify its economy and become a place where people want to live."
By definition: eco city
"An 'eco city' is a human settlement modelled on the self-sustaining resilient structure and function of natural ecosystems. The ecocity provides healthy abundance to its inhabitants without consuming more (renewable) resources than it produces, without producing more waste than it can assimilate, and without being toxic to itself or neighbouring ecosystems. Its inhabitants' ecological impact reflects planetary supportive lifestyles; its social order reflects fundamental principles of fairness, justice and reasonable equity."
That's a working definition of an eco city adopted in 2010 by the International Ecocity Standards advisory team and California-based Ecocity Builders, which has been working for more than 20 years to reshape cities for the long-term health of human and natural systems.
The non-profit organisation believes global climate destabilisation and the endangerment of entire ecosystems demand a restructuring of cities, towns, and villages worldwide for long-term energy efficiency and conservation.
While no ecologically healthy city exists exists now, there are hints emerging in today's solar, wind and recycling technologies, in green buildings and green businesses, in urban environmental restoration projects, urban gardening and organic farming, and in individuals using foot, bicycle and public modes of transportation in preference to the automobile.