This is what Agent Smith, the menacing face of the machine overlords in the film The Matrix, has to say about humanity as he interrogates Morpheus, one of its putative saviours: "Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment. But you humans do not.

"You move to an area and you multiply and multiply until every natural resource is consumed and the only way you can survive is to spread to another area."

Smith then tells Morpheus there's another organism that follows the same pattern. "A virus," he spits. "Human beings are a disease, a cancer of this planet." Harsh.

While humans have certainly left wonderful legacies on the planet, there's no question our needs and desires have also been harmful.

Our remedies involve changing such behaviours as driving gas-guzzlers or using harmful chemicals.

But tucked away at the end of Alan Weisman's The World Without Us is his call to think again about changing something far more fundamental: our numbers.

The world's population is expanding by the size of Toronto every eight days. So Weisman humbly suggests we "limit every human female on Earth capable of bearing children to one."

"I'm not trying to be sensationalistic or controversial," he says in an interview. "I'm trying to get us to think very hard about what the whole situation is.

"If we don't control ourselves, nature will do it for us. Every species that eats itself out of house and home experiences a population crash."

The startling conclusion reintroduces a controversial concept to a public discourse from which it has been lost for more than a decade.

According to Nigel Roulet, director of the McGill School of Environment and a contributing author to the Intergovernmental Panel on Climate Change, the idea was pushed out of mind by the West's declining birthrates and the admonishments of various religions and world governments.

People who study the environment, however, have never forgotten about it. "I think most people that think seriously about the environment and work on issues with the environment would argue that one of the most critical factors driving environmental degradation is overpopulation," Roulet says.

Arguing to reduce population creates a "visceral reaction" in people, Roulet reasons, "because it requires a reflection on ourselves."

So instead, "We think of carbon-dioxide emissions as the problem of climate change, but really it's the number of people whose lifestyles require the level of energy consumption and production that is 95 per cent based on fossil fuels."

Overpopulation, Roulet argues, can't be separated from the notion of lifestyle. "Even if population growth was zero, do we have the resources to sustain the 6.5 billion? I don't think we do with everyone having the same social well-being as we have now."