If we are an already overpopulated world heading for a major ecological collapse, the question has to be asked: Is having a large family environmentally irresponsible?

With the world's population hitting seven billion, campaigners are calling for open discussion about the number of children people have.

They include Sir David Attenborough, who earlier this year in a speech at London's Royal Society of the Arts urged the British to break the "absurd taboo" of the "strange silence" on population issues, while attacking the Catholic doctrine on contraception as being the major cause of the population growth problem.

"I meet no one who privately disagrees that population growth is a problem. No one - except flat-earthers - can deny that the planet is finite," he said in March.

"It remains an obvious and brutal fact that on a finite planet human population will quite definitely stop at some point. And that can only happen in one of two ways.

"It can happen sooner, by fewer human births - in a word by contraception.

"The alternative is an increased death rate - the way which all other creatures must suffer, through famine or disease or predation.

"That translated into human terms means famine or disease or war - over oil or water or food or minerals or grazing rights or just living space. There is, alas, no third alternative of indefinite growth."

UniSA International Grad- uate School of Business academic Dr Don Clifton agrees that there is a need for public discussion on the issue of overpopulation.

"We won't have those conversations publicly or face it," he said.

"We need to start talking about that openly and not just live under the assumption that it will just be okay."

Sir David was not trying to take away people's right to have as many children as they liked, but was calling for a cultural shift so they understood implications of having large families.

That view is shared by Dr Clifton, who said the expectation that people should have large families was a "socially created value set".

"There is nothing wrong with saying small families are good," he said, adding that women in particular were treated as though there was something wrong with them if they chose to have only one child, or indeed, remain childless.

He noted the world's population was expected to hit nine billion by 2050, but that the Earth's near seven billion inhabitants were already using three planets' worth of natural resources.

"We are in the middle of the sixth great planetary extinction - killing off other species at an alarming rate - and humans are the cause of this one," he said. "Are we heading for a major ecological collapse? The answer is probably yes.

"We are digging into the capital base and running it down. One day we are going to run out of capital ... it's a future generation problem that we are burdening them with. In the rich countries everything we need we can get our hands on without a problem."

Rich countries, including Australia, were taking resources from the poor, with an alarming number of the world's population not only living in, but dying because of, poverty, he said.

"If, for example, we were to look at the koala population on Kangaroo Island we look at consequences of population explosion and ask `What are we going to do about it?'," he said. "When we look at humans we don't look at it the same way ... we think we are immune to that for a bizarre reason."

While Dr Clifton was not advocating that people abstain from having larger families, he said a balance of big and small would increase the likelihood of a stable population, allowing us to "maintain ourselves as a society in the long term".

"Free choice has to come with a responsibility tag attached," he said.

However, demographer Bernard Salt said Australians should not be encouraged to have smaller families because the country was "already below replacement" when it came to population growth.

"The fertility rate is 1.9 meaning the average woman has 1.9 babies when the replacement rate is 2.1," he said.

"We are at sustainable levels now ... there is no need to reduce the birth rate."

Migration policy was where the attention should be focused, he said. While her predecessor Kevin Rudd had a "big Australia" world view, Prime Minister Julia Gillard has been vocal about her focus on sustainability "that preserves our quality of life and respects our environment".

Flinders University Associate Professor of Philosophy Ian Hunt also pointed out that smaller families would not prevent Australia from becoming overpopulated should the government increase immigration.

"Population depends on two things - sizes of families and level of immigration," he said. "A serious study needs to be done as to what sort of population can be supported in Australia.

"It would be unfair of us to look after ourselves at the expense of our children and grandchildren. We wouldn't have liked our ancestors to have looked after themselves at our expense."