They want government action to limit families to just two children and propose a crackdown on immigration to slash Scotland's population by a quarter.
But radical green group the Optimum Population Trust - branded "eco-fascist" by opponents - has now won the backing of some MSPs and will today stage a major conference at the Scottish Parliament.
SNP MSP Dr Ian McKee and Green MSP Patrick Harvie have given their support to the Holyrood conference, in which OPT members will call on Alex Salmond to launch a government campaign to limit family size, with the slogan "two's plenty".
Prominent supporters of the OPT include its patron, BBC naturalist Sir David Attenborough and scientist Dr James Lovelock, whose "Gaia" hypothesis suggests that the Earth is a single organism and humanity is destined to be wiped out unless it reduces environmental damage.
Government green adviser Sir Jonathon Porritt, who also briefs Prince Charles on environmental issues, is another top OPT figure.
Dr McKee - whose wife, Penny, is a co-organiser of the Holyrood conference - says he agrees with the group's aim of reducing the global population and encouraging smaller families.
But he yesterday denied their policies were "eco-fascist", saying: "The world has finite resources and we can't go on growing at the current rate. This is a taboo subject for politicians but I think it is something the Scottish Parliament should be discussing."
The Scottish Government has a stated aim of increasing the Scottish population, linking it to economic growth.
However, the OPT believes Scotland and Britain must do their part to stabilise the world's population, which could rise to more than nine billion within 40 years. They want the UK population to fall to as low as 30 million, with Scotland playing its part in the reduction.
Dr McKee said: "I think the government could encourage families to have less children.
"There is a lot of societal pressure to have children.I think it seems very sensible to put forward the case that people should consider carefully limiting the size of their family."
On immigration, the OPT believes there should be a zero cap on net immigration, meaning numbers would be strictly limited to replace those leaving the country to live elsewhere.
They claim that the restriction would have no "racial or ethnic" component, after critics accused them of promoting policies similar to the British National Party.
Conference organiser Penny McKee said it was important to raise these issues, although she said she personally is not "anti-immigration".
The conference has also attracted support from senior Scottish academics.
Government adviser Professor Peter Gregory, chief executive of the Scottish Crop Research Institute, will give a speech raising issues about food supply. Family planning pioneer Dr Libby Wilson will also be contributing to the debate.
Prof Gregory said: "I am not a member of the OPT, and I'm not taking part to talk about population matters. I'm going to be raising points about food security, both globally and in Scotland. The fact is Scotland is currently unable to fully supply its food needs, and that is something we should be debating."
Leading environmentalist George Monbiot is among the OPT's fiercest critics, claiming that global population increase "pales into insignificance when compared with the effect of increased consumption".
He said: "Many of the world's people use so little that they wouldn't figure in this equation. They are the ones who have most children. Around one-sixth of the world's population is so poor that it produces no significant emissions at all."
A recent paper by the OPT claimed that if couples had two children instead of three they could cut their family's carbon dioxide output by the equivalent of 620 return flights a year between London and New York.