Conserving biodiversity means limiting human numbers

A recent letter in Biological Conservation criticized TOP researchers for writing about how more people means less space for wildlife. We responded that without limiting our numbers, people will not be able to protect the natural world. Who’s right? Please share your thoughts and ideas in the on-line comments below.

by The Overpopulation Project

This summer, TOP researchers published an article titled “Overpopulation is a major cause of biodiversity loss and smaller human populations are necessary to preserve what is left” in the journal Biological Conservation. Eight researchers responded with a letter critical of our paper, accusing us of supporting colonialism, eugenics, and exterminating people. We reprinted that letter in our previous blog, along with readers’ responses to it, for which we are grateful.

Below we reprint our published response to the letter. You can see that unlike our detractors, we avoided personal criticisms. But in this less formal venue, we raise the question of whether the authors of that letter have a commitment to preserving biodiversity strong enough to meet the threats to it. We don’t think they would have advanced such obviously weak arguments if the issue was protecting something they really cared about. Once again, we invite your comments!


Conserving biodiversity means limiting our numbers: A response to Green et al. 2022

Green et al. 2022 invite readers to a fruitless discussion of whether overconsumption or overpopulation is driving global biodiversity loss. According to the 2019 Intergovernmental Panel on Biodiversity and Ecosystem Services Global Assessment, both are important. As summarized by Diaz et al. (2019), “the human impact on life on Earth has increased sharply since the 1970s, driven by the demands of a growing population with rising average per capita income.” “The world is increasingly managed to accelerate the flow of material contributions from nature to keep up with rising demand,” they write, as growing human economies displace wild nature. A similar analysis holds regarding climate change. IPCC’s 6th Assessment Report (2022) states, “globally, GDP per capita and population growth remained the strongest drivers of CO2 emissions from fossil fuel combustion in the last decade.”

Our paper (Cafaro et al., 2022) cites dozens of peer-reviewed research articles to support our contention that overpopulation is a major cause of biodiversity loss (not the cause, as Green et al. 2022 erroneously states). We document how population increase accelerates direct drivers of biodiversity loss, including habitat conversion, pollution, and overfishing. We provide examples where population decrease has facilitated successful ecological restoration projects.

Green et al. 2022 ignore all this evidence, instead arguing their case using an obviously flawed “simple arithmetic example.” Equating a population of 10 million consuming 1 unit/person with a population of 5 million consuming 2 units/person, they assert that if both populations added 5 million people, the larger one would add less total consumption. They say this shows “consumption rate matters more than human population numbers.” But if both populations increased per capita consumption by 1 unit/person, the larger one would increase its total consumption more, “showing” population’s greater importance. Using similar reasoning, one could prove the length of a rectangle is more important than its width in determining its area, or vice versa.

A simple model indicates how average consumption per capita (C) and population size (P) generally have similar multiplicative effects for total resource use (T). T can be specified by the eq. T = C × P, so that doubling either C or P will double resource use. For any particular resource use or environmental impact, this model is an oversimplification, but it is true at a rough level. Our paper invites researchers to explore more detailed models. Both C and P are likely to remain important in the more complex and accurate models we hope conservation biologists develop to explain biodiversity loss, just as both factor into the Kaya Identity atmospheric scientists use to explain changes in CO2 emissions.

The evidence we summarize in our paper suggests a world of 8 billion or more people is a world committed to crowding out the rest of life and extinguishing a large fraction of Earth’s species. For that reason, we call on conservation biologists to advocate for small family norms and universal access to modern, affordable contraception. Not eugenics. Not violence. Not colonialism, or neocolonialism. We do not advocate targeting specific groups, much less displacing or exterminating them.

In our paper, we explicitly call for smaller populations throughout the world, in both developed and developing countries, and smaller families among the wealthy as well as the poor. Coercion is unacceptable, whether forcing women to have fewer children, as in China and India during the 1970s, or forcing them to have more, as in the Philippines and the United States today. It’s important to recognize that voluntary family planning programs improved women’s lives, reduced hunger, and lifted hundreds of millions out of poverty during the past half-century, in many regions of the world (Engelman and Johnson, 2019). Unfortunately, some who see themselves as defenders of the poor and marginalised don’t know this history and misguidedly oppose such programs, inadvertently harming those they want to help.

We believe adult human beings have a right to freely and responsibly choose their family size (a right that relies on the availability of modern contraception). We also believe people have a right to adequate food, water, and shelter; and that other species have a right against untimely anthropogenic extinction (Cafaro, 2015). The evidence suggests all these rights cannot be accommodated simultaneously in a world of 8 to 12 billion people.

To have any hope of raising all people out of poverty while also preserving nature’s diversity, we must reduce our numbers. More broadly, we must replace our current economies, designed to provide ever more stuff for ever more people, with sustainable economies designed to provide a sufficiency for a limited number of people. Conservation biologists should join the environmental vanguard in advocating for such radical change, since the economic status quo is incompatible with preserving Earth’s remaining biodiversity.

Unlike Green et al. 2022, we trust conservation biologists and our fellow citizens to discuss these matters. Any people seeking to create a just and sustainable society should choose population policies that further the common good, rather than leaving those policies up to religious zealots, corporate lobbyists, or chance. We hope that going forward, nations around the world define the common good to include flourishing populations of other species, in addition to humans. Without strong, explicit commitments to limit human numbers and economic demands and share Earth more generously with other species, many of them are doomed to extinction.



Cafaro, P., 2015. Three ways to think about the sixth mass extinction. Biol. Conserv. 192, 387–393.

Cafaro, P., Hansson, P., Gotmark, F., 2022. Overpopulation is a major cause of biodiversity loss and smaller human populations are necessary to preserve what is left. Biol. Conserv. 272, 109646.

Diaz, S., et al., 2019. Pervasive human-driven decline of life on earth points to the need for transformative change. Science 366, 1327.

Engelman, R., Johnson, D., 2019. Removing Barriers to Family Planning, Empowering Sustainable Environmental Conservation: A Background Paper and Call for Action. Margaret Pyke Trust, London.

IPCC, 2022. Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.

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20 thoughts on “Conserving biodiversity means limiting human numbers

  1. Last night I read Putin’s Trolls, a book by Finnish journalist Jessikka Aro. Strongly makes that case that the game of science debates and politics is no longer unbiased or fair. The mindset that condemns efforts to reduce family size norms owes something to science denial and denigration of environmentalists by fossil fuel interests and other self-interested parties. And the Catholic Church’s medieval teachings on sex and reproduction. Much of the developed world has prematurely shifted policy aims towards raising low fertility rates. Actually, it is very good news that couples voluntarily have fewer than two children in Europe, Japan, America, 90+ countries in all, about half of world population. People who say “it’s consumption in rich countries that wipes out biodiversity” ignore forest clearing, bushmeat consumption, and overfishing by the world’s poor, high fertility countries. And they should, to be logical, if they think humans are harming biodiversity, call for lower fertility in rich countries. Their position also requires that the increasingly numerous poor much stay poor. That is not happening. The poor want to get richer. Need to get richer. So they also need to get fewer which has been a key element of successful economic development strategies everywhere. Cutting fertility rates is associated with rising incomes. People in the lowest fertility countries (TFR <1.3 have incomes 12 times higher than people in the highest fertility countries (TFR?4). And no hungry kids in low fertility countries.

    1. I agree but environmentalists have denigrated themselves, often, by refusing to set an example. I still remember the Al Gore movie in which he talked about climate change while driving around in the biggest car I had ever seen. Leonardo Di Caprio and yachts+caring for the Earth. Many climate and other researchers flying around the world to make their point that polluting is bad.
      It’s been very easy for the right-wing media to point at all this and discredit the whole movement.
      I think it’s time for environmentalists to start talking about resource redistribution. That’s the next big taboo to break.

    2. This misses the important point (made for the nth time by Cafaro et al in the opening sentence of their superb response to Green et all) that ALL must keep consumption/production levels extremely low AS WELL AS reduce numbers. It shows the usual fixation with a better standard of living – which often correlates with a lower standard of community cohesion, sadly. Yet community cohesion (“think globally, act locally”) is surely key to “saving the planet”?
      It also misses the statistical fact that it is Catholic and Orthodox nations (outside of South/Central America and the Philippines) that have the lowest birth-rates. Italy led the way, back in the 1970s. But other Catholic/Orthodox nations were not far behind. Some were overall poor like Romania and Russia. Some had pockets of poverty like Italy and Portugal. Whatever the drivers, the fact remains that there they are, clustered at the bottom of the TFI tables where they have been for years.
      By far the best contraceptive tool is a calendar – whether you are married or not, but monogamy and fidelity are also good aids. Along with the calendar, Catholics have C.I. which is permitted. Also, when I was a teenager I was an exchange student in an Austrian Catholic family who openly discussed whether the Pope would allow “gummisache” (I am afraid I did not have the opportunity to display this colourful vocabulary in my German exam). I could have told them the Pope does not – but anyway, a lot of Catholics do use them, especially now that you can get vegan, biodegradable, “fairtrade rubber” (?), etc.
      In my opinion, the best contraceptive of all is Fear of the Lord, just as it is the path to the getting of Wisdom. Clearly this is not a popular opinion – however it is borne out by the statistics from Catholic and Orthodox nations. Of course some Christian families rejoice in having many children – IF they can afford it. But they are a tiny minority, especially these days. The one thing Christians will do, is make sure they do not live beyond their means as individuals. I know many non-Christians do this too – but all the same, it helps to have it as your creed.

  2. I was recently called a “climate fascist” for suggesting that people need to control their breeding as the fundamental ‘best approach’ to climate impact mitigation. The unfortunate reality is that human beings cling to the beliefs that best align with their interests. In other words, people hell-bent on unbridled procreation think that people who practice a zero procreation lifestyle in consideration of future generations are somehow tools of political interest groups.

    The are incapable of understanding the basic mathematics of more people = less time for all of humanity.

    1. But I have to believe that basic math, clear reasoning and good examples can sometimes move people to face reality. We all have our ideological hopes and prejudices. But sometimes, reality is strong enough to demand recognition.

      1. One would hope Phil. But it is my opinion that the current ratio of human beings capable of voluntarily modifying their procreative behaviours by logic and scientific observation is lower than 50 percent, and worse, the wealthiest (governing) members of society are below the threshhold.

        As the convergence of ecological system failures accelerates, only brute force mass population reduction by natural forces as the organism Earth marshals its immune system responses (COVID, etc) will achieve the required re-proportioning of human special representation, I fear.

    2. The word “fascist” should be retire as it’s come to mean anything and its opposite, and mainly: “I don’t like you”. Actual fascists were all about boosting the birth rate. They even had a tax on bachelors.

  3. “Coercion is unacceptable”, why? Since the agency of most humans is to propagate without due deliberation and currently, as you state, nations are encouraging greater nativity this is a fantasy policy not based on current reality (and nativity numbers). Without China’s one-child policy at least one hundred million more children would probably have been born (Feng, Gu & Cai, 2016). Looking at China’s meat consumption per capita, at around 28 kg/year, that equals another 2,8 million tonnes of meat, further than what is now consumed (Li & Shangguan, 2012).

    If you not only consider anthropocentric ideals (such as rights of man) but a naturalist one you will find that unlimited propagation is not valid from a naturalistic imperative standpoint. It might be seen as valid based on the rights of man, not on the rights of the planet (therefore coercion must be viewed as anthropocentric and nothing else). One right, such as functional biodiversity in ecosystems, should be at least equal in justification for policies if not surpassing, based on the current situation which always justifies the anthropocentric ideal of destroying habitats based on the rights of man (as used as moral standpoint for unfettered capitalism).

    At what point does human propagation equal ecocide just from the viewpoint of consumption per capita? From a moral standpoint (i.e. naturalistic imperative, utilitarianism, virtue ethics) coercion is quite possible on the grounds of protection of rights to our own individual habitat (based on your used moral viewpoint of rights of man) as well as others whose unheard voices we must protect (since we have chosen to rule and not coexist).

    At what point is anthropocide viable as a point of self-defense for the planet (at some cutoff it should be, CO2 at 460 ppm or maybe 20% Phytoplankton left of 1950-levels)? Self-defense is constituted in the U.N. charter (such as article 51), why not encompass it to the whole world and enforce it based on a naturalistic imperative (i.e. the right of a being to exist without undue interference, such as Phytoplankton)?

    Feng, W., Gu, B., & Cai, Y. (2016). The end of China’s one‐child policy. Studies in family planning, 47(1), 83-86.
    Li, J. P., & Shangguan, Z. P. (2012). Food consumption patterns and per-capita calorie intake of China in the past three decades. J Food Agric Environ, 10, 201-206.

    1. Axel, I think those are entirely reasonable points to raise.

      The argument against coercion in family planning is a human rights argument. We condemn coercion in the name of human freedom and respect for human beings. But excessive human numbers can also lead to coercion and truncated human possibilities. At what point do the many costs, to people and other species, of excessive human numbers outweight our desire to maximize human reproductive freedom?

      These are the kinds of questions many proponents of improved family planning hate to see people raise, since discussing them provides fodder for our opponents. But we have created a world of 8 billion mass consumers and would-be mass consumers, and it doesn’t look like there is room on Earth that many of us. That leaves the way forward toward just and sustainable societies unclear.

      1. I kind of feel the same way. We do use coercion in other circumstances (I cannot get into your house and take what I like, or I will be arrested), why not this? To me, the main arguments *against* coercion are 1 that we have better options that could guarantee the same outcome and cause much less controversy, while also having side benefits 2 that, even though it’s obvious that crowding the Earth with humans destroys it for everyone, proving a direct link between each birth and this general effect would consume a lot of energies that could be better spent arguing for non-coercive measures
        In general, I believe in incentives and disincentives rather than outright forcing and prohibiting. We are not all the same; if one person wants to have the child I don’t want to have or take the one flight I was allotted but won’t board, I can in return get one extra dress or horse or whatever. Taxes and redistribution can guarantee this outcome better than: no, you cannot do that!

  4. With their Sustainable Development Goals (SDGs), already the UN had invented a superb vehicle, to drain global attention away from the obvious connection between the 8b humans, and the unrelenting habitat destruction and biodiversity crashes.

    But UN Net Zero Emissions takes the game to another level, with its alluring arithmetic of carbon sinks magically “netting out” human emissions – later on. A generation has gone down this rabbit hole. Why worry about the sixth wave of extinction, when you could be doing so many exciting manipulations and calculations, to tweak Australia to “43% reduction by 2030”?

  5. A very well-argued piece, Prof Cafaro. Regards the coercion debate: perhaps a better term is encourage. For example, we might provide child grants for the first two children but not for any more. People are still free to have more but the state will not fund them.

  6. OK. You failed to post my civil, and academically referenced comment. The highly learned network I am part of will now hear about this. So will public, social media. If you’d care to explain yourself before I do this tomorrow (10/5) email or call 207-239-8465

    Steve Kurtz
    Amherst MA

  7. I was contacted by Pernilla, and this has been resolved. Seems an automated spam filter stopped it, and nobody checked it even after my reminder post. It has now been posted as of Sept. 28, the original date. I’m pleased that this wasn’t a ‘cancelling’ event, as I’ve been an OP activist and educator foe 3 decades.

  8. Unfortunately, any subject science based or not are increasingly being seen on an emotional level “…accusing us of supporting colonialism, eugenics, and exterminating people. ” At the turn of the century scientists studying genetics discovered there is no such thing as race. “In a landmark paper based on the Human Genome Project, scientists showed that there are no “races” but a single human race—not in sociological terms, but according to biology.” Race is not real but racism is. However, in light of reality, it is not really about race, per se, but tribalism. Tribalism exists in all species and is a major force in nature. Tribalism is not about discrimination but competition for resources and, as such, is an environmental issue. A recent report on NPR discussed where growth and even, (yes) overpopulation has gotten the people of Florida. Their inability to recognize and properly manage growth management has led to this critical time of death and destruction in that state.

    I live in an island archipelago (some 150 named islands 4 of which are served by ferries) in an area known as the Salish Sea A group of us are battling the county over growth management and are frustrated by the same thinking that the counties in Florida went through. Despite a constant reminder of where overpopulation is affecting this planet we still think technology will save us and we can just keep going down the road we have been headed.An interesting TED talk about Urban Planning had a quick demonstration on the population/technology idea. Go to 13:30

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