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In which Shane argues with Marxists

October 16, 2012

I’ve gone and done it, haven’t I?

I’ve gone and gotten myself into an argument with a Marxist about population ecology.

Actually, you know what? It’s worse than that. I’ve gotten myself into an argument with three Marxists, because Marxists clump together and all want to defend Marxism. Egads!

Now, I don’t know enough about Marxism to tell you what Marx said on the topic of population growth and regulation. What I do know about is population ecology and evolutionary biology, and what the limits are to the growth of populations. And the Marxists chose to argue with me about that. So, let’s have some science.

The Reverend Thomas Malthus is, unsurprisingly, the chap who wrote down the Malthusian Dilemma. At its bare bones, the Malthusian Dilemma is this:

  1. Animals (including humans) are capable of reproducing geometrically. Any human could, if they weren’t worried about resources, have a dozen children pretty easily. And if those children were not worried about resources, they could each have a dozen children, and each of those grandchildren could have a dozen children…
  2. When they have surplus resources, animals (including humans) tend to have more offspring than when resources are limited.
  3. Animals (including humans) need food. Food production is finite, and doesn’t grow anywhere near as quickly as the human population can – and for most animal species, food production doesn’t grow at all.
  4. If 1, 2, and 3 are true, then population size is resource-limited. If such a population ever gets to a point where it is not resource-limited, it will quickly make itself resource-limited by reproduction.

Of course, since he was the Reverend Thomas Malthus, and was writing all of this in 1798, he bundled it up with some stuff about ‘vice’ and its control, in the grand theme of ‘the perfectibility of Man’. We population ecologists aren’t so worried about ‘vice’ in our study systems – If an Australasian gannet is going to engage in some ‘vice’, that’s sweet by us – so we kept the stuff about populations and dropped the stuff on ‘vice’.

After Malthus published, population biologists have continued to develop his ideas. And by and large, the ideas in the Malthusian Dilemma match up with reality. I’m yet to hear of a species that can’t reproduce geometrically (or pretty close to geometrically) in ideal conditions. Every species does need food (or some sort of resource) and damn near every species has more offspring when there are more resources. There are some pretty cool exceptions – if you’re a Snowshoe Hare, for instance, then it turns out that Canadian Lynx (which eat you, and nearly nothing else) have much more influence over your population density than the amount of grass. And sometimes the risk of disease (which increases when there are more people around for you to catch diseases off) acts like the lynx, and slows population growth before food limitation does. But resource limitation explains a really solid proportion of what we see in population dynamics. I explained all of these things to my Marxists as best I could.

But this was not enough! Malthus couldn’t be right, one Marxist argued – if his maths were sound, we’d have hit our Food Apocalypse ages ago!

I think this is what he meant

Now, this confused me. Malthus didn’t predict a food apocalypse – he predicted that there would be initial rapid population growth, but then the lack of resources would slow down population growth to nearly match the rate of increase in food production. I guess you could, in principle, generate an ‘apocalypse’ scenario if you assumed that society divided its food exactly equally, and everyone needed exactly the same amount and used it at the same rate, so everyone ran past the critical limit of ‘too little food’ at the exact same moment and dropped dead all at once – but Malthus didn’t think that. He was perfectly aware that there is an ‘upper class’ who aren’t ever in any serious danger of running out of food. He figured it would look more like this:

…and that’s what we see every time we drop an organism into a situation where resources are abundant relative to the starting population size: the population increases rapidly until it gets near the maximum supportable population, where the food requirements of the organisms matches the amount of food being produced in the system. When it approaches that, population growth starts to level off. Life gets really crap for the individuals with fewer resources – there’s a serious risk of starving to death, and some do, while others just don’t have offspring because they know they can’t support them. This keeps the population from rising above the sustainable level for very long, but if the population drops too far below the maximum sustainable level, there is surplus food and that pushes the population back up to the line.

And yet, still this was not enough for my Marxists! Human populations, they insisted, don’t grow geometrically! They shunt upwards in fits and starts, according to changes in production and the distribution of wealth in society!

And again, I was surprised. If you take a beaker with a sugar-eating yeast in it, and put 1 mL of sugar-water in per hour, the yeast population will grow until it’s consuming all that sugar every hour, but can’t grow past it – any extra baby yeasts would starve. If you increase the amount of sugar-water to 2 mL an hour, the yeast population will quickly double (as long as the beaker is big enough to support this much yeast) and then stabilise. And this is totally Malthusian. If a system is sitting close to its maximum supportable population – that is, its geometric growth phase is over and there is little free food in the system – then we expect population growth to very closely match any changes in food production.

Soft and tender baby yeasts, soon to be stripped of their innocence by the unspeakable cruelty of Nature!

But these are yeasts. Do we see the same patterns in humans? Well, yes. Where we should expect to see geometric growth is in newly-colonised areas with few people and abundant natural resources. New Zealand from ~1000AD is a great example. In that case, a very small founding population arrived, and quickly expanded in size. By about 1500AD, there were humans spread over most of the habitable land in New Zealand and the archaeological record starts to show evidence of resource competition: the largest, tastiest bird species were all hunted to extinction. Defensive earthworks started being constructed, used to keep people and food stores safe from raiders. This doesn’t prove Malthus right, of course (in science we never prove any theory right, we only fail to prove it wrong), but it certainly doesn’t offer any evidence against him!

Of course, this was not yet enough for my Marxists. How come, they argued, first world nations are increasing in population size so slowly? Surely it cannot be from lack of food!

And this looks like a very fair point. There are more available food resources in the first world, by a substantial amount. Why don’t people in the First World breed more?

The key part of the Malthusian Dilemma is that population growth is kept in check by resource availability. That can work directly, where individuals starve to death because there is not enough food for them. It can also work indirectly, where individuals don’t have offspring because their body condition is too poor to support a pregnancy or, in the case of humans, they don’t think they can afford to support offspring to adulthood. So one totally plausible explanation – that is totally compatible with Malthus – for the observation that fertility rates are lower in the first world than in the third world is that children in the first world cost more to raise than children in the third world. Another is that population is still kept in check by competition for resources, but in first world countries that competition is for some other critical resource that everybody needs, like housing – the maths is still exactly the same.

So food and housing are interchangeable in the Malthusian Dilemma, and what the dilemma predicts is that with people will limit their breeding based on their access to resources. Now, do we have any evidence that people in the first world limit the number of children they have because they can’t afford more children? I asked Google – it’s not a perfect data source, but it’s one that I had. I searched, in quote marks, the phrases “Can’t afford to have children”: 137,000 results. Then “Can’t afford to have any more children”: 16,000 results. The top stories here were about people getting vasectomies. Then “Can’t afford food”: 368,000 results. If people can’t afford food for themselves, it’s not a stretch to say they can’t feed themselves plus a child.

Malthus: still not disproven.

PS: I fear I paint rather a dim view of Marxists here, and I’d just like to say that hasn’t been my overall experience of them. Sorry Marxists! I think you’re mainly pleasant, interesting people – but I think that resource scarcity is an important factor controlling population size!

Once the details in this post were cleared up, one of the Marxists started a very pleasant conversation about the relative effect of resource scarcity and social structure on population – but those points weren’t misunderstandings of Malthus, nor proof against him. Malthus’ Dilemma still stands as explaining a huge amount of the observed variation in population sizes.


From → Science, Scientist

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