Biodiversity and the limits to growth

pleistocene.JPGWe hear frequently in the news these days that earth is in the midst of a mass extinction. To many people this is difficult to believe, thanks in part to the vigorous efforts at obfuscation by the likes of Bjorn Lomborg and conservative think-tank hitmen on their days off from distributing climate change misinformation.

The evidence for ongoing mass extinction is admittedly indirect (like the uncontroversial evidence that the world was round prior to 1969, when astronauts documented it photographically). But the evidence of the first wave of extinctions caused by humans is better documented from fossil data.

A new paper by Anthony Barnosky in PNAS reexamines this fossil data, in the light of some basic principles of ecology, and comes to some sobering conclusions about our place in the world, and who we will be capable of sharing it with in the future. Basically, he starts from the well-established premise that the biomass of all life on earth is ultimately limited by incoming solar energy, and then examines how that energy has been divvied up among the larger animals during the last few hundred thousand years.

barnosky2.jpgToward the end of the last ice age, between say 50,000 and 10,000 years ago, humans got restless and spread out over the wide world, along with rapid increases in hunting efficiency and other environmental impacts caused by deliberate fire setting. At around the same time, on every continent except Africa (where, not coincidentally, wild animals had evolved alongside humans for millions of years and presumably developed a healthy wariness), most of the world’s large vertebrates disappeared within a few thousand years. This general picture has been known for some time. The new twist is that the collective biomass of all these lost giants was essentially replaced with an equivalent biomass of people (see lower graph). In other words, we co-opted the share of the earth’s resources that formerly supported those creatures and basically substituted ourselves — and our domesticated livestock — for most other large animals in earth’s ecosystems.

But then our ingenuity allowed us to escape — temporarily — the limitations of incoming solar energy. Beginning with the industrial revolution, appropriation of fossil fuels began to subsidize exponential human population growth that has now reached far above what the earth can support once fossil fuels run out. As this energy supply dwindles, the human population will likely commandeer resources currently used by the remaining smaller animals, with sobering consequences for biodiversity.

barnosky.jpgThe punch line from the paper is that the rise of Homo sapiens starting in the late Pleistocene initiated a sudden and irreversible “regime shift” in the planetary ecosystem, that is a shift between two quite different ecosystem states, from one that supported a diverse array of large , relatively specialized animals (elephants and their relatives; grazing ungulates such as horses, camels, and their kin; big predatory cats and wolves; giant ground sloths, etc.) to one in which virtually the entire upper end of the global ecosystem’s biomass spectrum is made up by a single, hyper-generalized species: us.

A few interesting factoids:

1) Many of the huge mammal species of the Pleistocene weathered (literally) hundreds of thousands of years of climate change before going down the tube suddenly between 40,000 (in Australia) and about 10,000 years ago (in North America). During that long span of time, global climate and vegetation see-sawed several times between balmy and very cool conditions. This suggests that climate change alone could not have been responsible for the mass extinctions at the end of the Pleistocene. Suspiciously, the extinctions on each continent occurred very shortly, usually within a few thousand years, after Homo sapiens arrived on each continent. Yet these humans were sparsely populated by today’s standards and only had stone tools and fire. Makes you think.

2) Some megafauna, including mastodons, survived into the Holocene (i.e., modern, post-ice-age times) on isolated islands without humans until surprisingly recently. For example, on Wrangel Island in the Siberian Arctic, dwarf mammoth fossils have been dated as recently as 4000 years ago. That may sound like a long time ago, but consider this: The oldest pyramids in Egypt are dated at ~2600 BC, that is, 4600 years ago. In other words, ice age mammoths were still walking the earth by the time the great classical civilizations began to flourish.

Although the role of humans in megafaunal extinctions is already widely known in general outline, the approach from ecological energetics used in this paper highlights the fundamental physical constraints we face in attempting to conserve some semblance of wild nature. There is only so much to go around. Energetic considerations suggest it will be very difficult to maintain populations of other large vertebrates as long as we are using the lion’s share (so to speak) of the planet’s available energy. All of which reemphasizes the necessity to rethink Western society’s quasi-religious, ultimately destructive, cult of economic growth.

[Original source: Barnosky, A.D. 2008. Megafauna biomass tradeoff as a driver of Quaternary and future extinctions. Proceedings of the National Academy of Sciences of the USA 105, Supplement 1:11543-11548.]

About Emmett Duffy

I am a Natural Patriot and an ecologist with expertise in biodiversity and its importance to human society. My day job is Professor of Marine Science at the College of William and Mary in Virginia.
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7 Responses to Biodiversity and the limits to growth

  1. John Feeney says:

    A very important paper. Reading through it I’m left with two thoughts.

    First, it supports my suspicion that we’ve never so much increased carrying capacity for humans as learned, temporarily, how to overshoot it.

    Second, yes, it sure does make you think to consider this level of human-caused extinction taking place at the population levels we had that long before agriculture, much less the industrial revolution.

    My only complaint about the article is that, in addition to need for alternative energy sources, I’d like to see it take a bit less fatalistic stance toward future population growth and mention the need to slow and ultimately reverse that growth.

    Jeffrey McKee’s (and colleagues’) study (pdf) and his book, Sparing Nature, which elaborates on it, goes along well with this article!

  2. Emmett Duffy says:

    John: All good points. I am very interested to check out McKee’s article and bbok. Thanks for the heads-up.

  3. Lars Gamfeldt says:

    Hi Emmett,

    Thanks for highlighting this important paper on the Faculty of 1000. I would probably have overlooked it otherwise.

    I agree with John Feeney that one of the most daunting tasks of the future will be to try to limit and even reverse human population growth.

    But who am I to speak – I will hopefully get my second child next year, and my family will contribute to the overexplotation of our planet :(

    Keep up the good work with this blog, I thoroughly enjoy reading it.


  4. Emmett Duffy says:

    Thanks for your support Lars. And congrats on the new member of the family.

    Yes, John is right on the money that human population growth (and, of course, per capita resource use) is the elephant in the room for virtually all of the major problems we face as a global society. The challenge is that solving that problem goes against not only our most fundamental evolved instincts but also the basic tenets of western democracy.

    We need an effort on the level of the Manhattan Project to address realistic and humane approaches to getting human population under control.

  5. Very well researched and thick in information enough to open-up the minds of those who made this world in chaos. It’s us, humans, who little-by-little kill ourselves by the way we treat nature and ourselves in totality. Still a long way to go to put this “abuse” to a halt.

  6. Dave says:

    My thoughts:
    You guys are so self-infatuated that you can’t see past your own perceived intellects. It’s actually rather amusing to listen to a group of the ‘enlightened’ attempt to outdo one another with their sophisticated angles.

    You people actually believe that we’re so significant that we’ve done all this damage? Check the degree of damage that a volcanic eruption does to the atmosphere – we’ve survived a pretty long time despite these disasters. Did you know that if every car on the planet were left idling for a month that they wouldn’t even do as much damage to the atmosphere as a volcanic eruption? Do some scientific research instead of being so self-possessed. And Lars – if you were true to your statist beliefs, shouldn’t you have your wife agree to an abortion? Gotta look out for the planet, after all.

  7. Emmett Duffy says:

    Normally, Dave, I say thanks to my readers for the comments. But that’s hard to do in this case since your message is both transparently inaccurate and insulting to another of my readers. Believe it or not I am actually happy to hear and debate reasoned arguments from people who disagree with me on this site. but if you are only going to flame and produce heat and smoke without light, you won’t be welcome back.