We 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.
Toward 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.
The 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.]