The Sixth Extinction

Elizabeth Kolbert’s recent book The Sixth Extinction: An Unnatural History is science journalism of a high order. As with her earlier notable book on climate change, Field Notes from a Catastrophe, she includes lively narrative accounts of her visits to places around the world where scientists are at work and communicates the import of their work with clarity and intelligence. Well-informed background discussions on the general topic of extinction are woven into these narratives, in passages well pitched to the understanding of the general reader. Foreboding though the subject may be the book is a pleasure to read.

The phenomenon of species extinction has only begun to be understood in relatively recent times. Kolbert traces the discussions of the 19th century from the ground-breaking conclusion of Cuvier to the doubting Lyell and finally Darwin, whose theory of evolution necessarily involved the disappearance as well as the emergence of species.

In evolutionary terms the mass extinctions of the distant past are a special case, arising from relatively sudden events for which natural selection over long periods of time had not prepared many of the species which disappeared under the stress of a rapidly changed environment. Kolbert comments on the fact that just as we have recovered the story of these past events and identified five of them we have discovered that we are causing another. Whether it will reach the proportions of the Big Five is not yet known, but the indications are significant enough for it to be called the Sixth Extinction. She notes the estimation that one-third of all reef-building corals, a third of all freshwater mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles , and a sixth of all birds are headed toward oblivion. It’s no small matter.

Sometimes it is perfectly obvious what has caused the disappearance of a species. The Great Auk was simply hunted to final extinction by the 19th century, a fate similar to that of the Moa in New Zealand. The book notes that the disappearance of megafauna such as the mammoth and mastodon is increasingly seen as caused by human killing rather than climate change, though either count gives cause for worry.

Other extinctions or near-extinctions are more mystifying. Kolbert visits Panama to report on the fungus that is killing off large populations of frogs in many countries. She also describes a cold-loving fungus destroying large populations of hibernating bats in the north eastern United States. In the context of the bat chapter she embarks on an extended discussion of introduced and invasive species and their sometimes destructive effects in their new environments.  By transporting species “we are, in effect, reassembling the world into one enormous supercontinent”.

Ocean acidification, global warming’s “equally evil twin”, is occurring at what may well be a record pace in the geological scale of Earth’s history as a consequence of the increased carbon dioxide from burning fossil fuels. Acidification clearly played a significant part in at least two of the Big Five extinctions, and it is difficult to see how the current increases in ocean acidity can avoid serious loss of diversity in sea life.  Kolbert points out that oceans are now 30 percent more acidic than they were in 1800; under a business as usual scenario that will rise to 150 percent this century. The destructive visible effects of a more acid ocean on vulnerable species are described in a visit she makes to a research station on a tiny island near Naples close to a volcanic vent on the seabed.

The profound effect of acidification on coral reefs is highlighted in a chapter describing the author’s stay on a research station on a Great Barrier Reef island where a team headed by climate scientist Ken Caldeira was stationed. A recent paper by Caldeira and members of his team concluded that if current emissions trends continue, within the next fifty years or so “all coral reefs will cease to grow and start to dissolve.” Add the effect of warming oceans to acidification, and the future of coral reefs and all they mean for ocean life looks bleak indeed. Kolbert reports a group of British scientists saying reefs “will be the first ecosystem in the modern era to become ecologically extinct”.

I found myself wondering when reading this chapter what Australian prime minister Tony Abbott, making common cause with Canada’s Stephen Harper as I was writing this review, would make of such judgments. He seems to have dropped ‘crap’ from his climate change vocabulary, but he said recently that he could think of few things more damaging to Australia’s future than leaving coal in the ground and not  selling it. The Great Barrier Reef reduced to rubble looks a great deal more damaging, but I guess Abbott doesn’t read troubling books like Kolbert’s.

Migration or adaptation of finely-adapted species as temperature increases occur is in some cases likely to be essential to survival. One of Kolbert’s chapters tells of her visit to one of the world’s diversity “hot spots”, Manú National Park in Peru. Here she accompanied a forest ecologist Miles Silman who regularly checks the trees in seventeen plots which sit at different elevations and hence have different average annual temperatures. In the diverse world of the park this means that each plot represents a slice of a fundamentally different forest community.

Silman has found that global warming is driving the average genus up the mountain at a rate of two and a half metres per year, though the average masks a range of response. Kolbert points out that global warming is taking place at least ten times faster than it did at the end of the last glaciation, and at the end of all the earlier glaciations. To keep up, organisms will have to migrate, or otherwise adapt, at least ten times more quickly. It’s a big ask and so far in the monitored plots only the most “fleet-footed” trees are keeping pace. How many species overall will be capable of moving fast enough remains an open question, but  one  likely  to be answered within decades. The answer could be devastating.

In conclusion Kolbert faces up to the “transformation of the ecological landscape” for which our species has in many diverse ways already been responsible and which it looks set to continue through felling tropical forests, altering the composition of the atmosphere and acidifying the ocean. We are putting our own survival in danger, for we remain dependent on the earth’s biological and geochemical systems. Kolbert quotes Paul Ehrlich’s memorable words: “In pushing other species to extinction, humanity is busy sawing off the limb on which it perches.”