The Sixth Extinction II

The remainder of the book covers several additional topics, including forest ecology in the Amazon, bats with white-nose syndrome, the Sumatran rhino and the Neanderthals. The number of species per acre near the equator is much higher than anywhere else, and the extinction rate there is correspondingly higher. Many species are adapted to narrow temperature ranges at certain elevations in the tropical mountains, and global warming forces both animals and plants to migrate uphill over time. In more temperate regions without mountains, the migration is towards the poles. A chapter is devoted to the effects that humans have had on ecosystems by exposing them to organisms from which they had previously been isolated.

The discussion of invasive species interested me. Ordinarily we hear news about Asian carp or zebra mussels or Asian long-horned beetles, but these make up only a tiny fraction of them. It turns out that earthworms were extinct in New England following the last Ice Age and were reintroduced by colonists. If you look at your yard here, the grass isn't native and the dandelions came from Europe. Even Queen Anne's lace is European. The fact is that invasive species are ubiquitous, and not much attention is paid to them unless they are found to conflict with our preferences in one way or another.

The Anthropocene epoch, as Kolbert describes it, came about both directly and indirectly from the migrations of humans out of Africa that began about a hundred and twenty thousand years ago. The most obvious directly-caused extinctions were the result of hunting by humans of large animals that had slow rates of reproduction. Because of those slow rates, even without excessive hunting a species might become extinct over thousands of years. Other large species such as deer are able to reproduce fast enough to maintain a robust population. The indirect causes of extinction include the introduction of foreign species and the alteration of the atmosphere through the addition of greenhouse gasses. Kolbert says very little about greenhouse gasses or pollution and a lot about introduced species. Both frogs and bats are being killed by fungi that were inadvertently transported from one part of the world to another.

Kolbert discusses our relationship to the Neanderthals in some detail. Although we interbred with them, the evidence points to our causing their extinction. We also seem to have caused the extinctions of several other Homo species since we left Africa. Here Kolbert emphasizes the human resourcefulness that is not evident in any of our close relatives despite having almost identical genetic makeups. It appears that a few small mutations made all the difference.

Right up to the end of the book, Kolbert remains resolutely unphilosophical about the current extinctions. She goes as far as to quote Richard Leaky, who said "Homo sapiens might not only be the agent of the Sixth Extinction, but also risks being one of its victims," and Paul Ehrlich, who said "In pushing other species to extinction, humanity is busy sawing off the limb on which it perches." Kolbert herself says this:

Obviously, the fate of our own species concerns us disproportionately. But at the risk of sounding anti-human – some of my best friends are humans! – I will say that it is not, in the end, what's most worth attending to. Right now, in the amazing moment that counts to us as the present, we are deciding, without quite meaning to, which evolutionary pathways will remain open and which will forever be closed. No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy. 

That is as close as she comes to taking a position, and I would have preferred it if she had gone on to suggest altering some of our current practices: what about controlling our population growth and reducing further destruction to the Earth's ecosystems? Kolbert has made a contribution to the discussion of our environment, but this is hardly a call to action.