The Sixth Extinction

Kolbert's thesis is that we are living through a mass extinction event of the kind that has occurred five times previously, and that its cause is us — not a single activity but the aggregate of habitat change, chemical alteration, species transport and climate.

Her method is to establish each element through a specific species and a specific place, reported first-hand, rather than through aggregate statistics.

Each element is established through one species in one place, reported in person, rather than through aggregate numbers.

Devmark’s reading — not a quotation from the book

Her opening historical chapters, on Cuvier establishing that species could disappear entirely — an idea that had no place in the prevailing view of nature — and on the long argument between catastrophism and gradualism.

Her point is that extinction had to be discovered, and that the discovery required accepting that the past contained events unlike anything observed.

Her treatment of the end-Ordovician, late Devonian, end-Permian, end-Triassic and end-Cretaceous extinctions, with particular attention to the asteroid impact and how long it took for that explanation to be accepted.

The Alvarez story is her best illustration of scientific resistance: a correct explanation, with physical evidence, dismissed for years partly because it was catastrophist and partly because it came from outside palaeontology.

Her strongest scientific chapter, reported from volcanic seeps off Italy where naturally elevated carbon dioxide has already changed the water chemistry.

The mechanism is direct chemistry rather than modelling: absorbed carbon dioxide lowers pH, which impairs organisms that build calcium carbonate structures. Her observation of what actually grows near the seeps is more persuasive than any projection.

Her account of the chytrid fungus and of frog populations disappearing worldwide, including species that vanished between one field season and the next.

Her framing is that the fungus spread through human transport of animals, which makes this an extinction caused not by habitat destruction but by moving things around — a category of cause with no precedent at this scale.

Her term for the effect of global transport: species that were separated for millions of years are now routinely mixed, reversing the isolation that generated much of the world's diversity.

Her argument that this homogenisation is itself an extinction driver, and one that continues regardless of conservation effort elsewhere, is the most under-appreciated point in the book.

Reversing tens of millions of years of isolation is an extinction driver that conservation elsewhere cannot offset.

Devmark’s reading — not a quotation from the book

Her reporting from a long-running Amazon fragmentation experiment, where patches of forest of different sizes were isolated and monitored for decades.

The finding — that fragments lose species steadily over time, and that smaller fragments lose more — establishes that preserving a percentage of habitat does not preserve a proportional share of the species.

Her late chapters on human expansion and on the disappearance of other hominin species, and on the megafaunal extinctions that followed human arrival on each continent.

Her implication is that the current event is continuous with a much older pattern rather than a recent industrial aberration, which is the book's most uncomfortable argument.

Established: current extinction rates are far above background, and the drivers she describes are real and documented.

Contested: whether the event qualifies as a sixth mass extinction on the geological definition, since that requires a scale of loss not yet reached. Some ecologists regard the framing as ahead of the evidence, and Kolbert's case is that the trajectory rather than the current total is the point.

She reports rather than exhorts, and the book has no chapter of solutions and no call to action. Several reviewers criticised this.

The choice appears deliberate. The book's effect comes from accumulation rather than argument, and readers who want a programme will need to look elsewhere.