The Sixth Extinction Summary and key ideas

The Sixth Extinction, by Elizabeth Kolbert.

The book opens roughly two hundred thousand years ago, with a new species emerging in eastern Africa. It began in small numbers and may have passed through a near-fatal population bottleneck. It was not especially fast, strong, or fertile. Its unusual advantage was resourcefulness. Wherever it settled, it adapted. It crossed rivers, plateaus, and mountains, changed its diet and surroundings, and eventually reached almost every part of the globe. That species was Homo sapiens.

Then the pace changed. Human populations began doubling repeatedly within a single century. People deliberately cleared forests to feed themselves and, less deliberately, carried organisms between continents. By exploiting subterranean energy, they altered atmospheric composition, which changed climate and ocean chemistry. Some organisms could move uphill or toward the poles. Others became stranded. The book’s central question begins here: has one adaptable species become a force capable of changing the fate of most others? The author argues that no previous creature altered life in quite this way, although earlier eras experienced rare, enormous collapses of diversity.

That question requires a scale test. In ordinary geological time, extinction is rare and varies among groups. A rough mammal estimate implies about one disappearance every seven hundred years. For amphibians, fossils are too scarce for a firm baseline; an approximate estimate suggests that one species might disappear every thousand years. A mass extinction is something different: rapid, substantial, global loss of biodiversity, with a sharp rise far above background. It can remove organisms across whole branches of life’s tree and overturn which forms dominate. The five ancient catastrophes are grouped as the Big Five. The present crisis is called the Sixth Extinction by analogy, but it is still too early to know whether it will reach their scale.

Amphibians make this possibility concrete. They are ancient, predating dinosaurs, and remarkably diverse. Their shell-less eggs must stay moist, so many species remain tied to water, but they have evolved varied ways to live and reproduce. Their apparent similarity can conceal deep genetic differences. That makes the recent disappearances especially striking. Frogs vanished from relatively pristine mountains and forests as well as disturbed places. In Panama’s El Valle, the endemic golden frog was once abundant, even giving its name to Thousand Frog Stream. Then a wave of die-offs moved through rainforest sites, reached Santa Fe in two thousand and two, and appeared near El Valle by two thousand and four. Biologists planned to rescue a remnant population, but the wave arrived first.

Rescue became the work of EVACC, a sealed conservation facility. Its tanks provide golden frogs with running water that imitates their former streams, and the animals can breed there. Incoming frogs and materials are disinfected, visitors’ equipment is restricted, and all water is treated. The facility has preserved species that are extinct or nearly extinct in the wild. Yet it is also an uneasy kind of ark. The pathogen later named Batrachochytrium dendrobatidis, or Bd, can persist after susceptible amphibians disappear. Bleach may kill it, but an entire rainforest cannot be disinfected. Staff describe release as the goal while acknowledging that they have no workable way to return the frogs safely. Captive survival, in other words, is not the same as recovery.

The explanation for the wave emerged from an investigation of blue poison-dart frogs that suddenly died in a zoo. Researchers identified a previously unrecognized chytrid fungus. When healthy frogs were exposed to laboratory-raised samples, they became ill and died within three weeks. The fungus attacks through the skin, disrupting electrolyte uptake and producing a fatal collapse comparable to a heart attack.

Bd moves through water and runoff, which helps explain its eastward movement through Panama. That route cannot explain outbreaks on distant continents in roughly the same period. Two competing hypotheses connect those outbreaks to human transport. African clawed frogs used in pregnancy tests may have carried Bd while resisting it. North American bullfrogs moved or exported for food may have done the same. The passage does not choose between these explanations. Both imply that infected carrier frogs crossed oceans because people placed them on ships or planes.

That is why the amphibian case matters beyond frogs. The estimated amphibian loss rate may be as much as forty-five thousand times its approximate background rate, and serious risks appear among other groups as well. Wake and Vredenburg argued that this pattern signaled a mass extinction in progress. The book presents their diagnosis as an argument, not settled proof that the present event equals the Big Five. The stronger conclusion is already clear: Homo sapiens is both the observer of this crisis and one of its agents. The open question is how far the transformation will go.