The Sixth Extinction

Extinction rates today are 100 to 1,000 times the geological background rate of about one species per million per year, and the cause is humans. The framing — that we are in the early stages of a sixth mass extinction — was named in Richard Leakey and Roger Lewin's The Sixth Extinction (1995) and made canonical by Elizabeth Kolbert's 2014 book of the same title (Pulitzer 2015). The argument is striking enough that it has been argued every which way, and the data are uneven enough that the careful versions of the case are notably more cautious than the popular ones. What survives a careful reading is a trajectory, not yet a destination — and the policy question of the next century is whether that trajectory can be deflected before it arrives.

E.O. Wilson's HIPPO mnemonic names the drivers in rough order of importance: Habitat loss (the dominant cause — agricultural expansion, urbanization, mining), Invasive species, Pollution, (human) Population, and Overharvesting (fisheries, hunting). Climate change is now widely added as the rapidly-rising sixth driver. Habitat loss accounts for the lion's share of present-day extinctions: agricultural expansion has cleared roughly a third of the world's forests since the year 1700, and tropical hotspots have lost most of their original primary vegetation. The IPBES Global Assessment (2019) estimated that up to 1 million species are threatened with extinction in the coming decades, more than at any point in human history. The Living Planet Index maintained by WWF reports a 69% average decline in monitored vertebrate populations between 1970 and 2018 — a figure widely quoted but methodologically contested, since it tracks population sizes rather than extinction events and the choice of monitored populations introduces selection effects.

Three caveats survive scrutiny. First, most species have not been formally described, so measured extinction rates are necessarily for charismatic subsets — vertebrates, plants, butterflies — and the rate including microbes and unsurveyed invertebrates could be much higher or lower. Second, mass extinctions are conventionally defined geologically as a stratigraphic interval; current biodiversity loss is at a very early stage by that standard and could in principle be reversed before reaching mass-extinction magnitude. Third, the species concept itself is contested at the molecular level, and what counts as a species depends on definition. What survives all three caveats is the trajectory: present rates of habitat loss, range contractions, and population collapse, if continued, would produce a mass-extinction-magnitude event within a few centuries — a thousand times faster than any of the Big Five. The Sixth Extinction is therefore a forecast whose realization depends on the next century of land use, fisheries, and climate policy.