The Sixth Mass Extinction — What's Being Lost

In the history of life on Earth, there have been five mass extinctions — events severe enough to eliminate more than 75% of all species in a geologically brief period. The most recent was 66 million years ago, when an asteroid impact ended the Cretaceous and took the non-avian dinosaurs with it. Before that: the Permian-Triassic extinction 252 million years ago, the worst in Earth's history, which eliminated roughly 96% of all marine species. Each of these events reshaped the biosphere, and each was followed by millions of years of recovery and diversification — the empty ecological space colonized by survivors, eventually producing new radiations of life.

We are living through a sixth mass extinction. Not as a future prediction or a worst-case scenario — as a present observation. The data are unambiguous: current extinction rates are 100 to 1,000 times the background rate (the normal rate of species disappearance in the absence of major disturbance). Species that would have persisted for millions of years are disappearing within human lifetimes. The pace is accelerating. And unlike every previous mass extinction, this one has a known cause: one species, Homo sapiens, systematically transforming, consuming, and destabilizing the biosphere that all life depends on.

This article is about the biology of what we're losing, why it matters beyond the obvious, and what the science of extinction crisis actually looks like at the ecological and population level — because the full picture is considerably more alarming than the headline numbers suggest.

Part I — The numbers, and why they understate the problem

The IUCN Red List — the most comprehensive assessment of species' conservation status — currently lists over 44,000 species as threatened with extinction. But the Red List is biased toward well-studied groups: vertebrates, some invertebrates, and plants. Most of Earth's biodiversity — insects, fungi, soil microorganisms, marine invertebrates, nematodes — is barely assessed. The 44,000 threatened species almost certainly represents a small fraction of the true number.

The vertebrate population data is the most alarming signal we have, and it tells a more urgent story than species extinction counts alone. The Living Planet Index, maintained by WWF and the Zoological Society of London, tracks population sizes of monitored vertebrate species — not just whether they exist, but how many individuals remain. The 2022 report found an average 69% decline in monitored populations between 1970 and 2018. Some taxa are far worse: freshwater vertebrates (fish, amphibians, reptiles in rivers and lakes) have declined by 83% on average. Latin America and the Caribbean show a 94% average decline across monitored species.

This distinction between species extinction and population extinction matters deeply. A species can persist while losing 95% of its populations — technically "not extinct" while functionally devastated, having lost most of its ecological role and most of the genetic diversity that would allow it to adapt to future change. Paul Ehrlich and colleagues have called this "biological annihilation" — the collapse of biodiversity at the population level, largely invisible to species-count statistics, but equally or more consequential for ecosystem function.

Part II — The HIPPO drivers

The acronym HIPPO, coined by E.O. Wilson, names the five primary drivers of the extinction crisis in order of their overall impact. Understanding each mechanistically helps clarify why the crisis is so difficult to address and why it's accelerating.

To HIPPO, most ecologists now add a sixth driver: Climate Change — increasingly important and projected to become the dominant driver of extinction by mid-century. Climate change shifts species' geographic ranges faster than many can disperse; breaks ecological timing relationships (phenological mismatches — flowers blooming before their pollinators emerge, prey species breeding before their predators arrive); bleaches coral reefs through thermal stress; acidifies oceans by absorbing CO₂; increases drought and fire frequency; and interacts synergistically with every other HIPPO driver.

Part III — What's actually being lost

Numbers of species and percentage declines are abstractions. The reality of the extinction crisis has a texture that requires concrete examples to understand — not because individual charismatic species matter more than the broader pattern, but because the specific ecological roles being lost matter for understanding consequences.

Trophic cascades — the loss of predators

Large predators are among the most disproportionately impacted groups — they are the species most sensitive to habitat fragmentation, most vulnerable to human persecution (as competitors for livestock and game), and most affected by overhunting. They are also, ecologically, among the most consequential. The removal of a top predator from a food web triggers a trophic cascade — a chain of effects that reverberates through multiple trophic levels.

The reintroduction of wolves to Yellowstone in 1995, after 70 years of absence, is the most studied example. Within years, elk behavior changed: they avoided valleys and riverbanks where they were exposed and vulnerable. Vegetation on those riverbanks recovered, reducing erosion and changing stream geomorphology. Beaver populations increased, creating wetlands that supported fish, waterfowl, and other species. The wolves killed coyotes, reducing coyote pressure on small mammals, which affected vegetation through seed dispersal. The wolves' effect on the physical landscape was measurable in river GPS surveys. A handful of predators, allowed to reoccupy their ecological role, demonstrably changed the structure and function of an ecosystem. When we lose predators, we don't just lose predators — we lose the regulatory function they provide, often with consequences that propagate unpredictably through food webs.

Pollinators and the service economy of ecosystems

Roughly 87% of flowering plant species require animal pollination — mostly insects, but also birds and bats. These pollinators provide a service to agriculture estimated at $235–577 billion annually worldwide. The decline of pollinators — both wild bees (there are roughly 20,000 species of bee globally) and managed honeybees (hit by colony collapse disorder, Varroa mite infestation, pesticides, and habitat loss) — threatens food security in a direct, economically quantifiable way that most ecological services do not. In some regions of China where bee populations have collapsed from pesticide use, apple and pear trees are now hand-pollinated by human workers with paintbrushes.

Ecological tipping points

Perhaps the most alarming dimension of the extinction crisis is the possibility of ecological tipping points — thresholds at which the loss of species triggers nonlinear shifts in ecosystem state that are difficult or impossible to reverse. The Amazon rainforest is the most discussed example: evidence suggests the forest maintains its own rainfall through evapotranspiration (trees pump enormous quantities of water vapor into the atmosphere, which falls as rain elsewhere in the basin). Current models suggest that deforestation of 20–25% of the Amazon (we're currently at around 17–20%) combined with climate change-induced drought stress could trigger a tipping point beyond which the forest cannot maintain its rainfall regime and transitions from rainforest to savanna — releasing roughly 100 billion tonnes of carbon, devastating South American agriculture, and representing an irreversible shift in the Earth system. We don't know exactly where the tipping point is. We know we're approaching it.