Organisms disappear from Earth through extinction, a permanent process that has been occurring since life began roughly 3.8 billion years ago. At the current pace, species are vanishing about 1,000 times faster than the natural background rate, and that multiplier could reach 10,000 times in the coming decades. While extinction is a normal part of life’s history, the scale and speed at which it happens today is not.
Earth’s Five Mass Extinctions
Life on Earth has survived five catastrophic die-offs, each one reshaping which organisms dominate the planet. The worst was the End-Permian extinction around 252 million years ago, which wiped out 96% of all species. Oceans became nearly lifeless. The most recent, the End-Cretaceous event 66 million years ago, killed off the non-avian dinosaurs along with 76% of species when an asteroid struck what is now Mexico’s Yucatán Peninsula.
The full timeline of mass extinctions looks like this:
- End-Ordovician (444 million years ago): 86% of species lost, primarily marine invertebrates like trilobites and brachiopods, likely triggered by glaciation and falling sea levels.
- Late Devonian (372 million years ago): 75% of species lost across multiple pulses rather than a single event.
- End-Permian (252 million years ago): 96% of species lost, the largest extinction in Earth’s history, driven by massive volcanic eruptions in what is now Siberia.
- End-Triassic (200 million years ago): 80% of species lost, clearing ecological space that allowed dinosaurs to rise to dominance.
- End-Cretaceous (66 million years ago): 76% of species lost, caused by an asteroid impact combined with volcanic activity.
After each mass extinction, biodiversity took millions of years to recover. Following the End-Permian event, the worst on record, it took at least 5 million years for marine ecosystems to rebuild their species richness, and some analyses suggest full recovery didn’t happen until the Middle Jurassic, tens of millions of years later. Extinction is fast; recovery is not.
Why Species Go Extinct Today
The major international science body on biodiversity, IPBES, identifies five direct drivers pushing modern species toward extinction: land-use change, climate change, pollution, overexploitation of natural resources, and invasive species. Of these, converting wild habitat to farmland, cities, and infrastructure has the largest impact. When a forest is cleared or a wetland drained, every organism that depended on that habitat loses its home.
These drivers rarely act alone. A frog species might survive moderate habitat loss but collapse when a warming climate simultaneously allows a deadly fungal pathogen to spread into its range. Amphibians illustrate this compounding pressure clearly: 40.7% of the world’s amphibian species are now threatened with extinction, up from 37.9% in 1980. Disease, habitat destruction, and climate change are hitting them simultaneously.
What Happens When a Species Disappears
Losing one species can destabilize an entire ecosystem through a chain reaction called a trophic cascade. When a top predator disappears, the animals it once kept in check multiply, overconsuming the plants or smaller animals below them. A well-documented example involves sea otters off the coast of California and Alaska: when otter populations declined, sea urchins (their prey) exploded in number and devoured kelp forests, collapsing an ecosystem that supported hundreds of other species.
Sometimes the indirect effects are counterintuitive. The Iberian lynx, a predator of rabbits, actually helps rabbit populations survive. By feeding on mid-sized predators that kill far more rabbits than the lynx itself does, the lynx keeps rabbit numbers stable. Remove the lynx, and rabbit populations can crash rather than grow.
Certain species sit at critical bottleneck positions in food webs. In ocean upwelling zones, a single species of sardine, anchovy, or krill can be the only link between dozens of species at lower and higher levels. These “wasp-waist” species regulate energy flow in both directions. If they disappear, both the plankton below and the whales, seabirds, and fish above are affected.
Pollination and Economic Consequences
The disappearance of organisms doesn’t only affect wild ecosystems. Global pollination services, mostly provided by insects, are valued at roughly $1 trillion. A total loss of pollinators would shave 1 to 2% off global GDP in the short term, but the longer-term effects on food security would be far worse. About three-quarters of the world’s food crops depend at least partly on animal pollination. Without bees, butterflies, and other pollinators, yields of fruits, nuts, and vegetables would plummet.
Extinction Debt: Losses Still to Come
One of the most unsettling concepts in extinction science is “extinction debt,” the idea that species losses from past habitat destruction haven’t fully played out yet. When a forest is cut in half, not all the species that can no longer sustain viable populations disappear immediately. Some linger for decades or even centuries before finally dying out. The extinction is inevitable, but delayed.
Research tracking global forest-dwelling vertebrates found that extinction debts began accumulating around the mid-19th century, when industrialization drove widespread deforestation. The pattern is visible in the data: present-day species richness in a region correlates better with how much forest existed there 150 years ago than with how much forest exists there now. That gap between past habitat and current species numbers represents organisms living on borrowed time. And because deforestation and climate change continue to accelerate, these time-delayed losses are expected to compound in the future.
The Current Extinction Crisis in Context
Scientists increasingly refer to the present era as the sixth mass extinction. The comparison to the Big Five is not hyperbole in terms of rate, though the total number of species lost has not yet reached those historical levels. The current extinction rate of 1,000 times the natural background means species are disappearing at a pace that, if sustained, would rival the great die-offs over geological timescales.
Multiple planetary boundaries related to life on Earth have already been crossed. The “biosphere integrity” boundary, which measures how much of the planet’s biological productivity humans have co-opted, is now transgressed. Humans appropriate so much of Earth’s net primary production (the total energy plants capture from sunlight) that the biosphere’s ability to regulate itself is compromised. This isn’t a single species going extinct. It’s the underlying machinery of ecosystems being degraded.
The pattern across vulnerable groups reinforces the severity. Amphibians, often called indicator species because their permeable skin makes them sensitive to environmental change, have seen their threat levels climb steadily for four decades. Coral reefs, freshwater fish, and flying insects are all following similar trajectories of decline, each group responding to a slightly different combination of the five major drivers but all pointing in the same direction.