Biodiversity on Earth has followed a dramatic arc: rising over hundreds of millions of years, crashing during at least five catastrophic events, rebounding each time, and now declining again at a pace not seen since the last mass extinction 66 million years ago. Current extinction rates are tens to hundreds of times higher than the natural background rate, and monitored wildlife populations have shrunk by an average of 73% since 1970.
The Fossil Record: A Story of Growth and Collapse
The broadest view of biodiversity comes from the marine fossil record, which stretches back more than 500 million years. The overall pattern shows rising diversity in the early Paleozoic era, a long plateau through the middle of that era, and then a steep climb through the Mesozoic and into the present. But that upward trend was interrupted repeatedly by mass extinctions, each one wiping the slate partially clean and reshaping the kinds of life that followed.
Five events stand out. Around 440 million years ago, glaciation and falling sea levels killed up to 85% of shallow marine species, including trilobites, corals, and brachiopods. About 370 million years ago, another wave hit marine life hard, devastating reef-building corals and armored fish. The worst came 252 million years ago at the end of the Permian period: volcanic eruptions triggered global warming so severe that 96% of marine species and roughly 70% of land species vanished. Biologists call it the “Great Dying.” A smaller extinction 201 million years ago cleared the way for dinosaurs to dominate, and then the famous asteroid impact 66 million years ago ended the reign of non-bird dinosaurs, pterosaurs, and marine reptiles.
After each crash, biodiversity recovered, but recovery took millions of years and produced fundamentally different ecosystems. The groups that dominated before an extinction were rarely the ones that dominated after. Mammals, for instance, had existed alongside dinosaurs for over 100 million years but only diversified into their current range of forms after the asteroid impact removed the competition.
The Modern Decline in Numbers
The most striking recent data comes from WWF’s Living Planet Index, which tracks population sizes of more than 5,000 vertebrate species. Between 1970 and 2020, those monitored populations declined by an average of 73%. That doesn’t mean three-quarters of all animals are gone. It means that across thousands of tracked populations, the average group is about a quarter of the size it was 50 years ago. Some populations have collapsed almost entirely; others have held steady or grown. But the overall direction is sharply downward.
Freshwater species have been hit hardest, with an average population decline of 85%. Terrestrial species follow at 69%, and marine species at 56%. Freshwater ecosystems are especially vulnerable because they sit at the intersection of so many human pressures: water extraction, dam construction, agricultural runoff, and pollution all converge on rivers, lakes, and wetlands.
Insects, which make up the majority of all known animal species, are declining too. A meta-analysis of 16 studies found that insect populations dropped by about 45% over just 40 years. Because insects pollinate crops, decompose organic matter, and form the base of many food webs, those losses ripple outward through ecosystems in ways that are difficult to fully measure.
How Today Compares to the Background Rate
Species go extinct naturally. The normal pace, called the background extinction rate, is roughly 1.8 extinctions per million species per year for vertebrates. At that rate, about nine vertebrate species should have disappeared during the entire 20th century. In reality, 390 did. That’s more than 40 times the expected number.
The IUCN Red List, the most comprehensive inventory of species’ conservation status, has assessed more than 172,000 species so far. Of those, over 48,600 are threatened with extinction, about 28% of all species evaluated. Plants have not been spared either: nearly 600 plant species are confirmed extinct in modern times, a rate well above what would occur naturally.
Whether this qualifies as a sixth mass extinction is debated among scientists. The five previous events each eliminated at least 75% of species over geologically short windows. We haven’t reached that threshold yet, but the trajectory is heading in that direction faster than at any point in at least 10 million years.
What’s Driving the Losses
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) identifies five direct drivers of biodiversity loss, ranked roughly by their current impact. Land-use change is the biggest. Deforestation, agricultural expansion, mining, and urban development physically destroy or fragment habitats. When a forest becomes farmland, most of the species that lived there either leave or die. Habitat fragmentation can be just as damaging as outright destruction, because it isolates populations into patches too small to sustain them long-term.
Direct exploitation of wildlife is the second major driver. Overhunting and overfishing have occurred throughout human history, but the scale has accelerated sharply. In the ocean, the overexploitation of marine habitats has caused steep drops in commercially valuable fish species along with countless others caught as bycatch.
Climate change is a growing threat. Shifting temperatures and weather patterns force species to migrate, adapt, or die. Research suggests that rising temperatures could threaten as many as one in six species globally. Coral reefs illustrate this vividly: 14% of the world’s corals have disappeared since 2009 alone, and projections from the IPCC indicate that reefs would decline by 70 to 90% at 1.5°C of warming above pre-industrial levels. At 2°C, the loss reaches 99%. The planet has already warmed about 1.1°C, so those thresholds could be crossed by 2050 or sooner.
Pollution and invasive species round out the top five. Pollution hits freshwater and marine habitats especially hard, while invasive species introduced by human activity outcompete or prey on native wildlife. These drivers don’t operate in isolation. A forest fragment surrounded by agriculture, exposed to rising temperatures, and invaded by a non-native predator faces compounding pressures that are far worse than any single threat.
Which Ecosystems Are Changing Fastest
Freshwater ecosystems are losing biodiversity faster than any other major habitat type. Rivers and lakes cover less than 1% of Earth’s surface but support roughly 10% of all known species, including a third of all vertebrates. The 85% average population decline in freshwater species since 1970 reflects how heavily humans depend on, and alter, these systems.
Coral reefs, sometimes called the rainforests of the sea, support about a quarter of all marine species despite covering less than 1% of the ocean floor. Their sensitivity to water temperature makes them one of the ecosystems most immediately threatened by climate change. Even if warming is held to 1.5°C, the majority of reefs are projected to degrade significantly.
Tropical forests remain biodiversity hotspots but are shrinking due to agricultural conversion, logging, and fire. The loss of tropical forest doesn’t just remove trees. It eliminates layers of habitat, from the canopy down to the forest floor, that support vastly different communities of species.
What the Projections Show
Multiple studies estimate that 3 to 6 million or more animal and plant species face extinction risk even under intermediate climate scenarios. Under high-emissions pathways, additional losses of plant species richness by 2050 climb steeply compared to low-emissions scenarios, meaning the choices made about carbon pollution in the next decade will directly shape how much biodiversity survives into the second half of the century.
The relationship between climate change and biodiversity loss is not one-directional. Healthy ecosystems absorb carbon, stabilize weather patterns, and buffer coastlines. As biodiversity declines, ecosystems lose their ability to perform those functions, which accelerates climate change, which drives further biodiversity loss. This feedback loop is one reason scientists increasingly treat climate change and the biodiversity crisis as two faces of the same problem.