Animals go extinct when they can no longer survive or reproduce fast enough to maintain their populations. The causes range from catastrophic events like asteroid impacts to slow-building pressures like habitat destruction and climate shifts. Today, vertebrate species are disappearing at roughly 100 times the natural background rate, a pace that many scientists describe as the beginning of a sixth mass extinction.
What Drove the Five Mass Extinctions
Earth has experienced five major extinction events over the past 450 million years, each wiping out a significant percentage of all living species. The triggers fall into three broad categories: asteroid impacts, massive volcanic eruptions, and shifts in global climate driven by the evolution of plant life itself.
The most famous event, the extinction that killed the dinosaurs 66 million years ago, was triggered by an asteroid striking what is now Mexico’s Yucatán Peninsula. The impact released enormous amounts of carbon into the atmosphere, collapsing food chains worldwide. That event also coincided with a period of intense volcanic activity in what is now India, compounding the environmental damage.
Two other mass extinctions, at the end of the Permian period (252 million years ago) and the end of the Triassic (201 million years ago), were caused by colossal volcanic eruptions known as large igneous provinces. The Permian event, the worst in Earth’s history, was linked to the Siberian Traps, a volcanic system that flooded the atmosphere with carbon dioxide and toxic gases, killing roughly 90% of marine species. The two oldest mass extinctions, in the Late Ordovician and Late Devonian periods, appear to have been triggered by the evolution of land plants, which altered global weathering patterns and buried enough carbon to dramatically cool the planet.
The Five Human-Driven Causes Today
Modern extinctions look nothing like asteroid strikes or supervolcanoes. They’re driven by five dominant forces, all linked to human activity: habitat destruction, climate change, invasive species, pollution, and overexploitation. Among imperiled species in the United States, 82% are threatened by changes in land or sea use, 72% by climate change, 52% by invasive species, 34% by pollution, and 32% by overexploitation such as hunting or overfishing. Most species face several of these pressures simultaneously.
Habitat Loss
When forests are cleared for agriculture, wetlands drained for development, or ocean floors scraped by trawling, the species that depend on those ecosystems lose their food sources, shelter, and breeding grounds. Habitat destruction is consistently the single biggest driver of modern extinction because it doesn’t just affect one species. It dismantles entire ecological networks at once.
Invasive Species
Invasive species contribute to 60% of all recorded species extinctions globally, making them one of the most destructive forces in biodiversity loss. When animals, plants, or pathogens are introduced to ecosystems where they have no natural predators or competitors, they can outcompete, prey on, or infect native species with devastating speed. Island species are particularly vulnerable because they evolved in isolation and have no defenses against unfamiliar predators or diseases.
Overexploitation
Hunting, fishing, poaching, and wildlife trade push species past the point where their populations can recover. This pressure doesn’t need to wipe out every individual directly. Once a population drops below a certain size, it becomes vulnerable to inbreeding and random catastrophic events that finish the job. Research modeling over a thousand species found that a population generally needs at least 1,377 individuals to have a 90% chance of surviving 100 years, though that number varies widely depending on the species and its environment.
How Climate Change Pushes Species to the Edge
Rising temperatures don’t just make habitats hotter. They disrupt the precise timing that ecosystems depend on. Plants, insects, and animals have evolved to synchronize key life events: flowering, migration, egg-laying, and emergence from dormancy all happen on schedules shaped by seasonal temperature cues. When the climate warms unevenly, these schedules fall out of sync.
In eastern North America, researchers tracking 16 years of observations found that trees and butterflies responded to warming at similar rates, but migratory birds adjusted their arrival and breeding timing much more slowly, especially at higher latitudes. This creates a mismatch: birds arrive to find that the insects they depend on for feeding their chicks have already peaked. Hot droughts make it worse, pushing plants to flower earlier while delaying butterfly and moth activity. Over time, these mismatches can cause population crashes in species that depend on precise ecological timing to survive and reproduce.
Pollution’s Hidden Role
Chemical pollution doesn’t always kill animals outright, but it can cripple their ability to reproduce, navigate, find food, and learn. Pesticides reduce reproduction in solitary bees. Industrial chemicals called PCBs impair spatial learning in birds, making them slower and more error-prone at tasks they need for survival. Pharmaceutical residues in waterways alter the behavior and migration patterns of salmon and sparrows.
One of the starkest examples involves vultures in India. An anti-inflammatory drug used in livestock ended up in the carcasses vultures fed on, causing population declines greater than 95% in several vulture species. Synthetic chemicals like PFAS, often called “forever chemicals,” delay development and alter growth in amphibians. The cumulative effect of these pollutants is a slow erosion of population health that makes species increasingly vulnerable to other threats.
The Genetic Trap of Small Populations
When a population shrinks drastically, whether from hunting, disease, or habitat loss, it passes through what biologists call a genetic bottleneck. The surviving individuals carry only a fraction of the species’ original genetic diversity, and that loss compounds quickly. Genetic drift, the random loss of gene variants from one generation to the next, acts much faster in small populations.
This matters because genetic diversity is what allows a species to adapt. A population with rich genetic variation has a better chance that some individuals will carry traits suited to a new disease, a shift in food availability, or a changing climate. A bottlenecked population may have already lost the very gene variants it would need to survive new challenges. Inbreeding compounds the problem by increasing the likelihood of harmful genetic conditions. Many species that appear stable in number are actually genetically fragile, one environmental shock away from collapse.
When One Extinction Triggers Another
Species don’t exist in isolation. They’re woven into networks of dependence: pollinators and the plants they fertilize, parasites and their hosts, predators and the prey that structures entire food webs. When one species disappears, the species that depend on it can follow. These cascading losses, called co-extinctions, are likely the most common form of extinction, though they’re difficult to track because they happen after the initial, more visible loss.
A plant that loses its only pollinator will stop reproducing. A parasite that loses its host vanishes with it. A predator that relies on a single prey species faces starvation. Research into ecological networks shows that species assumed to be resilient based on their own individual traits can be unexpectedly compromised when a species they depend on disappears. Co-extinctions hasten the overall pace of biodiversity loss and reduce the functional diversity of ecosystems, making those systems less stable and more prone to further collapse.
How Fast Species Are Disappearing Now
The natural background rate of extinction, the pace at which species would vanish without human influence, is roughly two species lost per 10,000 species per century for mammals. Current rates dwarf that baseline. Since 1900, mammal extinctions have occurred at roughly 55 times the expected rate under conservative estimates. Amphibians are disappearing even faster, at up to 100 times the background rate. Across all vertebrate groups combined, the modern extinction rate since 1900 is between 22 and 53 times higher than normal, depending on how conservatively the numbers are calculated.
As of 2025, nearly 19,900 species are classified as critically endangered on the IUCN Red List, meaning they face an extremely high risk of extinction in the wild. These numbers capture only the species scientists have evaluated. Many more, particularly insects, fungi, and deep-sea organisms, have never been formally assessed.