Humans drive species to extinction through five major pathways: destroying habitats, overexploiting wildlife, spreading invasive species, polluting ecosystems, and changing the climate. Of these, habitat destruction is by far the most significant, acting as the dominant threat for over 71% of species currently being pushed toward extinction. The current extinction rate is estimated to be 1,000 to 10,000 times higher than the natural background rate, meaning species that would have taken 10,000 years to disappear are vanishing in a century.
Habitat Destruction and Conversion
The single biggest way humans cause extinction is by converting wild ecosystems into farmland, cities, and timber plantations. Of more than 20,000 species evaluated by researchers using IUCN data, 88% were negatively affected by habitat destruction, and for 71% it was the primary threat pushing them toward extinction. The majority of this conversion, at least on land, comes from food and fiber production. Forests are cleared for cattle ranching and soy farming, wetlands are drained for rice paddies, and grasslands are plowed for grain.
What makes habitat loss so devastating is that it doesn’t just remove one species at a time. When a forest is cleared, every organism that depended on that particular structure of vegetation, soil, moisture, and shade loses its home simultaneously. Specialist species, those adapted to narrow environmental conditions, are hit hardest. Generalists like raccoons or pigeons can move to nearby patches; a frog that breeds in a single stream system cannot.
Fragmentation compounds the damage. Even when some habitat remains, breaking it into isolated patches creates populations too small to sustain themselves over generations. This leads to something ecologists call extinction debt: species that appear to survive fragmentation but are effectively doomed by it. Research on European grasslands found that habitat-specialized plants were still going extinct 36 to 49 years after the fragmentation that sealed their fate. The species look present on the landscape, but their populations are slowly collapsing. This means the full cost of habitat destruction today won’t be visible for decades.
Overexploitation and Wildlife Trade
Roughly 27% of threatened species are affected by overexploitation, whether through hunting, fishing, or commercial trade. This is the driver responsible for some of history’s most dramatic extinctions. The great auk, a flightless seabird once abundant across the North Atlantic, was hunted for feathers, meat, oil, and eventually scientific specimens until the last pair was killed in 1844. The sea mink was hunted to extinction solely for its fur.
More recently, three rhinoceros subspecies have been driven to extinction or functional extinction by the international trade in rhino horn: the northern white rhinoceros, the western black rhinoceros, and the Vietnamese Javan rhinoceros. High-value sturgeon species like the beluga sturgeon have disappeared from parts of their range due to overfishing for caviar. The vaquita, a small porpoise in Mexico’s Gulf of California, is now the world’s most endangered marine mammal with fewer than a dozen individuals remaining. It isn’t even the target of the fishery killing it. Vaquitas die as bycatch in nets set illegally for totoaba, a fish whose swim bladder is prized in traditional medicine.
These examples reveal a common pattern: the rarer a species becomes, the more valuable it can be on the black market, creating a perverse incentive to hunt the very last individuals.
Invasive Species Introduced by Humans
Invasive species have played a role in roughly 60% of all known extinctions since 1500, making them one of the most destructive forces in extinction history. These are organisms transported by human activity, deliberately or accidentally, into ecosystems where they have no natural predators or competitors. Rats that stowed away on ships have decimated island-nesting bird populations worldwide. Cats introduced to Australia have contributed to the decline of dozens of native mammals. The brown tree snake, accidentally brought to Guam after World War II, wiped out nearly every native forest bird on the island within a few decades.
Invasive species don’t need to directly kill native wildlife to cause extinction. They can outcompete natives for food, introduce novel diseases, alter fire patterns, or change soil chemistry. Chestnut blight, a fungus introduced to North America from Asia around 1900, killed an estimated four billion American chestnut trees within 50 years, reshaping entire forest ecosystems.
Pollution Across Land and Water
About 18% of threatened species are affected by pollution, which takes many forms: pesticides, industrial chemicals, plastic waste, nutrient runoff, and excess carbon dioxide absorbed by the oceans. Agricultural runoff carrying fertilizer into rivers and coastal waters creates oxygen-depleted “dead zones” where fish and shellfish suffocate. Pesticides accumulate in the tissues of predators at the top of the food chain, a process that nearly drove bald eagles and peregrine falcons to extinction in the mid-20th century.
Ocean acidification is a less visible but increasingly serious form of pollution. As the ocean absorbs carbon dioxide from the atmosphere, seawater becomes more acidic. Under conditions projected for 2100 (roughly a 0.5-unit drop in pH), survival and calcification rates in marine organisms drop by about 27% on average. Corals, mollusks, and a group of tiny shelled plankton called coccolithophores are the hardest hit, with calcification reductions of 22 to 39%. Coral larvae show an even steeper decline, with settlement rates dropping by an average of 47%. Since coral reefs support roughly a quarter of all marine species, the knock-on effects could be enormous.
Climate Change as a Growing Threat
Climate change currently affects about 17% of threatened species, making it the smallest of the five major drivers today. But its share is growing rapidly. A large-scale analysis published in Science projects that 7.6% of all species face climate-driven extinction when averaged across all emissions scenarios. Under the highest-emission pathway, that number climbs to roughly one in three species globally. Extinctions are projected to accelerate sharply if warming exceeds 1.5°C above preindustrial temperatures.
Climate change threatens species in ways that are hard to escape. Shifting temperature zones force organisms to migrate to stay within their tolerable range, but many can’t move fast enough or have nowhere to go. Mountain-dwelling species are a clear example: as temperatures rise, their suitable habitat shrinks toward the summit until there’s no higher ground left. Polar species face the same squeeze. Changes in the timing of seasonal events, like when flowers bloom versus when pollinators emerge, can break apart relationships that species depend on for reproduction and food.
How These Threats Compound Each Other
These five drivers rarely act alone. A species in a fragmented habitat is more vulnerable to climate shifts because it can’t migrate. A population reduced by overhunting recovers more slowly when invasive predators are also present. Coral reefs stressed by warming water are less able to cope with acidification. This compounding effect is why conservation scientists often describe the extinction crisis as a set of interacting pressures rather than a list of independent problems.
The loss of a single important species can also trigger a chain reaction. When a top predator disappears from an ecosystem, the species it kept in check can explode in number, overconsuming plants or outcompeting smaller species. These trophic cascades have been documented in systems as varied as Alaskan kelp forests (where the decline of sea otters allowed sea urchins to destroy kelp beds) and Mediterranean scrublands (where the loss of the Iberian lynx increased populations of smaller predators that suppress rabbits). Each secondary extinction can trigger another, unraveling food webs in ways that are difficult to predict or reverse.
The Scale of the Current Crisis
The IUCN Red List now tracks 166,061 species, of which 46,337 are threatened with extinction. Trees alone account for more than a quarter of the list, and the number of threatened tree species is more than double the combined total of all threatened birds, mammals, reptiles, and amphibians. Over the last century, roughly 200 vertebrate species have gone extinct. At the natural background rate, those losses would have taken up to 10,000 years.
This pace has led many biologists to describe the current era as the sixth mass extinction, following the five great die-offs recorded in Earth’s fossil history. What distinguishes this one is its cause: it is driven almost entirely by the activities of a single species. And because of extinction debt, the numbers we see today undercount the true damage. Species that have already lost enough habitat to make their decline irreversible may persist for decades before the last individual is gone, meaning the extinctions playing out now reflect decisions made a generation ago, and the full consequences of today’s habitat loss are still ahead.