Plants and animals become extinct when they can no longer survive or reproduce fast enough to maintain their populations. Sometimes this happens because of a single catastrophic event, but more often it results from a combination of pressures: lost habitat, new predators, shifting climate, or dwindling genetic diversity. Today, species are disappearing at roughly 1,000 times the natural background rate, and that pace is expected to accelerate to 10,000 times higher in the coming decades.
The Natural Background Rate of Extinction
Extinction is a normal part of life on Earth. Species appear, persist for a few million years on average, and eventually disappear. Scientists measure this using a unit called “extinctions per million species per year.” The natural background rate sits around 0.1 to 1 on that scale. In other words, in a world untouched by human activity, you’d expect roughly one species out of every million to vanish each year. That slow turnover has been the baseline for hundreds of millions of years, punctuated by rare mass extinctions that reset the board.
Five Mass Extinctions in Earth’s Past
The fossil record shows five catastrophic die-offs, each wiping out a large fraction of life. The end-Ordovician (about 445 million years ago) and Late Devonian (roughly 370 million years ago) events reshaped marine ecosystems. The Late Devonian extinction may have been driven by something surprisingly biological: the rise of the first forests, which altered soil chemistry and ocean oxygen levels enough to collapse marine food webs.
The end-Permian extinction, around 252 million years ago, was the worst. Estimates of species loss range from 81% to as high as 96%, driven by ocean warming that starved the water of oxygen. The end-Triassic event, about 201 million years ago, was triggered by massive volcanic activity in what is now the central Atlantic, creating an oxygen-depleted ocean crisis that lasted roughly 50,000 years. The most recent, the end-Cretaceous event 66 million years ago, was caused by an asteroid impact that blocked sunlight for years and dropped temperatures by 26°C or more, with freezing conditions lasting up to 16 years. That’s the event that killed the non-bird dinosaurs.
Each of these mass extinctions had a common thread: rapid environmental change that outpaced the ability of most species to adapt. The same principle applies today, but the source of change is different.
Habitat Loss and Fragmentation
The single biggest reason species go extinct in the modern era is the destruction and fragmentation of their habitat. When forests are cleared for agriculture, wetlands are drained for development, or grasslands are converted to cropland, the species living there lose the space and resources they need. But it isn’t just about total area lost. How the remaining habitat is arranged matters enormously.
When a continuous forest is broken into small patches, several things go wrong simultaneously. The fragments are smaller, so they support fewer individuals. Smaller populations are more vulnerable to random bad luck, like a disease outbreak or a bad breeding season. The edges of each fragment are exposed to wind, temperature changes, and predators from the surrounding landscape, a phenomenon called edge effects that can degrade habitat quality deep into the remaining patch. Isolated fragments also make it harder for animals to move between populations, cutting off the gene flow that keeps populations healthy. For species whose range was already concentrated in a small area, even modest habitat conversion can wipe out the entire population.
Climate Change and Thermal Limits
Every species has a temperature range in which its body functions normally. When the environment regularly exceeds that upper limit, the animal or plant burns more energy just trying to survive, reproduction drops, and populations shrink. Research comparing the thermal comfort zones of birds and mammals worldwide found that about 15% of bird species and 16% of mammal species already experience maximum temperatures above what their bodies can comfortably handle. By 2080, those numbers are projected to jump to 36% of birds and 47% of mammals.
When researchers looked across entire species ranges rather than single locations, the picture was even starker: 63% of bird species and 76% of mammal species already face temperatures beyond their comfort zone somewhere in their range. Under projected warming, those figures rise to 83% and 96%, respectively. Mammals face a tougher challenge than birds overall because they tend to have narrower thermal tolerances.
Counterintuitively, tropical species are more vulnerable than polar ones, even though the Arctic and Antarctic are warming faster. Tropical animals already live near the upper edge of what they can tolerate, so even a small temperature increase pushes them past their limit. Polar species, while facing dramatic changes in their environment, often have a wider buffer between current conditions and their physiological ceiling.
Overexploitation and the Wildlife Trade
Humans have hunted, fished, and collected species to extinction for centuries. The great auk, a flightless seabird of the North Atlantic, was hunted for its feathers, meat, and oil, and even killed to supply scientific specimen collections, until the last pair was killed in 1844. The sea mink was trapped to extinction for its fur in the 1800s following European colonization of northeastern North American islands.
The pattern continues today. The northern white rhinoceros, the western black rhinoceros, and the Vietnamese Javan rhinoceros were all driven to extinction or functional extinction by poaching fueled by international trade in rhino horn. High-value sturgeon species, including the Atlantic sturgeon and the beluga sturgeon, have been fished to local extinction for caviar. The Yunnan box turtle was harvested for illegal wildlife markets, and the Burmese star tortoise became functionally extinct from collection for the pet and traditional medicine trades.
Overharvesting doesn’t always mean direct killing. The introduction of Nile perch into Africa’s great lakes for the commercial fishing industry triggered the extinction of around 200 species of native cichlid fish by predation, a case where exploitation of one species destroyed hundreds of others.
Invasive Species, Especially on Islands
Island species are extraordinarily vulnerable to extinction. Extinction risk increases 2.6 times faster on islands than on mainlands, largely because of invasive species. Rats, cats, mongooses, and other introduced predators have been implicated in 86% of all island species extinctions since 1500. The dodo is the most famous example, wiped out on Mauritius by a combination of human hunting and introduced predators that ate its eggs. Currently, 596 species of birds, mammals, and reptiles on islands are listed as threatened on the IUCN Red List due to invasive species.
The damage goes beyond direct predation. When invasive predators eliminate island-breeding seabirds, for instance, the loss of bird-derived nutrients alters soil fertility, which transforms plant communities and the entire below-ground ecosystem. A single invasive species can reshape an island’s ecology from top to bottom.
Pollution and Reproductive Failure
Chemical pollutants can drive species toward extinction not by killing individuals outright, but by preventing them from reproducing. Persistent organic pollutants, industrial chemicals that linger in the environment for decades, are the best-documented culprits. These substances can interfere with hormones that control growth and reproduction. In contaminated waterways, researchers have documented severe egg cell degeneration in mussels, with up to 75% of eggs affected in heavily polluted estuaries. When a large portion of a population can’t successfully reproduce, the population spirals downward even if no individual animal is visibly sick.
Bans on some of the worst legacy chemicals have allowed certain populations to recover, which confirms the link between pollution exposure and population decline. But newer chemicals continue to enter ecosystems faster than their effects on wildlife can be studied.
The Extinction Vortex
Small populations face a self-reinforcing cycle that biologists call the extinction vortex. As a population shrinks, individuals are forced to breed with close relatives. This inbreeding reduces genetic diversity, which makes the population less resilient to disease, environmental change, and random events. The weakened population shrinks further, leading to more inbreeding, and so on.
A widely used guideline called the 50/500 rule holds that a population needs at least 50 breeding individuals to avoid severe inbreeding in the short term and at least 500 to maintain enough genetic diversity for long-term survival. Research on freshwater fish found that minimum viable population sizes ranged from 42 to 320 individuals depending on the species, with an average of about 117. Below these thresholds, extinction becomes increasingly likely even if the original threat is removed. This is why conservation efforts that wait until a species is critically rare often come too late.
Cascading Extinctions
Species don’t exist in isolation. They’re woven into networks of relationships: pollinator and plant, predator and prey, parasite and host. When one species disappears, its partners can follow. In Singapore, the extinction of certain butterfly species has been linked to the decline and local extinction of their host plants. In the Netherlands and the United Kingdom, parallel declines in bee diversity and bee-pollinated flowering plants show the same pattern.
These cascading effects can ripple through indirect connections too. Two species that never interact directly may be linked through a shared partner, so losing one can destabilize the other through a chain of intermediaries. Predicting which species are most vulnerable to these cascading losses requires mapping not just direct relationships but the entire web of indirect paths connecting species in an ecosystem. The more specialized a species is, relying on one pollinator, one food source, or one host, the more exposed it is to co-extinction when its partner declines.