Extinction is natural, but the pace matters
A species is extinct when its last member has died and none remain anywhere. Extinction is a normal part of life’s history. The vast majority of species that have ever lived are no longer alive. Species have always come and gone, with new ones appearing through evolution and others disappearing as conditions changed.
What worries many scientists today is the rate. The fossil record shows five major mass extinctions in Earth’s history, when a large share of life disappeared over a geologically short time, caused by events such as huge volcanic eruptions, rapid climate shifts and, in the case of the one that ended the non-avian dinosaurs about 66 million years ago, an asteroid impact. Many researchers argue that current extinction rates are well above the typical background rate seen between those events and that human activity is the main reason. Exact numbers are uncertain, because many species, especially small ones, are not well known, and estimates differ in method and result.
The main pressures
Conservation biologists often group the causes into a few broad categories. In most real cases several act at once.
Habitat loss and fragmentation
The most widely cited cause of modern extinction is the destruction of habitat. Forests are cleared for farms, wetlands are drained, grasslands are plowed and land is built over. Even when some habitat remains, it can be broken into small, isolated patches. Fragmentation matters because small patches hold small populations, animals may not be able to move between them, and the edges are exposed to wind, heat, predators and invaders. Tropical forests, with their many species that have small ranges, are especially affected.
Overexploitation
Hunting, fishing and collecting can push a species down faster than it can reproduce. The passenger pigeon of North America, once extremely abundant, was hunted to extinction by the early twentieth century. Large whales were heavily reduced by whaling, and many fish stocks have been depleted. Slow-breeding animals such as sharks, tortoises and large mammals are particularly vulnerable since they replace their numbers slowly.
Invasive species
When people move species to new regions, intentionally or accidentally, some become invasive. They can prey on, outcompete or infect native species that have no defenses against them. Islands are especially vulnerable. Many extinct birds, including the flightless dodo of Mauritius, were affected by a combination of hunting and introduced animals such as rats, pigs and cats that ate eggs and young. Exact roles of each factor in the dodo’s case are still discussed by historians and biologists.
Pollution and disease
Pollution can poison animals directly or alter their habitat, for example through nutrient runoff that starves water of oxygen. Chemicals that build up through food webs can harm top predators. Diseases can spread, especially when animals are stressed or when a pathogen reaches a population with no resistance. The amphibian disease chytridiomycosis, a fungal infection, is linked to declines and extinctions of many frog species, though its spread and effects vary with other conditions.
Climate change
Changing temperature and rainfall can shift the habitats species depend on. Some animals and plants can move to cooler places or higher elevations, but others cannot move fast enough, or have nowhere to go, such as species already living on mountain tops. Climate change is also a threat to specialized systems like coral reefs, where warm water can cause bleaching. The Great Barrier Reef is a much-discussed example. It is generally regarded as a growing risk for many species, though the extent varies by species and region, and it works together with the other pressures rather than alone.
Why small populations are in danger
Even after the original cause of decline is removed, a small population can keep falling toward extinction for reasons of its own.
- Chance events. In a small group, a few bad years, a storm or an outbreak can wipe out most or all of the population.
- Loss of genetic variety. Small populations tend to lose genetic diversity and may become inbred, which can reduce survival and reproduction and leave them less able to adapt to changes. Genetic variation is the raw material for natural selection.
- Difficulty finding mates or cooperating, in species that depend on group living.
Conservationists sometimes speak of an extinction vortex, in which these factors reinforce each other and push a population lower.
Linked losses
Extinction rarely affects one species alone. A plant that depends on a particular pollinator may decline when the insect does, and a predator may starve when its main prey disappears. Researchers call these knock-on losses coextinctions. The loss of species can also reduce the variety that makes ecosystems resilient, a point explored in the article on species biodiversity.
Can extinction be prevented?
There are many examples where action has helped. Protected areas, bans on hunting, captive breeding programs, removal of invasive predators from islands and habitat restoration have all supported recoveries of particular species. Conservation organizations use categories such as vulnerable, endangered and critically endangered to describe how at risk a species is.
Success is not guaranteed, and it depends on the species, how much time and money are available and whether local people are involved and supported. Some species are lost before they are ever studied. The general message from the science is that the most effective step is usually to protect the habitat that species need before their numbers become very small, since recovery of tiny populations is slow, expensive and uncertain.