Human Impact and Conservation

What is biodiversity?

Biodiversity is the variety of life. It includes the number of different species in a place, the genetic differences within each species, and the variety of ecosystems, from coral reefs to prairies. On the Ecosystem Interactions page you saw how species depend on each other. When one species disappears, others that eat it, shelter in it, or are pollinated by it can feel the loss.

Conservation is the work of protecting species and the places they live, and of using natural resources in ways that can last. It mixes science, laws, and everyday choices.

How scientists keep count

The most widely used list of species at risk is the IUCN Red List of Threatened Species, kept by the International Union for Conservation of Nature. Experts check each species against the same rules, using things such as how fast its numbers are falling and how small its range is, and place it in one of the categories below.

In the Red List update of July 9, 2026 (version 2026-1), 175,909 species had been assessed, and 49,505 of them, about 28%, were threatened with extinction. The share differs a lot between groups: the Red List's own figures for version 2026-1 show about 41% of amphibian species and 11% of bird species are threatened.

Keep in mind that scientists have assessed only a small part of all the species on Earth. Groups such as mammals and birds are well studied, while many insects, fungi, and deep-sea animals have never been checked. That is one reason the Red List keeps growing.

The main threats

In 2016 a team led by Sean Maxwell looked at Red List data for 8,688 species that were threatened or near threatened, and counted how many species each kind of threat affected. Most species face more than one.

Habitat loss

When forests, grasslands, and wetlands are cleared for farms, roads, and towns, the plants and animals living there lose their homes and food. Farming affected 5,407 of the species in the 2016 study, and the growth of cities and towns affected 3,014.

Invasive species

An invasive species is one that people moved to a new place, where it spreads and causes harm.

• Cane toads. In 1935, 102 toads were brought from Hawaii to Queensland, Australia, to eat beetles in sugar cane fields. Thousands of their young were released that August. The toads are poisonous to many native predators, and they spread across Queensland and into the Northern Territory, New South Wales, and Western Australia. A 2006 study found that the front of the invasion was moving about five times faster than at first, partly because toads at the front had longer legs.
• Zebra mussels. These fingernail-sized mussels from Eurasia were first found in the Great Lakes, in Lake St. Clair, in 1988. They probably arrived in ballast water from ships. They filter out the algae that native animals eat, smother native mussels, and clog the water pipes of power plants.
• Kudzu. This vine from Asia was shown at the 1876 Centennial Exhibition in Philadelphia. In the 1930s the U.S. Soil Conservation Service grew more than 70 million seedlings to stop soil erosion. Kudzu is often said to cover 7 to 9 million acres, but a careful U.S. Forest Service survey found it on about 227,000 acres of Southern forest. It is still a pest, but a smaller one than its fame suggests.

Pollution

Pollution affected 1,901 species in the 2016 study. One famous pollutant was the insecticide DDT. It does not break down easily and is stored in fat, so it builds up as it passes along a food chain. This is called biomagnification.

Overharvesting

Overharvesting means hunting, fishing, logging, or collecting a species faster than it can replace itself. In the 2016 study it affected 6,241 species, about 72%, more than any other threat.

Climate change

In the same study, climate change affected 1,688 species, about 19%, and ranked seventh of the eleven threats measured. The authors said climate change still matters, but overharvesting and farming were harming far more species. Scientists are studying how the effects of climate change may grow in the future.

Gone for good: three lost birds

A species is extinct when its last member has died.

• The dodo. This flightless bird lived only on the island of Mauritius in the Indian Ocean. After sailors arrived, it was hunted, and animals people brought with them harmed it too. The last widely accepted sighting was in 1662. You can read how island life shaped birds like it on the Evolution and Adaptation page.
• The passenger pigeon. It was once common across eastern North America. Hunting for market and the clearing of forests wiped it out. The last known passenger pigeon, a female named Martha, died at the Cincinnati Zoo on September 1, 1914. Her body is kept at the Smithsonian.
• The Carolina parakeet. The only parrot native to the eastern United States lost its forest home and was shot as a pest and for feathers. The last known bird, a male named Incas, died at the Cincinnati Zoo on February 21, 1918, in the same aviary as Martha. The species was declared extinct in 1939.

Comebacks

The bald eagle

DDT made bald eagles lay eggs with shells so thin they broke during nesting. In 1963 the U.S. Fish and Wildlife Service counted about 487 nesting pairs in the lower 48 states. (Some of its pages give 417; the 1963 count is uncertain.) After Rachel Carson's book Silent Spring (1962) raised alarm about pesticides, the U.S. Environmental Protection Agency banned most uses of DDT, starting December 31, 1972. Eagles were also protected by law. Their numbers climbed, and on June 28, 2007, the bald eagle was announced as removed from the endangered and threatened list.

The California condor

By 1982 only 22 California condors were left, poisoned by lead from bullets in the carcasses they ate and harmed in other ways. In a hard decision, the last wild condor was captured on April 19, 1987, leaving 27 birds, all in zoos. Captive breeding worked, and releases began in 1992. At the end of 2024 there were 566 condors, 369 of them flying free. Lead poisoning is still their biggest threat.

The gray whale

Commercial whaling nearly wiped out gray whales. The eastern North Pacific population recovered so well that in 1994 it was taken off the U.S. endangered list, and in the mid-2010s NOAA Fisheries described it as nearing all-time highs, at about 27,000 whales. Recovery is not always a straight line: after a die-off from 2019 to 2023, the 2025 count was about 13,000, the lowest since the 1970s. Scientists think changes in their Arctic feeding grounds are the main cause, and they are watching closely.

The sea otter

The fur trade nearly wiped out sea otters. By 1911, when a treaty banned hunting them at sea, only small groups survived. One group of southern sea otters held on near Big Sur, California, and in 1913 California made them fully protected. Today there are about 3,000 southern sea otters. You met them as a keystone species on the Ecosystem Interactions page: by eating sea urchins, they help kelp forests grow.

Laws and protected places

• The Endangered Species Act (ESA). Signed by President Richard Nixon on December 28, 1973, it protects species listed as endangered or threatened in the United States and the habitat they need. The U.S. Fish and Wildlife Service handles land and freshwater species, and NOAA Fisheries handles most ocean species.
• CITES. The Convention on International Trade in Endangered Species of Wild Fauna and Flora was signed in Washington, D.C., on March 3, 1973, and came into force on July 1, 1975. Its 185 Parties control or ban trade across borders in species that are at risk.
• Protected areas. Parks, wildlife refuges, and marine sanctuaries protect whole habitats, so many species benefit at once.

Citizen science: how you can help

Scientists cannot be everywhere. In citizen science, ordinary people collect data that scientists use.

• The Christmas Bird Count. On Christmas Day in 1900, the ornithologist Frank Chapman proposed counting birds instead of holding a holiday hunt. That year 27 birders made 25 counts. The count, run by the National Audubon Society, still happens every winter.
• eBird. Started in 2002 by the Cornell Lab of Ornithology and the National Audubon Society, eBird collects checklists of the birds people see. eBird data helped the U.S. Fish and Wildlife Service estimate the bald eagle population in 2020.
• iNaturalist. It began in 2008 as a student project at the University of California, Berkeley. You photograph a plant, animal, or fungus, and other users help identify it.

What can you do? Share sightings, with an adult's permission, on iNaturalist or eBird. Clean mud and plants off boots, bikes, and boats before visiting a new place, so invasive species do not hitch a ride. Plant native plants, keep litter out of streams, and never release a pet into the wild.

Timeline

Key words

• Biodiversitythe variety of life, from genes to species to ecosystems
• Conservationprotecting species and habitats, and using resources in ways that can last
• Threatenedon the IUCN Red List, a species that is Vulnerable, Endangered, or Critically Endangered
• Extinctno members of the species are left alive
• Habitat losswhen the place a species lives is cleared, changed, or broken up
• Invasive speciesa species moved by people to a new place, where it spreads and causes harm
• Overharvestingtaking a species faster than it can replace itself
• Pollutionharmful substances added to air, water, or land
• Biomagnificationa chemical becoming more concentrated at each step up a food chain
• Captive breedingbreeding animals in zoos or centers to raise their numbers
• Protected areaa park, refuge, or sanctuary set aside to protect nature
• Citizen scienceresearch in which members of the public collect data

Game: Threat or Helper?

You will see twelve human actions, one at a time. Decide whether each one is a threat to living things or a helper. An action you sort right on your first try earns 1 point, for a total of 12. Your best score is saved on this device.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.