Biodiversity and Conservation

Biodiversity is the variety of life. National Geographic Education describes three levels of it:

• Genetic diversity: differences in genes among individuals of the same species. It gives a species the raw material to adapt to disease and changing conditions.
• Species diversity: the variety of species living in an area.
• Ecosystem diversity: the variety of ecosystems, such as forests, wetlands, grasslands, deserts and coral reefs.

Richness and evenness

Ecologists describe species diversity with two ideas, explained in OpenStax Biology 2e. Species richness is the number of species in an area. Species evenness is how equal the numbers of each species are. A meadow with 10 species in similar numbers is more diverse than a meadow with the same 10 species where one grass makes up almost every plant. Richness is generally highest near the equator and falls toward the poles.

How many species are there?

Nobody knows exactly. Scientists have named only a fraction of Earth's species, and different databases count them in different ways. These figures show the range:

Biodiversity hotspots

Some regions hold far more unique species than others. The Critical Ecosystem Partnership Fund (CEPF) explains that a biodiversity hotspot must meet two criteria: it has at least 1,500 species of vascular plants found nowhere else (endemic species), and it has lost at least 70% of its original native vegetation. There are 36 recognized hotspots. The newest, the North American Coastal Plain of the southeastern United States, was recognized in February 2016. About 2 billion people live in hotspots, and many rely directly on healthy ecosystems for their livelihoods. National Geographic Education notes that hotspots cover only about 2.5% of Earth's land surface but are home to more than half of the world's plants and 43% of land vertebrates, and that most plants in a hotspot are endemic.

Field scientists often sample with a quadrat, a square frame laid on the ground, and count every individual of each species inside it. To combine richness and evenness into one number, many use Simpson's diversity index, introduced by statistician E. H. Simpson in 1949.

First add up, for every species, n(n − 1), where n is the number of individuals of that species. Divide the total by N(N − 1), where N is the number of individuals of all species. That gives Simpson's D, the chance that two individuals picked at random (without putting the first back) are the same species. The diversity index is 1 − D: the chance that they are different species. It runs from 0 (only one species) toward 1 (many species in even numbers).

People depend on living things for far more than food. The benefits nature provides are called ecosystem services. OpenStax describes how soil organisms recycle nutrients that crops need, and how ecosystems support food production, drinking water and breathable air.

Resilience

National Geographic Education explains that diverse ecosystems tend to be more resilient: better able to recover after a disturbance such as a drought, a fire or a disease. If one species is lost, others may fill a similar role. In the early 1900s, chestnut blight killed most of the American chestnut trees in the eastern forests of North America; the forests survived because other trees were there to grow in their place.

Extinction is a normal part of life's history. Between big events, species disappear at a slow background extinction rate, which OpenStax gives as about one extinction per million species per year (1 E/MSY). Five times in the fossil record, though, a large share of life disappeared in a geologically short time. These are the mass extinctions.

Today's losses

In 2019 the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) released a global assessment compiled by 145 expert authors from 50 countries. Among its findings:

• Around 1 million animal and plant species are threatened with extinction, many within decades.
• The current rate of extinction is at least tens to hundreds of times higher than the average over the past 10 million years, and it is accelerating.
• At least 680 vertebrate species have been driven to extinction by human actions since the 16th century.
• The average abundance of native species in most major land habitats has fallen by at least 20%, mostly since 1900.
• Three-quarters of the land environment and about 66% of the marine environment have been significantly altered by human actions, and more than 85% of the wetlands present in 1700 had been lost by 2000.

What is driving the losses

IPBES ranked five direct drivers by their global impact so far, from largest to smallest:

1. Changes in land and sea use, such as clearing forests for farms or developing coastlines. Breaking habitat into pieces is called habitat fragmentation.
2. Direct exploitation of organisms, such as overfishing, hunting and logging (overexploitation). In 2015, 33% of marine fish stocks were harvested at unsustainable levels.
3. Climate change.
4. Pollution.
5. Invasive alien species. The number of invasive species per country has risen about 70% since 1970 in the 21 countries with detailed records.

OpenStax gives examples of invasive species and diseases. The brown tree snake, accidentally carried to Guam around 1950, has led to the extinction of three bird species and three to five reptile species found only on that island. A chytrid fungus is part of the cause of a global decline in amphibians, and white-nose syndrome, a fungal disease, infects cave-hibernating bats in eastern North America.

The IUCN Red List

Since 1964 the IUCN has kept the Red List of Threatened Species, which assesses each species' risk of extinction. Scientists place each assessed species in one category, based on criteria such as how fast its population is shrinking and how small its range is.

The July 2026 update (2026-1) includes 175,909 species, of which 49,505 are threatened, about 28% of all species assessed. The IUCN's goal is to assess 260,000 species, and the Red List is updated regularly, so these numbers will change. Some groups are much more at risk than others:

Source: IUCN Red List home page, checked October 30, 2026.

Species can move down the ladder too. In the same update, the numbat, a small termite-eating marsupial from Australia, improved from Endangered to Near Threatened. Introduced cats and red foxes had cut its numbers to around 300 animals by the late 1970s. After decades of predator control, captive breeding and moving animals to new sites, there are now 2,000 to 3,000 numbats.

The U.S. Endangered Species Act

The Endangered Species Act (ESA) became law on December 28, 1973. NOAA Fisheries explains its two main categories. An endangered species is in danger of extinction throughout all or a significant portion of its range. A threatened species is likely to become endangered in the foreseeable future. Two agencies share the work: the U.S. Fish and Wildlife Service is responsible for land and freshwater species (plus a few marine mammals such as manatees, sea otters and polar bears), and NOAA Fisheries for most marine species. The law protects listed species from harm and directs the agencies to work on their recovery.

As of October 30, 2026, the USFWS listed species summary shows 1,680 U.S. listings (1,259 endangered and 421 threatened): 743 animals and 937 plants. Including 706 foreign listings (species found only outside the United States), the total is 2,386. When a species recovers, it can be delisted.

Because habitat loss is the biggest driver, protecting habitat is central to conservation. A protected area is a clearly defined place, such as a national park, wildlife refuge or marine reserve, managed for long-term conservation. Protected Planet, the global database of protected areas, reported on October 30, 2026 that protected areas cover 17.58% of the world's land and inland waters and 9.82% of the ocean. Including other effective area-based conservation measures (OECMs), places outside formal protected areas that still conserve nature, coverage is 18.69% on land and 10.04% at sea.

Reserves as islands

In 1967 ecologists Robert MacArthur and E. O. Wilson published the theory of island biogeography. The number of species on an island settles where new species arriving balance species dying out there. Islands close to the mainland get more arrivals, and large islands have more habitats and lose fewer species. So large, near islands hold the most species, and small, far islands hold the fewest. OpenStax explains that nature reserves surrounded by farms and cities behave like islands too.

Designing reserves

OpenStax describes some general rules from conservation biology. One large reserve is usually better than several small ones of the same total area, because more of its habitat is core habitat unaffected by edges. For the same reason, square or round reserves do better than ones with long, thin arms. Conditions near an edge differ from those in the core; this is called the edge effect. Wolves, which range over hundreds of kilometers, need far larger reserves than butterflies, which may stay within ten kilometers. Buffer zones of less ideal habitat around a reserve let animals leave its edges without immediate danger.

Corridors and connectivity

The National Park Service explains that roads, development, croplands, energy projects and even fences can break up the landscape and block animals' movements. Wildlife corridors, strips of habitat that link patches, help reconnect them. Corridors are especially useful for mobile species with large territories, such as elk and mountain lions.

International agreements

The Convention on Biological Diversity (CBD) is a treaty with 196 Parties. The United States signed it in 1993 but is not a Party. In December 2022, at a meeting called COP15, Parties adopted the Kunming-Montreal Global Biodiversity Framework, with 4 long-term goals and 23 targets for 2030. Its Target 3, often called “30 by 30,” aims for at least 30% of land and inland waters and 30% of coastal and marine areas to be effectively conserved and managed by 2030, through protected areas and OECMs, while recognizing and respecting the rights of Indigenous peoples and local communities, including over their traditional territories. Each country decides how it will contribute, and people disagree about how areas should be chosen, managed and counted.

Some species that were close to disappearing have recovered with legal protection, habitat protection, captive breeding and, often, the removal of a specific threat. Each case is different, and several of these species still need monitoring or protection.

The bald eagle

USFWS explains that bald eagles were decimated by habitat loss, illegal shooting and the pesticide DDT, which contaminated the fish they ate and harmed their reproduction. In 1972 the Environmental Protection Agency banned the use of DDT and some related pesticides. Bald eagles were listed in 1978 as endangered in most of the lower 48 states and threatened in five (eagles in Alaska were never listed because populations there stayed robust), reclassified as threatened in July 1995 and removed from the list on June 28, 2007.