Earth is experiencing a sharp decline in biodiversity, sometimes called the sixth mass extinction because its rate rivals the five major mass extinctions in the geological record. The International Union for Conservation of Nature (IUCN) maintains the Red List, a global catalogue that categorizes species by extinction risk; species listed as endangered face a high risk of extinction in the wild. Two of the largest drivers of biodiversity loss are habitat destruction and invasive species, non-native organisms that cause ecological or economic harm. Habitat fragmentation, the breaking of continuous habitat into smaller, isolated patches, reduces populations' sizes and genetic diversity and increases their vulnerability to local extinction.
Conservation biology is the science of protecting biodiversity, and it operates at multiple levels. In-situ conservation protects species in their natural habitats, often through protected areas, designated regions managed for conservation; the ambitious 30 by 30 goal aims to protect 30 percent of the world's land and oceans by 2030. Wildlife corridors, which connect fragments of habitat, support movement and gene flow between isolated populations. Where habitats are too damaged to support wild populations, ex-situ conservation, including zoos and botanical gardens, provides backup populations. Seed banks such as the Svalbard Global Seed Vault in Arctic Norway preserve crop diversity as insurance against catastrophe.
Some species exert a disproportionate influence on their ecosystems and are called keystone species; the reintroduction of wolves to Yellowstone National Park is a classic example, where their predation reshaped grazing patterns and revitalized entire river valleys. Rewilding takes this further by restoring entire ecosystems to a wild state, including reintroducing locally extinct species. Restoration efforts also include reversing the damage done by invasive species and by overfishing, the removal of fish faster than populations can replace themselves. Bycatch, the unintended catch of non-target species, is a related problem often addressed through better gear design and fishery management. Aquaculture, the farming of aquatic organisms, is growing rapidly as a food source, but sustainable fisheries, those that maintain populations and ecosystems for the long term, depend on careful science-based management of wild stocks.