Acid rain Precipitation that becomes acidic due to acid-forming precursors put into the atmosphere by human activities.

Amphibian Members of a class of cold-blooded vertebrates who are aquatic in the larval stage, and breathe air as adults. Frogs, toads, and newts are examples of amphibians.
Anthropogenic Caused or influenced by human impact on natural systems.

Background rate of extinction The natural rate of extinction in the absence of human influence. Estimates of the background rate of extinction range from one to ten species per year, which is 100 to 1,000 times lower than the current rate.

Biodiversity (also known as
biological diversity)
The variety of living organisms. Biodiversity encompasses variation at all levels, from the genetic diversity within a species to the variation between higher level evolutionary groupings such as families and classes. It also includes the variety of ecosystems, habitats and the natural interactions of species in the wild.

Ecosystem An integrated group of biological organisms located in a particular type of habitat, and the physical environment in which they live. The ecosystem includes the living organisms, habitat structure, factors (such as temperature, wind, elevation, etc.) and their interactions.

Endemic Native to a particular, restricted geographic area.

Evolution The change in organisms over generations that gradually results in changes in populations and species.

The killing, capturing or collecting of wild organisms for human use.

Extinction (Also see the
introduction to Past Extinctions)
The state in which all members of a groups of organisms, such as a species, population, family or class, have disappeared from a given habitat, geographic area, or the entire world.

Extinction vortex The interacting factors that serve to progressively reduce already small populations, drawing them into extinction like an inescapable whirlpool.

Extirpation The complete removal of a particular type of organism from an area, usually a specified geographic area.

Food chain A sequence of steps through which food and energy move through the environment from the primary source (plants), through the animals that consume plants, up to the animals which consume other animals.

Fragmentation (also see
Spotlight on Island Biogeography [SP])
The disruption of large areas of habitat into smaller, separate units. Fragmentation involves both a total loss of habitat area and the isolation of remaining habitat patches, which prevents interaction between some organisms located in the fragments, and renders them effectively separate populations.

Genetic diversity Variation at the genetic level, within and between species, including the different forms of genes for particular traits.

Geographic range The geographic area within which the specified type of organism may be found.

Habitat The physical and biological environment in which an organism lives, including the arrangement of food, water, shelter, and sites for rearing young.

Introduced species (also known
as non-native, exotic, or alien species)
Species that humans transport to an area that was previously outside of that species’ geographic range. Introductions may be intentional, such as with domestic animals like sheep and dogs, or unintentional, such as with rats and other pests that live on ships.

Invertebrate The group of animals lacking a bony spinal column. Examples of invertebrates are insects, worms, starfish, sponges, squid, plankton, crustaceans, and mollusks.

IUCN (International Union for
the Conservation of Nature)
The IUCN, also called World Conservation Union, is an independent body founded in 1948 that promotes scientifically based action for the conservation of wild living resources. A union of nations, government agencies, and non-governmental organizations, the IUCN links a global network of more than 4,000 scientists who share information and develop cooperative plans for conserving endangered plants, animals, and ecosystems.

Marsupials The group of mammals whose young are born very undeveloped and must attach themselves after birth to the mother’s milk glands, where they are usually protected by a pouch. Australia is known for its wide variety of marsupials, such as kangaroos, wombats, and bandicoots. The opossum, found in North and South America, is also a marsupial. Marsupials are known in Europe, Asia, and Africa only through ancient fossils.

Mass extinctions Periods during which the rate of extinction is much higher than it is at other times, and a large percentage of the evolved biodiversity disappears in a geologically short amount of time. See the introduction to Past Extinctions for more information on episodes of mass extinction.

Metamorphosis An extreme change occurring between the stages of life, such as from a tadpole to a frog, or from a caterpillar to a butterfly.

The unique set of resources used by a species within an ecosystem.

Ozone depletion The reduction in the layer of ozone gas, found at the top of Earth’s atmosphere, which absorbs most of the ultraviolet radiation (UVR) coming from the sun. Ozone depletion decreases the absorption of UVR, which allows more of the harmful rays to penetrate to Earth’s surface.

Illegal hunting, capture, or collecting of wildlife. Poachers may target organisms that are protected from all hunting, such as elephants, or they may target animals outside of the regulated hunting season or inside the boundaries of a protected area.

Population (of a species) A subgroup of a species coexisting in the same time and area. Population may also be used in a different sense to refer to the number of individuals in a defined group.

Species A group of related organisms that are capable of breeding with each other to produce fertile offspring but are not capable of breeding with members of other species.

Subspecies A geographically isolated or physiologically distinct group within a species that is capable of interbreeding with other members of the subspecies but that usually does not.

Vertebrate Any of the group of animals that have a backbone. These include amphibians, birds, fish, mammals, and reptiles.

Viable population or viability A sufficient number of individuals of a species to make their continued existence possible. If the population dips below that number, the species will not be able to recover and will eventually become extinct. The number of individuals needed for a viable population will vary with the species, habitat conditions and other factors.

Wetland A land area with high amounts of moisture in the soils and characterized by plant communities that prefer that moist environment. Examples of wetlands are tide flats and marshes.