Threatened species have low or declining population numbers. Their populations are facing serious problems. The Endangered Species Act of 1973 defines a threatened species as one that is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range if factors contributing to their population decline or habitat degradation or loss continue.

Endangered species are still alive today but exist in numbers so low or are decreasing so rapidly that they are in immediate danger of extinction. The Endangered Species Act of 1973 defines an endangered species as any plant or animal species that is in danger of extinction throughout all or a significant portion of its range.

Extinct species are species that are gone forever. They once lived on this planet but have died out. Extinction has been occurring since life began on Earth. However, today extinction is occurring at a faster rate than ever. Species disappear because of either either changes in nature or the actions of people. In some instances a major natural event, such as a volcanic eruption, can kill an entire isolated species. In other cases, extinction will happen slowly as nature changes the world. For example, the dinosaurs disappeared because of a change in Earth's climate, and they could not survive the changing temperatures.

Increased human population growth causes problems for other life on this planet. As the human population grows, an increasing number of species become extinct. People change the habitats, or homes, upon which living creatures depend. Rapid development has destroyed or altered many of the natural environments to which individual species are adapted. When humans pollute, spray pesticides, use toxic chemicals, introduce non-natives species or poach, their actions affect what happens to other living things.

There are also plant and animal species that are generally referred to as candidate species. Scientific data exists to indicate candidate species populations are not stable, and, in some cases the viability of that species is in question. Species of concern face declining populations and/or habitat but not enough scientific data exists to propose an inclusion of species of concern on the state or federal threatened, endangered, or candidate species lists.