A group of related living organisms is known as a species.  The members of a species must share physical traits and interbreed.

Usually, a scientist will try to classify a species in the simplest possible way, often through physical features.  However, species often share a gene pool and a habitat, and have biochemical characteristics in common.

Sympatric species, the most common type, are species that live close together without mating with each other.  Examples include bullfrogs, wood frogs, green frogs, and pickerel frogs.  Though these species live in the same habitat, they do not mate and are thus unique species.

The other type of species is allopatric.  These animals live in isolated locations.  Often, one group of a species will move away and lose contact with the original group, forming its own unique characteristics and a new species.

Native species are those that originally inhabited an area or are part of the natural flora and fauna.  Sometimes, exotic species from outside the area are introduced.

Another type of species is known as an indicator species.  These animals are closely examined, because when they are in trouble, changes in their ecosystem are usually taking place. 

For example, some think frog populations have declined due to pollution and that the decline of songbirds means that forests are being destroyed.  In 1975, the decrease in the number of striped bass in Chesapeake Bay indicated that there was pollution there.

A keystone species is one on which the survival of other species is dependent.  Animals that are dying out are called endangered species.  In recent years, countless species of animals have gone extinct.