The ecological species concept defines a species as a group of interrelated organisms that occupy or adapt to a single niche. For example, if a population of birds exploits two different food sources, it represents two distinct species. By contrast, vastly different looking animals may be the same species if they fill the same niche.
Scientists do not have a universally agreed upon definition of “species.” The ecological species concept is one of several definitions scientists have proposed. Other species concepts include the biological species concept, evolutionary species concept and isolation species concept.
The biological species concept states that members of the same species cohabitate and can breed and produce viable, fertile offspring with each other. Horses and donkeys, for instance, are different species because their offspring are infertile. However, many scientists reject this definition, as some widely divergent species that would never encounter each other in the wild are capable of breeding and producing young.
The evolutionary species concept states that a species is a population of ancestor-descendant organisms that share the same evolutionary tendencies and fate. The isolation species concept states that populations only become species when they are reproductively isolated from one another. This isolation allows different forms to occur in formerly homogenous populations.