Opportunistic species are not resource‐limited. With favorable events they can increase rapidly in numbers. Such species populations are typically very unstable and may become extinct very quickly. In contrast, equilibrium species populations are resource‐limited and maintain stable population levels. The identification of explosive opportunists in the fossil record is an important means of recognizing animal communities which were primarily controlled by the physical, and not the biotic, environment. In young environments of high physiological stress where animal communities are physically controlled, most species are opportunistic. In old and biologically accommodated communities opportunists are relatively rare. Explosive opportunists can be recognized by means of distributional, relative abundance and fossil occurrence d