Opportunistic organisms commonly refer to animals and plants that tolerate variable environmental conditions and food sources. Some opportunistic species can thrive on almost any available nutrient source: omnivorous rats, bears, and raccoons are all opportunistic feeders. Many opportunists flourish under varied environmental conditions: the common house sparrow (Passer domesticus) can survive both in the warm, humid climate of Florida and in the cold, dry conditions of a Midwestern winter. Aquatic opportunists, often aggressive fish species, fast-spreading plankton, and water plants, frequently tolerate fluctuations in water salinity as well as temperature.

A secondary use of the term "opportunistic" signifies species that can quickly take advantage of favorable conditions when they arise. Such species can postpone reproduction, or even remain dormant, until appropriate temperatures, moisture availability, or food sources make growth and reproduction possible. Some springtime-breeding lizards in Australian deserts, for example, can spend months or years in...