a species with the capacity to live under different environmental conditions —note 1. when the term is used in reference to species occurrence in a wetland or upland habitat, the species is most likely (67 to 99 percent frequency) found in a wetland and called a facultative wetland species; if the species is most likely (67 to 99 percent frequency) found in an upland, it is called a facultative upland species; if the species is sometimes found in a wetland or upland (34 to 66 percent frequency), it is called a facultative species —note 2. facultative parasites can function as either parasites or saprophytes —note 3. facultative anaerobes can function in aerobic or anaerobic conditions —see obligateThis definition last updated 08/07/2008