Facultative vs. Obligate

Facultative

Facultative means “optional” or “discretionary” (antonym obligate), used mainly in biology in phrases such as:

Facultative (FAC), facultative wetland (FACW), or facultative upland (FACU): wetland indicator statuses for plants

Facultative anaerobe, an organism that can use oxygen but also has anaerobic methods of energy production. It can survive in either environment

Facultative biped, an animal that is capable of walking or running on two legs as well as walking or running on four limbs or more, as appropriate

Facultative carnivore, a carnivore that does not depend solely on animal flesh for food but also can subsist on non-animal food. Compare this with the term omnivore

Facultative heterochromatin, tightly packed but non-repetitive DNA in the form of Heterochromatin, but which can lose its condensed structure and become transcriptionally active

Facultative lagoon, a type of stabilization pond used in biological treatment of industrial and domestic wastewater

Facultative parasite, a parasite that can complete its life cycle without depending on a host

Facultative photoperiodic plant, a plant that will eventually flower regardless of night length but is more likely to flower under appropriate light conditions.

Facultative saprophyte, lives on dying, rather than dead, plant material

Obligate

As an adjective, obligate means “by necessity” (antonym facultative) and is used mainly in biology in phrases such as:

Obligate aerobe, an organism that cannot survive without oxygen

Obligate anaerobe, an organism that cannot survive in the presence of oxygen

Obligate air-breather, a term used in fish physiology to describe those that respire entirely from the atmosphere

Obligate biped, Bipedalism designed to walk on two legs

Obligate carnivore, an organism dependent for survival on a diet of animal flesh.

Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.

Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell

Obligate parasite, a parasite that cannot reproduce without exploiting a suitable host

Obligate photoperiodic plant, a plant that requires sufficiently long or short nights before it initiates flowering, germination or similarly functions

Obligate symbionts, organisms that can only live together in a symbiosis

Facultative (adjective)

occurring optionally in response to circumstances rather than by nature

“facultative adjustment in relation to competitive abilities”

Facultative (adjective)

capable of but not restricted to a particular function or mode of life

“a facultative parasite”