Essential; not facultative; limited to a single life condition; able to survive only in a particular environment or to assume only a particular role, as an obligate anaerobe.
(Of a life form or habitat requirement) restricted to this life form or habitat and not occurring as or in any other.
Restricted to a certain habitat or life history strategy.
Referring to an organism which exists and develops in only one way, e.g. a virus which is a parasite only inside cells.
Surviving only in a particular environment (e.g., an obligate parasite cannot survive without a particular host).
Describing an organism that is restricted to one particular way of life; for example, an obligate parasite cannot exist without a host.