fakultativ

Fakultativ is an adjective used in biology and related fields to describe traits, relationships, or organisms that are not strictly required or fixed; they can function under multiple conditions or switch between modes of life depending on circumstances. The term is the direct Norwegian, German, or Danish form of the English word facultative, tracing back to Latin roots meaning ability or opportunity.

In ecology and physiology, facultative describes alternatives that are possible but not obligatory. A facultative organism can survive in a range of environments or utilize different resources, rather than being limited to a single niche. For example, facultative anaerobes can grow in the presence or absence of oxygen, using aerobic respiration when oxygen is available but switching to fermentation or anaerobic respiration otherwise. In parasitology, a facultative parasite can live independently of a host but may become parasitic under suitable conditions, while an obligate parasite requires a host for survival.

The term also applies to interactions, such as facultative mutualism, where two species benefit from an association but do not rely on it for survival. In botany and weed science, facultative species or weeds can thrive in both disturbed or managed environments and more natural habitats, unlike obligate species that are restricted to a particular setting.

Overall, facultativ denotes flexibility and conditionality, contrasting with obligate or mandatory traits and relationships.