facultatives

Facultatives is the plural form of facultative, an adjective used in biology to describe traits, strategies, or organisms that can operate under alternative conditions or modes but are not obligatorily bound to one. The term emphasizes flexibility rather than essential dependence. In taxonomy and ecology, facultative relationships are non-exclusive and can change with environmental context.

In microbiology, facultative describes organisms that can perform a function but do not require it. A classic example is facultative anaerobes, which can grow in the presence or absence of oxygen; Escherichia coli and Bacillus subtilis are common examples. Some microbes are also described as facultative intracellular pathogens, meaning they can replicate inside host cells when needed but can also grow outside cells.

In parasitology and ecology, the term distinguishes facultative parasites from obligate parasites. Facultative parasites can obtain nutrients from a host when possible but can complete life cycles or survive in other environments without a host. Obligate parasites, by contrast, depend on the host for survival and reproduction.

In ecology and evolutionary biology, facultative interactions refer to relationships that are beneficial but not essential. Facultative mutualism occurs when species interact in a way that increases fitness but the interaction is not required for either to survive. Similarly, facultative predation or scavenging describes organisms that can prey on live prey or feed on dead matter depending on conditions.

Etymology: from Latin facultas, meaning "ability" or "opportunity."