Final answer:
A facultative interaction is optional and not essential for organism survival, unlike an obligatory interaction. It can occur in symbiotic relationships like commensalism where one species benefits and the other is unaffected.
Step-by-step explanation:
A facultative interaction is one that is optional and not essential for the survival of the organisms involved. This means that while the interaction may provide benefits to one or both of the species, they can survive without it. This type of interaction can be contrasted with an obligatory interaction, where the relationship is crucial for the survival of both species.
Facultative interactions could include certain symbiotic relationships like commensalism, where one species benefits while the other is neither helped nor harmed. For example, birds nesting in trees represent a commensal relationship; the tree is not harmed by the nest, while the birds gain protection.
Other types of symbiotic relationships include mutualism, where both species benefit, and parasitism, where one species is harmed while the other benefits.