Which term describes the ecological interaction in which both species benefit?

• A. Mutualism
• B. Parasitism
• C. Invasive species
• D. Keystone species

Answer: (A)

Mutualism is when two species gain fitness benefits from their interaction. In this kind of relationship, each partner provides something the other needs, and both are better off because of it. For example, pollinators like bees get nectar while flowers receive pollen transfer for reproduction, and mycorrhizal fungi trade nutrients with plant roots for carbohydrates. This is different from parasitism, where one species benefits at the expense of the other; invasive species describe a non-native organism that disrupts ecosystems, not a specific two-sided interaction; and keystone species are defined by their large ecological impact, not by the nature of a two-species interaction. So the term that best fits an interaction where both species benefit is mutualism.