Which term best describes a relationship in which both participants benefit?

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

Answer: (D)

In ecology, a mutualistic relationship describes an interaction in which both participants benefit. Each partner provides something the other needs—such as pollinators gaining nectar while helping plants reproduce, or nitrogen-fixing bacteria supplying usable nitrogen to plant roots in exchange for carbohydrates. Because both sides gain something valuable, this is the best term for describing a two-way benefit. By contrast, parasitism involves one organism benefiting at the expense of the other; invasive species are organisms that disrupt ecosystems rather than define a two-way beneficial interaction; and extinction is the disappearance of a species. Some mutualisms are obligate, meaning the partners rely on each other for survival, while others are facultative, where the relationship is advantageous but not essential.