Which of the following illustrates mutualism?

• A. A predator hunting prey
• B. A parasite harming host
• C. Competition for resources
• D. Nitrogen fixing bacteria living in plant roots which give nitrogen to the plant

Answer: (D)

Mutualism is a close, long-term interaction in which both species benefit. In the case of nitrogen-fixing bacteria living in plant roots, the bacteria convert atmospheric nitrogen into a form the plant can use for growth, providing essential nutrients. In exchange, the plant supplies the bacteria with carbohydrates produced through photosynthesis, giving them energy. This two-way benefit shows both organisms gaining from the relationship, which is what defines mutualism. Other scenarios describe different interactions: predation harms the prey while benefiting the predator, parasitism harms the host while benefiting the parasite, and competition reduces resources for both. The nitrogen-fixing plant-bacteria partnership clearly demonstrates mutualism because both participants benefit.