Which ecological interaction describes one species benefiting and the other harmed?

• A. Mutualism
• B. Commensalism
• C. Symbiosis
• D. Parasitism

Answer: (D)

Parasitism is an ecological interaction where one species benefits while the other is harmed. The parasite gains resources, shelter, or a habitat, often at the host’s expense, which can reduce the host’s energy, health, or reproductive success. This directly matches the description of one benefiting and the other being harmed. For example, a tapeworm inside a mammal steals nutrients from the host. In contrast, mutualism is when both partners benefit, commensalism is one benefits with the other unaffected, and symbiosis is a broad term for close living together that can be mutualistic, commensal, or parasitic; the specific situation described is parasitism.