What type of relationship involves one organism benefiting at the expense of another?

The concept of parasitism refers to a symbiotic relationship in which one organism, the parasite, benefits at the expense of another organism, the host. In this relationship, the parasite derives nutrients or some advantage from the host, often causing harm in the process. This type of interaction is characterized by the exploitation of the host, which may experience negative effects such as disease, nutrient depletion, or damage to tissues.

In contrast, mutualism involves both organisms benefiting from the relationship, while commensalism sees one organism benefiting without significantly affecting the other. Symbiosis is a broader term that encompasses various types of close biological interactions, including mutualism, commensalism, and parasitism, but does not specifically define the exploitative aspect that is characteristic of parasitism. Therefore, the correct identification of parasitism highlights the key feature of benefit versus harm in ecological relationships.