Final answer:
In positive interactions, such as commensalism, one species benefits while the other is not harmed. An example is the relationship between birds nesting in trees.
Step-by-step explanation:
In positive interactions, such as commensalism, the interaction does not cost either organism anything. In commensalism, one species benefits from the close, prolonged interaction, while the other species neither benefits nor is harmed. An example of commensalism is the relationship between birds nesting in trees. The bird benefits from protection, while the tree is not harmed by the presence of the nest.