A parasitic relationship in ecology is a type of interaction between two species where one, the parasite, benefits at the expense of the other, the host. This relationship often leads to harm or disadvantage for the host, which may experience diminished health, energy loss, or reproductive capacity. Examples include tapeworms in mammals and mistletoe on trees. Understanding parasitism is essential for grasping ecosystem dynamics, as it influences population sizes, community structure, and evolutionary strategies adopted by both parasites and hosts to survive and reproduce.