Commensalism is one example of a symbiotic relationship-where two organisms interact with one another in a feeding relationship. Commensalism can be thought of as a partnership in which one organism in the relationship benefits (+) while the other member is totally unaffected (0) by its presence. The plus sign denotes a positive effect or benefit and the zero denotes that the organism neither benefits nor is harmed.
A classic example of commensalism is when barnacles attach to the outer skin of whales. Barnacles are sessile organisms and filter feeders. By attaching to a whale it gains an advantage (+) of visiting new feeding grounds whenever the whale moves about in the ocean. The whale is neither helped nor harmed (0) by the presence of the barnacles.
Commensalism can be seen in the relationship between tall trees and epiphytes--plants that can grow on top of a tall tree in order to gain more sunlight or access to nutrients from the air. Rainforest trees will often have epiphytes growing on top of them including vines and certain types of orchids. The rainforest has very dense growth and smaller plants would have a difficult time reaching the area where the most light is present near the top. By wrapping around taller trees, the epiphyte will benefit (+) and the tree is not harmed (0) in any way.