Parasitism is when one organism benefits but the other is harmed. In the rain forest there ia a parasitic relationship between the leaf cutter and the phorid fly. When the ant is collectiong leaves is when the fly attacks. the fly plants its egs in the ants head, after the eggs hatch they new born flies borrow and feed on the ants body, thus killing the ant.
Commensalism is when one organism benefits and the other does not but it is also not harmed. The army ants walk all over the rain forest floor as the antbird flies above following the swarm of ants. The antbird follows the swarm of ants feeding on what ever the ants leave behind for the birds.
In the rain forest plants compete for sunlight so that have energy to grow and go through photosynthesis. Plants also compete for water to grow and go through the process of photosynthesis.