Beginner 8 minutes
Ecological relationships: who wins, who loses, and who never finds out
Mutualism, commensalism, parasitism, and competition, and how to recognize them underwater
A tiny fish swims into the mouth of a predator that could swallow it without effort, cleans its teeth, and comes out alive. Another lives among the tentacles of an animal that kills everything it touches. On the reef there are arrangements no intuition would predict.
In this article you'll see how relationships between organisms are classified according to who benefits, with observable examples of mutualism, commensalism, parasitism, and competition, and why the boundaries between those categories are blurrier than they seem.
It's the reading key that turns a pretty dive into an interesting one: almost everything that looks like coincidence (two animals together, a fish holding still when it should flee) is a relationship with its own logic.
How they're classified
The criterion is simple: who comes out ahead.
Symbiosis, broadly speaking, is any close, prolonged relationship between two species. Within it, three cases are distinguished according to the balance for each side.
Mutualism benefits both: each gets something and each pays something. Commensalism benefits one with no appreciable effect on the other. And parasitism benefits one at the other's expense, though without killing it right away, because a parasite that kills its host quickly loses its home.
Outside symbiosis lie two other fundamental relationships. Predation, where one eats the other, which is the engine of the food webs from the previous article. And competition, where two organisms need the same limited resource and both are harmed by the other's presence.
These categories are useful but not watertight compartments. Many relationships change sign depending on conditions: a mutualistic association can turn parasitic if one side is weakened, and the line between commensalism and parasitism often depends on a cost so small it's hard to measure.
The most important case in the ocean is also the most extreme: the relationship between coral and the algae living inside its tissues, described in the coral biology article. It's a mutualism so close that neither side builds a reef without the other, and its breakdown is what's called bleaching.
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