A number of different organisms can fill an ecological niche is called a what?

If you have more than one organism trying to fill the same niche, it's called competition - no two organisms can fill the exact niche at the same time, without one of them gaining some type of advantage over the other. That's called the competitive exclusion principle or "Gause’s principle" for G. F. Gause, the Russian scientist who demonstrated this idea by growing two species of Paramecium in the sme container, and counting the individuals every day. Eventually, one species always won out over the other until the one died out.