In an ecosystem, why can only one species fill an ecological niche?

In chemistry, the type of an atom (what element it is) is determined by: a) the number of protons it contains in its nucleus. b) the number of neutrons it contains in its nucleus. c) the number of protons it has in a cloud around the nucleus. d) the number of neutrons it has in a cloud around the nucleus. e) the number of electrons it exchanges with its neighbors.