Explain the competitive exclusion principle.

Two species of paramecium, *Paramecium aurelia* and *Paramecium caudatum*, were cultured in laboratory conditions. When grown separately in identical nutrient media, both species reached high population densities. When grown together in the same medium, *P. aurelia* population increased while *P. caudatum* population declined to zero by day 16. Use this information to explain the competitive exclusion principle.

Two species of aquatic beetle, *Dytiscus marginalis* and *Dytiscus semisulcatus*, were grown in laboratory tanks. When cultured separately, both species reached a population density of approximately 25 individuals per litre. When cultured together in the same tank with identical food and space resources, *D. marginalis* reached 22 individuals per litre while *D. semisulcatus* declined to extinction by day 18. Use this data to explain the competitive exclusion principle.

Two species of coral reef fish, Species X and Species Y, both feed exclusively on small crustaceans that shelter in the same type of branching coral. When researchers observe the fish populations over several months, they find that Species X maintains stable numbers while Species Y gradually declines and eventually disappears from that coral habitat. Species Y continues to thrive in nearby rocky crevices where Species X is absent. Which statement best explains this observation using the competitive exclusion principle?