Answer: The correct answer for the fill in the blank is- They try to share the same niche.
According to competitive exclusion principle, two species when present in the same area, try to compete for the same limiting resource like food. The species having better advantageous trait over the other survive and exclude the other one. In other words, the second species is either eliminated or shifts to some other niche.
This principle is also called Gause's principle.
Thus, competition occurs between two species when they try to share the same niche.