Three different mechanisms contribute to maintaining the unusually high numbers of yellow crazy ants on Christmas Island. The enemy release hypothesis, suggests that lack of pressure from co-evolved natural enemies may allow organisms to achieve a large colonial size. Another hypothesis relies on the fact that reduced intracolony aggression enables foraging ants to use the energy allocated to colony defense on expanding their colony. Finally, the superior efficiency compared to native species at exploiting a diverse array of resources enables the ants to maintain a high foraging rate and thus increase in numbers.