Unfortunately this question is incomplete as it does not provide any choices. However, there are certain ecological factors that can cause a population of mice to decrease quickly. One is an increase in predation. Many carnivorous animals prey on mice, including birds of prey, foxes and various other predacious mammals, as mice are small and high in number. This means a lot of mice are removed from the population through predation. Any increase in predator pressure could quickly disturb the balance in the mice population. Another factor is a disease. Some diseases can seriously deplete populations of animals, with the remaining hardier individuals developing immunity.
