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.
I believe the primary determinant of basal metabolic rate is the Lean Body Mass. Basal metabolic is the rate of energy expenditure per unit time by animals at rest. It comprises the processes that the body requires to function. It is the amount of energy per unit time that a person need to keep the body operational at a state of rest.
