Poison dart frogs live in the rainforest. They produce a poison by eating toxic fire ants. The only known predator is a snake, Liophis epinephelus. Which of the following factors is most likely a density-independent regulator of population growth of poison dart frogs in the rainforest?
a) clear cutting the rainforest
b) limited food source
c) susceptibility to disease
d) increase in population of Liophis epinephelus