Field observation suggests that populations of a particular species of herbivorous animals undergo cyclic fluctuations in density at 3 to 5 year intervals. Which represents a plausible explanation of these cycles? a. Periodic crowding affects the endocrine system, resulting in increased aggressivesness b. increases in population density lead to increased rates of predation c. increases in rates of herbivory cause changes in the nutritive value of plants used as food d. increases in population density causes more proximal infestations of parasites to host animals e. all of the above