This can occur directly sometimes as when a spring flood drives the muskrats out of their lodges and burrows and drowns many of their young. Other times, conditions work indirectly as when a cold spring kills oak blossoms, cutting the acorn crop in the fall and causing starvation for squirrels. Disease is another cause of mortality in the wildlife populations. Since many of the world's most virulent diseases have either been eradicated or are under control, we tend to think good health is the natural state and bad health is a departure from the norm. Just about every animal in the woods is host to a number of parasites and some of the parasites even have parasites.
Predators also take their slice of the population as well.
It is one of the most important causes of animal mortality in the wild. These and other influences join to reduce wildlife populations. How effective they will be depends to a great extent on the density of a population. As the population grows, more animals will die of weather-related causes, disease, starvation and predation. As the rabbit population increases in an area, food will gradually become scarcer and rabbits may die of starvation.