The population goes through a boom-and-bust cycle every four years. A small furry rodent found in eastern Greenland called the collared lemming ( Dicrostonyx groenlandicus) is a good example of how predation can be a density-dependent limiting factor. Scientists who added new nest sites in one expanse of forest saw the chickadee nesting population increase significantly, suggesting that nest sites are a density-dependent limiting factor. These little cavities are excavated and then abandoned by woodpeckers. Mountain chickadees ( Parus gambeli) compete for a special kind of nest site-tree holes. Competition and predation are two important examples of density-dependent factors. Density-dependent limiting factors tend to be biotic-having to do with living organisms. Other density-independent factors include hurricanes, pollutants, and seasonal climate extremes. Wildfire is abiotic (nonliving), and most density-independent limiting factors fall in this category. For example, a wildfire that sweeps through a dense forest in the Everglades has a big impact on every population in the community, regardless of the density of any one population. These names mean just what they say: Density-independent factors have an impact on the population, whether the population is large or small, growing or shrinking. Density Matters-Unless It Does Not Limiting factors fall into two broad categories: density-dependent factors and density-independent factors. Often, the population is affected by several limiting factors that act together. Any of these factors-food, shelter, breeding sites, predators, and more-may serve to limit the growth of a rabbit or toad population. But even if they avoid a hungry hawk or snake, they face other potentially deadly dangers, including diseases, forest fires, or drought. Both toads and rabbits have to watch out for predators. American toads eat insects and, though they often live in forest habitat, need ponds or puddles to lay their eggs. Cottontail rabbits need food to eat (grasses and other plants), water to drink, and a safe place to raise their young. So why are the meadows and forests of the eastern United States not literally hopping with rabbits and toads? In nature, the size of a population and the rate of population growth are influenced by what ecologists call “limiting factors.” Take It to the Limit Think about all the different resources that two common animals need to stay alive. A female American toad ( Anaxyrus americanus) can lay thousands of eggs every spring. Limits to Growth A female cottontail rabbit ( Sylvilagus floridanus) can give birth as often as seven times a year.
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