Density dependent factors are factors that affect a population's growth differently depending on the density of that population. A prime example of a density dependent factor is sickness, which limits a population's growth more severely when density is high, as illness spreads more easily when organisms are in close contact.
Food availability is another common density dependent factor, and its effects may ripple through several levels of a biome's food web. For example, if a mouse population's food source is reduced, the population may level off or possibly drop. As a result, the snake or hawk population is then limited because of the lack of prey. Thus, the factor that limited the mouse's population may also cause the snake or hawk population to slow in growth or level off. Other common density dependent factors are migration and predation.
Density independent factors, on the other hand, affect the population the same way regardless of the density of the population. Density independent factors tend to be abiotic factors, such as storms, floods or harsh winters. These phenomena have the same effect on the population whether individuals were in close proximity to one another or spread out. These factors can limit a population, such as an extreme winter, but also spur on growth, such as excessive spring rainfall.