What is Density-dependent and density-independent Factors?

Density-dependent : Ecologists identify population-regulating mechanisms whose functioning is related to density as density-dependent factors. For example, as the size of a population of deer increases, food consumption goes up, and the amount left available decreases. The food resources for the population as a whole may then not be sufficient to support the larger number of deer, and will start to limit the number in the population. In simple terms, without enough food, the individual will die, decreasing the population. Or, without enough food, the organism health may be weakened, making it more susceptible to disease, or less able to reproduce. These conditions would also result in a decrease in population size.

Individuals in a population that normally compete with others for resources will compete even more vigorously for the same resources as they become fewer and fewer. The same process can be seen happening with other environmental factors such as light, dissolved oxygen, nutrients, and other factors. In general terms, what happens is that density-dependent factors have heightened effects as the population size increases, which result in regulation of the population size.

Density-Independent Factors :  Density-independent factors, on the other hand, are not a function of population size. These factors serve to limit population size no matter what the size of the population, and therefore work independently. Examples of density-independent factors would be climate and weather. Hurricanes, tsunamis (often improperly named tidal waves, forest fires and drought usually result in unusual numbers of deaths, which reduce populations. Human activities are notorious for affecting sizes of natural populations. Note that these deaths would have occurred no matter what the population size was.