Populations vary in size over time. Decreases in population size can result from decreases in the number of births, increases in the number of deaths, or both. Factors that affect birthrates and death rates may be of two kinds: density-dependent and density-independent. The effects of density-dependent factors, such as food scarcity, contagious disease, and predation, increase as the population size increases. The effects of density-independent factors, such as drought, severe storms, unseasonable weather, and fire, are independent of population size. These factors may be just as devastating in small populations as they are in large populations.