Density-dependent factors are forces that affect the size of a population of living things in response to the density of the population, which is the number of individuals per unit area. These factors are often biological in nature and can be either positive or negative in their correlation to population density. Density-dependent factors can cause a populations per capita growth rate to change, typically to drop, with increasing population density. Examples of density-dependent factors include competition for limited food among members of a population, disease, and predation. These factors can lead to a logistic pattern of growth, in which a populations size levels off at an environmentally determined maximum called the carrying capacity. Some common examples of density-dependent limiting factors include competition within the population, parasitism, predation, disease, and migration. In contrast, density-independent factors affect per capita growth rate independent of population density, such as natural disasters like forest fires, and are often abiotic. The dynamics of most populations of living things are influenced by a combination of density-dependent factors and density-independent factors, and the relative effects of the factors vary among populations.