Density dependence refers to the regulation of population growth rates by the density of a population in population ecology/45%3A_Population_and_Community_Ecology/45.02%3A_Environmental_Limits_to_Population_Growth/45.2C%3A_Density-Dependent_and_Density-Independent_Population_Regulation). Density-dependent processes can be positive or negative. Positive density-dependence, also known as density-dependent facilitation or the Allee effect, describes a situation in which population growth is facilitated by increased population density. Negative density-dependence, or density-dependent restriction, describes a situation in which population growth is curtailed by crowding, predators, and competition. Density-dependent limiting 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. Density-dependent factors often arise from biological phenomena, rather than from physical and chemical phenomena. Examples of density-dependent factors include competition within the population, diseases, and predation. Density-dependent regulation can also take the form of behavioral or physiological changes in the organisms that make up the population. Density-independent factors, such as environmental stressors and catastrophes, are not influenced by population density change. Examples of density-independent factors include food or nutrient limitation, pollutants in the environment, and severe weather conditions such as fire/45%3A_Population_and_Community_Ecology/45.02%3A_Environmental_Limits_to_Population_Growth/45.2C%3A_Density-Dependent_and_Density-Independent_Population_Regulation).