Population limitations depend on many factors. Limits to human population growth may be density-dependent or density-independent. Density-dependent factors limit population growth because of the number of individuals in the area. An outbreak of cholera in a city could cause an increase in human mortality because the proximity of many people exposing each other to the pathogen causes the disease to spread quickly. Density-independent factors limit population growth primarily because of environmental stresses, such as catastrophes. A hurricane strikes an area, destroys homes, cuts off power, and limits food and clean water accessibility. If this situation is not resolved quickly, human mortality rates will increase because resources are not available. This disaster could happen on a small island or in a crowded city.Changes in human populations are not evenly spread throughout the world. In many developed countries, birth rates are declining and lifespans are increasing because of advances in medical technology. In developing countries, lack of access to birth control may continue to increase populations. In Canada, the fertility rate is 1.6 children per female, while Uganda and Zambia have fertility rates of more than 5 children per female. Lifespans also vary in different parts of the world, with Japan and Switzerland having projected lifespans of over 83 years, while the projected lifespan in Angola is only about 50 years. Food and water supplies fluctuate in many developing countries that struggle to feed their populations, while developed countries have an abundance of food and access to clean, drinkable water. Many criteria affect the demographics of humans: immigration and emigration and their causes, population density, age and sex ratios, and so on. Both natural and human events may seriously impact each of these criteria. Hurricanes, earthquakes, volcanoes, and drought may cause a shift in the locations of populations, and disease may also impact populations. For example, in the 1930s, drought conditions turned much of the Central United States into a "Dust Bowl." Families migrated west in massive numbers. Oklahoma lost 18.4% of its population as 440,000 people left to find better living conditions. Human-caused events can also create dramatic population shifts. In two large migrations, from 1916 through 1970, more than 6 million African Americans moved from rural southern states to cities in the Northern, Western, and Midwestern United States in search of better working and living conditions. As human populations increase, ecologists grow concerned about the demographics of various animal and plant populations. For example, the spotted owl living in the northwestern United States has lost much of its habitat due to logging. Increased human occupation reduces, eliminates, or fragments habitats. Human activity can result in increases of introduced or invasive species, which adversely affect native plant and animal populations. In the most extreme situations, human populations have caused extirpation of species in some areas and total extinction of other species.