Basic biological population dynamics are controlled by two kinds of factors: density-dependent and density-independent. Density-dependent factors include access to resources such as space, food, and potential mates. These resources become more limited as population grows. Density-independent factors include geographic limits such as the population's proximity to sources of water or to dangerous geological phenomena like volcanoes. These two thpes of factors also control human population growth. As the possibility of rapid human population expansion draws near, the question of resources becomes critical. Issues of food supply, water supply, and habitable space are now of global concern.