Organisms within a population interact with one another and their environment. Populations are controlled by factors like births, deaths, immigration and emigration. Carrying capacity is the maximum population size an environment can sustain without degradation. Limiting factors like disease, starvation and competition for resources prevent overpopulation. Population growth follows S-curves or J-curves. Succession over time leads to increasingly complex communities that stabilize at a climax stage in equilibrium with the environment.