"Mature populations tend to reach equilibrium – the carrying capacity – and then fluctuate around this equilibrium. If a population outgrows its carrying capacity, regulating factors come into play, such as famine, or emigration. If a population is below its carrying capacity, birth rates tend to increase, so the population grows. The common assumption is that carrying capacity is determined by the availability of food, water and land. While availability of food and water are important factors in determining the carrying capacity of populations, they cannot explain the unprecedented increases in population that have occurred in the last several hundred years. The availability of land has always been a factor in increasing carrying capacity. In the historic past, the Earth’s carrying capacity could be increased by expanding into sparsely occupied, or frontier, lands. In a fictitious future, carrying capacity could be increased by expanding outward to other planets or solar systems. At present, there is very little unoccupied, habitable land remaining on this Earth and no nearby habitable planets to release the pressure of population growth, so any increase in carrying capacity must be a result of other factors."