Resource availability is essential for the unimpeded growth of a population. Ideally, when resources in the habitat are unlimited, each species can fully realize its innate potential to grow in number, as Charles Darwin observed while developing his theory of natural selection. Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time. Darwin showed how even a slow-growing animal like the elephant could theoretically reach an enormous population if there were unlimited resources for its growth in its habitat. This is unrealistic in almost all situations (with exceptions, such as a laboratory); there is simply a finite quantity of everything necessary for life, and individuals in a population will compete with their own or other species for these finite resources. As the population approaches its carrying capacity, the rate of growth decreases, and the population trend will become logistic.
