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Population Life Cycle
According to Estrella Mountain Community College, all populations undergo three distinct life cycle phases. First there is population growth, which occurs when available resources exceed the number of individuals able to exploit them, the birth rate exceeds the death rate, resulting in a net increase in population. The second, and longest phase, is one of population stability. Population stability is usually followed by a "crash" as the growing population eventually exhausts its available resources. Thus the final phase is population decline, in which the death rate exceeds the birth rate, and may eventually lead to the extinction of a population.
Factors Influencing Population Growth
Limiting factors in the environment include food, water, minerals, predators, shelter and buildup of waste materials, as detailed by IUPUI Department of Biology, all of which prevent population sizes growing beyond certain levels. The carrying capacity refers to the maximum number of individuals (size of population) that be supported by the environment. Thus, populations will grow exponentially until the carrying capacity of their environment is reached. The two modes of population growth are summarized by Estrella Mountain Community College in two graphs, the first of which is called a "J-curve," representing exponential growth of a population, when there is no limit on population size. The second graph, a logistic "S-curve," shows the effect of the carrying capacity of the environment leading to population growth tailing off.
Human Population Growth
IUPUI Department of Biology describes how human population growth is different to the growth of other species' populations due to the way in which humans began "sidestepping" controls on growth rate. This first came about through the ability of humans to expand into new habitats. This was achieved due to the evolution of hunter-gatherers, proceeded by knowledge and language development, allowing for survival of humans in habitats across the world, and therefore greater numbers of people. Following on from this, around 12,000 years before the present, humans increased the carrying capacity of their environments. According to the IUPUI Department of Biology, this was achieved through a shift to agriculture and irrigation to provide food and water to meet the needs of larger populations.
Over-Population
Sometimes populations of species can over-grow beyond the carrying capacity of their environment. According to the IUPUI Department of Biology, over-crowding such as this can promote the spread of disease and pathogens throughout a population at greater speeds, leading to dramatic reductions in the population size as large numbers of species die out. Similarly, populations that have become too large can crash due to buildup of waste materials, pollutants or over-exploitation of resources. The ever-increasing rate of human population growth, however, has been sustained by technological advances that have enabled greater numbers of individuals to persist. These include medical advancements, improvements in agricultural yields, such as the green revolution, and general improvements in living conditions across the world. Factors such as this allowed humans to mitigate limiting factors on population growth common to all other species.