An account of factors affecting population density

The carrying capacity is the upper limit in the site of population that the environment can sustain. A density dependent growth is when the density of the population itself is affecting the growth rate and the population oscillates around the carrying capacity. Examples of density dependent factors are food, light, water and competition. Organisms themselves act as density dependent factors regulating the population size. Density independent growth curves show an exponential curve which stops abruptly as the environmental resistance becomes suddenly effective. The growth rate is not determined by the population density itself but of factors such as a sudden drop of temperature, a fire or pesticide.

Intra-specific competition is the competition between individuals of the same species and creates a cycle in the population density. Abundant food results in a high feeding rate, immigration and successful reproduction, which in turn results in numbers increasing rapidly. The food supply then becomes scare causing competition for food and eventually starvation and lack of resources so the numbers decline and the feeding rate declines until there is once again abundant food and the cycle continues. Inter-specific competition is between individuals of different species and graphs can show how two organisms in the same habitat affect the population of the other organism. In natural populations one species rarely out competes the other such that the population of the latter declines to zero because there are sufficient parts of the niches for two species to co exist.

A Niche is defined as the precise place in a habitat where an organism lives which includes its 'role' in the community. Predators and preys have an obvious affect on the population density. Prey numbers increase due to abundant food, which results in predators eating more prey and a greater predator reproduction rate. Predation causes increased mortality among prey and prey numbers decline, which subsequently means that predators emigrate, find new prey species or stave. This means that there is a reduced mortality of prey so their density increases and the cycle begins again. The population density of the prey species rises first followed by that of the predator. As the predator population rises it begins to limit the prey population which then also falls. As habitats become more complex the population density increases which leads to a natural emigration of prey from the defined area. With no sediment involved in the cycle it results in both population crashing as there is nowhere for the prey to hide so the predators have an advantage, consume the prey and then have a poor numbers of prey to feed on so starve or emigrate.

Many factors affect population density, which results in the population density constantly varying.

Rebecca Winter 13S