5.1 How Populations Grow

Describing Populations

There are certain facts you need to describe a population: Geographic Range, Density and Distribution, Growth Rate, and Age Structure.

Geographic Range: This is the whole range of places where a population can live; its habitat.

Density and Distribution: Density means how many organisms of a population are in a specific area. Distribution is how the organisms are placed within a population’s habitat. The distribution can either be Random (in no specific pattern), Uniform (with similar spacing, in a close to regular pattern, and alone) and Clumped (in small groups that are scattered irregularly and of different sizes).

Growth Rate: The growth rate of a population controls the future of the population; whether it will shrink, get bigger or stay the same.

Age Structure: This is how many male and female organisms there are of each age group within a population.

Population Growth

Population Growth is another important aspect to consider when looking at populations. Some major factors that affect the growth of a population is the number of births, the number of deaths, and the number of organisms that immigrate or emigrate to and from the population.

Population will grow during periods of time when there are more births than deaths, and the other way around. Note that “birth” in this context also refers to eggs hatching and seeds sprouting.

If new individuals come into (immigration) a population’s geographic range, they become part of that population, and the population will increase. The opposite will happen if organisms from a population leave (emigration) the population and go into a new geographic range.

Exponential Growth

Under the ultimate circumstances, with unlimited resources, a population will grow exponentially. In this context, this means that one generation will be larger than the one before it. Exponential growth comes into action because organisms will generally try to produce more offspring than one, so there will be more “children” than “parents”. This type of growth also means that the larger a population becomes, the faster it will grow. This type of growth cannot continue forever, since there are limited resources and space on this planet, and there are predators. Different species will reproduce at different speeds, but almost all will for some time, somewhere reproduce exponentially.

Logistic Growth

There are three phases of the growth of populations, and exponential growth is the first. Phase two is when the growth slows down due to limited resources, space, predators, etc. The population will continue to grow, but more slowly. In phase three, the growth stops at the so called carrying capacity (the largest number of organisms that a geographical range can support). This growth pattern is called logistic growth.

The population size will then remain close to this (carrying capacity) unless there are dramatic changes in the environment, such as natural disasters, disease, decrease or increase of the other species in its food webs, or major human interaction (hunting, feeding, industrial spills, etc).


Figure 5-1a, Logistic Growth Model



5-1a Logistic Growth Model. Digital image. NABT. Web. 01 Dec. 2012. <http://www.nabt.org/blog/2009/05/25/using-spreadsheets-to-introduce-the-logistic-population-growth-model/>.