A population consists of all the individuals of a species that live together in one place at one time. This definition allows scientists to use a similar term when speaking of different populations whether they are the human population, bacterial population or animal population.
Every population tends to grow because pairs of individuals have multiple offspring over a period of their lifetime. Predicting how the size of a population will grow is the job of demographers. Demographers study three key features of populations in order to predict population growth: size, density, and dispersion.
One of the most important features is the population size. The number of the individuals in a population, or population size, can affect the population's ability to survive. Smaller populations are at risk to become extinct due to floods, fires, or natural disasters.
Population density is the number of people that live in a given area. If the individuals of a population are few and spaced far apart, it is difficult for them to get together to reproduce lowering the reproduction rate.
A final important feature is the pattern of dispersion. Dispersion is the way the individuals of the population are arranged in a particular space. There are three main patterns of dispersion that are possible.
If they are randomly spaced the location of the individual is self-determined. If they are evenly spaced, they are located at regular intervals. If they are in a clumped distribution, they are located in clusters. Each of these patterns reflect the interactions between the population and its environment.
When demographers try to predict how a population will grow they make a model of the population. A population model is a theory that attempts to model key characteristics of a real population. By making changes and observing the outcome of the model, demographers can better predict the outcome in a real population. There are three basic models.
A Stage I population model describes the rate of growth as defined by the difference between the birth rate and the death rate. A Stage II population model is an exponential growth curve in which the population growth stays the same, but the population size increases steadily.
There are also density-dependent factors to measure growth rates. As a population grows the resources to sustain that population, such as food and water, decreases as a result. When this happens, the growth rate of the population decreases. They are density-dependent because the rate at which the resources become depleted depends upon the density of the population that uses them. This becomes the Stage III model of measuring population growth.
There are density-independent factors that are used to measure the growth rate of populations. When the growth rate is limited not by density-dependent factors, but by environmental factors such as weather or climate they are known as density-independent factors. For example, mosquito populations tend to grow in the summer, yet they decrease in the winter.
In summary, the growth rate of populations is complex and needs continued research and study to predict. Using various features demographers may better predict the future growth populations.
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