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Lesson 3: Population Ecology
3 Population Growth < Back | Next >

Populations grow due to an increase in the number of individuals within the population. Individuals are added to a population through birth and immigration and are removed from the population due to mortality and emigration. If the population gains more individuals than it loses, the population will grow. Population growth can only continue as long as enough resources are available to support the growing population. Thus, population growth depends upon the size of the population and how close it is to the carrying capacity of the habitat. The carrying capacity is the maximum number of individuals that can be sustained for a given area under a given set of conditions (Mackenzie et al 1998). The population density is defined here as the number of individuals per unit area.

Populations which are density dependent can often be explained by an “s” shaped curve. In the early stages of population growth when there are few individuals, nutrients are readily available and the death rate is minimal. This allows for reproduction and population growth. Growth will continue until the population size approaches the carrying capacity. At this point the growth rate declines to zero and the population stabilizes at the maximum carrying capacity. In the real world, examples have shown fluctuations around the carrying capacity as a result of environmental fluctuations.

It is also possible to have a population which is not density dependent. In this case, the population is not limited by resources. A density independent population will tend to grow exponentially with no limit to the population size. In other words, the mortality and birth rates will not be affected by the density of the population.

Density is just one factor which limits population growth. Other factors such as lack of food or nutrients, lack of water, lack of suitable habitat, adverse weather conditions, predators, disease, parasites and competitors also can influence the population density. For example within a given area there ma be only a small area which is suitable habitat for a given species, or in other cases the amount of suitable habitat may be plentiful but one species may be out competed by another to occupy that habitat.


Scott Bauer, USDA Agricultural Research Service, www.forestryimages.org
LESSON 3
1 Population Ecology
2 Population Characteristics
3 Population Growth
4 Types of Density Dependence
5 Population Dynamics
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