Population Ecology

A population is a group of interbreeding organisms found in the same place at the same time. Population ecology studies the dynamics of populations and how populations interact with the environment. There are a number of characteristics of populations that help ecologist and other scientists to monitor and manage wild populations. Population density, abundance, distribution, age structure and sex ratio are important characteristics which can be monitored for good population management.


Population density refers to the number of individuals in a given space. For instance, density may be measured as the number of trees per hectare or the number of wolves per square mile. The abundance of a population is different to the density as it does not deal with an area or volume but focuses entirely on the total population size in a particular ecosystem.

Abundance can be difficult to measure and there a number of methods used to estimate a population’s abundance if a complete count is not possible. Mark/recapture methods are common among large animal species where a number of individuals are caught, tagged and released; then an effort is made to recapture individuals from the same population and the percentage of caught individuals that are tagged is used to give an estimate of the population size for a given area.

Distribution of a population

School of fish - population ecologyThe distribution of a population tells us how large the home range is that individuals from a particular population can be found. Generally, populations are not distributed evenly and so density and distribution are often studied together.

The way in which a population is distributed often gives an insight into the relationships between individuals. If individuals are spread evenly across an area it is often due to negative relationships that cause each individual to be as far away as possible from others. Contrary to an even spread, individuals of a population may be attracted to each other and live in clumps causing a very uneven distribution of the population across the landscape.

Age structure

The age structure of a population can be helpful in determining the future growth rate of a population. Individuals can be categorized into three groups – pre-reproductive, reproductive and post-reproductive. Populations with large pre-reproductive and reproductive groups are likely to experience growth in the near future.

If large proportions of populations are post-reproductive it means recruitment of new individuals is proportionally low for the population. This is likely to lead to either zero growth or a decrease in the population’s size.

Age structure is very important in the management of fisheries and wild populations. In management schemes it is important to maintain significant levels of reproductive individuals and in harvested stocks knowledge of the populations age structure can be used to influence which individuals are important to keep in the population.

Sex ratio

The sex ratio of a population is simply the number of males relative to females. Sex ratio is often influenced with time and the life stage of individuals. The sex ratio at fertilization is usually around 1:1 but this has often changed by the time individuals reach a sexually active age, where females regularly have an increased abundance. At the post-reproductive stage, female abundance is also often higher than that of their male counter-parts. These are, however, only trends and differ from species to species.

Meta populations

Meta-populations are groups of separated populations which interact with each other at some level. There is a range of different functional structuring that meta-populations can be built upon. The structure of meta-populations depends on the level of isolation/connectivity between meta-populations and the size structure of each sub-population. The sub-population being a specific group of a meta-population, the meta-population is therefore made up from many sub-populations.

Last edited: 24 May 2015

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