What is population ecology?

Population ecology studies the distribution and abundance of plant and animal populations by analyzing a population, a group of interbreeding individuals of one species in a specific geographic area. Factors such as fertility, mortality, and migration influence population change, affecting attributes like age, sex, and geographic distribution. Population growth is also affected by the environment's carrying capacity , which is the maximum population size an environment can sustain based on available resources. Mathematical models, such as the Lotka-Volterra equations , assess species interactions and predict population dynamics.

What is a population in ecology?

In population ecology , a population consists of individuals of one species inhabiting a specific geographic area who interbreed, if they reproduce sexually. The geographic boundaries of a population can be easy to define, such as for plants or animals on islands. However, for species dispersed across large areas, pinpointing local population boundaries is more challenging. Populations range from closed (geographically isolated with no exchange) to open (varying degrees of connectedness). Population ecology studies why some species are rare while others are abundant and also examines the reasons behind population fluctuations. Factors affecting population size are density-dependent, such as competition and disease, and density-independent, such as weather. Organisms within the trophic network are arranged into populations and communities.

How do scientists measure population size?

Scientists measure population size using various vital rates and refined techniques. Crude birth and death rates, calculated per 1,000 inhabitants, indicate population changes, with the difference revealing the rate of natural increase or decrease. The marriage rate, though crude, records annual marriages per 1,000 inhabitants. Population density, while useful, is an imperfect measure that can obscure differences in settlement intensity within a unit of land. Studies that consider population density often use sampling to estimate the number of individuals in the area of interest and so are subject to sampling error.

What factors can cause a population to increase or decrease?

Population size is affected by the interplay of births, deaths, immigration, and emigration. A population increases through births and immigration and decreases through deaths and emigration. Populations are rarely closed, meaning that migration in and out can change population size, as do births and deaths. The rate of natural increase is the difference between births and deaths. Density-dependent factors intensify as a population increases in size, while density-independent factors, such as weather and climate, affect population size regardless of density. Food availability, competition, disease, and predation are ecological factors that curtail population growth.

How does competition affect population growth?

In population ecology, competition affects population growth through several ecological factors, including food availability and environmental stress. The life-sustaining resources in an environment are limited and can be reduced by greater population density and competition from other species. When population density is high, mortality tends to increase because of competition for resources, predation, or increased disease transmission, resulting in a plateau in population growth. The Lotka-Volterra equations can predict the effects of species interactions on population dynamics. Interspecific competition may lead to population stabilization for both species; however, one species may exclude the other over time.

Why is understanding population ecology important for protecting species and habitats?

Understanding population ecology is crucial for protecting species and habitats because it allows us to study the distribution and abundance of plant and animal populations. Population ecology helps us understand how species interact with each other and their environment, and how factors like birth and death rates affect population sizes. This knowledge is essential for conservation efforts. By understanding the factors that limit population growth and cause species to decline, conservationists can implement strategies to protect vulnerable species and restore habitats . Population ecology also helps us manage invasive species and prevent habitat loss , which are major threats to biodiversity.

population ecology

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A herd of wildebeest, including calves, moves across a dusty savanna in Africa with scattered trees visible in the background.

Graph showing three survivorship curves: Type I (red, with human figures) has high survival early and drops steeply at old age; Type II (blue, with a bird) declines steadily; Type III (green, with a plant) drops sharply early, then levels off. X-axis is age, y-axis is number of survivors on a log scale.

Galapagos cactus finch (Geospiza scandens)

Side-by-side line graphs compare exponential and logistic population growth. The exponential growth graph shows a curve rising steeply upward, while the logistic growth graph shows an S-shaped curve leveling off at the carrying capacity of the environment.

A line graph with red and blue shaded areas shows cyclical rises and falls in the populations of snowshoe hares and lynx from 1855 to 1925, illustrating how hare population peaks are followed by lynx population peaks. Small illustrations of a running snowshoe hare and a lynx appear above the graph.

invasive prickly pear cactus in Australia

biological control of invasive prickly pear cactus in Australia

population ecology

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Top Questions

What is population ecology?
Population ecology studies the distribution and abundance of plant and animal populations by analyzing a population, a group of interbreeding individuals of one species in a specific geographic area.

Factors such as fertility, mortality, and migration influence population change, affecting attributes like age, sex, and geographic distribution. Population growth is also affected by the environment's carrying capacity, which is the maximum population size an environment can sustain based on available resources. Mathematical models, such as the Lotka-Volterra equations, assess species interactions and predict population dynamics.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is a population in ecology?
In population ecology, a population consists of individuals of one species inhabiting a specific geographic area who interbreed, if they reproduce sexually.

The geographic boundaries of a population can be easy to define, such as for plants or animals on islands. However, for species dispersed across large areas, pinpointing local population boundaries is more challenging. Populations range from closed (geographically isolated with no exchange) to open (varying degrees of connectedness). Population ecology studies why some species are rare while others are abundant and also examines the reasons behind population fluctuations. Factors affecting population size are density-dependent, such as competition and disease, and density-independent, such as weather. Organisms within the trophic network are arranged into populations and communities.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do scientists measure population size?
Scientists measure population size using various vital rates and refined techniques. Crude birth and death rates, calculated per 1,000 inhabitants, indicate population changes, with the difference revealing the rate of natural increase or decrease. The marriage rate, though crude, records annual marriages per 1,000 inhabitants.

Population density, while useful, is an imperfect measure that can obscure differences in settlement intensity within a unit of land. Studies that consider population density often use sampling to estimate the number of individuals in the area of interest and so are subject to sampling error.
2 Britannica Sources
1. vital rates
2. population density: Problems in population density
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What factors can cause a population to increase or decrease?
Population size is affected by the interplay of births, deaths, immigration, and emigration. A population increases through births and immigration and decreases through deaths and emigration.

Populations are rarely closed, meaning that migration in and out can change population size, as do births and deaths. The rate of natural increase is the difference between births and deaths. Density-dependent factors intensify as a population increases in size, while density-independent factors, such as weather and climate, affect population size regardless of density. Food availability, competition, disease, and predation are ecological factors that curtail population growth.
7 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does competition affect population growth?
In population ecology, competition affects population growth through several ecological factors, including food availability and environmental stress.

The life-sustaining resources in an environment are limited and can be reduced by greater population density and competition from other species. When population density is high, mortality tends to increase because of competition for resources, predation, or increased disease transmission, resulting in a plateau in population growth.

The Lotka-Volterra equations can predict the effects of species interactions on population dynamics. Interspecific competition may lead to population stabilization for both species; however, one species may exclude the other over time.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is understanding population ecology important for protecting species and habitats?
Understanding population ecology is crucial for protecting species and habitats because it allows us to study the distribution and abundance of plant and animal populations. Population ecology helps us understand how species interact with each other and their environment, and how factors like birth and death rates affect population sizes.

This knowledge is essential for conservation efforts. By understanding the factors that limit population growth and cause species to decline, conservationists can implement strategies to protect vulnerable species and restore habitats. Population ecology also helps us manage invasive species and prevent habitat loss, which are major threats to biodiversity.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

population ecology, study of the processes that affect the distribution and abundance of animal and plant populations.

A population is a subset of individuals of one species that occupies a particular geographic area and, in sexually reproducing species, interbreeds. The geographic boundaries of a population are easy to establish for some species but more difficult for others. For example, plants or animals occupying islands have a geographic range defined by the perimeter of the island. In contrast, some species are dispersed across vast expanses, and the boundaries of local populations are more difficult to determine. A continuum exists from closed populations that are geographically isolated from, and lack exchange with, other populations of the same species to open populations that show varying degrees of connectedness.

Genetic variation within local populations

In sexually reproducing species, each local population contains a distinct combination of genes. As a result, a species is a collection of populations that differ genetically from one another to a greater or lesser degree. These genetic differences manifest themselves as differences among populations in morphology, physiology, behaviour, and life histories; in other words, genetic characteristics (genotype) affect expressed, or observed, characteristics (phenotype). Natural selection initially operates on an individual organismal phenotypic level, favouring or discriminating against individuals based on their expressed characteristics. The gene pool (total aggregate of genes in a population at a certain time) is affected as organisms with phenotypes that are compatible with the environment are more likely to survive for longer periods, during which time they can reproduce more often and pass on more of their genes.

The amount of genetic variation within local populations varies tremendously, and much of the discipline of conservation biology is concerned with maintaining genetic diversity within and among populations of plants and animals. Some small isolated populations of asexual species often have little genetic variation among individuals, whereas large sexual populations often have great variation. Two major factors are responsible for this variety: mode of reproduction and population size.

Effects of mode of reproduction: sexual and asexual

In sexual populations, genes are recombined in each generation, and new genotypes may result. Offspring in most sexual species inherit half their genes from their mother and half from their father, and their genetic makeup is therefore different from either parent or any other individual in the population. In both sexually and asexually reproducing species, mutations are the single most important source of genetic variation. New favourable mutations that initially appear in separate individuals can be recombined in many ways over time within a sexual population.

Chutes d'Ekom - a waterfall on the Nkam river in the rainforest near Melong, in the western highlands of Cameroon in Africa.

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In contrast, the offspring of an asexual individual are genetically identical to their parent. The only source of new gene combinations in asexual populations is mutation. Asexual populations accumulate genetic variation only at the rate at which their genes mutate. Favourable mutations arising in different asexual individuals have no way of recombining and eventually appearing together in any one individual, as they do in sexual populations.

Effects of population size

Over long periods of time, genetic variation is more easily sustained in large populations than in small populations. Through the effects of random genetic drift, a genetic trait can be lost from a small population relatively quickly (see biosphere: Processes of evolution). For example, many populations have two or more forms of a gene, which are called alleles. Depending on which allele an individual has inherited, a certain phenotype will be produced. If populations remain small for many generations, they may lose all but one form of each gene by chance alone.

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This loss of alleles happens from sampling error. As individuals mate, they exchange genes. Imagine that initially half of the population has one form of a particular gene, and the other half of the population has another form of the gene. By chance, in a small population the exchange of genes could result in all individuals of the next generation having the same allele. The only way for this population to contain a variation of this gene again is through mutation of the gene or immigration of individuals from another population (see evolution: Genetic variation in populations).

Minimizing the loss of genetic variation in small populations is one of the major problems faced by conservation biologists. Environments constantly change, and natural selection continually sorts through the genetic variation found within each population, favouring those individuals with phenotypes best suited for the current environment. Natural selection, therefore, continually works to reduce genetic variation within populations, but populations risk extinction without the genetic variation that allows populations to respond evolutionarily to changes in the physical environment, diseases, predators, and competitors.

population ecology

Genetic variation within local populations

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Effects of mode of reproduction: sexual and asexual

Effects of population size