7.1: Characteristics of Popualtions

Ecologists define a population as a group of individuals of a single species inhabiting an area with the potential to interbreed. Populations are dynamic entities. Their size and composition fluctuate in response to numerous factors, including seasonal and yearly changes in the environment, natural disasters such as forest fires and volcanic eruptions, and competition for resources between and within species. Population studies hold the key to solving practical problems such as saving endangered species, controlling pest populations, and managing fish and game populations. All populations share a number of characteristics including size, density, distribution, dispersion, and age structure. The statistical study of populations is called demography: a set of mathematical tools designed to describe populations and investigate how they change.

Population Size and Density

Populations are characterized by their population size (total number of individuals) and their population density (number of individuals per unit area). A population may have a large number of individuals that are distributed densely, or sparsely. There are also populations with small numbers of individuals that may be dense or very sparsely distributed in a local area. Population size can affect potential for adaptation because it affects the amount of genetic variation present in the population. Density can have effects on interactions within a population such as competition for food and the ability of individuals to find a mate. Smaller organisms tend to be more densely distributed than larger organisms.

Figure \(\PageIndex{1}\): Populations density decreases with increasing body size for populations of mammals. (CC BY-SA 4.0; via openstax)

Sampling techniques for determining population size and density

The most accurate way to determine population size is to simply count all of the individuals within the habitat. However, this method is often not logistically or economically feasible, especially when studying large habitats. Thus, scientists usually study populations by sampling a representative portion of each habitat and using this data to make inferences about the habitat as a whole. A variety of methods can be used to sample populations to determine their size and density including quadrat sampling, transects, mark and recapture, and camera traps.

For immobile organisms such as plants, or for very small and slow-moving organisms, a quadrat may be used (Figure \(\PageIndex{1}\)). A quadrat is a way of marking off square areas within a habitat, either by staking out an area with sticks and string, or by the use of a wood, plastic, or metal square placed on the ground. After setting the quadrants, researchers then count the number of individuals that lie within their boundaries. Multiple quadrat samples are performed throughout the habitat at several random locations. All of this data can then be used to estimate the population size and population density within the entire habitat.

For mobile organisms, such as mammals, birds, or fish, a technique called mark and recapture is often used (Figure \(\PageIndex{2}\)). This method involves marking a sample of captured animals in some way (such as tags, bands, paint, or other body markings), and then releasing them back into the environment to allow them to mix with the rest of the population; later, a new sample is collected, including some individuals that are marked (recaptures) and some individuals that are unmarked (). Using the ratio of marked and unmarked individuals, scientists determine how many individuals are in the sample. From this, calculations are used to estimate the total population size.

For rare organisms, such as top predators, camera traps can can capture wild animals on film when researcher are not present (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\)). Camera traps are typically equipped with a motion censor that is automatically triggered by movement in its vicinity. The great advantage of camera traps is that they can record very accurate data without disturbing the photographed animal. They minimally disturb wildlife and can replace the use of more invasive survey and monitoring techniques such as live trap and release. They operate continually and silently, provide proof of species present in an area, can reveal what prints and scats belong to which species, provide evidence for management and policy decisions, and are a cost-effective monitoring tool. Camera traps are helpful in determining behavioral and activity patterns of animals, or recording animal migrations.

Figure \(\PageIndex{1}\): A scientist uses a quadrat to measure population size and density (credit: NPS Sonoran Desert Network)

Figure \(\PageIndex{2}\): Mark and recapture is used to measure the population size of mobile animals such as (a) bighorn sheep, (b) the California condor, and (c) salmon. (credit a: modification of work by Neal Herbert, NPS; credit b: modification of work by Pacific Southwest Region USFWS; credit c: modification of work by Ingrid Taylar)

Figure \(\PageIndex{3}\): A camera trap, for taking pictures of game on trails (CC BY-SA 3.0; by Hustvedt via Wikipedia)

Figure \(\PageIndex{4}\): Image of a Leopard in Garhwal Hills of Western Himalaya, India taken with a trap carea (CC BY-SA 3.0; by winterline via Wikipedia)

Population Distribution

While there are few environments on earth without life, no single species can tolerate the full range of earth’s environments. Because all species find some environments too warm, too cold, too saline, and so forth, environment limits the geographic distribution of species. Population distribution is the geographic area where a particular population of a species occurs. This area of occupancy is determined by the local availability of appropriate habitat. Some species are habitat generalists and their populations spread out almost continuously across a landscape. American Robins, for example, can breed in parks, urbanized areas, farms, and the edges of forest. Other species are habitat specialists and only occur in specific places where a certain type of ecosystem occurs. Aquatic species also often occur in isolated populations because their habitat is necessarily bounded by the extent of the lake, wetland, or waterway they occur in.

The Northern Spotted Owl is a subspecies of spotted owl that is a habitat specialist that only occurs in old-growth forests in the Pacific Northwest of North America (British Columbia, Washington, Oregon, California). While much of the Pacific Northwest is still forested, most forests are less than 100 years old and don’t have the large, old trees that the owls nest in. The extent of each population of owls is therefore limited by the size of each patch of old growth forest.

A species' geographic range is the total geographic area occupied by a species. Bald Eagles can be found breeding in almost every state of the USA and most provinces of Canada. Its geographic range therefore encompasses almost all of North America north of Mexico. In contrast, Golden Eagles breed almost exclusively in western Canada, the western United States, and Mexico. The Golden Eagle's range therefore is restricted to the western part of the continent and Mexico.

A restricted range species refers to a species in which the range is so small, there is basically a single population. For example Kirtland’s warblers are habitat specialists only residing in the Jack Pine Forests of northern Michigan and a few adjacent states and Canadian provinces. This results in the population distribution and geographic range is essentially the same. A species' current geographic range is often very different from its historical range (Figure \(\PageIndex{5}\)). Factors such as habitat loss, hunting, and climate change can all reduce the distribution of populations and the species' overall geographic range. For example, lions previously ranged broadly over Africa, Western Asia, the Middle East and India.

Figure \(\PageIndex{5}\): Historical (red) and present (blue) distribution of lions (Panthera leo). Source: Tommyknocker (Wikipedia), based on a map created by 'The African Lion Environmental Research Trust (ALERT).

Population Dispersion

Further information about a population can be obtained by looking at the distribution of the individuals. Species dispersion patterns (or distribution patterns) summarize the spatial relationship between members of a population within a habitat at a particular point in time. In other words, they show whether members of the population live close together or far apart, and what patterns are evident when they are spaced apart. On small scales, individuals within populations are distributed in patterns that may be random, regular, or clumped. Patterns of dispersion can be produced by the social interactions within populations, by the structure of the physical environment, or by a combination of the two.

An example of random distribution occurs with dandelion and other plants that have wind-dispersed seeds that germinate wherever they happen to fall in favorable environments (Figure \(\PageIndex{6}\)). A clumped distribution, may be seen in plants that drop their seeds straight to the ground, such as oak trees; it can also be seen in animals that live in social groups (schools of fish or herds of elephants). Uniform distribution is observed in plants that secrete substances inhibiting the growth of nearby individuals (such as the release of toxic chemicals by sage plants). It is also seen in territorial animal species, such as penguins that maintain a defined territory for nesting. The territorial defensive behaviors of each individual create a regular pattern of distribution of similar-sized territories and individuals within those territories. Thus, the distribution of the individuals within a population provides more information about how they interact with each other than does a simple density measurement. Just as lower density species might have more difficulty finding a mate, solitary species with a random distribution might have a similar difficulty when compared to social species clumped together in groups.

Figure \(\PageIndex{6}\): Species may have uniform, random, or clumped distribution. Territorial birds such as penguins tend to have uniform distribution. Plants such as dandelions with wind-dispersed seeds tend to be randomly distributed. Animals such as elephants that travel in groups exhibit clumped distribution. (credit a: modification of work by Ben Tubby; credit b: modification of work by Rosendahl; credit c: modification of work by Rebecca Wood)

Video: Popualtion Dispersion

In this 3-minute video, see examples of the various types of distribution patterns populations exhibit across space (note that "aggregated" in the film is the same as clumped in the text above).

Attribution

This page is a modified derivative of:

• 106 Demographics and Population Growth, via Introductory Biology: Evolutionary and Ecological Perspectives by Various Authors; license CC-BY
• 45.1 Population Demography via Open Stax, license CC-BY
• Camera Trap by Wikipedia, license CC-BY-SA 3.0
• What is population Ecology via Open Stax, CC-BY-NC-SA