What Is a Population?

A population is an interbreeding group of individuals of the same species. In ecological terms, a population is considered to be a group of individuals of the same species living in a particular area. A survey of populations in a pond ecosystem might include determining the number of individuals of each species of fish, bird, deer, toad, and other animals living in and around the pond, as well as the plant species growing under, on top of, and beside the water. This determination can vary in difficulty, depending on how countable the individuals within the population are. For example, counting the population of nesting egrets or cranes, ducks, and geese in the pond ecosystem may be a simple task; determining the duckweed plant population may be far more complex, and pinpointing the number of the pond's dragonfly population may be even more challenging.

Population ecology is the study of how populations of plants, animals, protists, and other organisms change over time and how these living things interact with their environment. Population ecologists are interested in a population's size, density (the number of individuals of a population per unit of area), dispersion (the way in which individuals are arranged within the boundaries of a population), and growth patterns. If the number of individuals in a species is increasing, population ecologists may ask what factors have made that increase possible. In contrast, a decrease in population may give ecologists insight into future changes and may raise concerns about environmental stability. A typical ecological study might be done on a population of gray wolves. Ecologists may want to learn how population dynamics change, what the size of each population is, what territories the population covers, what the age distribution within each pack is, and what the rate at which the adult female reproduces is. They might wish to further study how a wolf population impacts its environment by preying on populations of elk, deer, and bison in a particular territory. Studies have shown how changes to large carnivore populations impact the biodiversity of plant and small animal populations. For example, in the late 19th and early 20th centuries, wolves were hunted in Yellowstone National Park so much that they died out completely. This led to an explosion of the elk population size, which had devastating effects on the ecology of the region because the elk destroyed large swaths of vegetation, eliminating food and shelter for many other species. The loss of vegetation also loosened soil, leading to erosion. Further, the loss of elk carcasses left as remains from wolf hunts led to an additional decrease in in scavenger populations, such as vultures and ravens. The reintroduction of wolves to Yellowstone, beginning in the 1970s, stabilized the region and restored the balance of the ecosystem.

Population density is the number of individuals of a population per unit of area. Human population density is easy to determine because the number of humans in a certain area can be determined. For example, If a city's population is 2.4 million people, all living in an area of 81 square kilometers, then the population density is calculated by dividing 2,400,000 by 81, which is about 29,630 people per square kilometer. Determining population densities of plants and nonhuman animals is not as simple because of the great numbers of plants and the difficulty of detecting some animals. Scientists have developed several ways to determine nonhuman animal and plant population densities based on mathematical calculations and various means of sampling. Two common techniques are quadrat sampling and mark-recapture methods, both of which provide an approximation of population density that is less accurate than human population density figures. With quadrat sampling, ecologists determine the size and shape of the entire area to be sampled, the number of samples to be collected, and the size and shape of each quadrat. A quadrat is a square that is part of a larger sampling grid. Quadrat sampling can be used by direct census or by extrapolating populations based on trace evidence left by animals, such as tracks, nests, burrows, or feces. Consider this example of an ecologist who wishes to determine population densities of species of native grasses and wildflowers in a 25-hectare field. Density is fairly even across the field, and a quadrat grid is drawn. Five quadrats in the field are randomly selected, and a census is taken, counting individuals of each type of grass and wildflower species per quadrat. By determining the average number of individuals of each species per quadrat, an estimated total population for the field is calculated. Mark-recapture is a method used to estimate the population size of migrating birds, butterflies, or ocean organisms. For migrating birds, capturing, tagging, and releasing the birds or counting the number of nests in an area gives ecologists a way to determine a population size. Some species have periods of idleness, which makes counting easier. Monarch butterflies, for example, are migratory through spring, summer, and fall but are stationary when they overwinter in a specific region of Mexico. The population size of monarchs is measured not by counting individuals but by determining the area that monarch colonies occupy during their winter rest. Marine mammals, such as dolphins, are captured in a specific area, tagged, and then released. After time, they are recaptured in the same area to see how many individuals remain in the population. With all species, a population size is, at best, an estimate, and it is sometimes given as a range of values (such as 10,000 to 14,000 blue whales).

The pattern of how various individuals are spaced within a population is called dispersion. Dispersion depends on several factors, including but not limited to the size of an ecosystem, the species, and the living and nonliving factors within the ecosystem. There are three basic types of dispersion patterns: clumped, uniform, and random.

The most common dispersion pattern is clumped. A clumped pattern is indicated by small groups of a particular species living together. For example, wolves within a population live in packs, and while they may move from one den to another, the pack remains intact. All social or community animals (such as herds of zebra or wildebeest) live in clumped dispersion patterns. Other examples of populations exhibiting a clumped dispersion pattern include mushrooms, meerkats, ants, bees, and gorillas, where individuals live in clumps according to the availability of resources, such as suitable soil conditions.

Uniform dispersion occurs when individuals of a species live in equally spaced sections of an ecosystem. The pattern often forms when resources such as sunlight or food are limited. Typical organisms that are found in uniform dispersion patterns include some nesting birds (gannets and penguins), smaller cacti or shrubs (barrel cactus or mesquite), and redwoods. Penguins typically have uniform dispersion because the distance between individual penguin nests is maintained by aggressive interactions. Gannets have limited space for nesting and limited materials for building nests; thus, they share space relatively evenly. Cacti compete for water, while redwoods compete for sunlight.

Random dispersion patterns tend toward unpredictability; each individual's position is independent of other individuals in the population. A dandelion produces seeds that are spread by the wind, so the location of new dandelions is heavily influenced by wind direction. Many spiders use aerial dispersion to seek new homes, casting a balloon-like structure that catches the wind. Some seeds may be dispersed by water movement, such as flowing rivers or flash floods. Seeds may also stick to an animal's fur and be dropped in new locations, where they germinate and produce new individuals. Seeds in fruit, such as berries, are dispersed through animal waste. The fruit is eaten, but the seeds pass through the animal's digestive tract and are eliminated as fecal matter. Lupines, geraniums, and violas disperse their seeds by explosion, literally flinging the seeds away from the parent plant.