28.1: What are Populations and Communities?
Populations are groups of individuals of the same species that inhabit a shared environment. Communities include multiple co-existing, interacting populations of different species. Metapopulations span multiple populations of the same species that occupy different areas. Metapopulations interact through immigration and emigration, providing genetic diversity that lends resilience to harsh environments. Population size and density can be estimated using quadrat and mark and recapture methods.
Populations Are Dynamic and Interactive
A population, or group of individuals, belonging to the same species and inhabiting the same general area, continuously changes in response to both biotic (living) and abiotic (non-living) factors. Influential abiotic factors include weather, elevation, latitude, soil and water composition, and pollution, among others. The biological study of how organisms interact with each other and their environment is called ecology.
Metapopulations include multiple populations of the same species that inhabit distinct areas. Metapopulations continuously exchange members through immigration, movement into an area, and emigration, movement out of an area. This exchange ensures genetic diversity, helping populations withstand unpredictable and unfavorable environmental conditions by increasing the likelihood that adaptive (i.e., helpful) traits will be naturally selected (i.e., emerge in the population).
Communities Are Combinations of Co-existing, Interacting Populations
An ecological community is comprised of multiple co-existing and interacting populations in the same habitat, and a community’s species richness is merely the number of species. The combination of ways a species uses environmental resources and interacts with other community members reflects the distinct niche the species occupies. In other words, a niche is like the “job” a species performs in its community.
Competition arises when species’ niches overlap. Bluebirds and woodpeckers both favor insectivorous diets and open areas with sparsely distributed trees. In an example of interspecific competition, these two species vie for limited food and housing resources. Bluebirds also compete with other bluebirds for these resources (intraspecific competition). Competition can be avoided by partitioning resources, or occupying different areas of a shared environment.
Predator-prey relationships, another important community interaction, resemble an evolutionary “arms race.” In prey animals, natural selection strongly favors features that help prevent predation. For example, Caligo (or “Owl”) butterflies have large eyespots on their wings that resemble owl eyes, which deter threatening predators. Predators also co-adapt to prey adaptations; both predator (e.g., leopard) and prey (e.g., deer) species use camouflage to avoid detection.
Populations Can Be Measured Using Quadrat and Mark and Recapture Methods
Populations are characterized by size and density. Population size (N) is simply the number of individuals. Population density refers to the number of individuals in a given area. Although counting individuals is the most accurate way to measure populations, it can be unfeasible in large habitats or for organisms that frequently move around. Thus, researchers often employ sampling methods to infer the total population size.
Quadrat samples are adequate for estimating population size and density of plants or very small or slow organisms. This method involves partitioning several randomly distributed sections of habitat with markers, such as string or stakes, and counting the individuals in each quadrat. The number and size of quadrats needed for accurate estimates vary according to species. For example, smaller organisms, like bacteria, require much smaller sampling areas than large organisms, such as trees.
Mark and recapture methods are more suitable for moving animals, like mammals, fish, and birds. First, a random sample of individuals from a habitat are captured, marked (e.g., with tags, paints, or bands) and re-released. At a later date, a second random sample is captured, which includes some of the marked animals from the first sample. The ratio of marked to unmarked animals is then used to estimate population size. Limitations of this method include assumptions that previously captured and uncaptured animals are equally likely to be caught in the second sample, and no animals died, were born, or moved between time-points.