Ecology: The Biology of Interactions. 4.02. Population Characteristics
Demographic characteristics of populations can be divided into two groups: static and dynamic. Static characteristics can be determined for a specific moment in time; an example is population size. Birth rate, mortality, and migration are dynamic characteristics. Their nature is such that they can only be measured over a spe...
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4.01. Populations and Their Properties
D. Shabanov, M. Kravchenko. Ecology: The Biology of Interaction Chapter 4. Population Ecology
4.03. Life Tables, Pyramids, and Survivorship Curves
4.02. Population Characteristics Populations consist of many individuals. The characteristics of their composition are studied by demography (literally, "people description"), a science whose methods were primarily developed during the description of the dynamics of human settlements' size and composition. Demographic characteristics of populations can be divided into two groups: static and dynamic. Static characteristics of populations can be determined at a specific moment in time; an example of such characteristics is population size. No matter how quickly this value changes, at any given moment, we can potentially determine how many individuals make up a particular population. In addition to population size, this category also includes its sex-age structure (the ratio of individuals of different ages and sexes within it), density (the ratio of population size to the area or volume it occupies), the characteristic distribution of individuals in space, and some other parameters. Population size can be determined in various ways. For determining the numbers of large, easily visible organisms that form aggregations in relatively small areas, direct counting is used. This is how bird nesting colonies (rooks, ducks) or ungulate herds (reindeer) can be counted. In some cases, the marking method is effective. Animals are marked and released back where they were caught. After some time, a new catch is made in the same location, and the population size is estimated by the proportion of marked individuals among the total number of caught individuals. This is how the population size of amphibians, small birds, mouse-like rodents, and many other animals is determined. However, it is often impossible to determine the total number of organisms by direct counting of individuals. In such cases, one has to rely on sampling and counting the number of individuals in the samples. Density is measured—the number of individuals per unit of space. The density of terrestrial organisms is expressed per unit area, and the density of planktonic organisms is expressed per unit volume of water. Sometimes density is estimated as the number of encounters along a route. For example, birds are counted by their songs in spring, and mammals by their tracks in winter. Another static characteristic is the spatial distribution of individuals and their groups. Three main types of spatial distributions are distinguished: random, regular, and clumped. In a random distribution, the location of each individual is independent of the location of other individuals. In nature, it occurs quite rarely. A regular distribution is observed when repulsive forces act between individuals (e.g., they defend individual territories). In a dense spruce forest, the trunks of individual trees are separated by a distance of no more than two crowns, and a solitary tree shades the space around its trunk, preventing the growth of seedlings and, consequently, the development of new individuals of this species. Such distribution is also common in artificially created ecosystems (parks, agricultural systems). In a clumped (patchy, aggregated) distribution, the probability of finding empty areas and areas with several individuals is much higher than in a random distribution. A vivid example is the distribution of herbaceous plants in a swamp, where they occupy elevated areas, forming "patchy" aggregations. Like the distribution of individual organisms, the distribution of groups can also be random, regular, or patchy. The list of static characteristics of populations is not exhaustive. For example, for species characterized by intraspecific hierarchy (pecking order), the hierarchical structure of populations—reflecting the status of individual organisms and their relationships with each other—is also important. What processes lead to changes in the static (i.e., not entirely unchanging!) characteristics of a population? Birth rate, death rate, and migration are dynamic characteristics. The nature of these parameters is such that they can only be measured over a specific period. The most important dynamic characteristics of a population can be combined into the following simple formula: Change in population size = (birth rate + immigration) - (death rate + emigration) When considering changes in population size, the lifespan of individuals in the population must also be taken into account. For organisms of each species, there is a certain maximum lifespan. This is most often needed as an extreme point when constructing survival curves and life tables. For example, in bacteria, it can be tens of minutes, while for woody plants, it can be tens of centuries.
4.01. Populations and Their Properties
D. Shabanov, M. Kravchenko. Ecology: The Biology of Interaction Chapter 4. Population Ecology
4.03. Life Tables, Pyramids, and Survivorship Curves