Ecology: Biology of Interaction. IV-02. Population Characteristics

IV-2. Characteristics of Populations. Populations consist of many individuals. The features of their composition are studied by demography (literally - "description of people"), a science whose methods were developed primarily from describing the dynamics of the number and composition of human settlements. Demographic characteristics of populations can be divided into two groups: static and dynamic. Static characteristics of populations (note: static, not statistical!) 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 are part of 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), 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 population of large, easily visible organisms that form aggregations in relatively small areas, direct counting is used. This is how bird nesting colonies (rooks, ducks) and ungulate herds (reindeer) can be counted. In some cases, marking is an effective method. Animals are marked and released back where they were caught. After some time, a new catch is made in the same place, and the population size is determined by the proportion of marked individuals in 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. To understand this approach, solve a simple problem. 100 individuals were marked in a population of common toads. The following year, in this population, 8 out of a sample of 120 individuals turned out to be marked. What is an estimate of the population size? And if we estimate the loss of individuals per year at one quarter of the total population of the spawning stock, how will the population size estimate change? For small organisms, a common approach to determining the number of individuals is to take samples and count the number of individuals in them. In this case, density is measured - the number of individuals per unit of space. The density of terrestrial organisms is determined per unit area, and the density of planktonic organisms is determined per unit volume of water. Sometimes density is estimated as the number of encounters on 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. With random distribution, the location of each individual does not depend on the location of other individuals. In nature, it occurs only in rare cases. Regular distribution is observed when repulsive forces act between individuals (for example, they are characterized by the defense of individual territory). 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 area around its trunk, preventing the growth of seedlings and, consequently, the development of new individuals of this species. Such distribution is often found in artificially created ecosystems (parks, agroecosystems). With clumped distribution (patchy, aggregated), the probability of finding empty areas and areas with several individuals is much higher than with 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 above does not exhaust the static characteristics of populations. For example, for species characterized by intraspecific hierarchy (pecking order), the hierarchical structure of populations is important - a reflection of the status of individual individuals and their relationships with each other. What processes lead to changes in static (i.e., not at all unchanging!) population characteristics? Birth rate, death rate, migration - these are dynamic characteristics. The nature of these parameters is such that they can only be measured for a certain period of time. 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 a given population must also be taken into account. For organisms of each species, there is a maximum lifespan. It is most often necessary as an extreme point when constructing survival curves and demographic tables. For example, in bacteria, it can be tens of minutes, and in woody plants - tens of centuries.