The population ecology or demoecology is the study of populations and their relationship with the environment. Its objective is to characterize the population in terms of birth, mortality, immigration and emigration, in addition to defining population parameters such as density, spatial distribution and age distribution of individuals..
A population is defined as a group of individuals belonging to a species that live at the same time in a common area. Members of a population use the same resources and interact with each other. The boundaries of a population can be natural (like fish in a lake) or can be defined by the researcher.
Research in population ecology may involve laboratory work, field work, and applying mathematical and statistical models to the study group..
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Population ecology can be distinguished from other similar science studies - such as the study of landscapes and ecosystems - by the scale and focus of the discipline. The main object of study is a group of organisms that are related from the taxonomic or functional point of view.
The concept of population ecology seeks to answer questions related to the environmental carrying capacity, the optimal population size, the causes and the mechanisms by which the size increases, such as the distribution of populations, among others..
Likewise, this body of knowledge seeks to understand intraspecific ecological relationships, call it competition or mutualism between individuals belonging to the same species, and interspecific relationships such as predation and coevolutionary processes..
When speaking of population ecology, it is necessary to define what a population is. In this context, a population is defined as a group of organisms with the ability to reproduce and that find themselves in a shared spatial area (that is, they are sympatric) at the same time. This concept is synonymous with biological population.
These individuals form a functional unit in which they interact with each other and can reproduce. Note that the concept of local population differs from the concept of species and the population of a species. In these cases, the concept of population is previously defined by the researcher and can be arbitrary..
Populations evolve by natural selection, which acts on heritable variations between individuals, changing the frequencies of various characteristics over time..
In the last twenty years the emphasis from population ecology has shifted to the ecology of "metapopulations".
This concept developed by Levins encompasses “populations of populations” and, following this vision, each local population is susceptible to extinction but can be balanced by immigration processes from other populations..
Population ecology focuses on studying certain properties of a group, mainly growth, survival and reproduction. The most important parameters are:
Population growth is determined by the combination of four processes: reproduction (whether sexual or asexual), mortality, immigration, and emigration..
A measure of population growth is the intrinsic rate of population growth, denoted by the letter r and is defined as the rate of growth per individual (or per capita) per unit of time in the population.
As discussed, the concept of population involves the variables time and space, so the population size and growth rates are calculated for a specific time and spatial unit..
There are several models of population growth: the exponential and the logistic. The first represents a population in an unlimited environment and according to the model, as the population increases the growth becomes faster. However, this pattern cannot be applied in the long term to any population..
In contrast, the logistic model is more realistic and incorporates the term “carrying capacity” - the maximum population size that the environment can support..
Populations can be described in terms of their density and dispersion. Density refers to the number of individuals per area or per volume - the number of plants per square meter or the number of bacteria per milliliters in a test tube. This parameter is dynamic.
Population density can be regulated by factors such as birth and death rates that slow down population growth, stabilizing it near its carrying capacity..
Dispersion is the spatial pattern that the population follows and can vary substantially depending on the local density and the ecological characteristics of the environment. It is logical to think that the most suitable regions for a certain species will be inhabited in a greater proportion.
Similarly, the social interactions of animals can also influence population dispersion..
The grouping of individuals in certain areas is the most common pattern of dispersal. For example, amphibians spend most of their time under rocks, as it provides a more humid environment than areas exposed to the sun, and thus prevents desiccation..
In the unlikely event that environmental conditions are homogeneous, the distribution of individuals will be random..
The uniform dispersion pattern is not common and when observed it can be a consequence of interactions between individuals. Some plants can produce chemicals that inhibit the germination of their companions in nearby areas or, in the case of territorial animals, they can alienate other individuals..
Population ecology integrates theory development, laboratory work, and field work.
However, with the modernization of the discipline and the arrival of computers capable of carrying out important statistical work, there is an enormous amount of data that can be used by population ecologists without the need for field work..
Knowledge of the number of individuals that make up a population (this value is known as “population size”) and their distribution are some of the main objectives of population ecology and can be estimated following various methodologies..
The most commonly used techniques to estimate parameters of relevance in population ecology will be described below:
The first approach - and the most intuitive - is the direct count of individuals. This technique can be applied to small populations where counting ensures an accurate value..
For example, if you want to study the number of domestic dogs in a region, the number of starfish in a shallow area, or the number of local university students.
However, when the research target is a larger group, direct counting is not a viable alternative..
In these cases, an indirect count of the members of the population is carried out. If the distribution of the study organism is very wide, the organisms can be counted in a delimited area and then extrapolated to the real area.
The number of individuals can also be estimated indirectly by evidence such as nests, burrows or fecal samples..
Finally, the capture and recapture method can be applied, which is widely used for the study of animal populations. The first step includes capturing the animals, marking them, and releasing them. Then they are captured again and the size is estimated in relation to the individuals captured and marked..
Population studies seek to characterize the population in terms of sex, stage of development of the individual, reproductive stage, among others..
To meet this objective it is necessary to know the approximate age of the organism. In the case of mammals, wear can be observed in the teeth, in other animal groups it can be inferred by the state of structures such as horns or feathers..
In the plant kingdom, growth rings can be counted on the trunk of trees. There are also molecular biology techniques that allow estimating the age of organisms.
In 1996 Trajano investigated the population ecology of the common vampire Desmodus rotundus (Chiroptera). Through capture and recapture experiments, he was able to conclude that the size of the colony varied monthly, which indicates that bats frequently move from cave to cave..
According to this study, the bat is capable of migrating to warmer regions when the climate warrants it. The minimum population density reported was 3.5 individuals per square kilometer.
Knowledge of population ecology is essential for wildlife and resource conservation and management biologists. To face problems related to the preservation of biodiversity, it is necessary to have accurate information on the population ecology of the study group..
For example, if you want to study what are the causes why amphibians are decreasing in number throughout the world or if the introduction of a foreign species is affecting a local species in some way, it is necessary to have population ecology data..
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