A population is a group of organisms of the same species that occupy the same territory for a long time, that is, the habitat. This term is used in biology, ecology, medicine and in other sciences.
Population density
This concept refers to the number of organisms, whether animals, fish or plants, calculated on any taken unit of volume or area of the territory where this population lives.
By "volume" can be meant the volume of water, air or soil, and by "area of territory" is the area of a reservoir or surface of the earth. The population density depends on many factors: whether the climate is favorable, is the distribution area wide, as well as whether there are representatives of other populations in this territory and how close contacts occur between species of two or more communities.
The most commonplace example: the population density of hares depends on the size of the forest zone, where it is convenient to get food. If a flock of wolves appears in this area, then hares, fleeing from them, try to expand their habitat - to go where contact with a hostile population can be avoided. This means that the wider the habitat, that is, the inhabited territory, the lower the density of the community. Again, this does not work if the population increases along with the habitat.
Not for nothing that the density of animal population was taken as an example. They are perhaps the most mobile individuals. Due to the constant search for prey, convenient places for feeding, or, conversely, flight from predators, animals are considered the most migratory on Earth. Of course, each population needs its own suitable climate and habitat, which is why elephants do not come to Siberia, and penguins do not come to visit Asia. But within their habitat, animals are in constant motion.
Population size
This concept denotes the total number of individuals of one species or another, a population on the ground, in water and in the air. That is, in this case, for the habitat they take not a limited area, for example, land or a reservoir, but the entire Earth, the entire World Ocean as a whole.
The population size depends on the difference between mortality and fertility of certain individuals of the same species. If over a certain period of time the birth rate is higher than mortality, the number of the population in question increases, if the birth rate is lower, it falls. Perhaps this is precisely the main difference between the population size and population density. If the first depends on many external factors, whether it be climate, emergency situations and natural disasters, or even human intervention, then the density depends largely on the number, and then on everything else.
Species population
A species is the basic and very first structural unit in a system of living organisms. Here, individuals are capable of crossbreeding, which produces prolific offspring. The species is widespread in a certain habitat and is subject to the influence of the external environment. Now the number of described various living organisms living on earth, in water and in the air, totals almost two million. The total number of living species is about nine million. The number of extinct over the entire existence of the planet, according to scientists, is almost half a million.
The species population is formed by individual individuals. They are capable of relationships, crossbreeding, cohabitation in a specific area. The viability of species depends on many factors, among which one can distinguish such as climate and the presence of competitors, that is, at least one other species living in the same territory and able to fight for food with its neighbors. The density of species populations on the Earth is very heterogeneous, especially for animals. If migration is very common in birds, for example, during the cold season, and it is easier for fish to change their habitat, drifting around the oceans, then animals are very dependent on the climate and topography of the territory where they live. "Convenient" sections of the earth's surface are very densely populated, but only certain species of animals can survive in the permafrost strip.
Individual
An individual is an organism or an individual having properties that distinguish it from non-living matter: metabolism, the ability to reproduce, preservation of heredity and its transmission to descendants. The species, respectively, and the species population are formed from individuals.
Sometimes individuals of various species can interbreed. For example, tigresses can mate with both male tigers and male lions, while producing offspring. Another example, but already with human intervention, is the crossbreeding of different types of plants, fruits, even animals to obtain something new, for example, as an attempt to adapt a species for life in different conditions. The population density of individuals of this species, that is, a mixture, is low, as this is the exception rather than the rule.
Natural and "unnatural" selections
If earlier only natural selection existed, now, in connection with the development of sciences such as genetics and selection, scientists are engaged in the breeding of various species on a very large scale. This contributes to the fact that there is an increase in the number and density of the population, for example, of some animals or rare plants that are placed in a different habitat to facilitate living conditions and reproduction.
Unfortunately, this does not happen everywhere and not always, an example of this is the Red Book, the volume of which is not decreasing, as one might expect, but increasing. Another minus of such human intervention in the life of nature is that individuals grown in unnatural conditions can only live under guardianship - in zoos, laboratories.
Animal populations
Before talking about a particular animal population, it is necessary to clarify what lifestyle its representatives lead. Some species are grouped into groups only by chance or for reproduction, while others lead a herd, group lifestyle, moving along the entire habitat only together.
Lifestyle, first of all, depends on two factors. The first is climatic conditions. In deserts, where there is little water and a hot climate, living alone is easier; there is no need to share water with representatives of your own species. In cold climatic zones, for example, at the pole, it is better to be in a group. Remember the penguins that survive in cold climates not only due to the "warm coat", but also due to the interaction, heating each other.
The second factor is the presence of predatory neighbors of other species that can encroach on the territory, food and water, and even on the life of the individual. Of course, it is easier to live in a group under such conditions - it is easier to fight back, to learn about the danger in advance. There are even species that maintain a "friendly neighborhood" in order to protect themselves from more predatory neighbors. For example, the neighborhood of antelopes, zebras and giraffes. The latter, due to growth, seeing lions lurking, raise an alarm, warning about the danger of everyone else. The population density of animals depends on these two factors - the climate and the presence of "neighbors".
Change in density and population
Above, we found out that a population is individuals of one species that are not related to one flock, herd, pride, and so on, but by common features that distinguish this species from all the others. It is they, one way or another, affect the fluctuations in the number and density of residence.
Usually there are three types of dependence of the population size on its density.
First: growth in numbers sometimes begins to decline with increasing density. At the same time, the habitat of this community should remain unchanged. This is a process of "self-regulation." To prevent overpopulation of a certain area, the species itself tracks the number of individuals it needs. The "surplus" is sometimes destroyed in a very cruel way, for example, adult perches feed on their offspring if it is born too much.
The second type is usually observed in species living in groups. With an average population density of its range in the population, a peak in population growth is achieved. No wonder everyone has enough space, water and food.
But the third type "follows" from the first. This is a sharper form. When the peak of the population is reached, overpopulation of the habitat, the change of the habitat begins. In other words, migration, which means an attempt to adapt to new living conditions, the irreversible death of many representatives of this species and, accordingly, a sharp decrease in the population.
Influence from the outside
All that was mentioned above is a natural effect on the number and density of the population. Now we will talk about unnatural influences that cannot be predicted or stopped. This effect on a certain type of any external factors. Remember from the school course, why did dinosaurs die? That's right, a meteorite fall and the onset of the Ice Age. Or, for example, a severe flood at the beginning of the 21st century of the Indian Ocean, not only people and cities were affected, but also animals. Here we include viruses and diseases, human intervention in nature and the like. This is an unnatural effect on the dynamics of the number and density of the population.
Population problems
Oddly enough it will sound, but the problem for humanity and any species on Earth is the same - overpopulation. Of course, first of all, the issue of overpopulation of the Earth concerns people. With a bad scenario, humanity will be able to “squeeze” animals out of the planet, but they will not force us to move. Resources, whether water, forest or minerals, are almost exhausted. Every year their consumption rate is growing, which means that it remains less for those who do not have the capabilities of mankind, that is, animals, fish and birds.
It is possible to monitor and control the population density of a human population , but I would like this to happen not by force, but by a completely natural way. But how? To find the answer to this question to scientists.