Populatıon

Populatıon

We all a group or organisms of the same species living together in given region a population. People in Antalya, trees in Bolu may be given as examples of different populations. In a population, in spite of there are individuals relating to only one species, individuals which are relating to one species, can make up populations more than one at different living spaces. Population is a biologic unit that has continuity. Because individuals of population are born, grow, make up new individuals and die. All the populations of different organisms within a given area make up a community. There are lots of species in a community. For example all frogs, fish, plants in around a see make up a pond community.

Factors Affecting Population Growth
In each community, the populations normally consist of producers, consumers and decomposers. The organisms in a community interact with one another as they compete for food and living space.
Populations can change in many ways. Catastrophic events, such as fire, floods, and drought can change population sizes dramatically. Many other events might cause the increase or decrease of the areas that populations occupy. Still other factors can vary population densities, distributions and male/female rations.
Population density is the number of individuals of a population living in a unit volume, or in unit area. The number of trees in a 1000 km² area is the tree population density the number of carps in 1m³ of water shows the density of polulation of the carps. The density of a population depends on many conditions such as the danger of predators, the availability of food, water, light, heat, and nesting places.
While population density tells us how many individuals live in an area, distribution tells us how those individuals are arranged in that area. Individuals of a population can be distributed in a random, uniform or clumped fashion.
Under ideal conditions, the size of a population of organisms would continue to increase. Ideal conditions are those in which nothing limits the growth of population. There is plenty of food, space, and other needed things for all organisms. The highest rate of reproduction of a population under ideal conditions is called the biotic potential.
A population that reaches its biotic potential can have devastating effects on itself, other members of the community, and perhaps the entire biosphere. For example, some bacteria divide about every 20 minutes. At this rate, the biotic potential is 72 generations every day. If this biotic potential were reached the bacteria would cover the entire surface of the earth to a depth of 20 cm in just one day.
To reach its biotic potential a population must have all food, water, and space to survive. It must also have an ideal climate and be free of predators, competitors, and disease. Of course no population finds such ideal conditions and therefore, a population never grows enough to reach its biotic potential. The biotic and abiotic factors that hold the biotic potential in check are called environmental resistance or limiting factors.
The size of a population is determined by birth rate and death rate. If the birth rate, the rate at which new organisms are born is greater than the death rate, the population will increase. If the death rate is greater than the birth rate, the population will decrease. If the rates of birth and death are equal, the population remains fairly stable. The age of a population is important for the ratio of birth to death in the population. In young populations generally the birth rate is high, but in old populations the death rate exceeds the birth rate.
Migration in population also affects its size. Immigration increases the size of population, but emigration decreases its size. For example, the migration of people from rural regions to the urban regions, increases the population size of humans in urban areas.
Thus the size of a population is determined by two factors: the
birth/death rate and migration. It can be formulated as follows:

The population size= (Birth rate + Immigration) – (Death rate + Emigration)

In Türkiye the age of human population is small. Therefore the population of Türkiye tends to increase in time. The situation is just the reverse in many European countries like Germany, France. So their size of populations tend to decrease because of the slow birth rate.
On very ideal conditions, increase of population starts quickly and continues in a series. This is called geometric increase.

Geometric Increase


Aritmetic Increase


Figure: Aritmetic and Geometric Increase

But the geometric increase can’t take long. Because of the change on environment conditions, speed of population growth decreases and becomes an aritmetic state. Even environment conditions can stop population growth, and start to decrease. Thus populations are balanced.
On ideal conditions all populations are balanced in a certain period. This period is different for all species. For some species one day, for some of them one season or one year, for some of them ten years, can form balancing period.

Number of individuals


Fixed Population
Dominant Population
Periodic Population
Time

Figure: Variation Kinds of Populations

Age dispersions of population which grow, shrink and balanced, are showed with age pyramids.

Age


Old


Adult


Young


I II III

Figure: I. A population which is growing up
II. A population which is retrograding
III. A population which is balanced

Age curved of population: The graps which show how much individuals live, in certain number of individuals during the biologic life process, are called age curved of population. When the age curveds of some of the different species organisms are organized, 3 kinds of curveds arise.

Number of individuals living

1000


100


10


1
Life Process

Figure: Different kinds of age curveds

I. The situation which has low death rate at different age groups, and most individuals can realize their biologic life in this situation. This kind living curved is seen at communities in developed countries (A curved).
II. For these curveds, death rate is same at every age groups approximately. But for different species, different ages and for factors which effect living rate, these curveds can be different. Because of this reason these curveds have variation approximately (B1, B2, B3 curveds).
III. The situation which has high death rate at early ages. This kind living curved is seen at fish, parasite organisms and see invertebrates such as mussel, oyster (C curved).

Speed of population growth: Number of the individuals that joined population at unit time with birth or immigration, is called speed of population growth.
If the number of individuals or logarithm of number of individuals that opposed to time is worked up on a graph, a curved arise. This curved is called curved of population growth. For populations, two kinds of growing curveds are seen.
1. S-shaped growth: If a few individual enter a space that isn^t settled in, growth is slow at first. This phase is called structure phase. Structure phase is followed by logarithmic increase phase. At this phase increase becomes geometric form.
Because of the increase of environmental resistance, growth becomes slower and negative increase period starts. After negative increase period, population reaches stable level.

Number of individuals
Carrying capacity

fast stable
grow slow
grow


setup


Time

Figure: S-shaped curved of population growth
At first, the number of individuals grows slowly because the population contains relatively small number of reproducing organisms. As the number of adults capable of reproducing increases, the population grows faster. However, after a certain amount of time the population growth levels off because of the counteraction of the limiting factors. At this stable level, the number of individuals of a population reaches its maximum. The maximum size of population an environment can support is called the carrying capacity of the environment. As the graph shows the population size fluctuates slightly but generally stays near the carrying capacity.
2. J-shaped growth: This growing curved is seen at some insects. The basic different between J-shaped growth and S-shaped growth is stable level. J-shaped growth hasn’t a stable level.


Number of individuals













Time

Figure: J-shaped curved of population growth

Limiting Factors
Limiting factors control population size and growth. Limiting factors may be external or internal. Availability of food, diseases, temperature, moisture… are external limiting factors. Ability to reproduce, the control of reproduction, social behaviour, innate factors are internal factors.
Space and food: Space is an important limiting factor for some plants and protists. A plant’s roots must extend far enough into the soil to obtain nutrients. Its leaves must be exposed to sunlight in order for photosynthesis to occur. As a result, only a certain number of plants can exist in a certain space. Therefore the size of a plant population may be limited by space.
Since animals can move, the space does not seem to be a limiting factor for animals. However, other factors, such as food, predators, or disease limit an animal population before space does.
The amount of food available may limit the size of a population. Suppose that a population of one thousand animals lives in a community and feeds on certain plants. Because the plant and animal populations are in balance, the animals do not eat the plants faster than plants can reproduce themselves. Thus there is a constant food supply for animals. However, if the animal population increases and the plant population does not, the plants would be eaten so quickly that they would have no time to reproduce. As a result some animals would starve to death.
Predation: Almost every organism is preyed on by some other organisms. Predation is a density-dependent factor. If at one time all the prey for a certain predator were eaten, all the predators would soon die. In a predator-prey relationship, each population continually determines the size of the other. An abundance of prey in an area leads to an increase in the number of predators. As the pradators feed on the prey, the number of prey begin to decrease. The number of predators also decrease because of the decrease in their food supply (prey). This predator-prey interaction creates a cycle like in figure.

Number of individuals

predator





prey



Time

Figure: Predator-Prey Cycle


Other Limiting Factors
Parasitism can also be a limiting factor when the number of parasites carried by a host becomes too large. Because too many parasites in or on a host might die the host.
Diseases: Population density is closely related with the spread of certain diseases. For example, malaria results from a parasitic relationship between plasmodium and humans. Malaria is density-dependent, that is, the denser human population, the greater the chance of the disease being passed to others. The spread of malaria is also dependent on the mosquito population density.
Malaria can be treated with certain drugs, but the best solution to malaria is to control the population of mosquitoes. Population control has been carried out in many places by destroying the swampy breeding grounds of mosquitoes.
Competition: If two populations compete for the same limiting factor, the competition may become a limiting factor. Competition may be interspecific or intraspecific. Interspecific competition is competition between populations of different species. Intraspecific competition is competition between populations of the same species. For example, the competition of two dogs for a piece of bone is an example of intraspecific competition, and the competition of a dog and a cat for a piece of flesh meat is an example of interspecific competition. To some degree, the intraspecific competition is good because the “more fit” can survive.
Temperature: Temperature often is a limiting factor for insect populations. The size of insect population begins to grow slowly in late spring, and then more rapidly through the summer. In autumn the size of the population decreases sharply because adults die as freezing weather begins. However, before they die the insects deposit their fertilised eggs in some “safe” place. The following spring the young adults emerge and begin another population “explosion”.
Oxygen: The amount of oxygen can be a limiting factor in aquatic communities. Oxygen is needed for respiration by almost all organisms. If the oxygen level drops too low, many animals cannot survive.
The oxygen content of water is affected by temperature. More oxygen can dissolve in cold water than in warm water. So the hot water discharged from factories or power plants may raise the temperature of the water to which it is added. The increase in the temperature of water decreases its oxygen-content. This may cause death of fish and other animals.

Experiments
These and similar conditions are showed with experiments on mouse populations.
1. Experiment ⇒ Every day 250gr. food was given to mouse colony. So mice increased quickly and population grew up.Number of mice increased so that food straits arised. As a result emigration started and populations were balanced.
2. Experiment ⇒ Daily food amount was fixed, but emigration in mouse colony was prevented. As a result birth speed decreased and populations were balanced.
3. Experiment ⇒ Daily food amount was increased, emigration was prevented. When the birth rate increased, space and appropriate nesting places for mice decreased. Thus population density increased. Later quarrels between mice and predations are seen. At this experiment death rate increased as much as birth rate too. Quarrels and deaths that arised by predation, balanced the population.

1. Experiment 2. Experiment 3. Experiment

Every day 250gr. Every day 250gr. Abundant
food food food

Population Population Population
increases increases increases

Food scarcity Food scarcity Migration is
starts starts prevented

Migration Migration is Living space
starts prevented narrows

Population is Birth rate Birth rate
balanced decreases decreases

Population is Population is
Balanced balanced