There are different factors influencing population size: immigration, natality, mortality, and emigration etc. All species of organisms have great capacity to reproduce. Without restraints, size of any population would increase at the exponential rate. Result is a J-shaped curve. As long as birth rates remain even slightly higher than death rates, population will grow exponentially.
Birth rate or Natality rate:
Natality rate is rate at which new individuals are added to the particular population by reproduction (birth of young ones or hatching of eggs or germination of seeds/spores). It is usually defined as number of births per 1,000 individuals of the population per year. Absolute, physiological or maximum natality, refers to theoretical maximum production of new individuals under perfect conditions. But it is never realized due to environmental resistance. Actual birth rate being attained under existing conditions that is much lesser than absolute natality is known as realized natality. Higher realized natality rate increases population size and population density.
Death or Mortality rate:
Mortality rate is rate at which individuals die or get killed. It is opposite of natality rate. Mortality rate is usually expressed as number of deaths per 1,000 individuals of the population per year. Lowest death rate for the given species in most favorable conditions is known as potential mortality while actual death rate being observed in existing conditions is known as realized mortality. Percentage ratio of natality over mortality stated in percentage is known as vital index.
Vital Index= Natality rate/Mortality rate * 100
Vital index finds normal rate of growth of population.
Age distribution (Age composition):
Relative abundance of organisms of different age groups in population is known as age distribution of population. With regard to age distribution, there are 3 types of populations.
i) Rapidly growing population is population, that has high birth rate and low death rate, so there are more number of young individuals in population.
ii) Stationary population is population that has equal birth and death rates, so population illustrates zero population growth.
iii) Declining population is population that has higher death rate than birth rate, so population has more numbers of older individuals.
Age Structure of Hypothetical Populations which are expected to increase, remain stable or decline with the passage of time Migration
In its most general sense migration is normally stated as relatively permanent movement of persons over the significant distance. A person who goes to another country and remains there for rest of his life, is migrant; and one who pays two-hour visit to nearest town is not. Between the two extremes lies a bewildering array of intermediate instances that can only partially be differentiated by more or less arbitrary criteria.
Permanence of movement:
With respect to international migration, recommendation of United Nations (and practice of the number of countries) is to state removal for one year or more as permanent, and therefore as migration, while stay for the shorter period is categorized as visit. When one speaks of migratory birds, or migrant laborers, or nomads, connotation is not of the permanent move from one area to another, but rather of a permanently migratory way of life, which often means a cyclical movement within a more or less definite area. Similarly, agricultural laborers frequently move with growing season, and shepherds alternate between high mountain pastureland in the summer and lowlands in the winter.
That migration is both associated to economic trends and yet not, in any simple sense, caused by them, must not occasion any surprise. Given sedentary population and inducement to leave home, typically some persons go and some stay behind. Push and pull factors, in short, don't exert their force evenly. Self-selection by which migrants distinguish themselves from sedentary population is known as migratory selection (or, by some authors, selective migration).
Potential migrants with particular skills go to places where there are openings especially for them. It is possible to examine migratory selection by the number of demographic and social characteristics additionally to occupation and race; and though conclusions from different studies differ extensively, some tentative generalizations are possible.
Effects on populations:
For two areas concerned, migratory selection finds importance of movement almost as much as number of migrants. Consider consequence of what can be taken as most primary question in migration theory: If X persons leave country A and migrate to country B, what changes occur in size of two populations. Common-sense answer, that country A is decreased and country B is increased by X, is accurate only in short run. If usually young migrants have their children in new country, its fertility rate may rise, whereas that of their native country falls. As remaining population of country A will then be older on average, its death rate may rise, whereas that of country B falls. In short, after generation transfer of X persons will in fact amount to X plus certain proportion based on migration's effects on population structures, and rates of population growth, of two countries.
Immigration is permanent entry of new individuals of same species in a population from outside. It increases size of local population.
Emmigration is permanent movement/departure of individuals of same species out of local population because of numerous reasons like lack of food, scarcity of space (overcrowding), etc. Emmigration decreases size of local population, but species spread to new areas. If more individuals are added than lost, then population will demonstrate positive growth. If more individuals are lost than added, then population will demonstrate negative growth. But if two rates are equal, then population will become stationary and is known as zero growth. Population growth = (Birth + Immigration) - (Death + Emmigration)
Carrying capacity (Resources):
Population density of area is mainly determined by available resources such as food, water and space in region. Maximum number of individuals of the population that can be given with essential resources for healthy living is known as carrying capacity of habitat. Size of the population increases till it reaches carrying capacity of habitat. When resources become short of requirement, population size starts to decrease through different determinants such as natality, mortality etc.
The drastic change in environment destabilizes or even exterminates the population. Natural calamities like earthquake, volcanic eruptions etc. cause drastic changes in environment leading to destruction of resources.
Abiotic and biotic factors:
Abiotic factors such as wind, temperature, rainfall, humidity, intensity of light has its effect on density of population. Biotic factors such as organisms of other species living in same area affect population, as they involve in different kinds of food relationships. Different populations have different skill to tolerate changes in weather, physico-chemical and biotic factors. This is known as resilience. In nature, factors such as predators, diseases, food scarcity etc. prevent the population to sour towards infinity. Sum of all these factors, that prevent the population from growing at maximum rate, is known as environmental resistance or population regulation.
Examples of Abiotic factors that affect population are:
Climatic and osmotic pressure places physiological constraints on organisms, like flight and respiration at high altitudes, or diving to deep ocean depths. These constraints affect vertical limits of ecosystems in biosphere as organisms are physiologically sensitive and adapted to atmospheric and osmotic water pressure differences. Oxygen levels, for instance, decrease with increasing pressure and are limiting factor for life at higher altitudes. Water transportation throughout trees is another significant ecophysiological parameter where osmotic pressure gradients factor in. Water pressure in depths of oceans needs that organisms adapt to such circumstances. For instance, mammals, like whales, dolphins and seals are particularly adapted to deal with changes in sound because of water pressure differences. Different species of hagfish give another example of adaptation to deep-sea pressure through particular protein adaptations.
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