Population

Population Dynamics

(1) Population density : Population density is the number of individuals present per unit area or volume at a given time. For instance, number of animal per square kilometer, number of trees per area in a forest, or number of plank tonic organism per cubic meter of water. If the total number of individuals is represents by letter N and the number of units of space by Letter S, the population density D can be obtained as D=N/S. Space is indicated in two dimensions (m²) for land organisms, and in three dimensions (m³) for aquatic organisms and for the organisms suspended in space.

(2) Birth rate or Natality : The birth rate of a population refers to the average number of young ones produced਋y birth, hatching or germination per unit time (usually per year). In the case of humans, it is commonly expressed as the number of births per 1000 individuals in the population per year.

The maximum birth rate of a species can achieve under ideal environmental conditions is called potential natality. However, the actual birth rate under the existing conditions is much less. It is termed realised natality.

(3) Death rate or mortality : The death rate of a population is the average number of individuals that die per unit time (usually per year). In humans it is commonly expressed as the number of death per 1000 persons in a population per year. Lowest death rate for a given species in most favourable conditions is called potential mortality, while the actual death rate being observed in existing conditions is called realized mortality.

(4) Survivorship curve : The death rate of population can be easily represented by survivorship curve. In this curve time is plotted against number of survivors. There are three kind of survivorship curves.

(i) Diagonal curve : If death rate of different age group organism are equal then the curve is represented or plotted as a straight line.

Example : Hydra, mice and many adult birds.

(ii) Convex curve : When organism completed their potential life spawn and died in old age then the curve is convex, the curves goes horizontal till potential life spawn and then decline rapidly.

Example : Men, rabbit and many mammals.

(iii) Concave curve : This kind of curve most found in such organism who die before their potential life spawn.

Example : Fish, Oysters and Invertibrates.

(5) Vital index : The percentage ratio of natality over mortality is known as vital index i.e. natality / mortality 󗄀. It determines the growth of a population.

(6) Immigration : It is permanent entry of additional person into the existing population of a country or region from out side. Example Many Nepalese and Chinese come to settle in India.

(7) Emigration : It is the permanent departure of some persons from the existing population of a region to a different state or a foreign country. Example Many Indians go to Western countries to settle there.

Immigration and emigration bring about redistribution of population, and are common in animals. These occur for various reasons, such as search for food, escape from competition due to overcrowding, need of shelter etc.

(8) Sex ratio : The number of females in a population per 1000 males is called sex ratio.

Sex ratio =$\frac{No.offemales}{1000(males)}$

(9) Age structure : The age structure of a population is the percentage of individual of different ages such as young, adult and old. Age-sex structure of a population can be shown by a pyramid-like diagram by plotting the percentage of population of each sex in each age-group. Figure shows the age-sex structural pyramids for India over the 20-year period from 1971 to 1991. These pyramids show that Indian population may still take many years to be stabilized.

(10) Age distribution : The relative abundance of the organisms of various age groups in the population is called age distribution of population. With regard to age distribution, there are three kinds of population.

(i) Rapidly growing or Expanding population : It has high birth rate and low death rate, so there are more number of young individuals in the population.

(ii) Stationary or stable population : It has equal birth and death rates, so population shows zero population growth. The pyramid of this type of population is �ll’ shaped.

(iii) Declining population :  It has higher death rate than birth rate, so the population of young members is lower than that of old members e.g. Japan (Ageing population). The pyramid of this type of population is ‘urn’ shaped.

Human population has three age groups : Pre reproductive, Reproductive, and post reproductive.

(i) Number of infants below one year of age and the older people as these have higher mortality rate than individuals of other age groups.

(ii) Proportion of reproductive active males and females in a population.

(iii) Number of females in active reproductive age (i.e. between 15 to 44 years)

(11) Change in Population Size and Growth Rate : Whether a population grows, declines or maintains it size depends upon the balance between the above factors:

The above expression in words may be represented in a simple way by a mathematical model. suppose N = population size and t = time. The Greek letter delta, $\Delta$, indicates change. We can now represent change in population as $\Delta$N, and time interval as $\Delta t$. The verbal equation can be written as $\Delta$N/$\Delta$t=(B+I) – (D+E) in which B is absolute number of births in the population during the time interval, and D =the absolute number of deaths during that interval I means immigrant and E, emigrants. I and E , being insignificant, may be ignored. Then the equation simplifies to $\Delta$N/$\Delta$t =B-D.

Migration is a two-way movement of a population for adjusting to seasonal changes. It occurs in some fishes (Anguilla, an eel), birds (Siberian crane), and mammals (fur seal). Migration is not considered a determinant of population size.

Annual average growth rate is the percentage of increase in population size per year. It can be calculated with the help of following equation :

$\mathrm{Annualgrowthrate}(\%)=\left(\frac{{\mathrm{P}}_2-{\mathrm{P}}_1}{{\mathrm{P}}_1\times \mathrm{N}}\right)\times 100$

Where P₁= Population size of previous census. P₂= Population size of present census.

N= Number of years between the two census.

Growth of Human Population

Population growth refers to the increase in its size. It is determined by the number of individuals added to the population and the number of individuals lost from the population. Addition occurs by births and immigration. Loss results from deaths and emigration. If more individuals are added than are lost i.e., the vital index is more than 100, the population will increase or show positive growth. If more individuals are lost than are added i.e., the vital index is less than 100, the population will decrease or show negative growth.  If addition and loss are balanced, i.e., the vital index is 100, the population will become stationary or show zero growth.

Malthus Theory of Human Population Growth : Thomas Malthus, a British political economist, put forward a theory of human population growth in 1778. Malthus in his "Essay on the principle of population" pointed out that population tends to increase in geometric progression while food supply increase only in arithmetic progression. Faster growth of population than of its requirements causes an imbalance between the two. When this imbalance reaches a certain limit, environmental factors like famine, epidemic of a disease, earth quake, flood, war etc reduce the population to a size, the available resources can support. The factors that control the population size were called positive checks by Malthus.

Natural Control of Population Growth : Growth of a population is controlled by an interaction between three factors : biotic potential, environmental resistance and carrying capacity of environment.

(1) Biotic or reproductive potential : Biotic potential is the natural capacity of a population to increase at its maximum rate under ideal environmental conditions and stable age and sex ratios. The biotic potential for all animals is very high. If unchecked, the numbers of any species will quickly over run the world. Biotic potential in the human female is estimated to be about 12 per female during its reproductive period between the puberty and the menopause period.

(2) Environmental Resistance : In nature full biotic potential of an organism or population is never realized, since conditions are rarely ideal. Various harmful environmental (abiotic) factors like non-availability of food and shelter, natural calamities like drought, cloud bursts, floods, fires, temperature fluctuations, accidents, etc. and certain biotic factors like pathogens, parasites, predators etc. check the biotic potential from being realized. The sum of all these inhibitory factors is called environmental resistance.

(3) Carrying capacity : It is defined as �ing capacity of an environment ofਊn ecosystem for a population of a species under provided set of conditions”. When a population reaches the carrying capacity of its environment, the population has zero growth rate. So the population generally stabilizes around the carrying capacity. The carrying capacity of the earth for human population is considered to be about 8 to15 billions. Carrying capacity of the environment for a population depends upon three major components :

(i) Productive systems which produce food and fibre e.g. croplands, orchards, etc.

(ii) Protective systems which buffer air and water cycles and keep moderate environmental temperatures e.g. ocean etc.

(iii) Assimilative systems which utilize the wastes produced by human activities e.g. waterways, wetlands, etc.

Productive system and protective systems collectively form the life-supporting capacity, while assimilative systems collectively form the waste assimilative capacity.

(iv) Population fluctuations and population cycles : The populations are not stable and do change due to a number of extrinsic as well as intrinsic factors. These variation in the population size are of two types :

(a) Population fluctuations or irruptive variations : In these changes, population density tends to fluctuate irregularly above and below some steady-state level. These are characterized by sudden increase in population in short time which is followed by equally quick decrease in population size. These are caused by random seasonal or annual changes in availability of resources (food or energy) or extrinsic factor (e.g., temperature, rainfall etc.) e.g., more birds during early summer due to their hatching period, more insects during summer months and more weeds in rainy season.

(b) Population cycles : These are regular changes in the population size. In these, population size is nearly constant over long period of time. These are caused by seasonal changes in environment e.g. population cycles (of 3 to 4 years) of lemmings of Tundras (Elton, 1942) Lemmings (Lemmus lemmus) (small mouse-like rodents found in arctic regions of Canada and Norway) increase in their number for a period of about 3 years when it reaches a peak beyond the carrying capacity of that area. They eat up all the available food. In the winter months, the lemmings migrate in large numbers in the sea and swim till they are drowned due to exhaustion. The surviving lemmings multiply and repeat the process.

Patterns of Population Growth : Growth of a population can be expressed by a mathematical expression, called growth curve in which logarithm of total number of individuals in a population is plotted against the time factor. Growth curves represent interaction between biotic potential and the environmental resistance.

Two basic types of growth curves

(i) Sigmoid or S-shaped growth curve : It is shown by yeast cells and most of organisms. It is formed of five phases :

(a) Lag phase.  In which the individuals adapt themselves to the new environment, so there is no or very little increase in population.

(b) Positive Acceleration phase. It is the period of slow increase in population in the beginning.

(c) Logarithmic or Exponential phase : It is the period of rapid rise in population due to availability of food and requirements of life in plenty and there being no competition.

(d) Negative Acceleration phase : In which again there is slow rise in population as the environmental resistance increases.

(e) Stationary (Plateau) phase : Finally, growth rate becomes stable because mortality and natality rates become equal to each other. So there is zero growth rate. A stable population is said to be in equilibrium, or at saturation level. This limit in population is a constant K and is imposed by the carrying capacity of the environment. S-shaped curve is also called logistic curve. Sigmoid growth curve was described by Verhulst, (1839)

(ii) J-shaped Growth curve : It is shown by small population of Reindeer experimentally reared in a natural environment with plenty of food but no predators. It has only two phases:

(a) Lag phase : It is period of adaptation of animals to new environment so is characterized by slow or no growth in population.

(b) Logarithmic or Exponential phase : It is characterized by rapid growth in population which continues till enough food is available. But with the increase in reindeer population, there is corresponding decrease in the availability of food and space, which finally become exhausted, which leads to mass starvation and mortality. This sudden increase in mortality is called population crash. Lemming of Tundra, some insect, algal blooms and annual plants also show J-shaped curves. The population growth curve is S- shaped in most of the organisms, Human population also shows S-shaped curve.

(iii) Human Population Growth Curve : The modern man (Homo sapiens sapiens) appeared about 25,000 years ago. For a very long time, the human population remained in the lag phase, having little or very slow growth. By the year 1 A.D., there were about 0.25 billion people in the world, and by 1600 about 0.5 billion. Thus, it took 1600 years for the population to become double. The exponential phase of growth of the human population started about 1750. Since then, the time taken by the population to become double has considerably shortened. It doubled in 200 years (1600-1800 A.D.), becoming 1 billion then doubled in 130 years (1800-1930 A.D.), growing to 2 billion then doubled in only 45 years (1930-1975 A.D.), reaching about 4 billion. At present, the world human population grows at a rate of 2 percent a year, and it has now reached 6 billion. If the present growth rate persists, there would be 8 billion people on earth by the year 2017.

World population gain is 2 persons every second 200,000 people every day 8 million every month and 70 million every year. The high rate of growth is often referred to as “population explosion” The word 𠇎xplosion” may be defined as a rapid and expansive change of state.

The future of human population is difficult to predict. It may stabilize and have S-shaped growth curve or decline rapidly and have J-shaped growth curve. The population will stabilize if the birth and death rates are balanced in the near future. It will rapidly decline if it overgrows the carrying capacity of the environment.