A population is a group of interbreeding individuals of a particular species occupying a particular area at specific time. In human demography, a population is a set of humans in a given area, as a village, town, city, state or country, Some of the ecologists, however, recognize two types of population:
(i) Monospecific population - It is the population of individuals of only one species.
(ii) Mixed or polyspecific population - It is the population of individuates of more. than one species. Polyspecific population is characteristic feature of a community. In which populations of different species share a common habitat interacting with each other. The subject of population ecology includes the following three major areas of study.
- Describing a population (population characteristics) - The various characteristics which are unique to the group (population) as a whole and not to an individual in the group are described including some of their numerical patterns.
- Population dynamics - the various theories to explain the population growth.
- Regulation of population - the factors which govern population density.
Characteristics of Population
The population has a definite organization and structure and exhibits various characteristics. In brief a population exhibits following characteristics:
1. Population Size and Density - Total size is generally expressed as the number of individuals in a population. More informative are estimates of density, the number unit area (or volume) of environment. Larger organisms as trees may be per expressed as 500 trees per hectare, whereas smaller ones like phytoplankton's (as algae) as 2 million cells per cubic meter of water. In terms of Weight, it may be 100 kg of fish per hectare of water surface.
Since, the patterns of dispersion of organisms in nature are different it becomes important to distinguish between crude density and specific (ecological) density.
(i) Crude density - It is the density (number or biomass) per unit total space.
(ii) Specific or ecological or economic density - It is the density (number or biomass) per unit of habitat space i.e. available area or volume that can actually be colonized by the population.
Population size is generally expressed as the number of individuals in a population, whereas the population density is the number of individuals per unit area or per unit volume. The unit of population density differs in different cases, as for example, the number of squirrels per sq. km. and the number of diatoms per liter of water. The population density is a measure of the population's success in a given region. Population density is quite variable but there exists upper and lower limits to species population sizes in nature. The upper limit is determined by the energy flow in ecosystem, tropic level of the organism and the size and rate of metabolism of the organism, whereas the lower limit is controlled by homeostatic mechanism.
Population density of organisms can be measured either by (i) total counts, or (ii) by sample plot method. The total counts is the simplest way in which all the individuals are counted over a measured distance. However, it is more practical in counting the plants than animals and provide good numerical picture of population density. The sample plot method consists of selecting several samples and counting the organisms per unit of the sample. Sample plots are distributed usually systematically in uniform size along the straight lines at equal intervals.
The population density can be computed by the following formula:
D = n/a / t
Where,
D = Density
N = Number of individuals
a = Area
t = the time unit
The study of density indicates the successful nature of population and suggests the future of birth rate and death rate. The increased density slows the population growth. The population density is affected by several factors. Nasality and immigration lead to an increase in density, whereas the mortality and emigration lead to a decrease in density.
2. Concepts of Growth Rates - Before dealing with other characteristics of population, it is necessary to have idea of growth rates. Since population is a changing entity, we are interested not only in its size and composition but also in nature of its change. As varying from place to place population density also varies in time. Population may remain constant, they may fluctuate or they may steadily increase or decrease (Fig.). Such changes are the main focus of population ecology. They are brought about by the interplay between four factors. Natality (production of offspring's) and immigration (individuals entering a population from elsewhere) lead to an increase in density, while mortality (death of individuals) and emigration (moving out of population) lead to a decrease in density. A rate is generally obtained by dividing the change by the period of time elapsed during the change. Thus growth rate of a population is the number of organisms added to the population per time. It is customary to abbreviate 'the change in something' by writing the symbol Δ (delta) in front of the letter representing the thing changing. Thus, if N represents the number of organisms and t the time, then,
ΔN = the change in number of organisms.
ΔN /Δt = The average rate of change in the number of organisms per time
This is growth rate. ΔN / NΔt = The average rate of change in the number of organisms per time, per organisms (growth rate divided by the number of organisms initially present or alternately by the average number of organisms during the period of time.) This is often called specific growth rate.
3. Natality (Birth Rate) - It is simply a broader term covering the production of new individuals of any organism. These new individuals are born, hatched, germinated, arise by division etc. In human population, however, the natality rate is equivalent to the 'bright rate'. Natality rate is the number of offspring's produced per female per unit time.
There are distinguished two types of the following nasality-
(i) Maximum (potential or physiological) Natality - It is the theoretical maximum production of new individuals under ideal conditions (i.e. no ecological limiting factors, reproduction being limited only by physiological factors). It is a constant for a given population. This is also called fecundity rate.
(ii) Ecological or Realised Natiality - It is also known simply as natality, which refers to population increase under an actual, existing specific condition. Thus it takes into account all possible existing is also designated as fertility rate.
Natality is expressed as,
ΔNn /Δt = the absolute natality rate (B)
ΔNn / NΔt = the specific natality rate (b) natality rate per unit of population
Where,
N = initial number of organisms
n = new individuals in the population
t = time
4. Mortality (Death rate) - It refers to death of individuals in the population. Like natality, mortality may also be of the following two types:
(i) Minimum Mortality - Also called specific or potential mortality, it represents the theoretical minimum loss under ideal or non-limiting conditions. It is a constant for a population. Thus even under the best conditions individuals would die of old age determined by physiological longevity.
(ii) Ecological or Realised Mortality - It is the actual loss of individuals under a given environmental condition. It is, like ecological natality, not a constant and varies with population and environmental conditions.
Like natality, mortality may be expressed as the number of individuals dying in a given périod (deaths per time), or as specific rate in terms of units of the total population.
A birth-death ratio (100× births/death) is called vital index.
For a population, the important thing is not which members die but which members survive. Thus survival rate are of much interest than the death rates. Survival rates are generally expressed by survivorship curves.
Survivorship curves - There are three general types of survivorship curves. Which represent different nature of survivors in different types of population. These are as follows
(a) Highly Convex Curve - Curve A in diagram is characteristic of the species in which the population mortality rate is low until near the end of the life span. Thus such species tend to live throughout their the span, with low mortality. Many species of large animals as deer, mountain sheep and man etc. show such curves.
(b) Highly Concave Curve - This curve (C in Figure) is characteristic of such species where mortality rate is high during the young stages. Oysters or other shell fish, oak trees etc show this type of survivorship.
(c) Diagonal Curve - In age specific survival is more nearly constant; the curve approaches a diagonal straight line (curve B₂ in Figure). It thus shows a constant proportion of organisms dying per unit time. Probably no population in the real world has a constant age specific survival rate throughout the whole life span. Thus a slightly concave or sigmoid curve (B3 in Figure) is characteristic of many birds, mice and rabbits. In these cases the mortality rate is high in the young but lower and more nearly constant in the adult (1 year or older).
5. Dispersion - Dispersion is the spatial pattern of individuals in a population relative to one another. Their are following three basic patterns.
- Regular Dispersion - Here the individuals are more or less spaced at equal distance from one another. This is rare in nature but is common in managed systems (cropland). Animals with territorial behavior tend toward this dispersion.
- Random Dispersion - Here the position of one individual is unrelated to the positions of its neighbors. This is also relatively rare in nature.
- Clumped Dispersion - Most populations exhibit this dispersion to some extent, with individuals aggregated into patches interspersed with nor or few individuals.
6. Age Distribution - Age distribution influences both, natality and mortality of the population, The ratio of the various age groups in a population determines the current reproductive status of the population, thus anticipating its future. There are three major ecological pages (age groups) in any population, which are, pre- reproductive, reproductive and post-reproductive. The relative duration of these age groups in proportion of the life span varies greatly with different organisms.
Age pyramids representing geometrically the proportions of different age groups in the population of any organism.
The three hypothetical pyramid types are as follows -
- A pyramid with broad base - It indicates a high percentage of young individuals. In rapidly growing young populations birth rate is high and population growth may be exponential as in yeast, housefly, paramecium etc. Under such conditions each successive generation will be more numerous than the preceding one and thus a pyramid with broad base would result.
- Bell Shaped Polygon - It indicates a moderate proportion of young to old. As the rate of growth becomes slow stable, i.e., the pre-reproductive and reproductive age groups become more or less equal in size, post reproductive group remaining as the smallest, there results a bell-shaped structure.
- An Urn Shaped Figure - It indicates a low percentage of young individuals. It the birth rate is drastically reduced the pre-reproductive group dwindles in proportion to the other two groups and it results in an unshaped figure (Figure C). which indicates that population is dying off. while density reveals little about differences between populations in developed and developing countries, age structure highlights some of the causes of rapid population growth in the latter. Developing countries have a much greater preponderance of young people, and the side of pyramids becomes concave. Such a shape means high population growth, not only at present bet also likely in future.
- Seasonal fluctuations
- Annual fluctuations
(b) Annual Fluctuations - The annual fluctuations may be caused: (a) annual difference in the physical environment or extrinsic factors (b) by population dynamics or intrinsic factors. The extrinsic fluctuations are irregular while the intrinsic fluctuations are cyclic or regular in nature.
7. Population Equilibrium - Initially a population increases in size rapidly, but after the initial attainment of maximum size, a population often maintains or tries of maintain the same level for a long period. If a population reaches its maximum growth rate (carrying capacity) in such a way that the supply of food and other necessities and also the removal of harmful byproducts keep pace with growth, then the population will keep itself at or near the equilibrium level by maintaining a constant growth rate.
At equilibrium level, A = M, where A is birth rate and M is mortality rate. Under these circumstances the reproductive rate may be high or low, but as long as the growth rate is exactly offset by mortality, the population equilibrium is maintained.
8. Biotic Potential - Each population has inherent power to grow. When the environment is unlimited (space, food, other organisms not exerting a limiting effect), the specific growth rate (i.e. the population growth rate per individual) becomes constant and maximum for the existing conditions. The value of the growth rate under these favorable conditions is maximal, is characteristic of a particular population age structure, and is a single index to the inherent power of a population to grow. It may be designated by the symbol r, which is the exponent in the differential equation for population growth in an unlimited environment under specific physical conditions.
The index r is actually the difference between the instantaneous specific natality rate (i.e. rate per time per individual) and the instantaneous specific death rate and may thus be expressed:
r = b - d
The overall population growth rate under unlimited environmental conditions (r) depends on the age composition and the specific growth rates due to reproduction of component age groups. Thus there may be several values of r for a species depending upon population structure. When a stationary and stable age distribution exists, the specific growth rate is called the intrinsic rate of natural increase or r max. the maximum value of r is often called by the less specific but widely used expression biotic potential or reproductive potential.
9. Life Tables - Information on natality and mortality in different ages and sex can be combined in the form of life tables. From these it is possible to estimate the growth or decline of population. As with survivorship curves, life tables are standardized to follow the progress of a cohort. In each table there are columns for age of individuals; number surviving to each age; the number dying in each age group; the proportion dying from the previous age category; fertility rate; and the number of young born by each group. The information obtained from these figures provides the net reproductive rate of the population i.e. offspring left by each individual. Similarly from life table. mortality in logarithmic form is also obtained. These are then used to calculate the rate of population growth.
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