Population ecology is the study of the number of organisms and what determines their abundance and distribution. The term population ecology is derived from the Latin word populus which means people. Originally, it was referred to as a group of people occupying a particular area. Like so many others, the term population has come to many things.
Ecologically, a population is a group of organisms of the same species or other groups within which individuals may change genetic information, occupying a particular space. According to Wynme Edcsards (1965), a population is an automated system.
Characteristics of Population
The characteristics of a population are,
- Population Density
- Dispersion
- Natality
- Mortality
- Life Tables
- Age Structure
Population Density
Population density refers to the number of individuals inhabiting a unit area or volume at a given time. Eg. The number of animals per sq. km or trees per hectare in a forest or planktons in a cubic meter per liter of water or diatoms and protozoan residues per liter per sq meter of water.
According to Henson, there are about 63000 earthworms on average in an acre of garden soil. The number of larger organisms is smaller. There are only 2-3 birds per acre and just 1-2 foxes for several 100 acres.
If the total number of individuals is reported by N and the number of units of space (area for land organisms and volume for water organisms) by S, the population density D is obtained as,
D = N/S
Space is indicated in two dimensions (m2) for land organisms and in 3D (m3) for aquatic organisms. Density is affected by several environmental factors such as natality, mortality, emigration, and immigration.
Dispersion
There are three types of dispersion patterns in a population.
- Uniform/regular: The organisms are regularly or uniformly spaced in the whole habitat or area. It occurs where the competition is severe throughout the habitat.
- Random: The tendency to aggregate is absent in random dispersion. The environment is uniform throughout the habitat and competition is also absent. This allows the organisms to be anywhere in the habitat creating a random pattern. Such dispersion occurs through seed dispersal by ait.
- Clumped: Here, the organisms grow in clumps or groups. This pattern can further be divided into uniform, random, and aggregated.
Natality (Birth Rate)
Natality is an expression of the production of new individuals in a population. It varies from organism to organism. The maximum number of births produced under ideal conditions of the environment, ie, ecological limiting factors, is called potential natality. This will be constant for a given population. When the actual number of births occurring under the existing conditions is much less, it is called seized natality.
The natality rate is computed per unit of time by the following formula.
Natality rate = B/T where B is the number of births while T is the total time in which the births have taken place.
Mortality (Death rate)
Mortality is the average number of individuals that die per unit of time. It is commonly expressed as the number of deaths per 1000 individuals in the population per year. This is not constant for the population and varies with the population and environmental conditions.
Mortality Rate = D/T, where D is the number of deaths and T is the time or year.
Usually, natality exceeds the mortality rate at low population density. When the population density is high, the death rate or mortality rate exceeds natality. When the two are equal, the population is stationary.
Birth over death ratio in a unit of time is called the vital index, which is expressed as,
Vital Index = Birth/Death x 100
Life Tables
Information on natality and mortality in different ages and sexes is given in the form of life tables. For this, it is possible to estimate the growth or decline of a population.
In each table, there are columns for the age of individuals, the number of surviving per age, the number of dying individuals in each age group, the proportion of dying from the previous age category, fertility rate, the number of young borns by each group, etc.
The information obtained from these figures provides the net reproductive rate of the population, i.e. offsprings left by each individual. Similarly, mortality in logarithmic form is also obtained from life tables.
Age Structure or Age Distribution
Birth and death rates of a population are considerably influenced by age distribution within a population. The stability of the population can be estimated from the age distribution.
Bodel Heimer (1958) has divided individuals of a population into three groups, pre-reproductive, reproductive, and post-reproductive. Depending on the proportion of the three age groups, a population can grow, stabilize, and diminish.
The graphic representation of different age groups with the number of pre-reproductive at the base, reproductive in the middle, and the post-reproductive group at the top is called the age pyramid. This age pyramid can be of three types- triangular, bell-shaped, and urn-shaped.
Triangular Age Pyramid
A triangular age pyramid is formed if there is a very high proportion of young or pre-reproductive individuals, a moderate number of reproductive or mature individuals, and a smaller number of post-reproductive individuals. Population growth is exponential and it denotes a young or growing population.
Bell-shaped Age Pyramid
When the number of pre-reproductive and reproductive individuals are almost equal, post-reproductive individuals are fewer; it forms a bell-shaped pyramid. It represents a stable population with no significant growth.
Urn-shaped Age Pyramid
When the proportion of reproductive individuals is high when compared with pre-reproductive individuals who are entering the reproductive phase, the size of the population will continue to decline. This forms an urn-shaped pyramid that shows a diminishing population.
Rules To Calculate Rate Of Population Growth
These are the rules to calculate the rate of population growth, regulation of population density, Key-factor analysis, and self-regulation of populations.
Regulation of population density
Population density can only be increased by natality or immigration while the decrease in population can only be caused by mortality and immigration. These factors may be density-dependent or density-independent in their effects.
As the population increases the density-dependent factors such as competition and predation are increased in their proportional effects. The rate of population growth will be increasingly depressed through intra-species competition.
On the other hand, the density-independent factors do not show much variation with the change in population density. The abiotic factors are usually density-independent but this may not be the case always. Yet, floods, fires, etc can damage the population density as would the cold weather in the tropics.
Both density-dependent and density-independent factors may interact in most populations to determine observed densities.
Key-factor analysis
It is a method to analyze mortality factors to find out which may be regulatory on the population. The K-value for each mortality factor together with the total k (the sum of all factors), are plotted for several successive generations.
The K-factor that most closely follows the pattern of k will be the key factor. For eg., in the insect population, the loss of adults through migration or death is the key factor.
Self-regulation of populations
Intraspecific density-dependent interactions regulate many populations in the lab and in that sense these are self-regulating. Accumulation of waste products may depress the population growth which is also considered as self-regulation.
Social behavior may also be self-regulatory. Another example is species with individuals or groups of individuals that differ from each other or have different breeding patterns. Death or failure to make new individuals are density dependent which can regulate population density.
Immigration, Emigration, Population Dynamics
Immigration is the entry of additional individuals into a population. Emigration is the departure of some individuals from a population.
Immigration and emigration are common with animals. For example, frogs may move from one pond to another. These processes do not occur actively in the population of land plants since they are rooted and are incapable of locomotion. However, an occasional addition to the plant population may occur through seeds brought by air, water, or animals from distant places.
Besides natality and mortality, immigration and emigration also affect the population density. These are features of dispersal where they move by individuals either singly or in groups, from one population to another. These migrated individuals may even die if no suitable environment is found.
Thus it is clear that several factors play a role in population regulation. The influence of any factor upon a population is determined by the time in the life cycle of the organism at which it is effective in its variability and its threshold and upper limit of vulnerability for the population.
Inter-competition occurs so that when one factor becomes ineffective in controlling the density of a population, another factor becomes more effective.
References
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- Verma, P.S., Agarwal, V.K. (1999). Cell biology genetics molecular biology evolution and ecology. New Delhi: S.Chand Co.(Pvt) Ltd.
- Tarsi, K. & Tuff, T. (2012) Introduction to Population Demographics. Nature Education Knowledge 3(11):3
