### Population ecology

The population is the total number of the collective group or the species living within the particular area with the various characteristics. It is dynamic and changes with the time. The ecology is the wide subject that deals with the population and the surrounding. Similarly, the population ecology is the branch of the ecology that deals with the variation in the population size and structure and also the population dynamics. It is also known as the population biology

#### Characteristics of population

Every moment the size of the population varies. So these can be expressed in the statistical form considering the characteristics of the individual in the group. The characteristics of the population include the following characteristics:

• Population size and density
• Dispersion
• Age Structure
• Natality or Birth rate
• Mortality or death rate
• Biotic potential
• Life table
• Growth Rate

#### 1. Population Size and density

Generally, the size of the population in relation to the space occupied by them is known as the population density. It can be also expressed in form of the biomass . The number of the individual population biomass per unit area or the volume of the environment is called population density. In biomass, it is expressed as 200 tress per acre, 200 pounds of fish per acre of the water surface. In this the larger living forms like trees may be expressed as 500 trees per hectare whereas the smaller ones like the phytoplankton may be expressed as 2 million cells per cubic meter of the water.

The population density is of two types

1. . Crude density

The density of the number of the species or the biomass per unit total space ( area, volume) is known as crude density.

$$Crude\ density = \ \frac {number\ or\ biomass\ of\ species} {Total \space\ occupied}$$

It includes all the land within the organism's land.

.b.Ecological density

The density (number or biomass) per unit of the habitat spaqce available is known as ecological density.

$$Ecological\density = \ \ frac {number\or\biomass\of\species} {habitat\space\occupied} #### 2.Dispersion and the pattern of dispersion Dispersion is the spatial pattern of the individual in a population relative to one another. The dispersal can also be defined as the means by which new or the depopulated areas are colonized and the equilibrium is established. Similarly, the population dispersion is the movement of the individuals or their propaguels( seeds, spores, larve etc) into or out of the population or the population area. The disoersion is the important component in gene flow and the process of the speciation. In general there are three different patterns of the dispersion: i. Regular Dispersion The regular or the even dispersion is relativwly rare in the naturewhere there is no tendency to aggregate. It is caused by the competition between the species . For e.g. territoriality between the animals , lack of the water in the desert soil etc. ii. Random dispersion The random distribution occurs when the environment is uniform with the evenly distributed resources and also available throughout the year, also theinteraction between the members of the population with out attraction or avoidance . iii. Clumped dispersion The clumped or the patches of the individuals is known as the clumped distribution. It is most common in nature. It is caused by microhabitat preference i.e. shaded or moist places, dispersal pattern (e.g. root shoots), social behavior of animals. #### 3. Age structure The individuals are always of the different ages in most populations. The proportion of the individuals in each group is known as age structure of that population. The natality and the mortality The individuals are always of the different ages in most populations. The proportion of the individuals in each group is known as age structure of that population. The natality and the mortality is dependent on the age structure of the population. Basically the ratios of the various age group in the population determines the current reproductive status of the ppulation.From the ecological point of the view, there are three major ecological ages: • Pre- reproductive • Reproductive • Post reproductive In human, three ages are relatively equal in length. Many plants and animals have a very long reproductive period and insects have extremely long pre- reproductive periods, a very short reproductive and no post reproductive period. 1. Age pyramids The representation of the geometrical proportion of the different ages in the population of any organism is called agr pyramid. There are three types of pyramids: a. A pyramid with broad base Such pyramid indicates the greater percentage of the yong ones. In the rapidly grieving young population the birth rate is high and population growth may be in the yeast, housefly , paramecium etc. b. A bell shaped polygon It indicates the moderate proportion of the young to the old ones. When the rate of the growth becomes stable and the slow , the pre- reproductive and the reproductive age group becomes more or less equal in size. The post reproductive group remains as the smallest. c. An urn shaped figure Such pyramid indicates the low percentage of the young individuals. If the birth rate is reduced, the pre reproductive groupsswipes away in proportion to the other two groups and it results in the urn shaped. #### 3. Natality Natlity is the inherent and the natural ability of the population to increase its proportion. Simply it is the broader term covering the production of the new ones by born, hatch, germinate or arise by division. It refers to the increase of the population under an actual or the specific environmental conditions. Generally it is expressed as the rate determined by dividing the number of the new individuals per unit of the time. Natality is expressed as: Absolute natality rate= ΔNn / Δt The specific natality rate = ΔNn / (nΔ t) where, N = Initial number of the organisms n = new individuals of the organisms t = time The natality increases upto the period of the maturity and slowly decreases and falls as the organism gets older but it differs in the tropical and the temperate region. The breeding and the clutch are the two important criteria. In the tropical areas with the dry periods, breeding is at least as seasonal as the temperate areas whereas the clutch size is smaller in the tropical region than in the temperate zone. #### 4. Mortality Simply the death of an organism due to the accidental case or by the age is known as the mortality rate. It is also known as the specific or the potential mortality. It is natural cause of the population decrease. The population would die of the old age determined by their physiological long life. It is also considered as the survival rate that is greater than the death rate. If the latter is expressed as a fraction. M then the survival rate is I-M. References: Santra, S. (2004). Environmental Science . India: New Central Book Agency (p) Ltd. Keller, E.A.Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company,Belland Howell Company, 1985. Mahapatra, G.B.Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992. • The population ecology is the branch of the ecology that deals with the variation in the population size and structure and also the population dynamics. • It is also known as the population biology. •$$Crude\ density = \ \frac {number\ or\ biomass\ of\ species} {Total \space\ occupied} 
• Absolute natality rate= ΔNn / Δt

The specific natality rate = ΔNn / (nΔ t)

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