The number of individuals of specie living in the same area at the same time is called population. Population has two aspects: number of individuals of a specie and area occupied by them. Clarke (1954) formed two types of populations:
- Monospecific: Population of only one specie living in the same area is called Monospecific.
- Mixed or Polyspecific: Populations of different species living in the same area are called Polyspec
Population has following characteristics:
- Density: The number of individuals living in a unit area is called density.. For example number of wheat plants in an acre etc. The unit of density varies in different species. Density has two types:
- Crude density: It is the number of individuals or biomass per unit of total area inhabited by the specie.
- Ecological density: It is the ‘number of individuals or the biomass per unit of that area actually inhabited by the individual of the specie.
- Natality or birth rate: The rate at which the new individuals are added to a population in a unit time is called Natality.
- Mortality or death rate: The rate at which the individuals are lost by death in a unit time is called mortality. If the birth rate is more than the death rate, then population is increasing. If the birth rate is less than death rate, the population is decreasing.
- Age distribution: The population,of individuals of different ages in the group is called age distribution. The age groups are prereproductive, reproductive and post reproductive.5. Dispersion or distribution: It is the random pattern of distribution of individual of a population over space. It may be of three types:
- Random: In this case. the individuals are distributed randomly. For example, tree of shisham grow randomly in the field.
- Uniform: These individuals are evenly spaced. Uniform distribution occurs when severe competition is present among the plants in a forest.
- Clumped: In this case. the individuals occur in scattered groups. Daily and seasonal w !tether and reproductive pattern causes clumping of trees.
- Population growth: The number of individuals of population or biomass increased in unit time is call population growth. There are different patterns of population growth in plants. Sonic plants have rapid growth rate. They increase rapidly and reached the carrying capacity of the field.Then their death starts due to competition and shortage of nutrients. Some populations have uniform growth rate. Their death rate is equal to birth rates. Therefore, the population remains uniform.
- Competition: The individuals compete with each other for space and nutrients. Intraspecific competition occurs within same population. Individuals of the same specie compete with each other. It is more severe competition because all the individuals have same ecological niche.
HISTORY AND BACKGROUND OF POPULATION ECOLOGY
Man is on this earth for about 40.000 years. Early man had a niche in the ecosystem. It was a primary consumer. It feeds on roots and fruits. Later it became secondary consumer and predator. It started hunting and killing his prey. Thus a new man evolved. He changed his niche in the ecosystem. Communities of plants and animals were also modified.
Human culture started over 5000 years ago. The development of industry, technology and modern medicine took place in the last 200 years. Modern man can modify his external environment. He constructs his home to live. He made cloths to wear. He releases heat energy from fossils fuel. Thus physical environment can not affect him. Therefore, he can lives in almost any climate.
The demand for food has increased with the increase of population. Thus Humans convert the natural ecosystem into artificially maintained agricultural ecosystem. The natural variety of plants and animals is destroyed and these plant and animals arc replaced with species useful to humans. Sometimes, these agricultural ecosystems are mismanaged. So the total productivity of the regions may fall below the original ecosystem. The dust bowl of North America, desertification in Africa and destruction of tropical rain forests are well known examples.
All these changes require ecological. studies. These studies are made in population ecology. In population ecology we study different factors involved in the rise and fall of the populations. It also gives how different populations tend to increase beyond their mean.
Population is the study of one species occupying an area. But a population depends on other resources for food and shelter. Therefore. population ecology interacts with community ecology. The characteristics of a population are shaped by their interaction with environment. Natural selection modifies these characteristics. Thus population ecology also has close relationship with the evolutionary ecology. Thus population ecology is not an isolated subject. It includes many other aspects of ecology.