What is Population Biology?

Population biology is the study of patterns in organism populations, specifically the growth and management of population size, population genetics, the evolution of life history, species interactions, and demography.

Population biology is an important category in science. It includes several terms such as Ecology, Species interactions, biological concerns of different organisms, their evolutionary history, and the population of different animal groups. Ecology is a branch of science (biology) that deals with the associations of different organisms to each other and the surrounding environment. In India, Ramdeo Misra is known as the Father of Ecology. He started the first Ecology course as part of his post-graduate studies. Biological concerns among organisms deal with the biotic and abiotic factors that influence their life patterns. Finally, the evolutionary history of organisms involves the variations (evolution) that occur in organisms' lives to make them fit in the environment they live in. Thus, population biology links ecology to mass evolution, the science of different creatures, and its genetic variations.

Under population biology, different organization levels, their evolution, extension and attributes, communities, and science of biomes are included.

Organisms and Associated Environment

The study of the ecology of the population is associated with the physiological environment and their adaptation to the physical environment. External environment such as annual changes in the temperature and precipitation leads to distinct seasons. These are further responsible for the formation of biomes such as Tundra, rain forests, and deserts. In each biome, regional and local variations occur, leading to the development of a wide range of habitats. Tropical rain forests, deciduous forests, deserts, and seacoast are the main biomes in India. Life exists in favorable as well as several extreme and harsh habitats. For example, in deep ocean trenches, polar regions, high mountain tops, stinking compost pits, scorching hot deserts, and thermal springs. The major factors responsible for the variations in chemical, physical conditions, and biology of habitats are abiotic factors such as temperature, light, water, soil, and biotic factors such as parasites, predators, pathogens, and competitors. In addition, there is a specific range of conditions that an organism can tolerate, a range in the resources that it uses, and a particular role in the ecological system; all these factors together form its niche.

Major Abiotic Factors

Temperature

It is the most important physical factor that affects the ecology of a specific population. The earth's mean temperature decreases as we move from plain areas towards mountains and the equator towards the pole. As a result, the temperature varies from sub-zero in high altitudes and polar areas to higher than 50°C in deserts (tropical) during the summer season. 

Water

The availability of water is a fundamental determinant of the presence of life. All the life forms starting from plants (their abundance and productivity) to higher animals, are entirely dependent on water. Water parameters such as pH, chemical composition, carbon dioxide concentration, and oxygen concentration play a major role in determining aquatic animals' lives. The salt concentration is less than 5ppm in the sea and is higher than 100 ppm in hypersaline lagoons.

Population Attributes

In a particular species, the total number of interbreeding individuals present in a geographic area that share and compete for the same resources is termed population. For example, in an abandoned dwelling, a total number of rats, trees in a forest, and total algae in a pond. 

Characteristics of Population

Birth rates and death rates

The number of births per capita is termed birth rate, and the number of deaths per capita is known as death capita. For example, if there are 20 hydrilla plants in a pond last year and eight new plants are added to the pond through reproduction, the present population of hydrilla plants will become 28. Therefore, the birth rate in this condition can be calculated as 8/20= 0.4 offspring per hydrilla per year. Similarly, death rates can be calculated. For example, if four hydrilla plants in a pond of 40 hydrilla plants died during one year, the death rate of the population during that one year is 4/40= 0.1 individual per hydrilla per year. 

Age distribution

There are three age groups in a population: pre-reproductive, reproductive and post-reproductive. Pre-reproductive: children and young adolescents (under 15 years old), Reproductive: working-age (15-64 years old), and the Post- reproductive: elderly (65 years and older).

Age Pyramids

Different age groups present in a community when represented graphically, at the base- pre-reproductive groups, a reproductive group in the middle, and at the top post-reproductive groups- are termed age pyramid.  Age pyramids can be one of the following types: 

• Triangular age pyramid- In this type of age pyramid, the largest number of pre-reproductive individuals is there, followed by reproductive and then post-reproductive. Such a pyramid shows the growing rate of individuals in a community.
• Bell-shaped age pyramid: There is an equal number of pre-reproductive and reproductive individuals in this age pyramid. Post-reproductive age group individuals are less in number comparatively. Such age pyramids show a stable pattern.
• Urn-shaped age pyramid: In this type of age pyramid, the number of pre-reproductive individuals is less than that of reproductive individuals. The number of post-reproductive individuals is also fewer. Thus, this type of age pyramid shows a negative pattern.

Population Growth

The size of the community is determined by several factors such as the carrying capacity of the environment, mortality, habitat, evolution concerning environmental conditions, the impact of the use of fertilizers, the effect of predators, and the natality of species. It is also termed population density (N). The size of individuals in a community is also determined by the factors such as weather conditions, surrounding environment, availability of food, and pressure exerted by predation. The number of individuals of a species present per unit area/space at a given period is referred to as N.

$$\text{Population Density }\left(D\right)=\frac{\text{Number of individuals }\left(N\right)}{\text{Space }\left(S\right)}$$

The basic factors that determine the number of individuals in a given community are:

• Natality: The number of births in a period under consideration in a population added to the initial density is termed natality. It increases the number of individuals in a given community.
• Mortality: The number of deaths in a given community during the period under consideration is called mortality. It declines the number of individuals in a given community.
• Immigration: The number of individuals of a specific species that shifted to the habitat from somewhere else during the time period under consideration is known as Immigration.
• Emigration: The number of individuals of a specific species that have moved from the habitat to somewhere else during the time period under consideration is known as emigration.

Growth Models

The number of individuals in a specific geographic area follows a particular expansion pattern. These are:

1. Exponential pattern: This pattern occurs when the number of individuals in a given area has an unlimited supply of food and space. In this situation, the number of individuals increases largely in a limited period of time, and the growth pattern is exponential.
2. Logistic pattern: When a group of individuals in a population has limited resources, individuals' expansion follows a logistic growth pattern.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors of Science. in Biology
• Bachelors of Science in Zoology
• Masters of Science in Zoology

Related Concepts

1. Evolution
2. Study of species interactions
3. Population biology
4. Study of Species growth and development
5. Biology of different species.
6. Biological aspects of species in a community.