CBSE Class 12 Biology Population and Population Growth : Chapter 13 Notes
Biology class 12 :- Population refers to the total number of individuals belonging to the same species, occupying a particular geographic area at a given time.
Characteristics of a population
- Populations are dynamic and change in response to environmental stress or change in environmental conditions.
- Populations change in their size (number of individuals), in density (number of individuals in a certain space).
- They can also change their pattern of dispersion in their habitat (i.e. the way in which individuals are spaced in a population).There are 3 general types of dispersion patterns: clumped, uniform, and random.
- Populations can also experience changes in their age distribution (proportion of individuals of each age in a population).
The above changes that a population experience is called as population dynamics which occur due to environmental changes.
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Population growth is the increase in the number of individuals in a population.
There are four variables/ causes that govern the population growth/population size:
- Birth rate(Natality): It is defined as a number of births per 1000 individuals in one year.
- Death rate (Mortality): It is defined as a number of death per 1000 individuals per year.
- Immigration: Entry of individuals in a given population.
- Emigration: Exit of individuals from a given population.
A population gains individuals by birth and immigration and loses them by deaths and emigration.
Thus, population change = (Births + Immigration) – (Deaths + Emigration)
Effect of Age Distribution on Population growth
In many organisms, the generations overlap. It is shown, from a theoretical point of view, that competition between the different ages classes in such organisms will have important population dynamics consequences. Strong competition between age classes can be either a stabilizing or destabilizing force depending on the stage of the life cycle—juvenile survival or adult survival/reproduction—at which density dependence occurs. When juvenile survival is density dependent, competition has a stabilizing effect. On the other hand, when reproduction is density dependent, strong competition tends to be destabilizing, i.e. it will cause population density to fluctuate. It is also shown that delayed effects on reproduction, caused by strong competition as the adults grow up, tend to be destabilizing. These findings have interesting implications for the population dynamic behavior in organisms with different types of life cycles, and these theoretical findings might be relevant to the interpretation of patterns of population dynamic behavior.
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Human Population Growth – Humans have escaped (for the time being) the bounds of nature; that is, most limiting factors. With medical advancements and abundant food supplies, the worldwide human population has increased dramatically. Since the industrial revolution, the human population has become less and less dependent on the ability to acquire resources directly from nature. The population growth is characterized by birth rate as well as death rate. So, lower birth rates don’t simply lead to lower population growth – if the death rates are also dropping then also the population might be growing.
Studies have shown that any country with many people below 15 years old (represented by a wide-based population structure diagram) has a powerful built-in momentum to increase its population size unless death rates rise sharply.The number of births rises even if women have only one or two children because of a large number of girls who will be soon moving into their reproductive years.
In 2002, 30% of the world’s people were under 15 years old, which means 1.9 billion young people are poised to move into their prime reproductive years. In developing countries, there are 33% of people in their prime reproductive years as compared to 18% in developed countries.
Population Geographers construct a population age structure diagram by plotting the percentages of numbers of males and females in the total population in each of three age categories
Pre-reproductive (ages 0-14), reproductive (ages 15-44), post-reproductive (ages 45 and up)
The above figure shows generalized population age structure diagrams for countries with: rapid (1.5-3%) growth rates/slow (0.3-1.4%) growth rates,zero (0-0.2%) growth rates/negative growth rates.
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