Biology 3 Marks Question

Reason for population growth : Following are the three main reasons for the growth of population of a country :
(1) High natality rate : In most of the countries, the birth rate is higher than death rate. Economic and social factors are responsible for high natality or birth rate.
(a) Economic factors : (i) agricultural dominance (ii) rural economy (iii) poverty 
(b) Social factors : (i) illiteracy (ii) religious beliefs (iii) child marriage (iv) superstition (vi) inevitability of marriage (vii) joint family and indifference towards family planning etc.
(2) Relatively low mortality : The number of people dying every year per thousand of the population is called death rate. The main reason for the population explosion is the continuously decreasing death rate. There has been a continuous decrease in the death rate in India in the last decades, which is an important factor for population growth. It has following reasons :
(i) Protection from natural calamities : Man started living by building houses for his own safety so that he could be protected from natural calamities like cold, heat, rain, storm etc.
(ii) Development of agriculture : Due to agricultural development, the production of food grains increased and it became possible to supply it in sufficient quantity to the increasing population.
(iii) Storage facility : Due to advance storage facility, food grains become easily available to humans throughout the year.
(iv) Advanced transportation : With advanced transportation system, it became possible to reach food grains to places where there is shortage of food grains with immediate effect and in a short time.
(v) Control on diseases : Due to control on various diseases and increase in health measures the death rate has decreased and human's average age has increased from 30 to 60 years.
Immigration : Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. It increases the population. This type of increase in population has been recorded due to the arrival of refugees and infiltrators from different countries in the borders of India. 

Both parasitism and predation are examples of negative interaction. In this type of interaction one species gets benefit but the other species is harmed.
In the process of predation members of one species capture, kill and eat up members of other species. Like tiger eats a deer. Here tiger is hunter and deer is prey. Herbivores eat plants. In this way predators control the prey population.
Parasitism is the relationship between two living organism of different species in which small organism (parasite) live inside or on the body of larger species (host) from which they obtain food and shelter. Parasites are host specific, they do not have alternates like predators. Parasites are smaller in size.
They have high biotic potential as compare to predators. Parasites require specific structures for dispersal to reach the host and to attack them. Whereas predators are movable and are capable to catch the pray

Commensalism : In this interaction only one of two is benefitted but no one suffers any loss. Epiphytes despite being autotrophic, grow on other plants. They absorb water through velamen roots and synthesize their own food by the process of photosynthesis. Examples orchids and benda. Green algae Basicladia grows on the shell of turtles found in freshwater. It is an example of epizoic.
Lianas are woody climbers which grow on the land in soil and climb up with the support of other trees and spread over their tops to get proper sunlight. Examples are tinospora, bignonia, bougainvillea.
Mutualism : In this interaction, both of the species are mutually benefitted and cooperation of both is necessary for their survival. Example—Lichen, symbiotic nitrogen fixing bacteria and mycorrhizal association. There is an association between the roots of higher plants and fungi which is called mycarorhizial association. Example : pinus, oak etc. In this association fungus absorb water and minerals and make them available to the plant and in return roots of plant provide food to fungi. Roots of such plants have lack of root hairs.

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The density of population in a given habitat during a given period of time fluctuates due changes in four basic processes. Natality and immigration contribute to an increase while mortality and emigration contribute to decrease in population. These processes are being defined as follows :
(i) Natality : Natality means the number of births in the population which adds to the initial density during a given period.
(ii) Mortality : This is the number of deaths in the population during a given time period.
(iii) Immigration : The number of individuals of a species who have arrived into a habitat from somewhere else during a given time period, is called immigration.
(iv) Emigration : The number of individuals of a species who have left their habitat and moved elsewhere during a given time period, is called emirgration.