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Class 12-science NCERT Solutions Biology Chapter 11 - Organisms and Populations

Organisms and Populations Exercise 204

Solution 1

(i)Population density

(ii)Natality or birth rate

(iii)Mortality or death rate

(iv)Population growth

(v)Sex ratio

(vi)Age distribution

Solution 2

Solution 3

Modification of leaves into thorns and development of spiny margins on leaves. Many plants produce and store chemicals which make herbivores sick. Example: Calotropis produces highly poisonous cardiac glycosides. Some other chemical substances such as nicotine, quinine, opium etc. are produced by plants and provide defence against grazing animals.

Solution 4

It is an example of commensalism, where the orchid gets space (benefitted) and the mango tree is neither benefitted nor harmed.

Solution 5

The ecological principle behind the biological control of pest insects is based on the ability of a predator to regulate the prey population in that habitat. Example: Gambusia fish prey upon the larvae of mosquito and acts as biological controller of malaria.

Solution 6

Population: A population is the collection of interbreeding organisms of a particular species living together in the same geographical area at a time.

Community: A community is a group of organisms belonging to several different species which live together in the same area or habitat and interact through trophic and spatial relationships.

Solution 7

(a) Commensalism: Commensalism is an interspecific interaction between two species where one species is benefitted and the other remains unaffected. Example: Orchid and mango tree.

(b) Parasitism: It is a relationship between two organisms where the larger animal is at harm and the smaller animal is benefitted. Example: Malarial parasite and human beings.

(c) Camouflage: Camouflage is the ability of animals to blend with the surroundings or background. In this way, animals remain unnoticed for protection or aggression. Example: Stick insect.

(d) Mutualism: It is relationship between two organisms where both organisms are benefitted. Example: Fungal symbiotic association with algae in lichens.

(e) Interspecific Competition: It is an interaction between individuals of two species where both the interacting species are affected. Example: Monarch butterfly and Queen Monarch.

Solution 8

Diagram: Population growth curve

a - When responses are not limiting the growth, the plot is exponential.

b - When responses are limiting, the growth plot is logistic. K is the carrying capacity.

Solution 9

(d) One organism is benefitted, other is affected.

Solution 10

Three important characteristics of a population are

i.Density: It is expressed as the total number of individuals per unit area or volume at a given time. The size of the population is determined by the available resources nutrients, water etc. at a given time and other group properties such as natality, mortality and age structure.

ii.Natality: It is the increase in the number of individuals in a population under given environmental conditions. Birth, hatching, germination and even vegetative propagation cause increase in the number of individuals.

iii.Mortality: The loss of individuals due to death in a population under given environmental conditions is called mortality.

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