Class 12-science NCERT Solutions Biology Chapter 11 - Organisms and Populations
Organisms and Populations Exercise 204
(ii)Natality or birth rate
(iii)Mortality or death rate
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.
It is an example of commensalism, where the orchid gets space (benefitted) and the mango tree is neither benefitted nor harmed.
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.
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.
(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.
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.
(d) One organism is benefitted, other is affected.
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.