Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Primary productivity varies in different types of ecosystems.
Reason (R): Productivity is expressed in terms of g m−2 yr−1 or (kcal m−2) yr−1 to compare the productivity of different ecosystems.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Productivity is the rate of biomass production. A dense tropical forest and a dry desert do not make organic matter at the same rate, so ecosystems have to be compared on a common scale: per square metre per year. The scale only measures the difference; it does not create it.
Assertion (A): Primary productivity varies in different types of ecosystems. — True. A forest, a grassland and an ocean differ in their plants, nutrients and conditions, so their productivity differs.
Reason (R): Productivity is expressed in terms of g m−2 yr−1 or (kcal m−2) yr−1 to compare the productivity of different ecosystems. — True. Giving productivity per unit area per year lets ecosystems of any size be compared.
Why R does not explain A? R only tells us the units in which productivity is written. Units are a way of measuring and comparing; they do not cause productivity to change. The cause of the variation lies in the plant species, environmental factors, nutrients and photosynthetic capacity, none of which R mentions. So R is not the correct explanation of A.
NCERT Extract:
Productivity – “… The rate of biomass production is called productivity. It is expressed in terms of gm–2 yr–1 or (kcal m–2) yr–1 to compare the productivity of different ecosystems.”
Productivity – “… Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems.”