Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): If the species-area relationships among very large areas, like entire continents, are analysed, the slope of the line is much steeper.
Reason (R): On a logarithmic scale, the relationship between species richness and area is a straight line.
In the light of the above statements, choose the correct 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)
Assertion (A): If the species-area relationships among very large areas, like entire continents, are analysed, the slope of the line is much steeper. — True. A is about the value of the slope (Z). For most regions Z lies between 0.1 and 0.2. When very large areas such as whole continents are compared, the slope is much steeper, with Z between 0.6 and 1.2.
Reason (R): On a logarithmic scale, the relationship between species richness and area is a straight line. — True. R is about the shape of the graph. Species richness plotted against area gives a rectangular hyperbola. On a logarithmic scale the same relationship becomes a straight line, log S = log C + Z log A, in which Z is the slope.
Why R does not explain A? R tells us only that the relationship is a straight line on a log scale. That holds for every region, small or large. A is about how steep that line is for continents. A straight line can be steep or gentle, so the fact that it is straight cannot tell us why the slope is higher for continents. So both statements are correct, but R is not the explanation of A.
NCERT Extract:
Patterns of Biodiversity – “… In fact, the relation between species richness and area for a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes) turns out to be a rectangular hyperbola (Figure 13.2). On a logarithmic scale, the relationship is a straight line described by the equation
log S = log C + Z log A
where
S = Species richness
A = Area
Z = slope of the line (regression coefficient)
C = Y-intercept”
Patterns of Biodiversity – “… But, if you analyse the species-area relationships among very large areas like the entire continents, you will find that the slope of the line to be much steeper (Z values in the range of 0.6 to 1.2). For example, for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is found to be 1.15.”
Figure 13.2 Showing species area relationship. Note that on log scale the relationship becomes linear