How do biologists arrive at a gross estimate of the total number of species on earth?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → By extrapolating the temperate-tropical species richness ratio of an exhaustively studied insect group to other groups
Published records tell us how many species have been recorded, but not how many exist on earth. Most of the species still waiting to be discovered are in the tropics, where inventories are less complete. So biologists take a group of insects that has been studied exhaustively, compare its species richness in temperate and tropical regions, and extrapolate this ratio to other groups of animals and plants. This gives a gross estimate of the total.
Option (1) By adding up all the species discovered and named in published records — Incorrect. Adding up published records only gives the number of species recorded so far. Many species are yet to be discovered and described.
Option (2) By extrapolating the temperate-tropical species richness ratio of an exhaustively studied insect group to other groups — Correct. The temperate-tropical ratio of a well-studied insect group is extrapolated to other groups to reach the gross estimate.
Option (3) By applying the species-area relationship of one continent to the land area of the whole earth — Incorrect. The species-area relationship shows how species richness changes with area within a region. NCERT does not use it to estimate the total number of species on earth.
Option (4) By adding the prokaryotic species identified with molecular criteria to the species already recorded — Incorrect. NCERT says these estimates give no figures for prokaryotes at all. Biologists are not sure how many prokaryotic species there are, since many cannot be cultured.
NCERT Extract:
How Many Species are there on Earth and How Many in India? – “Since there are published records of all the species discovered and named, we know how many species in all have been recorded so far, but it is not easy to answer the question of how many species there are on earth. … Considering that an overwhelmingly large proportion of the species waiting to be discovered are in the tropics, biologists make a statistical comparison of the temperate-tropical species richness of an exhaustively studied group of insects and extrapolate this ratio to other groups of animals and plants to come up with a gross estimate of the total number of species on earth.”