Correct Option
The correct option is 2 and 3 only.
Explanation
The assessment of global biodiversity involves comparing the number of species formally recorded by taxonomists (described species) against statistical estimates of the total number of species actually existing on Earth (estimated diversity).
Statement 1 is Incorrect:
According to the International Union for Conservation of Nature (IUCN) (2004), the total number of plant and animal species described so far is slightly more than 1.5 million. The figure is not less than 0.5 million. This represents only a fraction of the actual species present on Earth.
Statement 2 is Correct:
Estimates for the total number of species on Earth vary significantly, ranging from 20 million to 50 million. However, a more conservative and scientifically sound estimate made by Robert May places the global species diversity at about 7 million. This is currently considered the most widely cited baseline in ecological studies.
Statement 3 is Correct:
There is a significant disparity in the completeness of biological data across different latitudes. Taxonomists have historically studied temperate regions more intensively than tropical regions. While tropical regions harbor higher species richness (biodiversity), a vast majority of these species remain undiscovered. Therefore, species inventories are generally more complete in temperate zones.
Key Takeaway:
Current taxonomic records account for approximately 1.5 million species, whereas Robert May’s widely accepted estimate suggests a global total of 7 million species. This indicates that nearly 78% of all species, particularly in the tropics, are yet to be discovered.