Biology · The Living World · NEET
Biodiversity is the number and kinds of organisms present on Earth. It covers the huge variety of living things, from tiny bacteria to large trees and animals. NCERT says: if you look around, you see a large variety of living organisms. That variety is what we call biodiversity. For NEET, remember the one-line NCERT definition: biodiversity is the number and types of organisms present on Earth.
About 1.7 to 1.8 million species have been actually known and described. This is the exact NCERT number and it is the count that answers 'biodiversity'. The 7 million figure is different, it is Robert May's ESTIMATE of how many species may exist in total. So the number that has actually been described = 1.7–1.8 million. NEET often mixes these two to trap you.
'Known and described' means scientists have found the organism, given it a scientific name, and written down its features. That number is 1.7–1.8 million. 'Total species' means all species that exist, including ones not yet discovered. That number is unknown, so Robert May estimated it at about 7 million. Known = 1.7–1.8 million (a fact). Total = ~7 million (an estimate).
Many organisms live in places that are hard to reach, like deep oceans, thick rain forests, and soil. Many are microscopic and very small, so we cannot see them with the naked eye. New species are still being discovered every year. Because of this, we cannot count every species, so scientists give an estimate instead of an exact number.
In this NCERT chapter (The Living World), biodiversity is defined simply as the number and types of organisms present on Earth. So for this concept and for NEET Class 11 questions, focus on that species-count meaning: about 1.7–1.8 million described species. The wider meaning of biodiversity (genes, species, ecosystems) comes in a later chapter, so do not over-think it here.
It depends on the wording. If the question asks 'how many species are KNOWN and DESCRIBED', write 1.7–1.8 million. If it asks 'according to Robert May, the global species diversity / total number', write 7 million. Read the question carefully. The word 'described' points to 1.7–1.8 million; the words 'May' or 'global estimate' point to 7 million.
According to Robert May, the global species diversity is about:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
About 1.7 to 1.8 million species are known and described. This is the fact NCERT gives for biodiversity in the chapter The Living World.
Robert May estimated the global (total) species diversity at about 7 million. This is an estimate, not the counted number.
The known and described number, 1.7–1.8 million, is a real counted fact. The total of about 7 million is an estimate because many species are not yet discovered.
Because two numbers (1.7–1.8 million described and 7 million total) look similar but mean different things. NEET tests whether you can tell them apart by reading the question wording.
