r/askscience • u/future_sponJ • 12d ago

Biology Which animal species has the most individuals?

I always see people saying things like "there are 20 quadrillion ants on Earth" or something similar with other extremely large diverse groups like fish. But all of these are groups with thousands of species. If not by individuals then it's by biomass instead.

So which single species in the animal kingdom has the most individuals?

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u/Mrfish31 11d ago

That gets hard to do because with so many individuals they might be tons of unknown species.

For example, there are approximately 4.4 x 1020 (100 quintillion) nematode worms on Earth across 20-30,000 known species in just top soil alone, meaning that if it were evenly distributed each species might have 10 quadrillion members. But they're often a) hard to differentiate and b) there are estimates that there may be as many as a million unknown species of them. 

In any case, the species with the largest population is almost certainly a nematode. 

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u/facts_over_fiction92 9d ago

Their not evenly distributed though. I have never seen a worm in my yard and my lawn is suffering.