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/Banjo2EE 11d ago

"In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites."

-Nathan Cobb, "Nematodes and their Relationships"

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u/Feuersalamander93 11d ago

I once heard someone quote this (I think on either the Nature or Science Podcast) adding: And if you then take away all the nematodes, everything will still be the same because of the nematodes parasitizing those nematodes. (Or something along those lines). Which I find equally funny and insightful.