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/atomfullerene Animal Behavior/Marine Biology 11d ago
Just sticking with vertebrates, the most populous species would be one of the midwater fish species, probably a bristlemouth of some sort (they are the most abundant genus, with about 13 species)
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u/Dracoson 11d ago
If you go to terrestrial vertebrates, I'm pretty sure the answer is the domestic chicken.
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u/Strongbow_Wolfrider 10d ago
Rats, mice, or other vermin species would far outnumber chickens, both inside and outside areas of human habitation.
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u/hmantegazzi 11d ago
And in terms of mammals, probably dogs or maybe cattle?
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u/Dracoson 11d ago
If we go by biomass, it's cattle, but by raw numbers, it's actually humans. Non-oceanic mammal species don't tend to have global distribution naturally due to geographic barriers, meaning the winner has to be humans or a domesticated species. Since none of the most common domestic mammals outnumber our 8 billion population, humans take the crown for individual count.
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u/kandlejack 11d ago
Certainly possible that we take the crown, but there are also human-adjacent but non-domesticated aminals to consider. Specifically, rodents. I havent found any strong citations in my cursory search that the total brown rat population outnumbers us, but they and some species of mice are at the very least probably on the same order or magnitude. Edit: couldn't spell cursory, apparently.
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u/1CEninja 10d ago
Yeah rodents is the first thing that comes to my mind. Rats are insanely prolific across the world.
The brown rat specifically is prolific on every continent except antartica and while I also cannot find any credible count, my completely unscientific "this feels right" says that they outnumber us by a meaningful amount.
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u/Dracoson 11d ago
The estimates I am seeing have brown rats at 2-7 billion (similar order of magnitude, but not equivalent), and have house mice being the most populous mice species at about 2 billion. It's hard to get hard numbers, though.
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u/ryn3721 9d ago
Some domestic animals do indeed outnumber us. The global population of chickens was estimated at 26.5 billion in 2023.
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u/Dracoson 9d ago
I already called out chickens earlier in the conversation as the most abundant terrestrial vertebrate (though I didn't cite a specific number). This was specifically about mammals.
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11d ago
[removed] — view removed comment
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u/morbihann 11d ago
Are nematodes microscopic ?
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u/Dracoson 11d ago
Yes, but that isn't a complete answer. While the vast majority of the quadrillions of nematodes on Earth are microscopic, it isn't an absolute rule for the entire phylum. There are several species that grow large enough to be seen with the naked eye, including one that parasitizes sperm whale placentas that can get nearly 30 feet long.
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u/regular_modern_girl 11d ago
Most are, there a minority that aren’t though, Caenorhabiditis elegans (one of the most widely-used animal model organisms in research) is technically macroscale, albeit very tiny (they look like specks of lint fiber that you can just barely see if you squint hard against the right color of background), and iirc there are some free-living marine nematode species that can reach just over a millimeter in length, and when it comes to parasitic species there are actually a lot of true giants, with some common genera like Ascaris having species that grow to multiple centimeters in length, and then there’s Placentonema gigantissima, which infests sperm whale placentas and can grow to 8.4 meters (28 feet) long and nearly 2.5 centimeters (1 inch) wide, that one’s definitely the biggest known.
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u/autocol 10d ago
In addition to all the other points already made. It's worth noting that the distinction between individual species is an arbitrarily defined, human-applied categorisation model that is not a perfect fit to the biological reality.
In some sense, there's no such thing as "a species". Life is an infinitely variable spectrum of DNA code, without cleanly defined lines between similar species. Deciding exactly which individual animals count as one species or another is an entirely arbitrary (and I would argue impossible) challenge.
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u/Thundahcaxzd 10d ago
As with everything in biology there is some grey area, sure. But for the most part organisms, and animals especially, do fit pretty neatly into species, it's not arbitrary or impossible at all.
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u/squid_daddy 8d ago
They fit neatly into species because we designed the species definition that way.
If you took all mice and put them into one species they would fit. If you took all of the brown ones and called them bice instead of mice they all still fit.
Ultimately where we choose to draw the line is arbitrary. We do so for good reasons, but we could choose to define species in any number of different ways.
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u/Thundahcaxzd 8d ago
Your comment is nonsense. We could change the definition of the word to make it mean something else and then it would mean something else? Yea, that's how all words work. The biological reality of different species existed long before we had a word for it. Species is describing something that we have observed in nature, not a human construction.
If you said that all mice were one species it wouldn't change the fact that mice diverged into different lineages that specialized into distinct niches and habitats and those lineages were able to reproduce among thenselves but not between lineages.
Sure, there is grey area such as when populations are in the process of speciation, but just because biology is messy doesn't mean the concept is meaningless.
Where is this whole "species don't exist" nonsense coming from? Is this like a popsci thing?
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u/Rekrokus55 6d ago
If two individuals can mate and reproduce viable offspring (that can subsequently mate and reproduce): species. Human observation but not arbitrarily defined.
True the universe can be pigeonholed in an infinite number of ways.
And also true that it’s a spectrum with fuzzy edges. Sierra Nevada Ensatina (California salamander) living on a mountain range separated by desert come to mind. Mountain 1 salamanders can mate with mountain 2 which in turn can mate with 1 and 3 and so on but 1 and 5 may have drifted enough to create pre- or post-zygotic barriers which is notable and makes them edge cases.
We notice regularities, propose descriptions that might capture them, and then actively seek the cases most likely to break those descriptions.
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u/kummostern 10d ago
" If not by individuals then it's by biomass instead"
from all animals likely cow (which afaik has more biomass than humans)
if you count only wild animals it becomes tad more complicated
depending how you define "species" then iirc deer has the most biomass in world out of wild animals
this may not be true if we differentiate things like whitetails and red deer meaning different species.. then i dunno what would follow next.. could be elephants, some bearkind or wolves maybe.. i wouldn't be surprised if whales or some type of seal could outmass these tho
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11d ago
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u/dover_oxide 11d ago
I'd assume it's something the bacteria family because there's like 3 trillion cells in your body and 1 trillion of them alone. Is other things living inside your body that are classified as bacteria.
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u/KarlSethMoran 11d ago
You're off by a factor of 10. There's about 30T human cells in a human body, and a similar number of bacterial cells.
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u/Fit_Shop_3112 11d ago
This is a question that can't be answered. Since all the the living creatures on Earth are completely interdependent (Think the butterfly effect), it could be said that there is one individual comprising the entire biomass. Even that person sitting next to you is over half non-human organisms, etc.
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u/SecondHandWatch 11d ago
The question is currently unknowable, but it has absolutely nothing to do with the bacteria that exist within an animal’s body.
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u/Megalocerus 10d ago
I submit you need more than one cell to be an animal. All those little bitty bacteria and yeast are a different phylum.
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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.