by Brian Tomasik
First written: 2009; last edited: 31 Oct. 2014

Summary

I present some rough estimates of the numbers of wild animals on earth. This question is important because it determines how seriously we should be concerned about the suffering endured by animals in the wild.

Summary Table

I have so far been unable to find straightforward estimates of the total population of wild animals on earth. There is lots of good data on species diversity, but estimates of numbers of individuals are harder to come by. If readers are aware of good sources, please let me know. Still, the table below reports rough values for the best figures I have found.

I don't claim that all of these organisms can feel pain; indeed, for insects I think the evidence is mixed, and zooplankton sentience is even more unlikely (though these findings are at least interesting). Nonetheless, in view of the vast numbers of these organisms, it would be reckless to avoid giving some reduced weight to their possible suffering.

Animal Type World Population
Animals in Research Labs 108 (underestimate)
Humans 7 * 109
Livestock 2.4 * 1010
Land Birds 6 * 1010 to 4 * 1011
Land Mammals 1011 to 1012
Land Reptiles 1012 to 1013 (?)
Land Amphibians 1012 to 1013 (?)
Fish at least 1013
Dust mites more than 1016
Insects 1018 to 1019
Zooplankton 1018 to 1021
Nematodes 1022

Explanation of the Estimates

Animals in Research Labs

The number is roughly 50 to 100 million. This may not count "discarded" animals during breeding, nor does it include probably much larger numbers of invertebrates in research.

Livestock

The world livestock population in 2007 totaled roughly 24 billion (ignoring fish, lobsters, bees, and so on). This figure is calculated in the following table, which uses numbers copied from an FAOSTAT database.

World Livestock Populations Stocks
Total 24312162736
Chickens 17863376000
Ducks 1095805000
Geese and guinea fowls 343375000
Other Rodents 17425000
Turkeys 472635000
Animals Live Nes 5934816
Asses 42038633
Buffaloes 177247938
Camels 24246291
Cattle 1357183587
Goats 830391683
Horses 58641094
Mules 11840918
Other Camelids 6861765
Pigs 918278483
Sheep 1086881528

Birds

See the following table. The first two columns are mostly copied from "The main biomes," a geography module (though I was unable to find an original citation). I split off Tropical Forest as a separate category, using an estimated 7.75 km2 for their area, and taking the remaining 52.3 - 7.75 = 44.55 km2 to be temperate mixed forest. Of course, in reality, some temperate forests are rainforests, some are broadleaf forests, etc., but I've ignored those distinctions. Wild-bird densities by land type are reported in Gaverick Matheny and Kai Chan (2005), "Human Diets and Animal Welfare: the Illogic of the Larder" (p. 585), which cites a review study by Gaston et al. (2003). Data for the savannah were not given, so I've assumed they're roughly the same as for grassland. Figures were also not given for deserts and tundra, so I've assumed those as zero to keep the calculation conservative. Readers should feel free to play around with these numbers.

Biome Area (million km^2) Rough Bird Density (individuals / km^2) Notes
Tropical Rainforest 7.75 1250
Temperate Mixed Forest 44.55 800
Savannah 21.8 450 <--assumed same as grassland
Grasslands 8.8 450
Deserts 33.8 0 <--assumed due to no data and to make estimates conservative
Tundra 13.7 0 <--assumed due to no data and to make estimates conservative
Land birds (billions): 60
Land mammals (billions): 130 <-- assumed 2.25 times bird value
Land reptiles (billions): 500 <-- assumed 8 times bird value
Land amphibians (billions): 3000 <-- assumed ~50 times bird value

An alternate estimate comes from "How many birds are there?" by Kevin J. Gaston and Tim M. Blackburn. They estimate the number as 200-400 billion birds. 400 billion birds provides the basis for the upper-bound figures in the table at the top of this piece.

Land Mammals

Matheny and Chan (p. 585) report that a review of mammal densities similar to Gaston et al. (2003) has not been performed, but based on a British study by Gaston and Evans (2004) and Harris et al. (1995), they "assume the densities of wild mammals are 2.25 times those of wild birds for each land-use type," which I've done as well. Matheny and Chan (p. 585) note, "Applied to other continents, this is probably a significant underestimate, as Peters (1983, p. 167) records densities for some individual North American mammal species of over 10,000 individuals per square kilometer."

A separately calculated estimate is based on Derek W. Yalden's "A History of British Mammals". While noting the difficulty of estimating aggregate mammal populations, Yalden guesses a figure of 285 million mammals in Britain, compared with ~48 million adult humans. This implies about 6 wild mammals for every human in Britain. In other countries, particularly those with less development and more rainforest, I would conjecture that the mammal-to-person ratio is higher. And then we need to add the mammals in the ocean. Overall, given ~1010 humans, it seems plausible there are at least 10 times as many wild mammals: ~1011. This is the same as the lower bound based on the previous paragraph.

Land Reptiles

One study by Ishwar, Chellam, and Kumar (2001) assessed reptile densities in the tropical-rainforest floor of the Kalakad-Mundanthurai Tiger Reserve. Examining 25 m2 quadrats, the researchers found an average of 0.2559 reptiles per quadrat = 10,240 reptiles per km2 (p. 413). Assuming this is a typical density of reptiles in tropical rainforest, I naïvely divide this number against the Gaston et al. (2003) figure of 1,250 birds per km2 of tropical rainforest, yielding ~8 times as many reptiles as birds. I extrapolate this world population.

Vaclav Smil's Harvesting the Biosphere estimates the total (dry) mass of all land vertebrates on Earth as 10 million metric tons. Using the figures in the table above, this implies an average (dry) mass per land vertebrate of (1013 g)/(60 billion birds + 130 billion land mammals + 500 billion land reptiles + 3000 billion land amphibians) = 2.7 g. This may be reasonable, since, for example, humans are 70% water, so the wet mass might be more like 2.7/.3 = 9 g. If we still think the average is too small, one explanation could be that extrapolating ratios of herpetofauna vs. mammals/birds in rainforests does not work for other biomes, where intuitively, there are relatively more mammals/birds. Still, birds from the "Tropical Rainforest" biome of my table comprise 16% of all birds in the world, so even if there were no herpetofauna outside of rainforests, the herpetofauna numbers would still be 16% of my current estimates. Ignoring mammals/birds, this would very roughly give an average dry mass of 2.7/.16 = 17 g.

Land Amphibians

A study on amphibians in the Kalakad-Mundanthurai Tiger Reserve by Vasudevan, Kumar, and Chellam, parallel to the one on reptiles mentioned earlier, found densities of roughly 1 individual per quadrat = 40,000 per km2 (Fig. 2, p. 409).

Vasudevan, Kumar, Noon, and Chellam (2008), "Density and Diversity of Forest Floor Anurans in the Rain Forests of Southern Western Ghats, India," report frog-and-toad densities of 14,900 per km2 on the rainforest floor and over 30,000 per km2 near streams. Huand and Hou (2004), "Density and Diversity of Litter Amphibians in a Monsoon Forest of Southern Taiwan ," identified between 35,000 and 102,400 amphibians per km2 (p. 798). They cite (p. 799) other studies that had assessed densities of both amphibians and lizards: Allmon (1991), which measured 23,000-155,000 amphibians and lizards per km2 in a South American rainforest, and Heatwole and Sexton (1966), Scott (1976), and Inger (1980), which found 75,000 to 360,000 individuals per km2 in Costa Rica and Panama.

In general, it seems there are at least one to two orders of magnitude as many amphibians as birds based on these figures. In fact, Matheny and Chan note (p. 588) that on p. 510 of Reagan and Waide (1996), The Food Web of a Tropical Rain Forest, a table of animal densities by taxonomic group lists the density of reptiles and amphibians as up to 1000 times that of mammals and birds in some areas.

Fish

In "One Trillion Fish," I estimate crudely that "there are ~13 trillion (or ~10^13) wild fish in the oceans at any given time." This seems like a low-end estimate given how it was computed.

Dust mites

1016 is a lower bound on the number of dust mites because it only includes the dust mites supported by human skin. According to "Dust Mite Allergy": "An average adult person may shed up to 1.5 grams of skin in a day, an amount that can feed one million dust mites!" Given 1010 people in the world, that implies a minimum of 1016 dust mites.

Mayo Clinic reports 105 to 107 dust mites per bed. IndoorAir.com reports 107 mites per bed. National Geographic estimates 105 to 2 * 106 per bed.

According to Wikipedia, female dust mites live at most 70 days and lay 60-100 eggs in the last 5 weeks of life. On average in a stable population, all but 2 of those offspring will die, perhaps painfully, before reproducing.

In Dust Mites (p. 83), Matthew Colloff reports that adult female Dermatophagoides pteronyssinus dust mites have brains 30-40 micrometers in diameter. Say it's 35 micrometers = .0035 cm. Then assuming a spherical brain, its brain volume is (4/3) * pi * radius3 = 2 * 10-8 cm3. For comparison a human brain's intracranial volume is 1700-1900 cm3. Of course, I would conjecture that mites have smaller neurons and vastly more efficient architectures per neuron than humans.

Fortunately, a few animal supporters seem to take dust mites seriously:

Sadly, I couldn't find answers to these questions. To be safe, I daily flap out my bed sheet into an unused area of my house in an effort to remove some of the dead skin on it. But if dust mites can be crushed, I may still be injuring tens of them each night? (Likewise when I put pressure on clothing, wash clothing, step on floor dust, throw dusty trash into a garbage compactor, etc.)

Face mites

As an aside, mites live not just in our beds but on our skin as well. I couldn't find authoritative data on face-mite populations, but here's one very rough attempt. One study took mite samples from six facial locations with cumulative surface area of 10 cm2 (p. 444). Mite counts on normal subjects averaged 10.8 individuals (Table I, p. 445). This suggests roughly 1 mite/cm2. I doubt the sampling sites were perfectly representative of all areas of the skin on the human head, but assume they were. Assume the human head is a sphere with radius ~10 cm. Its surface area is then 4 * pi * radius2 = 4 * pi * 102 = 1256 cm2, which implies ~103 mites per face and ~1013 mites added over all human faces. I don't know if this is about right or way off. By comparison with the bed dust-mite numbers above, it seems somewhat low, but maybe vastly more mites can live on the copious quantities of dead skin in beds than on the small amounts of dead skin and oils on people.

Insects

The number of insects has been estimated to fall around 1018 or 1019.

Zooplankton

Most zooplankton are copepods (p. 23), which this source (p. 7) calls "the most abundant animals in the ocean, possibly the most abundant on Earth," and estimates the population at 1018. This is consistent with a comment on p. 2 of the introduction to Insect Biodiversity Science and Society by Robert Foottit and Peter H. Adler, which explains: "The number of individual insects on earth at any given moment has been calculated at one quintillion (1018) (Williams 1964), an unimaginably large number on par with the number of copepods in the ocean (Schubel and Butman 1998) [...]."

Page 23 of this source reports on one study that found 3 million copepods per m3 of ocean water. If such a density held uniformly up to some depth d meters in the ocean all over the planet's 361 trillion m3 ocean surface (ignoring freshwater environments, where copepods reside as well), the number of copepods would be ~(1021) * d.

Plankton Safari also suggests a figure on the order of 1021 by assuming at least one copepod per liter in each of the 1.347(1021) liters of ocean water. However, in reality, I would guess there are few copepods in the very deep ocean and many more than one per liter near the surface.

Nematodes

According to "Roundworm (Nematoda) research at CSIRO," "Nematodes are the most abundant and ubiquitous multicellular organisms on earth" and their number is 1022. Wikipedia says:

They are found in every part of the earth's lithosphere.[5] They represent, for example, 90% of all life forms on the ocean floor.[6] Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth [...].

It's unclear to me how much nematodes matter -- certainly much less than insects. Further reading: Nematode nervous system, C. elegans connectome.

Biomass Estimates

Comparing by biomass instead of individual count paints a different picture. The Earth's Biosphere: Evolution, Dynamics, and Change by Vaclav Smil features an Appendix F on p. 284, "Estimates of the biosphere's heterotrophic biomass," which can be viewed here. Further explanation can be found in Smil's paper, "Harvesting the Biosphere: The Human Impact," and in his Harvesting the Biosphere book. These results for mammals specifically were made famous in an xkcd comic. Note that land invertebrates still outweigh humans 10-25 times according to Smil's figures. Also, I haven't found any non-Smil sources for these estimates, so there's some chance of error or oversight in these numbers.

Brain mass should correlate roughly linearly with overall biomass, given that brain-to-body-mass ratios typically don't differ by more than 1-2 orders of magnitude across species, so the biomass estimates may be a decent approximation of brain size as well.

The individual-count figures vs. the brain-size figures represent two extremal positions for weighting the importance of animals. I think neither position is quite correct, and I would use some intermediate valuation, like maybe sqrt(brain size per organism). Let's approximate where this would leave us in comparing humans against, say, small mammals in relative direct importance. Smil estimates (Table 2) wild land mammals at 5 megatons of carbon, compared with 55 megatons for humans. Assume 1012 land mammals and 1010 humans, i.e., 100 land mammals per human. Most of the land mammals are small, weighing (5/55)/100 = ~1/1000th of a human. This jives with intuitive estimates: The average human weighs ~60 kg, and the average mouse weighs 20-40 g. The value of the small mammals using a sqrt(size) valuation is 1012 * sqrt(1/1000). The value of humans is 1010 * sqrt(1). The comparison reduces to 100 * sqrt(1/1000) vs. 1, i.e., ~3 to 1.

In general, if a group of N uniformly sized individuals collectively has mass M, the total importance of those minds is N individuals times sqrt(M/N) importance per individual, which equals sqrt(N*M).