by Brian Tomasik
First published: 2009; last edited: 26 Jan. 2017

Summary

This page offers some rough estimates of the numbers of wild animals on Earth. Collectively, wild land vertebrates probably number between 1011 and 1014. Wild marine vertebrates number at least 1013 and perhaps a few orders of magnitude higher. Terrestrial and marine arthropods each probably number at least 1018.

Introduction

"Attempts to assess the magnitude of global biodiversity have focused on estimating species richness. [...] The total number of individual organisms in the world[...] has been a largely ignored statistic."
--Gaston and Blackburn (1997), pp. 615-16

From a sentiocentric rather than ecocentric perspective, the number of animals in the world is actually much more important information than the number of species. Of course, if we're pessimistic about the net welfare of most wild animals, we may prefer for there to be fewer total wild animals.

Summary table

Animal Type World Population
Lab animals less than 108 (counting only vertebrates)
Humans 7 * 109
Livestock 2.4 * 1010
Birds ~1 * 1011 to 4 * 1011
Mammals 1011 to 1012
Reptiles 1011 to 1014
Amphibians 1011 to 1014
Fish 1013 to 1015 or more
Earthworms 1014 to 1017
Dust mites 1014 to 1017
Coral polyps 1015 to 1018
"Bugs" (insects, spiders, etc.) 1017 to 1019
Rotifers similar numbers as insects??
Gastrotrichs ~1019 (?)
Copepods 1018 to 1021
Nematodes 1022 (?)
Protozoa (not animals) 1025 (?)
Bacteria (not animals) 1029 to 1031

Explanation of the estimates

Lab animals

Wikipedia reports: "Accurate global figures for animal testing are difficult to obtain; it has been estimated that 100 million vertebrates are experimented on around the world every year,[70] [...] The Nuffield Council on Bioethics reports that global annual estimates range from 50 to 100 million animals. None of the figures include invertebrates such as shrimp and fruit flies.[72]"

Fraser and MacRae (2011) estimate that the total "census numbers" (i.e., instantaneous population) of (vertebrate) lab animals worldwide are less than 100 million (p. 582, Table 1). They explain regarding this figure: "The number given by Taylor et al (2008) is 115.3 million. This is the estimated number of animals used per year. Because many of the animals used in science are rodents that live for less than one year, the number alive at any given time will presumably be less than 100 million."

If we added on invertebrates used in research, the numbers would be higher.

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 24,312,162,736
Chickens 17,863,376,000
Cattle 1,357,183,587
Ducks 1,095,805,000
Sheep 1,086,881,528
Pigs 918,278,483
Goats 830,391,683
Turkeys 472,635,000
Geese and guinea fowls 343,375,000
Buffaloes 177,247,938
Horses 58,641,094
Asses 42,038,633
Camels 24,246,291
Other Rodents 17,425,000
Mules 11,840,918
Other Camelids 6,861,765
Animals Live Nes 5,934,816

Here's one simple check of the above numbers. Many broiler chickens in the US are killed at 5-7 weeks of age. Say it's 6 weeks. That implies (52 weeks per year)/(6 weeks per generation) = 8.7 generations per year. Given that ~9.2 billion meat chickens were killed in the US per year in 2010, the population of broiler chickens at any given time should have been (9.2 billion)/8.7 = 1.1 billion. Another 0.5 billion chickens were killed for eggs, and assuming ~1 generation of egg-laying hens per year, the total population of broilers + hens would have been 1.1 billion + 0.5 billion = 1.6 billion. FAOSTAT's database reports that in 2010, the US chicken population was 1,956,000,000, i.e., roughly 2 billion. So these estimates seem pretty close.

The global numbers don't match as well. Over 65 billion land animals were killed for food globally around 2010, compared against 10.2 billion in the US in 2010. If the proportions of types of animals globally are the same as in the US, this would imply that (9.2 billion)/(10.2 billion) * (65 billion) = 59 billion of the global land animals killed were broiler chickens, and (0.5 billion)/(10.2 billion) * (65 billion) = 3.2 billion were laying hens. If there were 8.7 generations of broiler chickens per year globally, that would imply a population of (59 billion)/8.7 = 6.8 billion at any given time. Combined with 3.2 billion laying hens, that suggests 6.8 billion + 3.2 billion = 10 billion total chickens, when in fact, the number in the table above for 2007 was ~18 billion (and the proper comparison, the 2010 number from the database, was ~20 billion). Either this discrepancy is a symptom of bad data, or global proportions don't match US proportions, or chickens outside the US live longer. Maybe meat chickens in developing countries tend to live for several months before slaughter?

Noah Strycker estimates that there are 24 billion domestic chickens worldwide.

Birds

Gaston and Blackburn (1997), in a paper titled "How many birds are there?", report: "we use a variety of methods to estimate the global number of individuals for a single taxon, birds. The different methods yield surprisingly consistent estimates of a global bird population of between 200 billion and 400 billion individuals" (p. 615). I think these numbers count all birds, not just breeding birds, based on two hints in the paper:

  1. The authors write (p. 617): "Numbers fluctuate both through and between seasons. We ignore such details and seek order of magnitude estimates only. To some (unknown) extent fluctuations in different parts of the globe will be out of synchrony, so that fluctuations in the overall total number of individuals will be less than fluctuations in regional totals."
  2. Table 2 on p. 619 specifically flags some population estimates as being "breeding" birds.

In contrast, Gaston et al. (2003) estimated the total number of breeding birds specifically. The "typical" estimate for the global breeding-bird population in 1990 is 86.70 billion, with a range "between 39.34 and 134.04 billion individuals" (p. 1296), as shown in the screenshot of Table 1 below. Gaston et al. (2003) caution (p. 1295): "These figures are based on breeding birds, and would be inflated by non-breeders during the breeding season, and by post-breeding individuals at other times. Seabirds are ignored, as they contribute little to the overall total number of breeding birds [...]." So probably the population of all birds (breeding + non-breeding), averaged over each month of the year, is some small multiple of 86.70 billion. Hence, the lower bound of 200 billion from Gaston and Blackburn (1997) still seems to me potentially reasonable. (That said, these figures might also be biased a bit high because "Areas chosen for study may well[...] be biased towards higher avian densities; few ecologists intentionally pick ‘poor’ study sites" (p. 1295).)

Gaston et al. (2003) also report (p. 1296):

Previous estimates put the total [number of birds] in the range 100–400 billion (Fisher & Paterson 1964; Wood 1982; de Juana 1992; Gaston & Blackburn 1997), but were based on unexplained methods or much cruder sets of data. Our calculations put the true figure at the lower end of this range.

This piece roughly agrees with the bird density for tropical forest specified above, since it says (p. 344) tropical rainforests have "a few dozen birds and mammals" per hectare. Two dozen birds per hectare is 2400 birds per km2.

Smil (2013) reports (p. 23) that biomass "for avifaunas usually do not surpass 0.05 g/m2 (Edmonds 1974; Reagan and Waide 1996)." Based on my reading of this page, it looks like backyard birds tend to have masses roughly in the range of 10 to 100 g. Then 0.05 g/m2 implies 10/0.05 = 200 m2 to 100/0.05 = 2000 m2 per bird, which implies a bird density of 500 to 5000 per km2. This seems reasonable in light of Gaston et al. (2003)'s Table 1.

Mammals

Matheny and Chan (2005), after discussing Gaston et al. (2003)'s bird estimates, report (p. 585) that "A similar review has not been conducted for mammals." So Matheny and Chan (2005) estimate a rough multiplier (p. 585):

Based on a British study by Gaston and Evans (2004) and Harris et al. (1995), here we assume the densities of wild mammals are 2.25 times those of wild birds for each land-use type. 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.

Multiplying a global bird population estimate of 200 to 400 billion by 2.25 gives a number close to 1 trillion mammals.

A separately calculated estimate is based on Derek W. Yalden's article "A History of British Mammals". While noting the difficulty of estimating aggregate mammal populations, Yalden guesses a figure of 285 million wild mammals in Britain in the spring pre-breeding season, compared with ~48 million adult humans. (Yalden compares against Britain's adult human population rather than its entire human population "for fairer comparison with just the breeding populations of other mammals".) This implies 285/48 = 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. In fact, this ratio of ~10 wild mammals per human on Earth may be too conservative. Gaston and Blackburn (1997) estimate that there are roughly 40 to 60 birds per human worldwide, while "in Britain there are approximately 2 to 3 birds per head of human population" (p. 623).

Great Britain has a land area of 209,331 km2. So Yalden's figures suggest roughly (285 million wild mammals) / (209,331 km2) = ~1400 wild mammals per km2. Smil (2013) roughly agrees with this number (p. 21): "the expected density of animals weighing 10 g would be about 1,000/km2", where I assume that 10 g is intended to denote roughly the smallest classes of mammals.a It's not completely clear from Smil (2013)'s text, but I think the "1,000/km2" number may be a general approximation based on allometric scaling. Smil (2013) adds regarding this and other abundance estimates (pp. 21-22): "This would be only the best-fit values as the actual densities for every mass category range over at least two (and even three) orders of magnitude."

Smil (2013) also presents the following tableb of mammal biomass on p. 24:

However, it's important to point out that the high zoomasses of large herbivores are reported in Africa. Smil (2013) says (p. 22) that "in Africa's savannas and rain forests [...] the mean mass of mammals is significantly greater than in Amazonia (Cristoffer and Peres 2003)." Therefore, "aggregate mammalian densities decline when expressed as averages for large regions or for biomes: at those scales, only savannas and some tropical forests harbor in excess of [20 kg/ha], while the mammalian zoomass in most equatorial and montane rain forests as well as in temperate woodlands is below [10 kg/ha] (Prins and Reitsma 1989; Plumptre and Harris 1995)" (p. 23).

Smil (2013) also gives the following rough figures (I think expressed as live weight?) as inputs to "a liberal estimate of the total zoomass of wild terrestrial mammals at the beginning and end of the twentieth century" (p. 227): "1 kg/ha in croplands, 2 kg/ha in low-productivity ecosystems (in both cases dominated by rodents), and 5 kg/ha (dominated by large herbivores) in the richest grasslands and forests". It's not clear if these numbers are based on specific sources or if they're generic estimates that Smil (2013) is partly making up based on his experience. 2 kg/ha = 200,000 g/km2, which would imply ~20,000 mammals per km2 if the mammals are 10 g each. That does indeed sound like a "liberal" estimate, although including larger mammals in this calculation would make it more reasonable.

Smil (2013) estimates (p. 227) "25 Mt of live weight [...] in the year 2000" for all wild mammals. 25 Mt = 25 * 1012 g, so my global wild-mammal population estimates of 1011 to 1012 imply 250 to 25 g per mammal. Of course, these numbers aren't very meaningful because much of this zoomass comes from large mammals.

Fittkau and Klinge (1973) suggest (Fig. 2, p. 8) the following biomass composition of mammals for "a central Amazonian rain forest near Manaus, Brazil" (p. 2). I added common names in purple based on Wikipedia. You can see that rodent biomass comprises a much bigger share of the total than in Smil (2013)'s Table 2.3 above. (Perhaps Smil (2013) himself would agree with this, given that he says (p. 78): "in contrast to grasslands and open woodlands, hunting in tropical rain forests is much more difficult because most of the resident zoomass is relatively small[ and for other reasons].")


Fittkau and Klinge (1973) explain regarding their animal-biomass numbers (p. 7):

The values have been derived from our general observations over the last 10 years and from observations of others, but not from actual counting and weighing [...]. Excepted are: soil fauna, total fresh biomass (84 kg per hectare; Beck 1970, 1971), and certain arthropods and vertebrates which we collected in the biomass estimation plot.

Reptiles

Ishwar, Chellam, and Kumar (2001) assessed reptile densities in the tropical-rainforest floor of the Kalakad-Mundanthurai Tiger Reserve. Examining 5 m x 5 m quadrats, the researchers found an average of 0.2559 reptiles per quadrat = 10,240 reptiles per km2 (p. 413).

Reagan and Waide (1996) compiled research done on "a 40 ha area of forest around the El Verde Field Station in the Luquillo Experimental Forest of Puerto Rico" (pp. ix-x). Appendix 14.B (p. 510) reports the following vertebrate densities. Adding up the four reptile species gives 3.5 reptiles per m2 = 3.5 million per km2.

Looking at Table 1 from Gaston et al. (2003) above, we see that in 1990, the global area of tropical regions ("tropical woodland" + "tropical forest") was 5.88 million km2 + 8.61 million km2 = 14.49 million km2. Multiplying by ~104 to ~106 reptiles per km2 gives ~1011 to ~1013 reptiles in all tropical regions. This tropical land area is only (14.49 million km2) / (134.12 million km2) = 11% of the world's land area, but reptile densities are probably lower outside of the tropics. This book says (p. 28): "Being ecotherms, more lizard species live in the warm tropics than in temperate zones." So these estimates may be reasonable worldwide population estimates. Even if reptile densities per km2 outside the tropics were as high as in the tropics, the worldwide total number of reptiles would be only an order of magnitude higher.

Smil (2013), p. 21: "any large-scale averages of biomass densities of small ectothermic vertebrates (mainly frogs and snakes) are just statistical artifacts, as their densities are highly variable. But reptiles and amphibians can dominate the vertebrate zoomass in some tropical rain forests, where their overall [biomass] density may rival that of invertebrates (Reagan and Waide 1996)." Adding up reptile biomasses in the above Reagan and Waide (1996) table across the three species for which densities are reported gives 10.8 + 6.2 + 38.7 = 55.7 g/m2 = 557 kg/ha of wet weight.

Amphibians

Vasudevan, Kumar, and Chellam (2001) studied amphibians in the Kalakad-Mundanthurai Tiger Reserve. They 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.

The Reagan and Waide (1996) table in the "Reptiles" section above shows a total amphibian density, adding up each species, of 2.23 per m2 = 2.23 million per km2.

However, these trends may not extend to non-tropical biomes. One person wrote me the following comment: "Amphibians need to be close to water. [...] Quite a few birds in cold regions but not many amphibians." Sören and Carl expressed similar concerns about "extrapolating too much from studies in rainforests" regarding herpetofauna population numbers. For this reason, I'll stick to the conservative approach I used for reptiles of only calculating the number of tropical animals. Based on the above studies, there seem to be ~104 to ~106 amphibians per km2 of tropical forest. Multiplying this by a tropical area of 14.49 million km2 gives ~1011 to ~1013 amphibians. Being generous about amphibian populations outside tropical regions might push the upper bound toward 1014.

Fish

In another piece, I estimate crudely that "there are ~13 trillion (or ~1013) wild fish in the oceans at any given time." This seems like a low-end estimate given how it was computed.

This article claims that bristlemouth fish alone may number 1014 to 1015.

This video explains that previous fish numerosity estimates had been ~3 * 1014, while newer estimates are ten times higher.

Earthworms

Brady (1974) reports (p. 118):

Because of their sensitivity to soil and other environmental factors, there is a wide variation in the numbers of earthworms in different soils. In very acid soils under conifers, an average of fewer than one organism per square meter is common. In contrast, more than 500 per square meter have been found on rich grassland soils. The numbers commonly found in arable soils is from 30 to 300 per square meter [...]. The biomass or wet weight for this number would range from perhaps 100 to 1,000 pounds per acre.

Based on the above, let's assume that the world average number of earthworms per m2 is between 1 and 1000 on biologically productive land (and zero elsewhere). Sundquist (2004) suggests that "87.35 million [km2] is something on the order of the amount of the world's reasonably biologically productive land." Rounding this to 100 million km2 implies 1014 m2.

Dust mites

Estimates of dust mites per person range from 105 to 107:

  • 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!"
  • 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.

However, dust mites are not present in all houses. This paper says "A 4-yr survey of mites in bedroom and family room carpets and couches in 252 homes of asthmatics located in 8 different geographical areas of the United States found that most homes (81.7%) were co-inhabited by both D. farinae and D. pteronyssinus (Arlian et al. 1992)." I'm uncertain how prevalent dust mites are in other parts of the world.

Dust mites may also be mostly absent (or at least quiescent) in colder, less humid months of the year or if home humidity is sufficiently low during summer months. This page says, "Well-ventilated homes in dry climates contain few dust mites." This paper echoes: "Field studies report that homes located in dry climates, such as those of the mountain states or upper Midwest, have few mites and little mite allergen present."

So a conservative estimate of the average number of dust mites per human on Earth might be several times lower than the lower bound of 105 dust mites per person mentioned above. Let's say the range is 104 to 107 dust mites per person. Multiplying this by ~1010 humans gives the range in the summary table at the top of this piece.

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.)

And if dust mites build up in pillows, is this an argument for replacing pillows regularly so that your head doesn't crush lots of mites? Or maybe most of the accumulated mites in pillows are dead?

This video shows dust mites under a microscope.

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.

Coral polyps

One paper found coral polyp densities of 0.5-2 coral polyps per cm2. Since I haven't found other data on polyp densities, let's assume that the average density of polyps across all coral reefs is roughly the same as that: ~1 per cm2.

But is this density too low to be plausible? Polyps are typically only 1-3 millimeters in diameter, so it should be possible to fit several of them in a square centimeter. If polyps were 1 millimeter in diameter and stacked as tightly as possible, then it would be possible to fit 100 of them in a square centimeter. So in principle the density could be as much as 100 per cm2. More realistically, let's guess that it could be as much as, say, ~30 per cm2.

This implies ~1010 to ~3 * 1011 per km2. Note that a km2 of coral might occupy less than a km2 of land area because the coral has folds. So I'll guess that the density per km2 of flat land area is roughly between 1010 (lower bound) and 3 * 1012 (upper bound).

Coral reefs occupy an estimated 255,000 km2 of the Earth's surface, or about 3 * 105. That suggests a total population of coral polyps of roughly 3 * 1015 (lower bound) to 9 * 1017 (upper bound), which I rounded off as 1015 to 1018 in the summary table.

Note that the number of polyps should plausibly be less than the number of copepods, because polyps eat copepods (among other things). Also, because polyps are sessile, they probably have less developed nervous systems than most other animals.

"Bugs" (insects, spiders, earthworms, etc.)

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

Gaston and Blackburn (1997) explain (p. 617):

As far as we are aware, the only explicit attempt to calculate the overall number of individuals in a large taxon is that of Williams (1960). He estimated that the global insect fauna consists of around 1 x 1018 animals, based in part on the assumption that the global insect fauna is around three million species, which is probably unrealistically low.

The Brady (1974) table here shows densities for "Other fauna" (i.e., non-nematode invertebrate animals) in soils as 103 to 105. I assume this means mostly "bugs" (mites, springtails, ants, etc.)? As noted earlier, Sundquist (2004) suggests roughly 1014 m2 of biologically productive land area on Earth. This suggests 1017 to 1019 non-nematode invertebrates on land.

This book says: "Microarthropods mainly include mites and collembolans and are found in most types of soils. Hundreds of thousands of individuals [...] may be found within a square meter forest floor."

TODO: CALCULATE BIOMASS BASED ON SPRINGTAILS AND COMPARE WITH SMIL'S ESTIMATE IN THE HARVARD THING

Rotifers

This page reports:

Population densities often reach greater than 1,000 individuals per liter. [... Rotifers represent] more than 50 % of the zooplankton production in some freshwater systems.

Rotifers may also be very abundant in the interstitial water of soils reaching densities up to 2 million per square meter.

This book agrees: "Depending on the soil type and its moisture level estimates of their densities range from about 32,000 to more than 2 million per square meter."

This book says that while rotifers are usually aquatic, "Rotifers may also be found in tens of thousands per square meter in unsaturated soils."

This book notes that rotifers are usually only found on land when the soil contains a significant amount of water films. One agricultural site in Costa Rica had tens of thousands of rotifers per m2. Rotifers "may reach numbers exceeding 105 per square meter in moist, organic soils (Wallwork, 1970)."

The estimates are variable, and I don't know how many hectares of soil has what density of rotifers. Rotifers are abundant in freshwater, though this covers only 1.77% of Earth's surface, compared with 30% for land. Until I get better numbers, I'll squint and say that rotifer densities seem maybe comparable to bug densities per square meter of soil, which suggests maybe similar abundances?

Gastrotrichs

This source includes the following table of gastrotrich density estimates (p. 166):


In case some of these estimates are biased high (because people will tend to study locations where the objects of study are more abundant), I'll use an estimate on the lower end: 50,000 gastrotrichs per m2 across all freshwater bodies.

This page reports that, excluding the Caspian Sea, "The world’s lakes have a combined surface area of about 5 million square kilometers".c That's 5 * 1012 m2. Assuming 50,000 gastrotrichs per m2, we have 2.5 * 1017 gastrotrichs worldwide in fresh water.

How about marine gastrotrichs? This page says:

In marine sediments, gastrotrich density may reach 364 individuals/10 cm2; typically they rank third in abundance following the Nematoda and the harpacticoid Copepoda, although in several instances they have been found to be first or the second most abundant meiofaunal taxon. In freshwater ecosystems population density may reach 158 ind/10 cm2 making the taxon rank among the top 5 most abundant groups.

The estimates in that quote are maximum densities, not average densities, so we can't draw immediate inferences about average densities. Still, it seems plausible that average marine densities per unit area are in a similar ballpark as average freshwater densities per unit area.

Earth's oceans cover about 360 million km2. Again assuming 50,000 gastrotrichs per m2, that implies (360 * 106 km2) * (106 m2 / km2) * (50,000 gastrotrichs / m2) = 1.8 * 1019. However, it's plausible this number is too high, since I'm unsure whether gastrotrichs can live throughout the oceans, including in the deepest parts??

Gastrotrichs can also live on land:

They inhabit the interstitial spaces between particles in marine and freshwater environments, the surfaces of aquatic plants and other submerged objects and the surface film of water surrounding soil particles on land.[4] They are also found in stagnant pools and anaerobic mud, where they thrive even in the presence of hydrogen sulphide.

I haven't counted terrestrial individuals, assuming they contribute less to the totals than marine individuals.

Copepods

Most zooplankton are copepods (p. 23). This source (p. 14) calls copepods "the most abundant animals in the ocean, possibly the most abundant on Earth" and estimates their 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 361.9 trillion m2 ocean surfaced (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.

Note that Antarctic krill numbers may also be quite high:

Planktonic copepods [...] are usually the dominant members of the zooplankton [...]. Some scientists say they form the largest animal biomass on earth.[11] Copepods compete for this title with Antarctic krill (Euphausia superba).

Nematodes

According to this page, "Nematodes are the most abundant and ubiquitous multicellular organisms on earth", and they number around 1022. I'm suspicious whether this number is too high, since except maybe for copepods, my numerosity estimates for other animals have been a few orders of magnitude lower. However, the Brady (1974) table here shows 102 to 103 times as many nematodes in the soil as "Other fauna". So assuming 1019 non-nematode invertebrates (mites, springtails, etc.), 1022 nematodes might be possible.

Wikipedia says:

Nematodes have successfully adapted to nearly every ecosystem from marine (salt water) to fresh water, to soils, and from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts and oceanic trenches. They are found in every part of the earth's lithosphere,[7] even at great depths (0.9–3.6 km) below the surface of the Earth in gold mines in South Africa.[8][9][10][11][12] They represent 90% of all animals on the ocean floor.[13] Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.[14]

This article affirms that "Four out of every five animals on Earth is a nematode."

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

Protozoa and bacteria

Protozoa (a somewhat outdated taxonomic group) are animal-like creatures that don't fall under the formal kingdom Animalia. We might extend very small amounts of ethical significance to these tiny creatures.

The Brady (1974) table here suggests that in soil, there are ~103 times as many protozoa as nematodes. Assuming this ratio holds true in non-soil environments as well, the world protozoan population would be about 103 times the world nematode population, i.e., about 1025.

The Brady (1974) table also shows bacteria as outnumbering protozoa by a multiple of 104, which gives an approximate world total of 1029. This estimate based on a chain of multiplication factors is reassuringly close to a more direct estimate of the number of bacteria in the world: 5 * 1030.

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 Smil (2013). 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.

Moreover, I'm skeptical of some of the numbers in that source. It lists fish as having a mass of less than 40 million metric tons of carbon, with whales adding another 5 - 15 million. Let's generously consider this as 40 + 15 = 55 million metric tons of carbon. Since carbon makes up 18.5% of the human body by mass, that translates to roughly 55/0.185 = 297 million metric tons wet mass. However, another paper found that the biomass of marine life bigger than 1 g in size was 197.97 + 176.03 + 156.51 + 139.16 + 123.73 = 793.4 million metric tons wet mass. For comparison, global catch from wild fisheries in 2005 was 93.2 million metric tons.

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 jibes 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. Moreover, this comparison ignores the fact that smaller, more r-selected animals probably suffer more per individual because a (much) bigger percentage of their lives is composed of the agonizing experience of dying.

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).

See also

Footnotes

  1. For example, a mouse has an average body weight of ~20 g.  (back)
  2. As shown in red, there was a typo in two of the estimates, where the table said something different than Smil's text on p. 23. I think Smil's text, not his table, is correct, based on my skimming of Table 2 of Smil's source: Hayward, O’Brien, and Kerley (2007). There are a few other small typos in Smil (2013)'s book, such as

    • writing "m2" instead of "m2" (p. 22)
    • writing "after1850" instead of "after 1850" (p. 68).

    These are helpful reminders that we shouldn't believe everything we read, even in published works by respected authors.  (back)

  3. Sundquist (2004) reports a similar number: "A table in Section (se3-C) gives 4-6 million km2 of swamps, marshes, bogs, peat lands, lakes and streams."  (back)
  4. In practice, plankton densities vary a lot throughout the ocean, so this assumption isn't valid.  (back)