by Brian Tomasik
First written: 2009; last edited: 17 Nov. 2016
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.
- 1 Summary
- 2 Summary table
- 3 Explanation of the estimates
- 4 Biomass estimates
- 5 Beyond animals
- 6 See also
- 7 Footnotes
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 (?)|
|Fish||1013 to 1015 or more|
|Dust mites||1014 to 1017|
|Coral polyps||1015 to 1018|
|"Bugs" (insects, spiders, earthworms, etc.)||1018 to 1019|
|Rotifers||similar numbers as insects??|
|Copepods||1018 to 1021|
Explanation of the estimates
Animals in research labs
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|
|Geese and guinea fowls||343375000|
|Animals Live Nes||5934816|
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.
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 million km2 for their area, and taking the remaining 52.3 - 7.75 = 44.55 million km2 to be temperate mixed forest. Of course, in reality, some temperate forests are rainforests, 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 savanna 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 km2)||Rough Bird Density (individuals / km2)||Notes|
|Temperate Mixed Forest||44.55||800|
|Savanna||21.8||450||<--assumed same as grassland|
|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|
|Amphibians (billions):||500||<-- assumed ~8 times bird value|
This piece roughly agrees with the bird density for tropical forest specified above, since it says tropical rainforests have "a few dozen birds and mammals" per hectare. A dozen birds per hectare means 1200 birds per km2.
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.
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.
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 + 500 billion amphibians) = 8.4 g. Humans are ~1/2 to ~2/3 water by mass -- let's assume 60%. Then the average wet mass of a terrestrial vertebrate might be roughly 8.4/0.4 = 21 g. This seems roughly correct, given that a mouse has an average body weight of ~20 g, and most terrestrial vertebrates are probably mouse-sized. If we think the 21-gram 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 wet mass of 21/.16 = 131 g.
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 in tropical regions 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.
However, these trends may not extend to non-tropical biomes. One person wrote me the following comment: "Amphibians need to be close to water. There are relatively few amphibians in grasslands and savanna but bird density only drops by half or 3/4. Quite a few birds in cold regions but not many amphibians." For this reason, I deflated the multiplier for amphibians relative to birds worldwide to just 8, the same as the reptile multiplier, until I have more data.
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.
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 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."
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:
- "Will I crush dust mites when I sleep at night?"
- "Question: Do you crush dust mites when you walk or are they like ticks, you can step on them but it will have no effect?"
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.
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.
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.)
A later section of this piece contains a table showing densities for "Other fauna" in soils as 103 to 105. I assume this means mostly "bugs" (terrestrial arthropods, earthworms, etc.). The Earth's land area is 1.5 * 108 km2. Suppose for simplicity that only 2/3 of this has nontrivial insect populations: 108 km2. That's 1014 m2. Then multiplying by 103 to 105 bugs per m2 gives 1017 to 1019 bugs on Earth.
This book agrees: "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."
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?
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". 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. 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.
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 surfacea (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. Copepods compete for this title with Antarctic krill (Euphausia superba).
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, even those of similar size as nematodes, have been a few orders of magnitude lower.
They are found in every part of the earth's lithosphere. They represent, for example, 90% of all life forms on the ocean floor. Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth [...].
This article affirms that "Four out of every five animals on Earth is a nematode."
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.
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).
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 following table is scanned from p. 116 of The Nature and Properties of Soils, 8th editionb:
If these trends for numbers of organisms per square meter apply worldwide, they would suggest ~103 times as many protozoa as nematodes, which would give a world population of 1025. And the number of bacteria would be another ~104 times as many, giving a world total of 1029, which is reassuringly close to a more direct estimate of the number of bacteria in the world: 5 * 1030. The comparison between "Earthworms" and "Other fauna" vs. "Nematoda" in the above table is also broadly consistent with my estimate of 103 to 104 times as many nematodes as insects. (Of course, earthworms aren't strictly insects, but their sizes and abundances are probably on a similar order of magnitude as those of insects.)c
If you want to determine an individual organism's mass in order to calculate its per-individual ethical importance, you can invert the "Number per gram" column. If you weigh individuals linearly in their mass, you can just look at the "Biomass" column to see the cumulative ethical importance of the organisms in a given row of the table. I probably give bacteria less ethical importance than their biomass alone would suggest but still a nonzero amount of importance.
Following is another figure (from this book) also showing relative abundance of organism types in soils.
This figure shows earthworms at a few thousand per square meter, slightly but not dramatically higher than in the previous figure. And this figure shows that populations of other (non-nematode) fauna are in the range of thousands to hundreds of thousands, consistent with the previous figure.
- Wikipedia's "Lists of organisms by population" has very incomplete population numbers.
- "What’s the most common animal species?"
- In practice, plankton densities vary a lot throughout the ocean, so this assumption isn't valid. (back)
- I'm a bit puzzled by the numbers in this table. If we divide "Number per square meter" by "Number per gram", we get about 105 grams = 100 kg per square meter for each of the rows. That by itself seems okay, since the different rows have biomass numbers that aren't too far apart. But 100 kg seems too much for a single square meter. The volume of a square meter 15 cm deep is (15 cm)(100 cm)(100 cm) = 1.5 * 105 cm3. Assuming the soil has a density not far from water's density of 1 g per cm3, that suggests 1.5 * 105 g = 150 kg of soil total. So then how can 100 kg for each of the organism groups in the table fit in that amount of soil??
The biomass numbers in the third column are also weird. "AFS" is an "acre furrow slice", i.e., 4046.86 square meters. Thus, using the upper bound of ~4000 bacteria pounds per AFS gives about 1 pound per square meter, or about half a kilogram. That's a far cry from the 100 kg per square meter calculated in the previous paragraph. (back)
- The Nature and Properties of Soils adds this information about earthworm numbers (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 [as in the above table ...]. The biomass or wet weight for this number would range from perhaps 100 to 1,000 pounds per acre.
Assuming a world average of ~50 earthworms per m2 over ~1.5 * 1014 m2 of land on Earth implies almost 1016 earthworms worldwide. Considering that earthworms are bigger than the typical bug and are just one of many kinds of bugs, the estimate in the main text of 1018 to 1019 as a world population of all bugs continues to seem plausible. (back)