by Brian Tomasik
First written: 2009; last edited: 26 Jul. 2014
I present some rough estimates of the number 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.
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|
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|
See the following table. The first two columns are mostly copied from 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),
|Biome||Area (million km^2)||Rough Bird Density (individuals / km^2)||Notes|
|Temperate Mixed Forest||44.55||800|
|Savannah||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|
|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.
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.
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.
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),
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.
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.
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.
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.
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.
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 [...].
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).