Summary. 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.

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: (webmaster [at] utilitarian-essays.com). 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.

Explanation of the Estimates

Animals in Research Labs

The number is roughly 50 to 100 million.^{[Orlans, p. 400]}

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.

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.

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.

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.

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

See the explanation in the top post on Felicifia's One Trillion Fish discussion.

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.

[Orlans] Orlans, F. B. (1998) History and Ethical Regulation of Animal Experimentation: An International Perspective, in A Companion to Bioethics, Kuhse H. and Singer P., eds. Oxford: Blackwell, 1998.