Introduction

For such a straightforward question, the answer to how many individual animals exist is surprisingly unexplored in scientific literature. A variety of reasonable estimates have been made for the number of animal species, but means of gathering data on animal abundance in different environments are so varied that the actual number of individual animals is relatively unexamined. Here, we discuss some estimates that approach this number, and assess their accuracy.

Number of species

A 2014 paper (Caley, Fisher, & Mengersen, 2014) describes how global species richness has been estimated repeatedly over the years, but that estimates have consistently wildly fluctuated, and are still not converging on a standard value. The values have also been logically inconsistent, i.e., number of marine species outranking number of total species, or number of species in coral reefs outranking number of marine species.

A 2011 study expects that there are 7.77 x 106 animal species, of which only 9×105 have been characterized (Mora, Tittensor, Adl, Simpson, & Worm, 2011)

Determining the average number of individuals in a species is difficult, because some species are fairly common and some are quite rare (Fisher, Corbet, & Williams, 1943). Estimates that have been made to relate these too generally focus on a small range of species, such as flying insects in a limited geographic area.

Insects

The Smithsonian Institution’s online encyclopedia (“Encyclopedia Smithsonian: Numbers of Insects,” n.d.) reviews several literature sources regarding the number of individual insects. It estimates 1019 insects worldwide and refers to studies conducted in fields in North Carolina and Pennsylvania.

A textbook estimate puts the number of insects at 4 x 107 per acre. If this applies globally, this implies there are 1.47 x 1018 insects worldwide (Pedigo & Rice, 2014). The exact figures used to calculate this number are not given, such as whether the relative insect abundance in deserts or rainforests is considered.

The Manduca project (“The Manduca Project,” n.d.) suggests that there are 1 x 1018 insects, although this figure is not cited.

Pelagic zooplankton

One study, covering a long swathe of the Atlantic ocean that stretched over a range of latitudes, found an average of about 20 individual zooplankton per m-3, with significantly more than that at certain latitudes, at a depth of 10 M (Clark, Aazem, & Hays, 2001). If we assume that this density is constant throughout the euphotic zone (“photic zone | oceanography,” n.d.), the first 80 meters of the earth’s ocean should contain 600 zooplankton per square meter, giving us a low bound for the entire number of pelagic zooplankton, for 5.78 x 1017 plankton in the top 80 meters of the earth’s oceans.

This does not include zooplankton deeper in the ocean, about which data is scarcer. Benthic fauna decreases logarithmically with depth. Additionally, copepod populations, which make up the majority of zooplankton samples, can increase tenfold during seasonal blooms (Atkinson, Ward, Hunt, Pakhomov, & Hosie, 2012).

Copepod researcher Geoff Boxshall makes a rough estimate of one copepod per liter of seawater in the earth’s oceans, which would result in 1021 copepods worldwide in the world’s oceans, although he suggests that one copepod per liter is probably an underestimate (“COPEPODS | Plankton Safari,” n.d.). Brian Tomasik points out that copepods are probably much more abundant than that near the surface, and much less abundant at greater depths (“How Many Wild Animals Are There? | Essays on Reducing Suffering,” n.d.). Zooplankton abundance drops sharply below 80 meters or so (May & Ser., 1982) (Grice & Hulsemann, 2010), and 90% of animal life lives in the euphotic zone (“Seaweek 2010: Oceans of Life ours to explore; ours to restore”,” n.d.), however, 90% of the ocean’s volume is in the aphotic zone (“Ocean Zonation,” n.d.), so more precise data on zooplankton abundance at greater depths would result in a more accurate estimate.

Animals in tree canopies

Large numbers of animals live in tree canopies. Satellite surveys of earth suggest that there are 3 x 1012 trees on the entire planet (Ehrenberg, 2015). Using the number of arthropods one study on the subtropical Australian Argyrodendron actinophyllum tree (Basset & Arthington, 1992), and an average crown width for rainforest trees of 17.5 M (“Rainforest Primer « Rainforest Conservation Fund,” n.d.), we can generalize to 6.06 x 1019 arthropods in the leaves of all subtropical and tropical trees, and presumably between 7 x 1019 and 1.4 x 1020 arthropods on the leaves of all trees. This is a very rough estimate, since it relies on all trees being versions of one species covered in one study, as well as a rough estimation of tree leaf volume from only crown width. The study referred to also looked at animals on canopy leaves, and may have excluded animals living on trunks.

Nematodes

Studies from the 1980’s and before suggest that there are several million nematodes per square meter of oceanic and terrestrial soil, which is an order of magnitude greater than other animals (Wharton & Surrey, 1994). This implies that there are about 2.5 x 1021 nematodes on earth’s surface, and 2.75 x 1021 total animals on or in soil. The number on a detailed examination of more modern surveys, such as the 2010 Census of Marine Life, may make it possible to revise these calculations for better results.

Benthic fauna

A 2006 study (Rex et al., 2006) looked at compiled ocean floor fauna data from 128 studies, and observed that the number of individual animals in the sample decreased logarithmically with depth. They generated several equations that can be used to determine the amount of animals present per decimeter of ocean floor, for animals in three categories. Using the percent of the earth’s surface which occurs at a given depth (Wright & Rothery, 1998), we can extrapolate this to estimate the total number of animals living on the ocean floor.

The three categories of animals, which were analyzed separately:

Meiofauna : Microscopic nematodes, harpacticoid copepods, very small members of other groups. Total: 9.03 x 10 21 animals on the floors of all of the ocean.

: Microscopic nematodes, harpacticoid copepods, very small members of other groups. Total: 9.03 x 10 animals on the floors of all of the ocean. Macrofauna : Polychaete worms, pericarid crustaceans, other mollusks. Total: 2.06 x 10 19 animals in this class on the entire ocean floor.

: Polychaete worms, pericarid crustaceans, other mollusks. Total: 2.06 x 10 animals in this class on the entire ocean floor. Megafauna: Fish, echinoderms, cnidarians, and crustaceans (all from 1 cm to 1 dm in diameter.) Total: 2.41 x 1015 animals in this class on the entire ocean floor.

This adds to a total of 9.05 x 1021 animals living on the ocean floor.

These equations were generated from an n of 150-650 each over a wide range of depths. The sampling sites used were from multiple oceans and latitudes, but especially concentrated in the Atlantic Ocean and adjacent seas, and under-represented samples from the Indo-Pacific ocean and Southern Hemisphere (Rex et al., 2006). Further studies of standing stock in underrepresented areas could make these estimates more accurate.

Soil

A 2007 study (Uk, n.d.) estimated the number of invertebrates living in the top 8 cm of soil across Great Britain at 1.28 x 1016. Extrapolating from the area of Great Britain to the land area of Earth, we’d expect 8.07 x 1019 invertebrates living in soil. This is likely to be much, much lower in deserts, especially Antarctica, and higher in tropical rainforests.

Total

One of the most comprehensive available summaries of all animal life is from Brian Tomasik, who reviews a variety of sources to estimate the population of various animal groups, including degrees of uncertainty (“How Many Wild Animals Are There? | Essays on Reducing Suffering,” n.d.).

2.2 x 1010 livestock

1013 – 1015 fish, or more

1018 – 1021 copepods

1017 – 1019 insects

(?) 1019 gastrotrichs

(?) 1022 nematodes

Total: ~1020 – 1022 animals.

Estimates given for various groups in this piece overlap: for instance, estimates for the total number of insects include both insects living in soil, and insects living in tree canopies, as well as other estimates. Because each estimate is made with different methods, their accuracies should be assessed individually. A rough attempt at number of total animals would include summing best guesses from non-overlapping categories with by far the greatest number of animals, such as: pelagic zooplankton + benthic fauna + animals living in tree canopies + animals living in soil = 5.78 x 1017 + 9.05 x 1021 + 1020 + 9.08 x 1018 = 9.16 x 1021 animals. Due to uncertainty, the actual total may be within a few orders of magnitude of this.

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