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How Many Species? (Part II)

August 26, 2011

In a previous article (How Many Species? June 15, 2010), I reviewed a statistical estimate of the number of species on Earth.[1-3] The generally accepted estimate of the total number of animal and plant species on Earth has been between 10-30 million, although only a few more than a million animals and 300,000 plants have been cataloged.[4]

The earliest known estimate of the number of species was done in 1833 by John Obadiah Westwood, a British entomologist. As can be guessed by his being an entomologist, he based his estimate on the number of insect species. His estimate, 400,000, was too low. About a million insect species have been cataloged to date, and there is no indication that the rate of discovery of new insects has slowed.[7]

One recent, composite, estimate of the total number of species, was based on the opinion of taxonomy experts. The number is between 3 and 100 million species,[5] which is not a very enlightening finding. Of course, the Earth is overrun by bacteria, and it's estimated that there are 5 x 1030 bacteria on Earth. It's hard to estimate the number of bacterial species, so these estimates range from as few as 10 million to one billion.[4]

Robert M. May, a zoologist at Oxford University, made an estimate in 1988 based on the idea that large animals are easy to find, but smaller ones are not. May reasoned that he could estimate the total number of species based on how many were seen at each size level. Then he could adjust the number for the species yet to be seen. May's estimate was 10 - 50 million species of land animals.[4] The discovery rate for species is now about 15,000 per year.[2]

Wrote May in a recent article,
"It is a remarkable testament to humanity's narcissism that we know the number of books in the US Library of Congress on 1 February 2011 was 22,194,656, but cannot tell you -- to within an order-of-magnitude -- how many distinct species of plants and animals we share our world with."[6]

ChimeraIt's hard to miss a species as large as this. The idea that large species are easily counted, but smaller ones aren't, is the basis of one estimate of the total number of species.

A floor mosaic (1213) depicting a chimera. The mosaic is at the Basilica of St. John the Evangelist, Ravenna, Italy.

(Via Wikimedia Commons).

A recent article in PLOS Biology[8] summarizes the various methods that have been used to estimate species numbers, as follow:

Macroecological Patterns
•  Body size frequency distributions (May's approach). There are fewer large species, and many smaller species. Extrapolation from the frequency of large to small species gives an estimated 10 - 50 million species of animals.
•  Latitudinal gradients in species. An extrapolation is made from the better sampled temperate regions to the tropics. This method gives 3 - 5 million species of large organisms.
•  Species-area relationships. Based on seafloor samples, an extrapolation shows that the seafloor could harbor up to 10 million species.
Diversity Ratios
•  Ratios between taxa. A 6:1 ratio is assumed between the numbers of fungi and vascular plants. Since there are 270,000 species of vascular plants, there's an estimated 1.6 million fungal species.
•  Host-specificity and spatial ratios. There are 50,000 known species of tropical trees. A 5:1 ratio is assumed between the numbers of beetle species and trees. Looking at ratios between beetles and all arthropods results in an estimate of 30 million arthropod species in the tropics.
•  Known to unknown ratios. If all bugs are sampled in a region, 62.5% are found to be of unknown species. This leads to an estimate of between 1.84 and 2.57 million insect species.
Taxonomic Patterns
•  Time-species accumulation curves. Extrapolating from the rate of discovery, there's an estimated 19,800 species of marine fish and 11,997 species of birds.
•  Authors-species accumulation curves. Modeling the number of authors of papers that describe species as a function of time shows that only 13% - 18% of flowering plants are yet to be discovered.
•  Analysis of expert estimations. This meta-analysis gives 5 million species of insects and 200,000 marine species.

That article in PLOS Biology by scientists at the Department of Biology, Dalhousie University (Halifax, Nova Scotia), the Department of Geography, University of Hawaii (Honolulu, Hawaii), the United Nations Environment Programme World Conservation Monitoring Centre (Cambridge,UK) and Microsoft Research (Cambridge, UK) presents a new estimate for the number of species.

The PLOS team found that the taxonomic tree for each species (the assignment of species to phylum, class, order, family, and genus) follows a pattern that allows an estimate of the number of species in each taxonomic group.[8] A similar analysis was done about a decade ago by a team at the University of Rome.[9]

After first validating their method against well-known taxa, the PLOS authors applied it to all Earth species. They predict 8.7 ± 1.3 million eukaryote species, 2.2 ± 0.18 million of which are marine. Since only 1.2-1.3 million species have been cataloged, this research implies that there are quite a few still to be discovered. The study estimates that 86% non-marine and 91% marine species are still not cataloged. A detailed summary appears in the table.[8]

SpeciesCatalogedPredicted±Std Error
Eukaryotes
Animalia953,4347,770,000958,000
Chromista13,03327,50030,500
Fungi43,271611,000297,000
Plantae215,644298,0008,200
Protozoa8,11836,4006,690
Total1,233,5008,740,0001,300,000
Prokaryotes
Archaea502455160
Bacteria10,3589,6803,470
Total10,86010,1003,630
Grand Total1,244,3608,750,0001,300,000

Robert May wrote a commentary on this study in which he stated, "This is higher than my earlier 'best guess', but I like the simplicity of this new method."[6] May is quoted further in The New York Times as saying, "I think it is an interesting and imaginative new approach to the important question of how many species actually are alive on earth today."[7]

However, some critics think that even these numbers are too low. David Pollock of the University of Colorado thinks that the fungal numbers are far too small. Pollock thinks that the number of fungal species is about 5.1 million, far higher than the 611,000 estimate in the PLOS paper.[7]

The PLOS authors do admit that their bacterial estimate is likely too low, since scientists have just started to investigate bacterial abundance. There are many thousands of species of bacteria in a cubic centimeter of soil, and many of these are not cataloged.[7]

References:

  1. Andrew J. Hamilton, Yves Basset, Kurt K. Benke, Peter S. Grimbacher, Scott E. Miller, Vojtech Novotný, G. Allan Samuelson, Nigel E. Stork, George D. Weiblen and Jian D. L. Yen, "Quantifying Uncertainty in Estimation of Tropical Arthropod Species Richness," American Naturalist, vol. 176 (May 2010), pp. 90-95.
  2. Meghan Miner, "Earth holds less biodiversity than thought," Cosmos Online, May 31, 2010.
  3. Emily Sohn, "Animal, Plant Species Less Diverse Than Once Thought," Discovery.com, June 7, 2010.
  4. R. May, "Tropical arthropod species, more or less?" Science, vol. 329, no. 5987 () 41–42.
  5. How Many Species of Bacteria Are There? (Wisegeek.com).
  6. Robert M. May, "Why Worry about How Many Species and Their Loss?" PLOS Biology, vol. 9, no. 8 (August 23, 2011), Document No. e1001130.
  7. Carl Zimmer, "How Many Species on Earth? It's Tricky," The New York Times, August 23, 2011.
  8. Camilo Mora, Derek P. Tittensor, Sina Adl, Alastair G. B. Simpson and Boris Worm, "How many species are there on Earth and in the ocean?" PLOS Biology, vol. 9, no. 8 (August 23, 2011), Document No. e1001127.
  9. C. Ricotta, M. Ferrari and G. Avena, "Using the scaling behaviour of higher taxa for the assessment of species richness," Biological Conservation, vol. 107, no. 1 (September 2002), pp. 131-133.