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How Many Species?

June 15, 2010

This year, 2010, is the International Year of Biodiversity [1,2]. As far as we know, the Earth is the only abode of life in the universe. We've been searching for life elsewhere in the universe, and we continue to catalog life here on Earth. 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. The historical 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. [3]

Looking at the insect world, about 855,000 arthropod species have been cataloged, but many more are thought to exist. A recent paper [4] claims that the historical estimate of the number of arthropod species, 30 million, is far too large [5,6]. Statistically, it appears that there is less than a 0.00001 probability that there are more than 30 million arthropod species. The new estimate is about five million.

Hydrous piceus, adult, dorsal view

Hydrous piceus

So, fewer vermin exist. Is that really bad? The problem here is that arthropods are thought to encompass about 80% of known species if bacteria aren't counted. Extending that logic, we are left with the idea that nearly all plant and animal species have been cataloged. Earth's estimated biodiversity has taken a plunge by an order of magnitude!

Of course, all estimates are prone to error. In this case, the one that claims that arthropods comprise 80% of known species should be examined more closely.


  1. The United Nations International Year of Biodiversity Web Site
  2. UNESCO International Year of Biodiversity Brochure (PDF File).
  3. How Many Species of Bacteria Are There? (Wisegeek.com).
  4. 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..
  5. Meghan Miner, "Earth holds less biodiversity than thought," Cosmos Online, May 31, 2010.
  6. Emily Sohn, "Animal, Plant Species Less Diverse Than Once Thought," Discovery.com, June 7, 2010.

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