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| The Erwin Study |
Prior to 1982 most biologists thought that the number of undescribed species was roughly comparable to, or perhaps a few times as many as, the number already described. Thus, pre-1982 guesses of the total number of animal species were on the order of several million. But no one really knew.
In 1982 Terry Erwin published a provocative report in which he estimated the number of species of insects to be not several million but an order of magnitude higher - several tens of millions. Erwin reasoned that because the tropical forests appeared to contain vast unexplored areas of biodiversity he would sample there. Erwin, an expert on beetles, fogged the canopy of several trees of the species Luebea seemannii with a pesticide. The fogged insects then fell to the ground, allowing Erwin to sample them. As he sampled the beetles, Erwin kept finding new undescribed species. From the canopy of a single species of tree (L. seemannii) Erwin found more than 1,100 species of beetles.
How did Erwin arrive at a global estimate for the number of species from his "kill 'em and count 'em" experiment? He first estimated that 160 of those species were specialized to the canopy of that particular species of tree. Considering that beetles represent two-fifths of species diversity of insects, there should be about 400 (160 x 5/2) species of insects specialized to the canopy of L. seemannii. This inference assumes that beetle diversity is representative of insect diversity for that species. Erwin assumed that about two-thirds of the insect species were in the canopy and the rest were elsewhere. Based on that assumption, there should be 600 (400 x 3/2) species of insects specialized to L. seemannii. There are an estimated 50,000 species of trees in tropical forests. If each tree has 600 species of insects specialized to it, there should be 30 million species of insects in tropical rain forests.
Many authors expressed criticism and reservations about Erwin's extrapolations and inferences. Moreover, there have been only a few similar studies, none on the same scale as Erwin's. Much of the criticism revolves around Erwin's initial guess that 160 of the species he collected were specialists. If Erwin had overestimated the proportion of specialists, he would be overestimating the total number of species. Likewise, had he underestimated the proportion of specialists, he would have underestimated the total. Nigel Stork noted that Erwin could well be vastly underestimating biodiversity, given that he did not know how much of the diversity of beetles from the L. seemannii he had sampled. Suppose Erwin had only sampled one-third of the beetle diversity; all of his estimates would be three times too low. Could there be 80 million species of animals? 100 million? In actuality, two decades after Erwin's report, most biologists have revised their estimates for the total number of species downward toward the 10 million range, in part due to studies suggesting that Erwin overestimated the proportion of specialists. Still, nobody really knows how many species are on Earth.
Another factor that adds to the uncertainty about overall global diversity is our lack of knowledge about smaller organisms. There may be hundreds of thousands or millions of mites and fungi that we have literally overlooked. Even less is known about microbes. There are about 5,000 known species of prokaryotes, but scientists estimate that true diversity could range between 400,000 and 4 million species.