How many species of arthropods? Erwin's estimate revised

Erwin's much debated estimate of 30 million species of arthropods is revise d. The original estimate is based on the evaluation of host specificity of guilds in beetle samples, and subsequent hierarchical ratio extrapolations. The growing number of studies including mass sampling of arthropods have p rovided several data sets suitable for obtaining an empirical basis of this estimate. The structure in this modified version is somewhat changed compa red to the original estimate in order to make each hierarchical step more e asily testable. Plant species are separated into different growth forms, an d host specificity measures are based only on phytophagous species. Effecti ve specialization is applied as a measure of host specificity to correct fo r the fauna shared between plant species. A between community correction fa ctor is applied to correct for differences in host specificity at different spatial scales. There are still great uncertainties attended with such est imates. The largest problems refer to the between community correction fact or and the proportion of canopy species to total species. Further work on h ost specificity and the least known hyperdiverse groups are also needed. Th e revised version of the estimate does not support hyperestimates of 30-100 million species. Rather, it compares nicely with estimates derivied from o ther estimation methods, indicating a global arthropod species richness of 5-10 million species. (C) 2000 The Linnean Society of London.