After collating data on the numbers of species in each major taxon, May (1988) concluded that our knowledge of vertebrates far exceeded that of invertebrates and protists. The principal reason for the deficient quantitative assessment of diversity in invertebrates and protists was the limited number of trained taxonomists (especially in the tropics, where most of the world’s biodiversity resides). Although strides have been made to build capacity in these areas over the last 20 years (Janzen, 1994; Smith and Rogo, 2005), the number of taxonomists working in the museums of most tropical countries today is roughly comparable to the number that worked in Sweden’s museums 250 years ago (in Linnaeus’s time, at the dawn of taxonomy). Consequently, classifying and naming species continues to proceed at a slow and uneven rate.

Erwin’s (1982) work on beetles in tropical forest canopies provided a dramatic illustration of our lack of comprehension of how many extant species exist. Erwin’s initial estimates suggested there might be as many as 30 million species of beetles in the world’s tropical forests [considerably more than the 20,000 species initially estimated by John Ray (1627–1705) and cataloged by Linnaeus in Systema Naturae (Linnaeus, 1735)]. Erwin’s estimate of global insect diversity stimulated a series of articles that used a variety of different approaches to estimate total species numbers. Erwin (2004) recently reviewed this literature, and his summary table is illustrated in Fig. 4.1. Two key patterns emerge. First, estimates of global spe-

FIGURE 4.1 Estimates since the time of Linnaeus of the number of metazoan species. Data are from Erwin (2004), and the dates for Linnaeus (1735) and John Ray (1691) were estimated from time of publication of their major books on this topic (Erwin, 2004). The most recent sets of estimates sometimes provide a range, or an upper bound, and less frequently a “best estimate” of total species numbers.