New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods

Abstract

It has been suggested that we do not know within an order of magnitude the number of all species on Earth [May RM (1988) Science 241(4872):1441-1449]. Roughly 1.5 million valid species of all organisms have been named and described [Costello MJ, Wilson S, Houlding B (2012) Syst Biol 61(5):871-883]. Given Kingdom Animalia numerically dominates this list and virtually all terrestrial vertebrates have been described, the question of how many terrestrial species exist is all but reduced to one of how many arthropod species there are. With beetles alone accounting for about 40% of all described arthropod species, the truly pertinent question is how many beetle species exist. Here we present four new and independent estimates of beetle species richness, which produce a mean estimate of 1.5 million beetle species. We argue that the surprisingly narrow range (0.9-2.1 million) of these four autonomous estimates--derived from host-specificity relationships, ratios with other taxa, plant:beetle ratios, and a completely novel body-size approach--represents a major advance in honing in on the richness of this most significant taxon, and is thus of considerable importance to the debate on how many species exist. Using analogous approaches, we also produce independent estimates for all insects, mean: 5.5 million species (range 2.6-7.8 million), and for terrestrial arthropods, mean: 6.8 million species (range 5.9-7.8 million), which suggest that estimates for the world's insects and their relatives are narrowing considerably.

Keywords: Coleoptera; biodiversity; body size; species richness.

Fig. 1.

Cumulative log_e mean [with 95% confidence interval (CI)] of British beetle body size for each year from 1758 to 2014, and for mean body size (with 95% CI) for a sample of beetles from the Natural History Museum, London (ALL). Note that the horizontal axis is arbitrary for ALL but that the dotted horizontal lines indicate where the 95% CIs intercept the years for the British data (see also Fig. S1).

Fig. S1.

Cumulative log_e mean (with 95% CI) of British beetle body size against cumulative beetle species number. The data points are the same yearly values plotted in Fig. 1 but with the x axis scaled to the cumulative number of described species instead of time. ALL indicates mean body size (with 95% CI) for samples from the Natural History Museum, London, of all beetles; note that the horizontal axis is arbitrary for ALL but that the horizontal lines indicate where the 95% CIs intercept the accumulative number of species for the British data.

Fig. S2.

Data on the number of animal species in different countries of Western Europe were downloaded from the Fauna Europaea website (www.faunaeur.org) and are plotted against the area of the countries concerned (BE, Belgium; CZ, Czech Republic; DE, Denmark; FI, Finland; FR, France; GE, Germany; GR, Greece; HU, Hungary; IR, Ireland; IT, Italy; NE, The Netherlands; NO, Norway; PO, Poland; PT, Portugal; SL, Slovakia; SP, Spain; SU, Switzerland; SW, Sweden; UK, United Kingdom). There is a significant linear relationship between area and the number of species (P = 0.008). Generally, higher-latitude countries are below the line and lower-latitude countries are above the regression line, likely reflecting the influence of climate on number of species. The data point for the United Kingdom is close to the regression line, suggesting that the size of the UK/British fauna is typical of a midsized and midlatitude Western European country.