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.biodiversity | body size | Coleoptera | species richness B eetles account for roughly 25% (350,000-400,000 species)(1) of all described species (∼1.5 million species), making this the most species-rich order known on Earth and supporting the philosopher Haldane's famous observation that God has "an inordinate fondness for beetles" (2, 3). Therefore, because this is a single lineage, an understanding of their global species richness, and that of the insects and other arthropods of which they form a part, is particularly important. There have been several reviews that discuss the merits of different estimates of the species richness for these taxa as well as of other organisms (1, 4-9), but none have been able to use these to derive mean estimates with some measure of error associated with these means. Here we compare global species estimates for beetles, insects, and terrestrial arthropods from eight different methods of estimation (here called methods 1-8). One of these (method 8) we introduce here, called the "body size and year of description" method, is based on the observed tendency for larger species of organisms to be described and named before smaller species, resulting in a decline in the mean body size of named species over time (10,11). We use data for beetles from the Natural History Museum (NHM) world collection in London and the British fauna to test this method. There has been some discussion as to whether global species estimates are converging (12) or not (13), and here we test this further. M...

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