ReviewThree reasons to re-evaluate fungal diversity ‘on Earth and in the ocean’
Highlights
► Fungal diversity is extremely complex; there have been several attempts to quantify it. ► These attempts have often involved scaling approaches applied to taxonomic structures. ► Incomplete taxonomic and molecular sampling preclude an accurate estimate of global diversity.
Introduction
Recently, Mora et al. (2011) estimated the total number of species on Earth using a predictive algorithm based on generalizing the ‘strong’ correlation between number of higher taxa and taxonomic rank across all of life. This approach relies on the fact that the number of higher taxa is much more complete and consistent than the total number of species. For each taxonomic level from phylum to genus they fitted asymptotic regression models to the temporal accumulation curves of higher taxa, then the predicted number of taxa at each taxonomic rank down to genus was regressed against the numerical rank, and the fitted models used to predict the number of species (Mora et al., 2011). This approach was shown to work best where the higher ranks were most completely known and stable, and their number was at or close to asymptote, and where species were clearly and reproducibly defined and their diversity relatively well investigated. Consequently, it works particularly poorly for microbes, especially prokaryotes, in which new evolutionary groups are steadily being described (Pace, 1997, Rappe and Giovannoni, 2003) and where ‘species’ are often not the unit of research focus and therefore under-developed as a concept, and also tend to be more genetically inclusive than in many eukaryote groups (e.g. Doolittle, 2008, Gevers et al., 2005, Koeppel et al., 2008, Konstantinidis and Tiedje, 2005, Staley, 2006, Vandamme et al., 1996).
Unlike in animals and plants, where the algorithms used by Mora et al. (2011) can be argued to have a historical validity in that taxa at all levels have been described using similar approaches and in a relatively consistent manner over the period of their analysis (1800–2000), fungi (along with protists and prokaryotes) have not enjoyed such consistency. Here we expand upon the caveats outlined in Mora et al. (2011) regarding the estimation of total fungal diversity and identify three main reasons why the total number of species is almost certainly much higher than their predicted figure of 611,000 +/− SE = 297,000: (1) uncertainty about the number of described fungal species; (2) the revelations provided by molecular biology and environmental probing; and (3) the expanding number of higher fungal taxa.
Section snippets
How many fungal species are actually known?
Most effort in describing and cataloguing fungal species has been directed towards the larger and more obvious forms. The current (10th) edition of the Dictionary of the Fungi gives a figure of 98,128 species (Kirk et al., 2008 – Fig. 1), excluding all fungal analogues (Richards et al., in press). The majority of this 98,128 are terrestrial ascomycete and basidiomycete species (Kirk et al., 2008). This figure may be artificially inflated by synonyms created by separately described anamorphs and
Molecular inventories of fungal diversity and the cryptic majority
Small and cryptic organisms are difficult to distinguish according to the techniques of classical taxonomy, but their diversity is potentially massive (Horton and Bruns, 2001, Rappe and Giovannoni, 2003, Sogin et al., 2006). One clear illustration of this is the effect of the molecular biology revolution on diversity estimates in microbial groups: environmental sequencing has revealed a far higher microbial species-level diversity than is suggested by their morphological diversity. The
The implication of adding new groups at the highest taxonomic grades
The approach employed by Mora et al. (2011) relies heavily on a stable census of higher taxonomic groups for the species estimation methodology to be reliable. Insights from molecular techniques have implications at all taxonomic levels in fungi, not just around the species level. Blackwell (2011) states that ‘there cannot be any doubt that ascomycetes and basidiomycetes (Dikarya) comprise the vast majority of fungal diversity’. This is certainly true in terms of described species, but is not
Conclusion
In summary, we assert that there is at least one order of magnitude more fungal species than are currently known. The predicted 611,000 of Mora et al. (2011) should be seen as a call-to-arms for a concerted and continued effort to improve our knowledge of fungal diversity by all means available. It is essential that modern taxonomic methods and databases are employed and maintained so that improvement in our knowledge of this all-pervasive and hugely influential group of organisms is embedded
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