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Fungal Diversity Revisited:
2.2 to 3.8 Million Species
DAVID L. HAWKSWORTH
1
and ROBERT LÜCKING
2
1
Department of Life Sciences, The Natural History Museum, London SW7 5BD, United Kingdom,
and Comparative Plant and Fungal Biology, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS,
United Kingdom;
2
Botanischer Garten und Botanisches Museum, Freie Universität Berlin, 14195 Berlin, Germany
ABSTRACT The question of how many species of Fungi there
are has occasioned much speculation, with figures mostly
posited from around half a million to 10 million, and in one
extreme case even a sizable portion of the spectacular number
of 1 trillion. Here we examine new evidence from various
sources to derive an updated estimate of global fungal diversity.
The rates and patterns in the description of new species from
the 1750s show no sign of approaching an asymptote and
even accelerated in the 2010s after the advent of molecular
approaches to species delimitation. Species recognition
studies of (semi-)cryptic species hidden in morpho-species
complexes suggest a weighted average ratio of about an order
of magnitude for the number of species recognized after and
before such studies. New evidence also comes from
extrapolations of plant:fungus ratios, with information now
being generated from environmental sequence studies,
including comparisons of molecular and fieldwork data from
the same sites. We further draw attention to undescribed
species awaiting discovery in biodiversity hot spots in the tropics,
little-explored habitats (such as lichen-inhabiting fungi),
and material in collections awaiting study. We conclude
that the commonly cited estimate of 1.5 million species is
conservative and that the actual range is properly estimated
at 2.2 to 3.8 million. With 120,000 currently accepted species,
it appears that at best just 8%, and in the worst case
scenario just 3%, are named so far. Improved estimates
hinge particularly on reliable statistical and phylogenetic
approaches to analyze the rapidly increasing amount
of environmental sequence data.
BACKGROUND
In 1825, Elias Magnus Fries (1794–1878) predicted that
the fungi would prove to be the largest group in the
vegetable kingdom, analogous to the insects in the ani-
mal kingdom. Notwithstanding that fungi are not actu-
ally part of the plant kingdom, how right he has proved
to be as the bicentenary of his prediction approaches. By
the 1960s a few mycologists were speculating that there
might be as many fungal as plant species, but almost
no attempts to calculate estimates from the available
data were made. As concern over the conservation of
biodiversity in general grew in the subsequent decades,
culminating in the signing of the Convention on Bio-
logical Diversity in 1992, more precise figures on species
numbers of all kinds of organisms were required. A se-
ries of estimates of the number of fungi settled on figures
ranging from 500,000 to almost 10 million species, with
1.5 to perhaps 5 million receiving most support among
mycologists. A recent study even predicts up to a trillion
species of microorganisms globally (1); how many of
these are supposed to be fungi is not specified, but if
this estimate holds true and only 1% of these were fungi,
the global estimate of fungal diversity would be a thou-
sand times higher than the current highest estimate of
10 million species.
Different extrapolation techniques have been used
to arrive at global fungal species richness estimates, in-
cluding publication rates of new taxa (2), plant:fungus
ratios (3,4) similar to plant:insect ratios first used in
Received: 24 May 2017, Accepted: 1 June 2017,
Published: 28 July 2017
Editors: Joseph Heitman, Department of Molecular Genetics and
Microbiology, Duke University Medical Center, Durham, NC 27710;
Timothy Y. James, Department of Ecology and Evolutionary Biology,
University of Michigan, Ann Arbor, MI 48109-1048
Citation: Hawksworth DL, Lücking R. 2017. Fungal diversity revisited:
2.2 to 3.8 million species. Microbiol Spectrum 5(4):FUNK-0052-
2016. doi:10.1128/microbiolspec.FUNK-0052-2016.
Correspondence: Robert Lücking, r.luecking@bgbm.org
© 2017 American Society for Microbiology. All rights reserved.
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Erwin’s famous study (5), quantitative macroecological
grid-based approaches (6–8), methods based on envi-
ronmental sequence data including plant:fungus ratios
(9,10), and ecological scaling laws (1).
This article provides updated background informa-
tion on the number of described species and explores
new evidence obtained from rates of species discovery,
extrapolations from fungus:plant ratios, and molecular
sequence data from environmental samples. We con-
clude by indicating where undescribed species are likely
to be found and recommend a working number suitable
for general use in estimates of global and regional species
richness.
We focus here on the numbers of species in the king-
dom Fungi, that is, inclusive of the Cryptomycota and
Microsporidia, and also including lichen-forming fungi.
We do not, however, consider other organisms that
mycologists study as fungi, such as the fungal analogues
Mycetozoa (as an unranked supergroup or in the king-
dom Protozoa, phylum Amoebozoa) and Oomycota and
Hyphochytriomycota (in the kingdom Heterokonta =
Straminipila). However, we caution that some data sets
and papers discussed in the following sections are based
on fungi in the broad sense and thus include informa-
tion from the analogue phyla. While readers should bear
this in mind, the numbers of known taxa in these groups
are relatively modest, so their occasional unavoidable
inclusion will not materially affect the interpretations
presented here.
EVIDENCE FROM PUBLICATION
RATES OF NEW TAXA
Numbers of Described Species
The number of new species described in each decade,
as recorded in the Index Fungorum database (http://
www.indexfungorum.org), shows three distinct phases
(Fig. 1): an ascending phase in which progress was fast
in the 1750s to 1860s, a steep phase as microscopy came
into general use and there was intensive collecting in
hitherto barely explored parts of the world in the 1870s
to 1880s, and then a relatively constant phase from the
1890s to the present day. The partially higher figures
in the first decades of the “constant phase”reflect con-
tinuing exploration and a number of prolific individ-
ual mycologists. The rate of description over the past
40 years has, however, remained fairly constant at
around 1,300 per year (Fig. 2), with peaks generally
relating to particular major monographic works.
Of special note is the somewhat steep increase in the
rate since 2010 to around 1,800 per year. This resur-
gence in species description is largely attributed to the
increasing use of molecular techniques for species de-
limitation and is of particular note because this increase
followed, and even accelerated after, the ending of the
separate naming of morphs of the same species in 2011
(when the provision for a Latin diagnosis or description
was also eliminated). Prior to that date, the totals for
annually described “species”included separate names
given to different morphs of the same species.
FIGURE 1 Numbers of newly introduced species names of fungi for each decade from
1750 to 2010. Based on data from the Index Fungorum database provided by P. M. Kirk.
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The number of newly named species does not, how-
ever, equal the number that have been truly described for
the first time. It is much easier to name a fungus as new
to science rather than to ascertain that it has previously
been named. This is not simply a matter of comparing
material to that of previously recognized species in the
same genus but requires meticulous detective work, be-
cause not uncommonly, species were described in genera
they do not belong to. Further, species concepts change,
for example, in genera where morphologically identical
fungi were regarded as different species if they occurred
on different plant species. The number of accepted fungi
can only be obtained by counting those recognized as
“good”in each genus. This has been done since 1943
through the 10 editions of Ainsworth & Bisby’s Dic-
tionary of the Fungi and now annually through Species
Fungorum inputs to the Catalogue of Life (http://www
.catalogueoflife.org/annual-checklist/). An analysis of
these data shows that the total number of existing spe-
cies names runs at around 2 to 3 times that of currently
accepted species (Fig. 3), a figure consistent with the
ratio of 2.6 derived from an analysis of names in
monographic works (11). The number of described good
fungal species currently stands at around 120,000.
Unlike the situation in many other groups of organ-
isms (12), the number of good fungal species continues
to rise steeply decade by decade, with no indication of
leveling off. The steepness of the curve would be even
greater if there were more mycologists actively involved
in species discovery, using either morphological or mo-
lecular approaches.
The number of species being recognized is necessarily
impacted by the species concepts used. While in the
premolecular era, species circumscriptions were based
almost entirely on morphological and sometimes bio-
chemical and cultural features, the incorporation of
molecular information has led to a refining of species
concepts. There is, however, a plethora of species con-
cepts in biology (13,14) and no single objective crite-
rion that can be applied universally. Because the purpose
of names is to provide a means of communication, a
pragmatic species concept is essential, which may be
defined as the following: “a species is what it is useful to
give a species name to.”(15). There have been numerous
attempts to apply mechanistic systems for species rec-
ognition in phylogenetic trees, but the consolidated spe-
cies concept has much to commend it and ultimately
provides a broad consensus on how to recognize fun-
gal species. The consolidated species concept adopts a
polyphasic approach combining morphological, eco-
logical, and phylogenetic species concepts (16) and is
being increasingly taken up by mycologists. While in
some situations this may lead to more species being
recognized than otherwise would have been the case,
in others it results in formerly separately distinguished
species being united. Therefore, over- and underesti-
mations in particular cases are usually balanced out. A
rarely considered notion is the time scale: a proportion
FIGURE 2 Numbers of newly introduced species names of fungi for each year from
1975 to 2015. Note that the data for 2015 were incomplete when this work went to
press. Based on data from the Index Fungorum database provided by P. M. Kirk.
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of “species”is in active speciation, with incomplete
lineage sorting, yielding it difficult to resolve good spe-
cies (17); in such cases, determining the number of spe-
cies is philosophical, since that number is in active flux.
A large number of new fungal species continue to be
described with no molecular data, either due to the age
of the underlying material (e.g., unveiled in older col-
lections) or by taxonomists who do not have access to
molecular techniques; assessing the proportion of good
species in such taxa is therefore difficult.
Patterns in Taxon Discovery
Some predictable patterns in the recognition of pub-
lished taxa in the rank of genus and above have been
recognized and applied to a broad set of organisms, in-
cluding fungi (2). By combining the full classifications of
around 1.2 million species of all groups of organisms,
these authors found, as might have been expected, that
accumulation curves of higher taxa over time showed
that these were more completely described and in many
cases approached an asymptote. The use of the higher
taxon approach was then validated by comparing the
actual number of known species with that predicted
from the accumulation curves. The authors then fitted
asymptotic regression models to the actual curves for
different groups to provide estimates of undiscovered
species numbers. In the case of fungi, this led to a total
number of expected fungal species of 611,000 (standard
error ± 297,000). This conclusion was, however, mis-
leading, because the analysis was based on the assump-
tion that there were only 43,271 catalogued fungal
species, rather than the nearly 100,000 accepted at
that time (18). Intriguingly, a recalculation using their
method with the latter species number generates a figure
of around 1,400,000—not far out of line with estimates
derived from extrapolations based on plant:fungus ra-
tios (see below).
EVIDENCE FROM SPECIES
RECOGNITION STUDIES
Unknown fungi are now considered to come mostly
from two sources: (i) newly discovered species by means
of traditional inventory methods in little-studied areas
and habitats and (ii) newly discovered lineages through
environmental sequencing. A further, neglected but sig-
nificant source of unrecognized fungal diversity is the
restudy of known taxa, applying species recognition
techniques to molecular data, using either the fungal bar-
coding locus ITS (19) or multilocus approaches. Exem-
plar taxa are the fly agaric, Amanita muscaria, and the
golden chanterelle, Cantharellus cibarius, which contain
numerous, often regionally distributed species (20,21).
The record is currently set by the basidiolichen fungus
FIGURE 3 Growth in the total catalogued number of species names of fungi by decade
from 1750 compared with the global number of accepted species. Based on figures
adopted in the 10 editions of Ainsworth & Bisby’s Dictionary of the Fungi for 1943–2008
and data in the Index Fungorum and Species Fungorum (Catalogue of Life) databases
provided by P. M. Kirk.
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Cora glabrata, which has been shown to contain at least
189 species (6,22). We surveyed 45 such studies pub-
lished between 1998 and 2016, including studies of non-
lichenized and lichenized forms, and computed the ratio
of species recognized in the target group before and
after the study (Table 1). Most studies started out with a
single or few species, whereas two looked at complexes
of up to 18 species—Protoparmelia (23) and Usnea (24)
—and one revised a complex of 71 species accepted prior
to the study, in the genus Coprinellus (25). The posterior:
prior ratios in the number of species recognized ranged
from 0.67 (from 18 to 12 species in a group of Usnea
[24]) to 189 (from 1 to 189 species in the basidiolichen
Cora glabrata)(
6,22). Other high ratios were found for
Dictyonema with 59 (26), Cladosporium cladosporioides
with 22 (27), Sticta weigelii with 15 (28), Microbotryum
violaceum with 11 (29), and Fusarium graminearum
with 7 (30). Environmental sequence and genomic data
revealed high ratios for the genera Archaeorhizomyces,
with 145 (31), and Cyphobasidium with 26 (32), as well
as for the still barely explored Cryptomycota,with138
(33,34), in which the true figure could be much higher.
The median ratio over all evaluated studies was 3.0, and
the arithmetic mean was 14.5 (Table 1).
To obtain a more reliable picture, we adjusted these
ratios by geographic area covered. If a species complex
had a global distribution prior to the study, and if based
on material from a single geographic area, it was found
to represent several species with presumably restricted
distribution, the detected ratio was adjusted by the fac-
tor 2 (expanding to both hemispheres in cases in which
one hemisphere was covered), the factor 3 (for tropical
fungi for which a single tropical region was covered),
and up to the factor 6 (for cosmopolitan fungi for which
a single region within a single continent was studied).
These factors were derived from selected global studies,
for example, in the genera Cladosporium (27), Cora (6,
22), Fusarium (30), and Sticta (28). In some cases, we
added unpublished data to the adjusted values; thus,
for Archaeorhizomyces we currently recognize at least
500 species (B. E. Smith and R. Lücking, unpublished
data), and for Cyphobasidium, we recognize about
100 species from deposited GenBank sequences (32).
The thus adjusted ratios ranged from 0.7 (Usnea) to 435
(Archaeorhizomyces), with a median value of 10.5 and
an arithmetic mean of 39.3.
To predict the global species richness of Fungi using
these raw and adjusted ratios, we employed two further
adjustments. First, we weighted the ratios according to
the number of species recognized in a genus in the 2008
Dictionary of the Fungi (18). For instance, a genus in
which a single species had been recognized received a
low weight (1.0), since even a high number of newly
recognized species will only contribute minimally to
global fungal species richness. This adjustment avoided
a high impact of extraordinary ratios such as in the
previously monotypic genus Cora. In contrast, for
groups in which a large number of species had already
been recognized, the ratios based on a study of one or a
few species were upweighted, e.g., in the genus Puccinia
(35), with 4,000 accepted species. In addition, to ac-
count for ecological bias in the evaluated studies, we
applied a group weight according to the major lifestyle
attributed to each genus; to that end, we divided the
approximately 100,000 known fungi in 2008 as fol-
lows: 40,000 saprotrophic (weight 4.0), 20,000 phyto-
pathogenic (weight 2.0), 20,000 lichenized (weight 2.0),
8,000 ectomycorrhizal (weight 0.8), 5,000 endophytic
(weight 0.5), 3,000 entomopathogenic (weight 0.2),
2,000 aquatic (weight 0.2), 500 endomycorrhizal (weight
0.05), 500 marine (weight 0.05), 400 soil (weight 0.04),
and 100 human pathogenic (weight 0.01). The cate-
gories aquatic, marine, and soil are used here for cases
where the habitat is known but not the biology. These
proportions denote estimates based on known fungi in
2008, not estimated proportions of extrapolated global
species richness, in which, presumably, categories such as
plant pathogens and soil fungi will have much higher
values. Using these weights, the overall arithmetic mean
of the posterior:prior ratio for all evaluated studies
amounted to 11.3. This suggests that even without in-
cluding important sources of new species discoveries,
the already accepted Fungi may contain up to 10 times
as many species as currently recognized, resulting in more
than 1 million estimated species. Remarkably, this is
about 4 times higher than the currently estimated syn-
onymy ratio of 2.6, suggesting that a large proportion of
these unrecognized species have no names available.
EVIDENCE FROM EXTRAPOLATIONS
BASED ON PLANT:FUNGUS RATIOS
The widely cited and accepted plant:fungus ratio-
derived global estimate of 1.5 million species of fungi
was based on information from several independent
sources (3): (i) the numbers of fungi reported from all
habitats in the United Kingdom and sites within the
United Kingdom compared with the number of plant
species present, (ii) the numbers of host-specific fungi on
particular plant species, (iii) percentages of new species
being discovered, and (iv) extrapolations to the global
level based on conservative estimates for the number of
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TABLE 1 Selected species recognition studies in different groups of fungi
Taxon Genus
a
Lifestyle Coverage
b
Range
c
Ratio
d
Factor
e
Adjusted
f
Weight
g
Reference
Acantholichen pannarioides 1 Lichenized Neotropical Neotropical 6 1 6 8 100
Amanita muscaria 500 Ectomycorrhizal N. Hemisphere Global 6 2 12 6,000 20
Archaeorhizomycetes 0 Soil fungus N. Hemisphere Global 145 3 500 500 31
Aspergillus flavus 266 Saprotrophic Australia Global 2 6 12 3,192 101
Aspergillus fumigatus 266 Human pathogenic Global Global 2 1 2 532 102
Auxarthron zuffianum 15 Saprotrophic Global Global 2 1 2 30 103
Beauveria 9 Entomopathogenic Global Global 4 1 4 36 104
Blastomyces dermatitidis 1 Human pathogenic N. America Global 2 1 2 2 105
Botrytis cinerea 54 Phytopathogenic Europe Global 2 6 12 648 106
Calonectria pauciramosa 34 Phytopathogenic Global Global 3 2 6 204 107
Cenococcum geophilum 1 Ectomycorrhizal N. America Global 5 6 30 30 108
Cladosporium cladosporioides 150 Saprotrophic Global Global 22 1 22 3,300 27
Coccidioides immitis 3 Human pathogenic N. America N. America 2 1 2 6 103
Coniophora puteana 20 Saprotrophic Global Global 3 1 3 60 109
Coprinellus 100 Saprotrophic Global Global 1,2 1 1.2 120 25
Cora 1 Lichenized Subglobal Global 189 1 189 189 6,22
Corella 0 Lichenized Central/S. America Central/S. America 11 1 11 11 26
Cryptomycota 0 Aquatic N. America Global 39 6 234 234 33
Cyphellostereum 1 Lichenized Central/S. America Global 15 3 45 45 26
Cyphobasidium 0 Lichenicolous N. America Global 26 4 100 100 32
Dictyonema 6 Lichenized Subglobal Global 59 3 177 1062 26
Fusarium graminearum 111 Phytopathogenic Global Global 7 1 7 777 30
Grosmannia clavigera 1 Phytopathogenic N. America N. America 2 1 2 2 110
Hymenoscyphus albidus 155 Phytopathogenic Europe Europe 2 1 2 310 111
Lasiodiplodia theobromae 9 Phytopathogenic Pantropical Pantropical 3 1 3 27 112
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Letharia 2 Lichenized N. Hemisphere N. Hemisphere 3 1 3 6 113
Lobariella 5 Lichenized Neotropical Neotropical 3.8 1 3.8 19 114
Metarhizium anisopliae 9 Entomopathogenic N. America Global 2 6 12 108 115
Microbotryum violaceum 87 Phytopathogenic Europe/N. America Global 11 4 44 3,828 29
Neofusicoccum parvum/ribis 13 Phytopathogenic Regional/host Global 2 60 150 1,950 116
Paracoccidioides brasiliensis 1 Human pathogenic S. America S. America 4.0 1 4 4 117
Parmelia 95 Lichenized Europe/N. America N. Hemisphere 3 1.5 4.5 428 118
Parmelia saxatilis 1 Lichenized N. America Global 4 3 12 12 119
Parmotrema reticulatum 348 Lichenized Subglobal Subglobal 4 1 4 1,392 120
Phialocephala fortinii 1 Endophytic Europe Global 7 6 42 42 121
Protoparmelia 20 Lichenized Global Global 1.6 1 1.6 32 23
Puccinia monoica 4,000 Phytopathogenic N. America Global 1.8 6 10.5 42,000 35
Rhizoplaca melanophthalma 11 Lichenized N. America Global 2.5 6 15 165 122
Stachybotrys chartarum 44 Saprotrophic N. America Global 2 1 2 88 123
Sticta fuliginosa 114 Lichenized Global Global 15 1 45 5,130 28
Sticta weigelii 114 Lichenized Global Global 23 1 23 2,622 28
Strobilomyces 20 Ectomycorrhizal Asia Global 3.5 6 21 420 124,125
Tricholoma scalpturatum 200 Ectomycorrhizal Europe Global 2 6 12 2,400 126
Uncinocarpus reesii 3 Saprotrophic Global Global 2 1 2 6 103
Usnea 338 Lichenized N. Hemisphere N. Hemisphere 0.7 1 1 338 24
a
Number of species recognized in the corresponding genus in reference 18.
b
Origin of material used in the study.
c
Global distribution of the group.
d
Ratio of the number of species recognized after versus before the study.
e
Geographic adjustment.
f
Adjusted ratio based on raw ratio and geographic adjustment.
g
Weight based on adjusted ratio and original number of species recognized in the corresponding genus in reference 18 (in some instances [Acantholichen,Sticta,Usnea] further adjusted based on unpublished data).
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plant species, making allowances to discount separately
named morphs of the same species. The 1.5 million fig-
ure was considered conservative because, while num-
bers of fungi from all sources, including organisms
other than plants and soil, were included in the species
totals for particular places, no allowances were made
for undiscovered fungi in and on the millions of insects
predicted to exist.
Some figures used in these early extrapolations have
subsequently been revised upward. Ten years later, it
was pointed out that the number of plant species and the
fungus:plant ratio were too conservative and the vast
number of insect fungi had not been considered. As a
consequence, some revised estimates ranged between
2.7 and 9.9 million fungal species (4).
The total inventory of fungal species found in field
surveys in a particular site continues to increase year by
year, even with regular visits spanning several decades,
while the number of plant species remains more or less
unchanged. Esher Common (Surrey, United Kingdom) is
by far the most investigated site by field mycologists in
the world, and to date about 3,400 species have been
found (B. M. Spooner, personal communication). Be-
cause the number of vascular plant species remains at
420, this now gives a fungus:plant ratio of 8:1.
Some preliminary results from next-generation se-
quencing of soil samples from a wood within the second
most field-surveyed site in the world, the Slapton Ley
National Nature Reserve in southwest England, are
illuminating in relation to the fungus:plant ratios (G. W.
Griffith, B. S. Dentinger, and D. L. Hawksworth, un-
published). The wood studied has 1,136 species of fungi
and 88 vascular plant species recorded during field sur-
veys, a ratio of 12.9:1. However, just 686 sequence-
based species (accounting only for taxa represented by
five or more sequences) were recovered from soil sam-
ples (and a further 153 taxa were represented by one
to four sequences). What is intriguing is that of the 164
named genera represented in the next-generation se-
quencing data set, only 32 were among the 549 genera
collected in the fieldwork, which included fungi on plant
parts and plant litter, not just sporophores emerging
from the soil. The actual ratio in a site may therefore
be substantially higher than indicated by traditional in-
ventory techniques.
The interpretation of some data sets needs to be
considered in the light of such long-term investigations.
Ratios comparing fungal sequences recovered from en-
vironmental samples with plants growing on the sample
sites have given some surprising ratios. For example,
data from a forest in North Carolina gave fungus:
plant ratios of 19:1 (491 fungi and 26 plants) and 13:1
(616 fungi and 48 plants), leading to a suggestion that
there may be 3.5 to 5.1 million fungal species worldwide
(9). A more modest ratio of 7.5:1 (1,500 fungi and <200
plants) was obtained from boreal forest soils in Alaska
(36). In contrast, Tedersoo et al.’s(37) impressive study
of 365 globally distributed soil samples using metabar-
coding molecular methods (see below) led the authors
to conclude that fungal species richness had actually
been overestimated by 1.5 to 2.5 times from that based
on plant:fungus species ratios. Any such extrapolation
from soil samples alone to the total fungal biota at a
site is, however, unsound as evidenced by the above data
from the Slapton reserve. Much of the actual fungal
species diversity in a site will inevitably be missed in
soil sampling, because most is above ground, on and in
plants and animals of all kinds, in water, in lichens, on
rocks, etc.
Collections made in brief smash-and-grab collecting
trips or even extending over several years will only start
to reveal what fungi may actually be present at a site.
Studies such as that of Piepenbring and collaborators
(38), in which 567 species of fungi and 311 species of
plants were found along a 500-m pathway in Panama
over 2 years, giving a ratio of just 1.8:1, have to be in-
terpreted in this context. The ratio resulting from that
study would be expected to rise significantly if the period
over which it was conducted had been 2 decades rather
than 2 years, and with increased attention to micro-
scopic fungi.
When considering fungi of all biologies and in all
ecological niches in a site, the ratio can be expected to
vary depending on the geographical location, as recog-
nized some 25 years ago (3). It is not surprising, there-
fore, that the studies of fungus:plant ratios in a site,
obtained by field survey and molecular approaches, have
generated ranges from 1.8 to 19.1:1. The mean of the
figures cited in this section yields a ratio of 9.8:1, sug-
gesting that the ratio of around 6:1 arrived at in 1991
may be conservative. With the currently accepted num-
ber of approximately 380,000 vascular plant species
(39), this adjusted ratio would predict a number of
nearly 3.8 million fungal species.
EVIDENCE FROM ENVIRONMENTAL
SEQUENCING TECHNIQUES
The advent of obtaining taxon-specific, molecular se-
quence data from environmental samples, most com-
monly soils but also water and plant tissues, has
generated a new source of data for estimating global
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species numbers. It is commonly found that numerous
sequences thus obtained do not have any matches with
those from named fungi in GenBank (see below), and
this has led to speculations that previous estimates have
been too low.
Generalizing data from environmental samples is
problematic for several reasons, including that the
broader geographic context of detected operational
taxonomic units (OTUs) is usually unknown. For ex-
ample, using 454-sequencing technology, two 0.25-g
soil samples from an Alaskan forest collected just 1 m
apart had only 14% of fungal species in common (36).
In a pyrosequencing study in the Andes, 1,839 species-
like taxa were found, of which 25% were most similar to
other unidentified environmental samples, with signifi-
cant differences between forests at different altitudes
(40). The most comprehensive study of soil samples to
date is that of Tedersoo et al. (37), who analyzed 2-g soil
samples from pooled soil in 5-cm deep cores taken from
365 global sites. They obtained 80,486 fungal OTUs,
used a 98% sequence similarity for species recognition,
and did not consider almost half in further analyses be-
cause they were singletons and potentially artifacts; in that
study, plant:fungus richness declined toward the poles.
However, in concluding that extrapolations from plant:
fungus ratios were unjustified (see above), the authors did
not take into account fungi other than those in soil, such
as those in and on aerial parts of plants, in decaying
vegetation, lichen-fungi, and entomogenous fungi.
Especially instructive is a study of 928 swabs of dust
samples across the continental United States by direct
PCR and using high-throughput sequencing (HTS) to
analyze them (41); not only were 38,473 fungal taxa
detected, but there were clear geographic patterns with a
predictive value of placing a sample within 230 km. Of
the 38,473 taxa, however, sequences of about 40%
could not be matched with any named species (R. R.
Dunn, personal communication).
Just how many unidentified sequences generated from
environmental samples represent undescribed species is
uncertain. Compared with the 120,000 formally recog-
nized species (see above), as of 25 November 2016, there
were only 34,878 named fungal species in GenBank, but
there were a further 94,059 species-level OTUs with no
names (C. L. Schoch, personal communication); these
figures have increased by 68% and a staggering 117%,
respectively, in the past 5 years. However, only a minute
fraction of environmental sequences are deposited indi-
vidually in GenBank; the vast majority is placed in the
Sequence Read Archive (SRA) (42). Currently, the SRA,
on the query “(fungal OR Fungi) AND (internal tran-
scribed spacer [or, ITS] OR ITS1 OR ITS2),”returns
179 studies, 1,822 biosamples (i.e., environmental sam-
ples), and 14,334 experiments or HTS runs, containing
928 million fungal ITS reads, with an average length of
353 bases (SRA: https://trace.ncbi.nlm.nih.gov/Traces
/sra/sra.cgi?view=search_obj; accessed 12 March 2017).
In contrast, using the same query, GenBank (43) returns
993,987 sequences obtained predominantly through
Sanger sequencing (GenBank: https://www.ncbi.nlm.nih
.gov/genbank; accessed 12 March 2017). Thus, at pre-
sent there are almost 1,000 times more HTS reads than
Sanger sequences. Only 3 years ago, this ratio was 18:1,
which means it has increased by a factor of more than
50 in this short period.
Obviously, a proportion of the unidentified se-
quences can be expected to represent known but as yet
unsequenced species, especially considering that about
85,000 of the currently accepted fungal species have
no sequence data available. In the case of Mortierella,a
reference set of type and other strains was sequenced and
then compared with unnamed sequences in GenBank
(44); the authors modeled the effects of increasing type
strain sequencing and found a linear relationship with
the number of strains that could be identified, and they
predicted a number close to that of the already described
species.
One of the most significant problems with using en-
vironmental sequence data to extrapolate global fungal
species richness is the approach to define OTUs. Due
to the volume of data generated and the usual broad
taxonomic range detected, alignment-based methods
that would allow us to critically define species as phy-
logenetic lineages are impossible to use as routine tech-
niques. Thus, environmental sequence data are analyzed
using clustering techniques, and OTUs as equivalents
to species are defined on thresholds, ranging between
95 and 99% depending on the study. This approach has
several shortcomings. First, species do not exhibit fixed
sequence thresholds; in contrast, sequence divergence
and hence the barcoding gap depend on the evolu-
tionary age of the species complex. Second, commonly
used thresholds of 95 to 97% are unrealistic. Assuming
that the ITS barcoding locus has a length of, e.g., 500 to
600 bases, 5% divergence would correspond to 25 to
30 bases. In most studied species, infraspecific ITS var-
iation is much lower, with an average of about 1%.
Theoretically, a more realistic threshold of 99% would
greatly increase the number of OTUs detected in a data
set. However, this effect is more than outweighed by
the sensitivity of clustering methods to variation in
the data, including the common CAFIE (carry-forward-
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incomplete-extension) sequencing error common to all
base flow technologies (45) (see below).
This error and other problems with clustering tech-
niques can lead to an overestimation of the number
of OTUs at several orders of magnitude, which may
result in serious bias when using such estimates for
global extrapolations. We tested this effect with SRA
data of the soil fungus Archaeorhizomyces (Smith and
Lücking, unpublished). Through a blast script, we ob-
tained 106,563 ITS reads from the SRA that were rea-
sonably close to the type species, Archaeorhizomyces
finlayi. Clustering analysis using USEARCH (46) re-
sulted in the following numbers of OTUs depending on
the set threshold: 2,658 at 95%, 5,793 at 97%, 10,630
at 98%, and 28,435 at 99%. Thus, the difference be-
tween the 95% and the 99% threshold is an order of
magnitude. Alignment-based phylogenetic analysis sug-
gests the presence of approximately 500 species-level
lineages in this data set. Thus, depending on the thresh-
old level used for species-level separations, clustering
could overestimate the actual species richness in this case
by a factor of between 5 and 50.
Lücking and colleagues (45) showed that severe prob-
lems with estimating OTUs through clustering emerge
from even minor sequencing errors. The CAFIE error is
common to all next-generation sequencing technologies,
because it represents a statistical error of the distribu-
tion of bases among wells on a plate through a given
base flow. Through careful adjustment, this error can be
held to a minimum of less than one erroneously phased
insertion per read, i.e., about 0.3% assuming an ITS1
or ITS2 read length of approximately 300 bases. This
proportion is well within the expected infraspecific
variation even at a high level of 99% similarity and thus
theoretically should disappear as background signal.
However, the random distribution of this error across
the entire sequence, which cannot be recognized by
quality filtering methods, especially not in environmen-
tal samples where no expected sequence patterns exist
for comparison, causes clustering methods to interpret
these differences as taxonomic, leading to serious over-
estimations by distribution sequences of the same taxon,
but with different minor errors, into separate clusters.
This is because clustering methods work with local
alignments of small, highly similar sequences, whereas
phylogenetic methods require alignment of sequences
of a data set to template sequences obtained from
Sanger sequencing. As a consequence, in a broad align-
ment including Sanger reference sequences, erroneous
and phased indels will be located in mostly gapped
columns, which have practically no influence on the
resulting topology, whereas in clustering methods, they
will frequently be aligned with non-homologous base
calls and falsely interpreted as substitutions (45).
In the above (45), a proportion of less than 1%
erroneously phased indels caused by CAFIE errors in
the ITS of a single species caused overestimation of the
number of taxa by a factor of 35 at a 95% threshold
level, 137 at 97%, and 980 at 99%. Thus, instead of
disappearing as background signal, the sequencing error
had a tremendous effect on OTU evaluation when in
that case it was known that all sequence data came from
a single target species. Even after removing all erroneous
indels, clustering continued to overestimate species rich-
ness by a factor of 4 at a 95% threshold level, 8 at 97%,
and 116 at 99%, due to minor remaining infragenomic
variation and other sequencing errors. In contrast,
alignment-based phylogenetic analysis of a broad set of
nearly 2,000 reads recovered a single species, even with
all sequence errors included. Thus, while environmental
sequencing continues to accumulate a vast amount of
important data on unrecognized fungi, extrapolations
from these data must be taken with great care when
based on clustering methods. An extreme example is
a recent study which predicts up to a trillion species
of microorganisms globally based on OTUs derived
from environmental sequence data (1), using ecological
scaling laws which, put in simple terms, predict how the
global, limited number of Nindividuals would divide
into Sspecies, taking into account resource competi-
tion based on body size and other factors. This number,
while certainly intriguing, appears to be exaggerated,
and it would be interesting to see what alternative pre-
diction would be obtained when defining the underlying
taxonomic entities in this analysis through alignment-
based phylogenetic methods.
In spite of these shortcomings, it is clear that envi-
ronmental sequencing is now the major source of dis-
covering novel fungal taxa, at least in the same order
of magnitude as taxa recovered in species recognition
studies or perhaps much higher. It follows that to prop-
erly catalog these fungi, a naming system for voucherless
sequences must be developed, and proposals on how to
do that are currently under consideration (47–51).
In addition to discovering new species, environ-
mental sequencing may reveal unknown higher fungal
taxa up to the class level. The best-known example is
the recently discovered class Archaeorhizomycetes (31,
52). In Tedersoo and colleagues’study (37), ∼6% of
all fungal OTUs below the phylum level could not be
assigned to any known class. Further clustering of un-
identified fungal sequences at 70% sequence similarity
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revealed 14 distinct taxonomic groups comprising >7
OTUs, suggesting that there are several deeply divergent
class-level fungal lineages that have not yet been de-
scribed or previously sequenced. A follow-up study by
the same group (53) also revealed numerous new line-
ages at least at the order level.
WHERE ARE THE UNDESCRIBED FUNGI?
If there are at least 1.5 million or more fungi on Earth,
and we know of just 120,000, where can the missing
1.38 million species be? As outlined, three major sources
for unrecognized fungal diversity seem to emerge: (i)
geographic areas and ecological habitats that are largely
understudied, particularly in tropical regions and bio-
diversity hot spots, (ii) ecologically cryptic fungi that
occur in the environment but usually do not manifest
themselves via discernible structures other than micro-
scopic hyphae and mycelia, and (iii) so-called cryptic
species hidden under well-established names.
Biodiversity Hot Spots
It is generally recognized that missing species of all
groups of organisms will be found in biodiversity hot
spots (54). A novel quantitative method to identify hot
spots of unrecognized species richness is the grid ap-
proach (6–8), which relies on linear interpolation and
hence gives more reliable estimates than nonlinear ex-
trapolation. The area in question is divided into grids,
and observed richness per grid is linearly correlated
with predictive macroenvironmental parameters and
sampling effort, using at least one well-sampled grid for
calibration. This method can be applied to both tradi-
tional inventories and data based on environmental
sequencing, as long as species are reliably delimited
and macroenvironmental predictors can be reason-
ably derived from the data. Thus far, the method has
been employed for lichen-forming fungi in the families
Graphidaceae and Trypetheliaceae and the genus Cora,
resulting in estimates ranging between 1.5 and 4 times
the number of known species.
South America is generally recognized as having
major hot spots, and various studies point to high levels
of novelty there. By combining data from basidiomes
and sequence data from ectomycorrhizal roots in a
Dicymbe corymbosa forest in Guyana, Henkel and col-
laborators (55) estimated that over 250 species were
associated with this one tree species in this single
site. López-Quintero and colleagues (56) found that of
632 macromycete species in Colombian Amazonian
forests, 52% could not be identified to the species level,
a significant proportion likely representing undescribed
species. Truong and coworkers (57) collected 1,430
basidiomes during just four collecting expeditions in
southern South American Nothofagaceae-dominated
forests; they generated 439 OTUs out of 957 specimens,
of which 308 (ca. 70%) did not match any in the UNITE
dynamic database at 97 to 99% similarity and thus did
not correspond to any “species hypothesis”currently in
the database.
Little-Explored Habitats
There are an enormous number of potential sites for
fungi in any locality, and in the case of the tropics at least
31 ecological niches meriting study can be recognized,
many requiring different techniques and specialists to
explore (58). While no site on Earth can yet be consid-
ered to have a complete inventory of the fungi present,
some habitats stand out as particularly underexplored.
The situation can be exemplified by the fungi which
obligately grow on lichens, the lichenicolous fungi. With
a few notable exceptions, this niche was largely over-
looked by lichenologists and other mycologists until the
mid-1970s. In 1976, there were just 457 species known
worldwide (59), but by 2016 that number exceeded
1,800 (http://www.lichenicolous.net/); that represents
a 394% increase over 40 years, or 98.5% per decade.
This growth is reflected at the more local level, where in
Great Britain and Ireland the number known rose from
just 218 species in 1983 to 403 in 2003, an increase of
85% over 20 years (60); the number has now passed 500
and continues to rise (D. L. Hawksworth, unpublished).
That figure compares with around 1,900 lichenized spe-
cies in the same region, implying by extrapolation to
the global scale that there may be as many as 25% as
many lichenicolous fungi as lichenized species, i.e.,
around 5,000 species. This hypothesis is supported by
studies of these fungi in areas where they have not pre-
viously been investigated; for example, of 189 species
reported from Isla Navarino in Chile on the basis pri-
marily of collections made in just 1 year, 6 genera and
60 species (32%) were new to science (61). And these
figures do not include the so-called endolichenic fungi
that can be cultured from lichen thalli or are only known
from sequence data (e.g., 62,63). In addition, emerging
studies suggest the presence of highly specific, yet mor-
phologically cryptic, basidiomycete yeasts and other
basidiomycetes in the cortex of lichen thalli, which add
yet another dimension to unknown fungal diversity (32,
51).
The experience with lichenicolous fungi appears or
can be expected to be paralleled in other little-explored
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habitats, including bryophytes (64–66), algae (67,68),
endophytic fungi inside vascular plants (69–71), tropical
foliicolous and fungicolous fungi (72), mammal guts
(73), insect guts (74–77) and exoskeletons (78,79), on
and in rocks (80,81), and in deep sea and ocean sedi-
ments (82), to name just a few.
Cryptic Species
Biologically distinct species which are morphologically
indistinguishable, so-called cryptic species, were recog-
nized 2 decades ago as one place where some of the
predicted “missing fungi”might be discovered, since
physiological and other biological divergence evidently
often precedes morphological divergence in the evolu-
tion of fungal species. Based on studies of selected com-
plexes then available, it was suggested that the number
of known fungi might rise by a factor of 5 or more for
this single reason (83). Molecular studies reveal that
cryptic speciation is widespread through diverse groups
of fungi, including plant pathogens, clinically impor-
tant fungi, lichen-forming fungi, and mushrooms. Re-
cognition of such cryptic speciation is indeed playing
an increasingly important role in species discovery, as
outlined above, and the ratio derived from current
studies, 11.3:1, is about twice as high as estimated by
Hawksworth and Rossman (83).
Existing Reference Collections
It has been estimated that over half of the projected
70,000 undiscovered plant species have already been
collected and await description (84). Similarly, most
reference collections of fungi, whether of dried speci-
mens (fungaria) or living cultures (culture or microbial
resource collections), include material that is not named
to the species level, and in some cases not even to the
genus level. Further, some collections and isolates re-
ferred to named fungi not uncommonly prove,on critical
examination, to be different species. It has been sug-
gested that around 20,000 fungal species have been
collected but are as yet undescribed (83), but that is likely
to be an underestimate because many mycologists have
large backlogs of material awaiting formal description.
One of the challenges in the linking of newly obtained
sequence data with named material in collections, par-
ticularly those that are type material, is fixing the ap-
plication of the species name. Currently, the 34,878
species with sequences in GenBank (see above) repre-
sent just 29% of the 120,000 known species. Obtaining
sequences for the missing 71% of named accepted spe-
cies is essential to determine the novelty of recovered
sequences from the environment, specimens, or isolates.
While some researchers have had considerable success in
recovering DNA from dried specimens (85), even from a
dried Hygrophorus basidiome collected as far back as
1794 (86), this is not always possible (87). Aged dried
material generally exhibits high levels of DNA degra-
dation, making access through conventional extrac-
tion and PCR methods difficult (88,89). In view of the
serendipity of obtaining satisfactory results, collection
curators are often unwilling to allow parts of often
scant, irreplaceable type specimens to be destroyed for
DNA extraction. Several attempts are now being made
at using high-throughput sequencing technologies to
obtain sequence data from highly fragmented DNA
(88). One of the main problems in this respect is con-
tamination, since even for highly variable loci, it will
be a challenge to piece small fragments together with-
out the risk of generating artificial chimeras. Therefore,
efforts should be made to identify highly diagnostic re-
gions in the ITS or other loci that allow the identification
of a taxon even when only short fragments are at hand
(89), as exemplified by comparing the ITS1 and ITS2
subregions (90).
There is a particular problem regarding the type
species of generic names. An impressive 5,317 generic
names of fungi are represented in GenBank (C. L.
Schoch, personal communication), but in many cases
the sequences are not from the name-fixing type spe-
cies. A concerted attempt has been initiated to sequence
the type species of genera currently not represented in
DNA databases. This involves recollection and/or iso-
lation from the geographical area of the original material
and from the same host or substrate when no DNA has
been recovered from the type material (91). Sequenced
material of species can then in some cases be designated
as an interpretive type—an epitype (92). Such an effort
involves global collaboration of sequencing laborato-
ries with colleagues in countries where high unknown
diversity is located.
TOWARD A WORKING NUMBER OF
GLOBAL FUNGAL SPECIES RICHNESS
There is general acceptance among mycologists that the
global number of fungal species is a seven-digit figure, in
the range 1 to 5 million (10), with “at least”1.5 million
now predominating, though a possible order of mag-
nitude higher has been hinted at (93) and a minimum
figure of 611,000 to 712,000 was arrived at (2,94),
leaving aside the figure of potentially billions of fungi
extrapolated by Locey and Lennon (1). A recalcula-
tion of the number computed via Mora and colleagues’
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approach (2) gives a figure of about 1.4 million (see
above), close to the 1.5 million estimate which has come
to be widely accepted by other biologists (54,95–97),
notwithstanding a few estimates that fail to appreciate
the difficulty of inventorying fungi (98) or that equate
numbers detected in soil alone as indicative of the total
number at a site (37).
Unfortunately, the different techniques used to esti-
mate global species richness are in part additive and in
part overlapping or redundant, so that a combination of
estimates from these studies is difficult. Thus, the revised
estimate of nearly 3.8 million fungal species based on an
updated fungus:plant ratio of 9.8:1 and 380,000 vas-
cular plant species should include fungal species of all
sorts with all types of ecologies and detected by different
methodologies, including environmental sequencing and
species delimitation methods. This number should then
be congruent with approaches such as publication rates
or scaling methods, but these approaches result in widely
disparate estimates. Alternatively, an additive approach
would be based on the three major sources of unrecog-
nized fungal species richness, which can be reasonably
sorted in descending order into (i) environmental se-
quencing, (ii) cryptic speciation, and (iii) other novel
discoveries through traditional field work. For cryptic
speciation, the above survey of established literature
results in a weighted ratio of about an order of mag-
nitude, i.e., more than 1 million additional species if
based on 120,000 previously known species. If novel
species from environmental sequencing are at least as
high in number, this would add at least another million.
We conservatively assume that other novel discover-
ies (excluding species delimitation methods and envi-
ronmental sequencing) will occur at a proportion of at
best 10% of the latter two approaches. This assumption
is based on the rate of 1,300 newly described species
per year until about 2010, before the onset of rigorous
species delimitation methods and environmental se-
quencing (see above). At this rate, it would take nearly
100 years to double the number of known species. Thus,
while environmental sequencing and species delimita-
tion methods would contribute about 2 million new
taxa, other novel discoveries would add at best another
120,000 taxa within a reasonable time frame, for a total
of a little over 2.2 million. This would give a range of
between 2.2 (additive approach) and 3.8 million (global
ratio approach), very much in line with the previously
updated estimates of between 2.3 and 3 million (4,99)
and precisely narrowing down the range of 1 to 5.1
million given by Blackwell (10) by 1.2 to 1.3 million on
either side.
We therefore propose to replace previous estimates
of global fungal species richness with this updated
range of 2.2 to 3.8 million. This estimate is likely to
be further improved on when reliable statistical ap-
proaches to analyze the huge amount of environmental
sequence data become available. An interesting ap-
proach would be to combine the following techniques:
(i) a geographically and ecologically broad sample of
environmental sequence data, (ii) alignment-based spe-
cies recognition methods to properly estimate OTU di-
versity, (iii) species-based niche modeling to establish
macro- and microecological patterns, and (iv) a grid-
based interpolation of global species richness, taking
into account sampling effort. We predict that such an
analysis will result in estimates that might lie well above
the revised conservative estimate of 2.2 million species
and likely even beyond 3.8 million species. But even if
continuing to adopt the previous, now appearing highly
conservative number of 1.5 million species, the discovery
and formal description of the missing fungi remain a
daunting task.
ACKNOWLEDGMENTS
We thank Robert R. Dunn, Gareth W. Griffiths, Conrad L.
Schoch, and Brian M. Spooner for providing previously unpub-
lished data. We are especially indebted to Paul M. Kirk for pro-
viding the raw data from Index Fungorum, on which Figs. 1 to 3
were based.
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Fungal Diversity Revisited
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