Everything is not Everywhere (Fig. 6)
To sum up, our analyses discussed above suggest that the great majority
(72%, 46/64) of Milnesium species have restricted distributions,
indicating that Everything is not Everywhere and that natural LDD
events are rare. Overall, only 28% (18/64) species indicate LDD, either
ancient (17%, 11/64) or recent (11%; 7/64); see Tables 4 and 5 for
details. Moreover, after discarding examples that are likely to be the
results of anthropogenic dispersal (species #1, #3, #26, and #54,
all LDD), and cases of dispersal between localities close to the borders
of adjacent zoogeographic realms (i.e. the Afrotropic and the Madagascan
realm in Africa, the Neotropic and Panamanian zones in South America,
and the Australian and Oriental realms) and treat them as examples of
SDD rather than LDD (species #33, #40, #46, #53, and #55), then we
end up with only 14% (9/64) species exhibiting likely natural LDD: 12%
(8/64) species with evidence for ancient LDD (species #16, #19, #23,
#54, #55, #56, #57, and #61) and 2% (1/64) species with evidence
for recent LDD (species #58). Thus, the great majority of these LDD
cases are examples of ancient dispersal and half of them happened in the
tropics and subtropics, meaning that LDD is generally rare and most
likely to occur in the zones affected by trade winds. In other words,
there seem to be evidence for natural (mainly ancient) LDD in only 14%
(9/64) species (Fig. 6).
The fact that we found multiple climate types within the majority of the
main clades (Figs 3–5), indicates that Milnesium species may
quickly adapt to new climates or are capable of dwelling in a wide
spectrum of environments. Either way, this indicates that geographic
ranges of Milnesium species should be even wider than the EiE
predicts (i.e. not limited to a single climate type). Thus, since our
data suggest that the great majority of species have limited geographic
ranges, climate is not the main factor limiting species distribution. In
other words, this underlines that Milnesium species exhibit
limited dispersal abilities and their geographic ranges are determined
mainly by paleogeography.
Similar patterns seem to emerge also in other tardigrade groups. For
example, the first phylogeny of an echiniscid genus BryodelphaxThulin, 1928 (Heterotardigrada) uncovered two clades: one restricted to
the Western Palaearctic and the other to the Orient (Gąsiorek et al.,
2020). Furthermore, Gąsiorek et al., (in review) who analysed 64
populations representing 25 species of another echiniscid,Pseudechiniscus Thulin, 1911, from 6 continents (8 realms) found
that the great majority of species are limited to single zoogeographic
zone and only two were found to have pantropical distributions. Finally,
among the several known species of the Echiniscus virginicusRiggin, 1962 complex, only one has a wide distribution and, again, it is
pantropical in character (Gąsiorek et al., 2019b). Thus, in line with
our study, works on heterotardigrades suggest that, in general,
tardigrades may have limited dispersal abilities and geographic ranges.
Moreover, if a tardigrade species happens to exhibit a broad geographic
distribution at all, it is usually a pantropical range, which indicates
that trade winds might be the key factor needed for LDD of these
animals. However, any categorical general conclusions would be premature
at this stage, because the accurate delineation of species geographic
ranges and detection of LDD requires thorough sampling, whereas detailed
molecular faunistic datasets concerning different geographic regions are
extremely limited for tardigrades. Thus, even though the present study
offers the largest sample size in terms of the number of sequenced
species collected in various zoogeographic realms of any tardigrade
genus to date, enlarging the dataset both in terms of taxonomy and
geography may alter some of the current key conclusions. For example,
the proportion of ‘inclusion species’ and species with broad geographic
ranges may increase and some of the ‘inclusion species’ may turn out to
be species that exhibit wide geographic ranges and lose their current
status. If this were indeed true, then it would mean that LDD is more
frequent than inferred from this study.
Finally, our results should be compared to other microscopic animals
with similar ecology, ideally coexisting with limnoterrestrial
tardigrades in the same habitats, such as moss and lichen-dwelling
rotifers and nematodes. Unfortunately, global biogeographic analyses are
not yet available for nematodes, but several studies concerning rotifers
have been conducted (e.g. Mills, Lunt, & Gómez, 2007 or Fontaneto et
al., 2008). The most influential study, Fontaneto et al. (2008), based
on worldwide sampling (although with a European bias) indicated that
rotifer species generally do not conform to the ‘Everything is
everywhere’ hypothesis, with the isolation by distance being present and
significant. However, many species were found to be widespread and some
were classified as cosmopolitan. In contrast to many widespreadMilnesium species, wide ranges of rotifer species are not
hypothesised to be human mediated (Fontaneto et al., 2008). Moreover,
whereas widespread tardigrade species are more often found to have
pantropical distributions (Gąsiorek et al., 2019b; Gąsiorek et al., in
review; this study) rather than disjunct temperate ranges in the
northern and southern hemisphere (Gąsiorek et al., 2019a), the
precedence of pantropical distributions has not been found in rotifers
(see Fontaneto et al., 2008; Jaturapruek, Fontaneto, Meksuwan,
Pholpunthin, & Maiphae, 2018), as some temperate widespread species
were found, e.g., in Europe, Africa and New Zealand. Importantly, we
have not identified such disjunct temperate distributions inMilnesium that are likely to be natural (see subsectionMacroevolutionary patterns above). It is premature to explain why
bdelloid rotifers have apparently greater LDD potential than apochelan
tardigrades, but most obvious could be body size (ca. 200 vs 700–1000
µm) and oviposition strategy (eggs laid freely into the environmentvs eggs laid in clutches into exuviae).