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).