Species with broad geographic ranges – evidence for recent LDD?
Of the 27 species represented by more than one population in our data set, seven (26%) species were found in more than one zoogeographic zone (Figs 3–5 and Table 5). Such species with wide geographic ranges signify recent LDD events, but the question is whether these LDD examples represent natural or human-mediated dispersal. In our opinion, four (57%) of them are likely a result of anthropogenic dispersal:M. tardigradum (species #1, clade A), M. berladnicorum(species #3, clade A), M. inceptum (species #26, clade B), and a preliminarily delineated new species #54 (clade E). Specifically, the presence of Palaearctic species (#1, #3, and #26) in the Republic of South Africa is not surprising, as hundreds of invertebrate species have been unintentionally introduced from Europe to South Africa by humans (e.g. see Janion-Scheepers et al. 2016). The anthropogenic origin of these populations is even more probable, as M. tardigradum andM. berladnicorum were found in a single sample (ZA.040) and all four African populations of these two species were collected on the roadsides in the Western Cape Province, where the number of invasive species is among the highest in the RSA (e.g. Janion-Scheepers & Griffiths 2020). Interestingly, M. inceptum seems to be an example of a cosmopolitan taxon, as it has been reported from five continents and an oceanic island (Europe and Far East Asia in Morek et al. 2019b, Australia in Morek & Michalczyk 2020, and South Africa and New Zealand in the present contribution). In all five cases,M. inceptum was found in urban or rural areas, thus possibly it is a synanthropic species whose dispersal is facilitated by human activity, and it would not be surprising if the species is found in other localities around the World that are associated with human settlements. Finally, species #54 (cade E) seems to be a predominately Oriental taxon, with one of the six analysed populations recorded in the Oceanic realm. Importantly, however, the Oceanic locality (ID.711) comes from Indonesia, as further three populations do (ID.432, ID.940 and ID.950), although these are classified within the Oriental realm. Given that Indonesia is one of the most densely populated countries, with high rates of human travel between the numerous and closely arranged islands, the ID.711 population is likely to be an example of anthropogenic dispersal (from the Oriental to the Oceanic realm). Alternatively, given that the locality in the Oceanic realm is only 650 km away from the closest known locality in the Oriental realm, this could be also a case of natural SDD. Thus, we consider the four abovementioned species as most probably artificially dispersed and therefore not supporting the EiE hypothesis despite their wide geographic ranges.
The remaining three species with broad geographic ranges, i.e. preliminarily delineated new species #40 (clade C), #55 and #58 (both in clade E), could be examples of recent natural dispersal. Except sample ZA.360, they were collected in sparsely populated and/or places that are not popular among tourists (e.g. French Guiana or North Argentina), further decreasing potential of human mediated dispersion. However, populations representing species #40 and #55 were found in adjacent zoogeographic realms: the Afrotropic and the Madagascan realm (ca. 2000 km apart), and the Neotropic and the Panamanian realm (ca. 3500 km apart), respectively. Moreover, there is no geographic barrier in the latter case. Thus, even though the geographic ranges of species #40 and #55 span more than one zoogeographic realm, they probably should not be considered as examples of LDD, but rather as cases of SDD.
Importantly, determining whether recent dispersal events are natural or anthropogenic is notoriously difficult to test in tardigrades (Gąsiorek et al. 2019a), thus mechanisms behind recent dispersal events of all seven species discussed above (Table 5) should be treated as working hypotheses.