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.