Trechus eremita, Schmidt, 2009

Schmidt, Joachim, 2009, Taxonomic and biogeographical review of the genus Trechus Clairville, 1806, from the Tibetan Himalaya and the southern central Tibetan Plateau (Coleoptera: Carabidae: Trechini) 2178, Zootaxa 2178 (1), pp. 1-72 : 65-67

Trechus species range extensions and range evolution

The distribution maps indicate the limited distribution of all wingless Trechus species ( Figs. 98–99 ). It is apparent that only a few species of the T. thibetanus group have ranges larger than 100 km in extent that span several extensive high valleys ( Fig. 98 , numbers 1–7 in white circles). The ranges of all other Trechus species seem to be restricted to the slopes and upper valleys of their respective rivers. In several instances the ranges are extremely restricted, i.e., the closely related species T. eremita sp. n., T. franzianus Mateu & Deuve, 1979 , and T. muguensis sp. n. of the T. franzianus group occur in the Western Nepal Himalaya allopatrically in three adjacent high valleys on the north-western slope of the Kanjiroba Massif ( Fig. 98 , numbers 10–12 in white squares). That these ranges are actually this small and not an artifact of collecting bias can be ascertained by the extensive field work focused on the beetle fauna of Western Nepal. None of the species have been found sympatrically. Events of in situ speciation following the geographical separation of the range of the ancestral species can therefore be postulated. This is most probably true for all species of the T. franzianus group since the members exhibit a strong geographical vicariance ( Fig. 98 , white squares).

With the exception of the T. thibetanus group the same can be said for the Southern Central Plateau. All Trechus species occurring between the Yarlung Zhangbo Valley and the main mountain chain of the Nyainqentanglha Shan were only found in a single study area ( Figs. 99 , 100 ). Whether the ranges of these species are actually restricted to a single side valley along one of the main rivers on the plateau or occur also in adjacent side valleys should be investigated with additional field work close to sites already sampled. What can be said with certainty is that none of the ranges of any species investigated extend beyond the slope of the massif. If this had been the case field work would likely have discovered them in at least one other field site, which has not happened once south of the main chain of the Nyainqentanglha Shan.

This degree of narrow endemicity is apparently not expressed by species on the central parts of the plateau, which can be exemplified by the position of the collecting sites of T. antonini Deuve, 1997 . This species is the north-easternmost species of the T. antonini group in the Nyainqentanglha Shan, which occurs on the northern slope of the massif as well as at the nearby Lhachen La pass ( Fig. 100 , number 1 in white circles). In this part of the mountain chain the ridge of the Nyainqentanglha Shan reaches lower elevation so that the passes over it are situated in the alpine zone. The present range of T. antonini might therefore be the result of a Holocene range expansion from the southern slopes of the Nyainqentanglha Shan to its northern slope.

Effective barriers for an expansion of strictly edaphic Trechus species might have occurred along the southern slope of the western parts of the Nyainqentanglha Shan for quite some time. Strong vicariance of the allopatric ranges is exhibited for the proposed sister species pairs T. budhaensis sp. n. - T. yeti sp. n. and T. astrophilus sp. n. - T. lama sp. n., the two geographical subspecies of T. yak sp. n. as well as the terminal species group of the T. antonini group: T. folwarcznyi Deuve, 1997 - T. rarus sp. n. - T. singularis sp. n. - T. tsampa sp. n. This can be explained by in situ speciation caused by geographical separation and lack of subsequent range expansion.

With the exception of the T. thibetanus group a distinct local endemism can be found in the Tibetan Himalaya as well as on the southern Tibetan Plateau. Since it has been shown during the carabid beetle surveys between the Yarlung Zhangbo Valley and the main mountain chain of the Nyainqentanglha Shan that each of the relatively randomly selected collecting sites harboured a single or several distinct Trechus species , one may conclude that there are likely many more of these locally endemic species on the southern Tibetan Plateau. Whether this is also true for the western and central parts of the plateau is questionable.

That species of the T. thibetanus group show a larger distribution in general when compared to other Himalo-Tibetan Trechus species groups can be explained by different habitat preferences. Trechus species with the exception of the T. thibetanus group are more directly associated with wetter conditions in small snow-covered valleys, damp and shady slopes, and at the edge of springs or snow-melt channels. In the higher alpine zone these species occur on the summits because the dampness is still high due to temporary snow cover during the summer months. In the lower alpine zone these species live deep in the soil where they can be found in damp crevices of the rubble under large stones. More extensive eye reduction and depigmentisation of several species would support this hypothesis. The data indicate that the ranges of these Trechus species are much more fragmented, at least on the plateau, as this is known for species of the T. thibetanus group. The latter species are more tolerant of drier conditions because they have been found on lower alpine mats at a considerable distance away from water where they were found locally abundant under smaller stones together with phytophagous species of the Amara subgenus Bradytulus Tschitschérine, 1894 .

The potential habitats are larger and less isolated along the mountain slopes. In addition, one could speculate that, like the Amara species the imago spends a considerable time nocturnally on the surface. Species of the T. thibetanus group could therefore have a stronger dispersal potential than species of other Himalo-Tibetan Trechus species groups.

Figures

FIGURE 98. Map of central parts of Tibet and the Himalaya showing distribution of species of the Trechus thibetanus group (white circles), of the Trechus pumoensis group (white squares), of the Trechus rolwalingense group (black circles), and of the Trechus stratiotes group (black squares). 1, T. namtsoensis sp. n. 2, T. dongulaensis sp. n. 3, T. thibetanus Jeannel, 1928. 4, T. glabratus sp. n. 5, T. thorongiensis Schmidt, 1994. 6, T. eutrechoides eutrechoides Deuve, 1992. 7, T. eutrechoides mondaensis Deuve, 1997. 8, T. pumoensis Deuve, 1997. 9, T. tilitshoensis Schmidt, 1994. 10, T. franzianus Mateu & Deuve, 1979. 11, T. muguensis sp. n. 12, T. eremita sp. n. 13, T. aedeagalis sp. n. 14, T. sculptipennis sp. n. 15, T. rolwalingense rolwalingense ssp. n. 16, T. rolwalingense daldunglana ssp. n. 17, T. stratiotes stratiotes ssp. n. 18, T. stratiotes malikasthana ssp. n.

FIGURE 99. Map of Southern Central Tibet showing distribution of species of the Trechus wrzecionkoi group (white circles), of the Trechus dacatraianus group (white squares), of the Trechus solhoeyi group (black circle), and of the Trechus chaklaensis group (black square). 1, T. wrzecionkoi Deuve, 1996. 2, T. martinae sp. n. 3, T. korae sp. n. 4, T. damchungensis Deuve, 1997. 5, T. hodeberti Deuve, 1997. 6, T. bastropi sp. n. 7, T. mieheorum sp. n. 8, T. solhoeyi sp. n. 9, T. chaklaensis sp. n.

FIGURE 100. Map of Southern Central Tibet showing distribution of species of the Trechus antonini group (white circles). 1, T. antonini Deuve, 1997. 2, T. religiosus sp. n. 3, T. astrophilus sp. n. 4, T. budhaensis sp. n. 5, T. rarus sp. n. 6, T. yeti sp. n. 7, T. folwarcznyi Deuve, 1997. 8, T. yak yak ssp. n. 9, T. yak shogulaensis ssp. n. 10, T. tseringi sp. n. 11, T. singularis sp. n. 12, T. tsampa sp. n. 13, T. lama sp. n.