Caddisflies (Trichoptera) of the Yukon, with Analysis of the Beringian and Holarctic Species of North America
GLENN B. WIGGINS and CHARLES R. PARKER
The Trichoptera recorded from the Yukon Territory now number 145 species, constituting 11 per cent of the North American fauna north of Mexico. Present distribution known for each species in the Yukon is outlined,
and biological information at familial and generic levels is briefly summarized. For biogeographic analysis, evidence bearing on the distribution of the species is considered under 4 categories. Members of category I are wholly Nearctic in distribution (98 species, 68 per cent of Yukon Trichoptera) and, in the absence of evidence to the contrary, are considered to have repopulated the Yukon and other northern areas from glacial refugia to the south of the Laurentideand Cordilleran ice sheets of Wisconsinan time.
Species of category II are Holarctic, and are now more or less widely distributed in Eurasia and northern North America (28 species, about 18 per cent of Yukon Trichoptera). These species could have passed the last glacial period in unglaciated Beringia, or to the south of the ice, or in both areas.
Category III is composed of Palaearctic species which, from evidence available, are now confined in North
America mainly to unglaciated Beringia or somewhat beyond (13 species, about 10 per cent of Yukon Trichoptera). Several represent a paradox of Beringian distribution—widely distributed Palaearctic species, evidently successful colonizers when they entered North America but, with retreat of the ice, have not extended their Nearctic range. Geological and biological factors underlying this paradox are discussed. Two Palaearctic species are recorded from North America for the first time: Rhyacophila mongolica Schmid, Arefina and Levanidova and Limnephilus diphyes McLachlan.
Category IV comprises 8 species (including 2 additional species expected from the Yukon), about 4 per cent of the fauna, known mainly from the Yukon or from adjacent areas of Alaska or the Northwest Territories; these species are considered to be Beringian endemics or glacial relicts. Finally, because almost all of the Holarctic Trichoptera now recognized in North America are reviewed in the foregoing groups, the remaining Holarctic species that do not occur in Beringia are considered briefly in a fifth category, although they have not been recorded from the Yukon and most do not appear to be species of far northern latitudes. The origin of the Trichoptera of Greenland is also discussed.
Ecological factors influencing the northern penetration of Yukon and Beringian Trichoptera are considered with an analysis of lotic and lentic-dwelling species through a latitudinal gradient of 49° to 70°N—from the southern border of British Columbia to the Arctic coastline of the Yukon. At latitude 60°N, the southern boundary of the Yukon, diversity has declined by almost 50 per cent from levels obtaining in British Columbia, 49° through 60°N.
The main depletion occurs in the Spicipalpia and filter-feeding Annulipalpia; case-making caddisflies of the
Integripalpia show less reduction. Similar trends are continued through the Yukon from 60° to 70°N, where species diversity in the Trichoptera declines by another 59 per cent. Although most North American Trichoptera occur in running waters, there is a marked reduction of species in these habitats with increasing latitude. Of 60 species recorded in the Yukon north of the Arctic Circle (67°–70°N), 81 per cent are Integripalpia with case-making larvae living mainly in lentic habitats. Factors underlying the decline of lotic species, and the proportional increase of lentic species at higher latitudes are considered. Trichoptera of lentic habitats were much more successful in crossing the Bering land bridge than were species dependent on lotic waters. Taxonomic changes resulting from this study include suppression of Grammotaulius subborealis Schmid as a junior subjective synonym of G. alascensis Schmid. The status of Limnephilus fumosus Banks is clarified as a species distinct from Limnephilus santanus Ross, and a lectotype is designated for L. fumosus; L. isobela Nimmo is recognized as a junior subjective synonym of L. fumosus Banks. Goera radissonica Harper and Méthot, described from northern Quebec, is recognized as a junior subjective synonym of Goera tungusensis Martynov, originally described from Siberia. A morphological variant of Ceraclea nigronervosa (Retzius) is described.