ORIGIN

Southern Ontario and southern Québec, like most of Canada, were covered with ice during the last (Late Wisconsinan) glaciation. Since there has been no speciation during the Pleistocene in any group of temperate or arctic beetles (Campbell 1980), we must assume that the present distributional patterns of the fauna of the Mixedwood Plains Ecozone result from changes in range following retreat of continental ice. Several authors (see Campbell 1980) have proposed the presence of an off-shore East Coast refugium during the last glaciation to explain the current distribution of some insects. Even if this refugium existed, there is no evidence to suggest that any of the carabids present in the Ecozone migrated from there. Undoubtedly, the most important refugium for the insect populations of southern Ontario and southern Québec was south of the ice sheet, in United States, east of the Rocky Mountains. As the ice margin retreated with climatic warming, the boreal elements and then the temperate elements shifted northward into Canada.

The species of Carabidae found in the Mixedwood Plains Ecozone have various patterns of distribution and their general distributional ranges may vary considerably. In general, at least four different patterns may be recognized among native species.

1. Eastern elements. These are more or less restricted to the deciduous forest formation of eastern North America. About half of the present fauna of the Mixedwood Plains Ecozone (240 species) belongs to this type. The Canadian distributional ranges of these species vary considerably. Some species are restricted to the Carolinian zone (e.g. Carabus goryi Dejean; Scarites subterraneus Fabricius; Clivina bipustulata Fabricius; Loxandrus velocipes Casey; Myas coracinus Say; Pterostichus permundus Say; Pterostichus atratus Newman; Amara exarata Dejean; Oodes amaroides Dejean; Chlaenius pusillus Say; Dicaelus purpuratus purpuratus Bonelli), some reach their northern limit on the continent in the Mixedwood Plains Ecozone (e.g. Brachinus janthinipennis Dejean; Schizogenius lineolatus Say; Bembidion americanum Dejean; Lophoglossus scrutator LeConte; Pterostichus stygicus Say; Oodes americanus fluvialis LeConte; Dicaelus politus Dejean; Cymindis americana Dejean), while other extends north of the Ecozone (e.g. Notiophilus aeneus Herbst; Cicindela sexguttata Fabricius; Sphaeroderus canadensis Chaudoir (Fig. BE-1); Sphaeroderus stenostomus lecontei Dejean; Clivina americana Dejean; Pterostichus coracinus Newman; Olisthopus parmatus Say).
   

   Most of the species in this category are rather widely distributed within the limits of the deciduous forest, but some are more or less restricted to the Appalachian Mountains (e.g. Scaphinotus viduus Dejean; Bembidion simplex Hayward; Bembidion basicorne Notman; Pterostichus diligendus Chaudoir). In the Mixedwood Plains Ecozone, Appalachian species are usually restricted to southern Québec.

2. Transcontinental elements. About 20% of the fauna of the Mixedwood Plains Ecozone are transcontinental species, or almost so, being distributed without major gaps from the Atlantic Coast to the Rocky Mountains or to British Columbia and Alaska (e.g. Trechus Motschulsky; Bembidion salebratum LeConte; Pterostichus luctuosus Dejean; Platynus apicalis mannerheimi Dejean; Agonum mutatum Gemminger & Harold; Agonum metallescens LeConte). These species are mainly associated with the boreal zone. Eleven of the transcontinental species (e.g. Dyschirius politus Dejean, Bembidion transparens Gebler, Amara lunicollis Schiødte, Agonum thoreyi Dejean) are also represented in the Palaearctic region and may be considered as genuine Holarctic elements. I know of no species of Carabidae that reaches their southern limit in the Ecozone; most of these species display southern extensions, often both in the east and in the west.

3. Western species. The species in this category are broadly distributed in western North America; they invaded, most of them only recently, the Mixedwood Plains Ecozone (e.g. Agonum corvus LeConte; Harpalus fuscipalpis Sturm; Elaphropus dolosus LeConte; Bembidion rupicola Kirby; Bembidion obscurellum Motschulsky, Bembidion bifossulatum LeConte, Chlaenius purpuricollis Randall). At least one of these species, Harpalus fuscipalpis , is Holartic in distribution. One western species, Cymindis planipennis LeConte, could have a relict population in the Mixedwood Plains Ecozone. The species is widely distributed in the west, east to southern Manitoba. There is one record from southern Québec based on a single specimen. Although the identification is beyond doubt, it is nevertheless possible that the specimen from Québec is mislabelled.

4. General elements. About 120 species of Carabidae occurring in the Mixedwood Plains Ecozone belong to this category; they represent 25% of the entire fauna of the Ecozone. The distributional ranges of these species are so extensive in North America that their origin cannot be recognized.

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