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The biogeography of Pacific Northwest butterflies

by timber press on May 8, 2018

in Natural History

Arctic Fritillary (Boloria chariclea) alights and delights

Animals and plants occur where they do, and don’t where they don’t, because of particular characteristics of the landscape in concert with their own adaptations and ecological amplitude. Learn more about the butterflies unique to the biogeographic regions of the beautiful Pacific Northwest with help from Robert Michael Pyle and Caitlin C. LaBar, authors of Butterflies of the Pacific Northwest.

“Rainfall, temperature, insolation (incoming solar radiation), soil moisture and topography [are] believed to be the main abiotic factors influencing the survival, growth and range limits of insects” (Singh Bais 2016). Our region has plenty of each. Distribution is not random or arbitrary, and it is not always easily understood. Why the Woodland Skipper (Ochlodes sylvanoides) and the Purplish Copper (Lycaena helloides) are nearly ubiquitous throughout Cascadia, while the American Copper (L. phlaeas) and the Mardon Skipper (Polites mardon) occupy minute portions of the region, can be answered only through close attention to these species’ preferences, habits, and limiting factors alongside the region’s ecological offerings, geological record, and human history. Biogeography, to me, offers one of the most engrossing and adventuresome avenues for butterfly study.

The natural subdivisions of the Northwest have been described geologically (McKee 1972) and botanically (Franklin and Dyrness 1988). In 1974 (WWB), I proposed a series of butterfly zones that roughly equated with geographic parameters. Later, I examined and tested a series of butterfly provinces based on analysis of some 10,000 distributional records (Pyle 1982). With the 50,000+ records on which the atlases of Oregon and Washington butterflies are based (Hinchliff 1994, 1996), and thousands more accumulated since, a reanalysis might suggest refined and somewhat altered patterns. But the general idea holds up: Cascadia consists of a mosaic of landscapes, each with its own particular face and influences in terms of butterflies.

Close-up of a butterfly’s head

Such studies reveal units of the countryside with distinctive, though overlapping, butterfly faunas. By picturing these, we can look for species that “ought” to be in a given area, or ask why expected elements might be missing, and thereby discern important conservation priorities. The makeup of a province’s butterfly assemblage is determined by a delicate blend of adaptation, ecology, geology, and human use, which together constitute its biogeography. Therefore, the North Cascades will exhibit a fauna tolerant of high elevation, heavy precipitation, and a history of logging, fire, and grazing, with species drawn from Canadian, Far Northern, and to a lesser extent, Rocky Mountain and Sierran elements; whereas the Columbia Basin fauna tolerates drought, wind stress, and still more grazing and agriculture, and has a Great Basin complexion to its makeup.

Ecogeographic Provinces of the Pacific Northwest

Vancouver–San Juan Islands

While northern Vancouver Island contains heavy forests and high mountains like the British Columbia Coast Range, the southern portion is similar to Washington’s San Juan Islands. Lying in the rain shadow of the Olympics, these islands are much drier than surrounding mainland areas and proportionately more productive of butterflies. Garry oak, madrona, and Douglas-fir woodlands grow among extensive grasslands with mossy balds, floriferous headlands, and open summits.

Elements largely extirpated around Victoria, such as the Island Marble (Euchloe ausonides insulanus), still survive in parts of the San Juans. Others, like the Great Arctic (Oeneis nevadensis gigas), persist on Vancouver Island but have not been seen in the San Juans for decades. With nearby Whidbey Island, these islands stand in need of further energetic survey.

Oregon Silverspots (Speyeria zerene hippolyta)

North Cascades

Beginning somewhat arbitrarily at Snoqualmie Pass and extending north beyond Manning Provincial Park in British Columbia, the North Cascades span the crest of the range into the foothills on both the wet west side and dry east side. Mt. Baker and Glacier Peak stand out as volcanoes in a region largely made of ancient sedimentary deposits, folded and metamorphosed, with intrusions. Mt. Stuart imposes a very large granitic batholith.

The topography shows its age in the deeply dissected, low-gradient river valleys, with high relief all around. Extreme glaciation manifests in U-shaped valleys such as the larch-rimmed Methow, and scraped ridges ranging from 5,900 to 8,500 feet, with timberline reached between 5,500 and 7,000 feet. Douglas-fir/western hemlock forests underlain by salal on the west side grade into spruce/fir/whitebark pine highlands, then the rain shadow drops into ponderosa pine and eventually basin big sage on the eastern flank. Arctic-alpine habitats are narrow west of the crest, broader east.

Northern elements absent elsewhere in the region come into the North Cascades, such as the Labrador Sulphur (Colias nastes), Astarte Fritillary (Boloria astarte), and Lustrous Copper (Lycaena cupreus). Slate Peak is well known, but vast areas (e.g., the Picket Range and most of the Pasayten Wilderness) remain unexplored by lepidopterists. West of the Fraser River rise the biologically similar Coast Range mountains of British Columbia, sharing Vidler’s Alpine (Erebia vidleri) with the North Cascades.

Astarte Fritillary (Boloria astarte) at Slate Peak (North Cascades)

Okanogan Valley and Highlands

The Okanogan River makes a major corridor from the southern interior of British Columbia (where it is spelled Okanagan) into Washington. To the west, the land climbs into the Cascades. To the east rise the Okanogan Highlands, a mountainous expanse drained north-south by the Sanpoil, Colville, and Columbia rivers. Their valleys form aridland incursions into wetter uplands largely 3,000 to 4,000 feet in elevation, punctuated by a number of peaks over 7,000 feet. Partly Pre-Cambrian, outcrops are composed of a complex variety of volcanic, sedimentary, metamorphic, and intrusive rocks and their derivatives. Glaciation has been extensive. Ponderosa pine, lodgepole pine, and Engelmann spruce with true firs dominate the forested slopes, from bottom to top. Exotic species of weeds have overtaken many areas, but there is also much high country in largely natural condition.

The extensive wildlands have been little explored for butterflies, which are particularly diverse and numerous in the province. Lake Roosevelt has flooded the riparian Columbia shores, but many canyons remain. Pale Crescents (Phyciodes pallida), Meadow Fritillaries (Boloria bellona), and Tawny-edged Skippers (Polites themistocles) are Okanogan specialties.



Noted lepidopterist and writer Robert Michael Pyle is the founder of the Xerces Society for Invertebrate Conservation and the founding chairman of the IUCN/SSC’s Lepidoptera Specialist Group. A Yale-trained ecologist and a Guggenheim fellow, he is a full-time biologist and the author.



Pacific Northwest native Caitlin C. LaBar was born with a fascination for insects, which has developed into an interest in studying the habitats and life histories of butterflies. A geographer and GIS technician by training and a conservationist by nature, she enjoys photographing and collecting local butterflies and working on various butterfly mapping projects.

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