Range Expansion

Atalopedes campestris has a dynamic biogeographic history (Fig. 1.1). Its historical northwestern range is documented with collection

'CobtMiUiun by County

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300 400 600 goo Ifg) Kilometers

Figure 1.1. Range map of Atalopedes campestris in the western United States. The stages of range expansion are depicted by shading. The range map is modified (shading and dates added) from Opler (1995) by incorporating collection records from J. Hinchliff, J. Emmel, A. Shapiro, J. Pelham, and L. Crozier.

records since the mid-1800s, with which I have re-created the history of the western part of the range. In the first half of this century, the northern range edge lay in the Sacramento Valley region in California. Oregonian lepidopterists documented the invasion of Oregon in the 1960s (Dornfeld 1980). A. campestris first established at the base of the Willamette Valley (Eugene and Corvallis, Oregon) in 1967; it appeared in Salem, Oregon, in 1978, and finally Portland, Oregon, in 1986 (J. Hinchliff, collection records). Instead of continuing directly northward west of the Cascade Mountain Range, A. campestris moved eastward 335 km along both sides of the Columbia River (via White Salmon, 1990) to the Tri-Cities,1 Washington (first record of a population was in 1993). Only two areas in Washington maintained populations at the beginning of my research in 1996:Vancouver in southwestern Washington contiguous with Portland, Oregon, and the Tri-Cities in southeastern Washington (pers. obs., Hinchliff 1996).Vagrant individuals appeared for the first time in many other far northern sites in 1991, including Wisconsin, Minnesota, North Dakota (McKown 1992), and Manitoba, Canada (Taylor 1993), but failed to establish populations. Evidence of longdistance travel is particularly important in the context of rapid climate change, because the paleoecological record shows that long jumps to new colonies may be necessary to explain rapid migrations that occurred in the past (Shapiro 1993, Pitelka and Plant Migration Workshop 1997, Clark et al. 1998).

In September 1998, at the peak of the flight season, I conducted a survey for Atalopedes campestris along the 125 km route northwest from the Tri-Cities to Yakima,Washington. Establishment in Yakima would mark the next major advance in the range expansion. Starting from a known population in the Tri-Cities, I stopped every 8 to 16 kilometers along Interstate Highway 82 to search for Atalopedes. I found substantial numbers of both males and females dispersing along the entire route, producing many first records of this species in Yakima County. Physical barriers apparently do not prevent this species from colonizing Yakima.

Range-limiting factors tend to fall within several categories (for recent reviews see Gaston 1990, Hoffmann and Blows 1994, Brown 1995, Brown et al. 1996). I will address specific hypotheses in each of the three major classes: geographical barriers to dispersal, biotic interactions, and abiotic factors.

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