Sonora skipper (Polites sonora) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

Guppy and Shepard (2001) show only three (two overlapping) distribution dots for the entire Canadian range of P. sonora. Layberry et al. (1998) show four distribution dots, of which one (Merritt) is unconfirmed and is considered by Guppy and Kondla to be an error. Guppy and Shepard (2001) omitted the “Hope Mountains” record from their distribution map because of lack of locality precision, while Layberry et al. (1998) apparently mapped those specimens as being from “Hope”. Therefore there are four confirmed locations that have not been recently checked, two of which are on Crater Mountain. Kondla found two additional locations through focused search in 2003 (detailed data and search effort information not available), for a total of six locations.

The few Canadian records are from an area of southern British Columbia that has been frequently visited for purposes of recreational and research-related insect collecting for more than 100 years. There are many thousands of specimens and specimen records of butterflies for this area, in stark contrast to the few records of Polites sonora. It is therefore reasonable to conclude, solely on the basis of historical information, that Polites sonora is not a common species that has been consistently overlooked by many observers over many years. In contrast, Anna’s Blue (Plebejus anna or Lycaeides anna (W.H. Edwards, 1861), depending on taxonomic preference) is a common species in the same area as the Sonora Skipper and occupies natural mesic meadows and logged areas. Its flight period also overlaps that of the Sonora Skipper. Guppy and Shepard (2001) show 15-20 distribution records for Anna’s Blue in the same area as the known distribution of the Sonora Skipper. These distribution records are evidence that observers were present in the right habitats at the right time of year to have noticed the Sonora Skipper if it had been present.

In 2003, Kondla conducted presence/absence searches (no dates or location information provided) on 11 days during the flight period within the plausible range of this species. Searches were conducted in 10 areas comprising more than 40 sites in conditions suitable for butterfly flight activity. Two additional areas with multiple sites were searched under marginal conditions for butterfly flight, so presence or absence of the species in these areas is inconclusive. Search methodology consisted of walking through potential occupied habitat, stopping periodically, disturbing herbaceous and shrubby vegetation and observing. Flowers and puddling sites were also examined. Searches were made of moist, mesic and dry non-forested habitats. Sites were selected based on access practicalities and presence of suitable vegetation structure. One ‘site’ consisted of a 6-km trail transect through natural habitat with ample visually suitable habitat patches.

This focused searching resulted in confirming the presence of Sonora Skippers in two areas at four sites. This low frequency of occurrence within potentially suitable habitat is consistent with the lack of records and supports the interpretation that the Sonora Skipper is a highly localized species within its extremely limited Canadian range.

Abundance

There are no empirical data on the population sizes of this species in Canada or elsewhere. In 2003, at least 15 adult Sonora Skippers were seen at the Wolfe Creek site and at least 15 were also seen at the collective Placer Creek sites. All past collections are of one or a few individuals; no large populations are known. Kondla and Guppy did not do any quantitative survey work, but based on their experience and knowledge of butterflies in general, believe the total plausible population size in Canada to be in the order of 3,000 to 5,000. This is their best guesstimate and is not based on any “hard” data.

Fluctuations and trends

There is insufficient information to identify empirical or even estimated population fluctuations or trends for P. sonora in Canada.

Rescue effect

The Sonora Skipper is known to occur in Canada at six locations, each of which may represent a population that is more or less isolated from the other Canadian populations by areas of unsuitable habitat. Two of these populations are quite close together. The others are separated from the closest population by a distance of 10 to 65 km. Because of the fragmented nature of the species’ distribution, there is likely little or only limited exchange of individuals between most of the population clusters in Canada.

There are only a few known locations occupied by the Sonora Skipper in the northern Cascade Mountains of Washington (Hinchliff 1996), and there is an apparent 50-km disjunction between the nearest known of these populations and the nearest Canadian location. This apparent disjunction may be explained by the possible existence of inaccessible and unsurveyed suitable habitat in the 50-km area. Despite insufficient survey effort to determine whether the apparent disjunction is a sampling artifact, there is indirect evidence that the disjunction is real. A review of United States Geological Survey (USGS) topographic maps and satellite images indicates that the area of the apparent disjunction is entirely forested below the subalpine zone (about elevation 1900 m). Therefore the apparent lack of suitable habitat for the Sonora Skipper in the 50-km area indicates that there is a low probability of the skipper occurring at other, more northern sites in Washington State. South of the few known localities in the northern Cascade Mountains of Washington, there is another apparent disjunction of approximately 130 km in the known distribution of the Sonora Skipper in the US (Fig. 3). Consequently, natural re-establishment of Sonora Skippers from the known Washington locations, in the event that the species were to become extirpated from Canadian locations, is unlikely to occur.

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