Half-moon hairstreak (Satyrium semiluna) COSEWIC assessment and status report: chapter 9

Population sizes and trends

Search effort

The plausible distribution of the British Columbia populations has been the subject of considerable historical field time by many people over more than 100 years (Guppy and Shepard 2001). Thirty-three apparently suitable sites were searched over 11 days by Kondla in 2003 (detailed site and search effort information not available) as part of presence/absence surveys conducted specifically for this species. Some areas were searched more than once before Hairstreak presence was noted. These searches resulted in Hairstreak presence being detected at only seven sites. Because a cumulative historical total for S. semiluna of only 11 sites is known, it appears that the British Columbia populations are highly localized and that the dearth of known locations is not a result of the species being persistently overlooked.

The area of occupancy of S. semiluna in Alberta is also within an area that has received a substantial amount of historical field time (Bird et al. 1995). In 2003 and 2004, Kondla examined more than 40 Alberta sites (detailed site and search effort information not available) and detected only one population in Waterton Lakes National Park.

Abundance

A preliminary estimate of the Alberta population of S. semiluna, based on counting butterflies and professional opinion, suggests a population in the range of 3 000 to 10 000 adults in 2003 (Kondla 2003a). In 2004, only about 250 adults were counted in the same population (Kondla 2004b).

The sizes of the seven British Columbia populations are unknown. The limited available information suggests that populations vary in size and that the total adult population could plausibly be in the order of 5 000 to 15 000, but this is a very crude estimate. The numbers of adults seen in British Columbia populations in 2003 are: Anarchist Mountain – 5, Blind Creek – 9, Kilpoola – 5, Richter Pass – 47, White Lake –56. Dennis St. John repeatedly surveyed the White Lake site in 2005, but found only two adults (O. Dyer, pers. comm.). Museum specimens are also of few individuals from each site. The examination of collection data from 37 Canadian collections and one USA collection revealed a total of 40 specimens, of which 19 are located in one collection. Thirty-three of the museum specimens are from Anarchist Mountain. Five of the remaining seven specimens may also have been collected on Anarchist Mountain, but the exact collection site of these specimens is unknown.

Fluctuations and trends

There is insufficient data to identify fluctuations and trends for the British Columbia population. The only suggestive information comes from historical and recent collections on Anarchist Mountain where the species has been found on a number of occasions. The number of specimens collected from this location over the years suggests that the species may have been more abundant there in the past. In 1975, a search by Guppy for a few hours on one day yielded nine adults. Although Guppy returned in 1976 and 1988, he did not find any adults in a few hours of searching each time. In 2003, intensive searching by Kondla on four days yielded only five adults.

For the British Columbia population as a whole, it is reasonable to suggest that infrastructure development, reduction in host plant abundance by livestock grazing, weed invasion and forest ingress into open areas subsequent to wildfire control has resulted in a smaller population of S. semiluna compared to pre-settlement times.

The Alberta population was examined by the same observers in 2003 and in 2004. In 2003, there were so many individuals that they could not be counted in any practical manner. A conservative population estimate is several thousands of individuals. In contrast, only about 250 adults were located in four days of field time in 2004. This massive reduction may have been caused by an unusually late killing frost in the area in the spring of 2004 (Kondla 2004b) (Appendix 1). Stochastic weather events may cause order of magnitude population size fluctuations in the Alberta population.

Rescue effect

Dispersal abilities for Satyrium butterflies are thought to be quite small, a few kilometres and generally less. There are populations of S. semiluna in Washington State that are apparently secure (S4) and contiguous with the British Columbia population (Guppy and Shepard, 2001) so that rescue of the latter could be considered possible. However, if the extirpation of population from a site is caused by habitat loss/degradation, then that site will not be recolonized until suitable habitat becomes once again available, and even then it may not happen if the source population is more than a few kilometres away.

Rescue is very unlikely for the Alberta population because there are no known nearby locations. There is a record of the species in Montana on the other side of the continental divide. However, this record is questionable because the area of the record is not typical habitat for S. semiluna (S. Kohler, pers. comm.). In any case, movement of S. semiluna across the continental divide is not plausible. The nearest known population of the butterfly along the east front of the Rocky Mountains in Montana is some 300 kilometres distant, and it would not likely be a natural rescue source for the Alberta population of S. semiluna.

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