Half-moon hairstreak (Satyrium semiluna) COSEWIC assessment and status report: chapter 7

Habitat

Habitat requirements

In 2003, the British Columbia population of S. semiluna was found only in sagebrush/bluebunch wheatgrass habitat with silky lupine (Lupinus sericeus) present, at an elevation from 600 m to 1100 m. Extant populations were found in areas that, at the landscape scale, had reduced relief in comparison to surrounding steeper areas (Figure 4). All sites where the butterflies were present also had flowering plants of common yarrow (Achillea millefolium) and umbrella-plant (Eriogonum heracleoides).


Figure 4. Typical Half-moon Hairstreak habitat in British Columbia– White Lake basin

Typical Half-moon Hairstreak habitat in British Columbia– White Lake basin.


Because the British Columbia population has not been found in all visually suitable habitat patches that have been searched, the population appears to be fragmented. Clusters of sites in the White Lake, Richter Pass and Anarchist Mountain areas are each considered to represent one population. Each of the other three sites is considered to represent a separate population. There is, however, substantial connectivity of sagebrush habitats which could allow movement between occupied sites. It is possible that these sites represent part of one large, fragmented meta-population. The one known exception is the White Lake basin which has a block of known and potential habitat that is disjunct at the landscape scale from other known habitat patches and is therefore not part of that meta-population.

Multiple sites were examined in the White Lake basin in 2003. A robust population of S. semiluna was found in lightly grazed sagebrush/grass habitat. In contrast, no Hairstreaks were found just across the road in very heavily grazed sagebrush/grass habitat. Hairstreaks were also not seen in areas where lupine was present but sagebrush was absent. It appears that the presence of sagebrush is an important habitat component in British Columbia. The minimum amount of lupine needed as a habitat component is unknown. No Half-moon Hairstreaks were seen at any British Columbia site with extreme livestock grazing.

The single Alberta population of S. semiluna occupies level grassland habitat at an elevation of about 1300 m on a coarse-textured alluvial fan with abundant Lupinus sericeus and flowering low goldenrod (Solidago missouriensis) and yellow umbrella-plant (Eriogonum flavum). Numerous areas within and near Waterton Lakes National Park at low to high elevations and with Lupinus present were searched but no additional Hairstreaks were found except for three individuals on hummocky moraine near the densely populated fan habitat (Kondla 2003a, 2004b). Furthermore, these three individuals outside the breeding habitat on the nearby alluvial fan were seen only in 2003 under very windy conditions. Examination of the same area in 2004 under calm conditions did not reveal any Hairstreaks. It is possible that the three individuals had been blown away from their normal habitat. Habitat in Alberta is not fragmented because there is only a single population of S. semiluna in a single habitat patch.

There may be an obligatory association of the larvae in the two Canadian populations of S. semiluna with specific species of ants. This potential relationship, if present, may be a key component of the habitat requirements of S. semiluna. Ants have specific habitat requirements, which may be different from butterfly habitat requirements, and hence two interacting sets of habitat requirements may determine the suitability of an area for S. semiluna.

Habitat trends

Except for periodic flooding and reworking of portions of the alluvial deposits, the Half-moon Hairstreak habitat in Waterton Lakes National Park is stable. The habitat is being invaded, however, by knapweed (Centaurea maculosa). Continuation of this process will alter the habitat either directly through plant competition or as a consequence of weed control measures.

Information on site-specific habitat trends is lacking for the British Columbia populations, except to note that one occupied site on Anarchist Mountain was burned by a wildfire in July of 2003. The condition of sagebrush sites at lower elevations was observed by Kondla to be generally poorer than at higher elevations. The quantities of six types of sagebrush grasslands where the Half-moon Hairstreak may occur in the Okanagan valley as well as the trends for these habitats are presented in Table 1. Many of these habitats have undergone substantial declines, ranging from 40% (for big sagebrush – bluebunch wheatgrass, the habitat type for the Half-moon Hairstreak) to none. However, several of the sites where the Hairstreak occurs are on lands recently acquired for protection purposes. Such protection will partially offset the trend of loss of sagebrush grasslands, only 50% of which remain compared to what occurred historically in the south Okanagan Valley (T. Lea, pers. comm.). The general increase in land use intensity in the south Okanagan-Similkameen area will likely cause further losses and degradation to the condition of the small amounts of these sagebrush grassland habitats that persist outside protected areas at both lower and higher elevations.

 

Table 1. Habitat trends for sagebrush grassland habitat types that potentially support S. semiluna. (Dyer and Lea, unpublished data, 2005)
Habitat type Total area occurring in 1995
(ha)
Total area occurring in 1800
(ha)
Change from 1800 to 1995
(ha)
% change
Big sagebrush-selaginella; very shallow soil 4 546 4 536 10 ~0%
Big sagebrush-bluebunch wheatgrass; mesic 2 000 3 321 -1 320 -40%
Bluebunch wheatgrass-arrow-leaved balsamroot; deep soil 7 671 7 776 -105 ~0%
Bluebunch wheatgrass-Sandberg’s bluegrass; deep soil 4 400 4 314 86 ~0%
Bluebunch wheatgrass-junegrass; mesic 1 829 2 454 -625 -25%
Bluebunch wheatgrass-selaginella; shallow soil 12 563 12 456 107 ~0%

Habitat protection/ownership

The Waterton Lakes National Park population of S. semiluna is protected by virtue of its location, and the significance of this population is now known to park officials, but the habitat is susceptible to the effects of knapweed invasion.

Most of the known British Columbia populations of the butterfly are located on private land where they do not receive any legal protection; however, some sagebrush grasslands that potentially support the butterfly are within conservation lands (Table 2). Some confirmed sites for S. semiluna occur within recently acquired sites, including White Lake and Mount Kobau (D. Fraser, pers. comm.). Some of the White Lake population is on federal land belonging to the National Research Council and leased to a private ranching family that is cooperating with the Biodiversity Ranch Program of The Nature Trust. Management of the land does not presently include specific consideration of S. semiluna habitat requirements, but the Ranch takes an integrated approach to managing ranching and other land uses to balance the habitat requirements of the many species at risk that exist there. Some of these may be complementary to the needs of the Half-moon Hairstreak (CWS, Pacific and Northern Region, pers. comm., 2006).

 

Table 2. Current ownership and conservation status of sagebrush grassland habitat types that potentially support S. semiluna (Dyer and Lea, unpublished data, 2005).
Habitat type Total
(ha)
Lands in conservation
(ha)
Provincial crown land
(ha)
Indian reserve
(ha)
Private land
(ha)
Big sagebrush-selaginella; very shallow soil 4 546 1 163 1 112 944 1 327
Big sagebrush-bluebunch wheatgrass; mesic 2 000 457 273 329 941
Bluebunch wheatgrass-arrow-leaved balsamroot; deep soil 7 671 966 1 589 2 628 2 488
Bluebunch wheatgrass-Sandberg’s bluegrass; deep soil 4 400 471 780 851 2 298
Bluebunch wheatgrass-junegrass; mesic 1 829 374 238 368 849
Bluebunch wheatgrass-selaginella; shallow soil 12 519 2 547 3 794 2 532 3 646

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