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Recovery Strategy for Multi-Species at Risk in Maritime Meadows associated with Garry Oak Ecosystems in Canada (Proposed)


Species Descriptions

1.10 Island marble euchloe ausonides insulanus

Common Name: island marble

Scientific Name: Euchloe ausonides insulanus Guppy & Shepard

Status: Extirpated

Reason for designation: This butterfly was formerly found on two islands off the west coast, but has not been seen in Canada since 1908. It is presumed to have been extirpated by 1910.

Canadian Occurrence: British Columbia

Status history: Extirpated by 1910. Designated Extirpated in April 1999. Status confirmed in May 2000. Last assessment based on an existing status report.

1.10.1 The species

Euchloe ausonides subspecies insulanus is a well-delineated taxon as described in the COSEWIC Status report (Shepard 2000b). The species was recognized as a separate subspecies as described in Guppy and Shepard (2001).

The island marble is a white and greyish black butterfly with a marbled texture to the underside of the hindwings and black markings at the tips of the forewings. The veins on the hindwings are emphasized as yellowish lines, giving the species a marbled appearance. The sexes are similar but the females have darker yellowish marbling on the hindwings, and a yellow ground colour, compared to the white of the males. The body is covered with whitish-yellow hairs giving it a fuzzy appearance.

Figure 1. Approximate global and Canadian distribution of island marble
(Extant populations shown as stars, extirpated populations as triangles)
Figure 1. Approximate global and Canadian distribution of island marble (Extant populations shown as stars, extirpated populations as triangles)

1.10.2 Distribution

The subspecies Euchloe ausonides insulanus is isolated from the rest of the species' range. The species as a whole is found from Alaska south to California and Colorado and east to Ontario and Minnesota.

The current global range of the subspecies, insulanus consists of one known population in Washington State (S1), identified in 1998 at San Juan Island Historical Park, American Camp, San Juan Island, Washington (Figure 1) (J. Fleckenstein per. comm. 2002). Other than this single population, the subspecies insulanus has not been documented in the United States. The subspecies has been extirpated from British Columbia (Hinchliff 1994, 1996; Layberry et al. 1998).

1.10.3 Population and distribution trend

The island marble is known from 14 historical records in Canada suggesting the existence of five naturally fragmented populations distributed among the Greater Victoria area (Langford, Beacon Hill Park and James Bay), and Nanaimo (Wellington) on Vancouver Island and Gabriola Island (Figure 1). The earliest record, from the 1860s, was a specimen labeled simply 'Vancouver;' this was most likely a reference to Vancouver Island. The species is presumed extirpated by about 1910, with the last record in Canada being in 1908 on Gabriola Island. Recent survey efforts have not located any new populations of this species in Canada.

Extirpation of the island marble in Garry oak ecosystems in Canada occurred prior to substantial habitat loss of these ecosystems. It is suspected that grazing by sheep and/or cattle prior to, or during World War I eliminated the larval foodplant (suspected to be hairy rockcress [Arabis hirsuta]) and acted as the primary cause leading to extirpation of this species from British Columbia (Shepard 2000b). The overall reasons for extirpation are unknown.

There is no published estimate of global abundance for the population on San Juan Island. At least 20 individuals were observed in 2002, (Fleckenstein pers.comm. 2002; Miskelly pers. comm.2004), and at least 100 in 2003 (Pyle pers. comm.2003), but no population estimate was completed. There is no indication of the former total number of individuals in Canada.

1.10.4 Biotic and abiotic features of habitat

Since the island marble has not been recorded in Canada since 1908, habitat requirements are based on information from the San Juan Island population in the United States and current descriptions of sites with historic records.

The San Juan Island population lives in a large (> 100 ha) open area that contains three distinct habitats: mesic, disturbed grassland; sand dunes; and shorelines. The area primarily faces west and is devoid of large trees. The areas used by the butterflies are not steep: slopes range from flat to up to 50% (Miskelly pers. obs. 2004). Introduced mustards (Brassicaceae) are patchy but abundant in the grassland and sand dunes, while native foodplants are abundant along the shorelines. Native and introduced weedy mustards (Brassicaceae) usually reach their highest densities in moderately disturbed areas. The mustard plants in the grasslands grow in areas disturbed by burrowing mammals (Fleckenstein pers. comm. 2002; Miskelly pers. comm. 2004; Pyle pers. comm. 2003). There are no similar habitats of this scale in Canada.

The extant population on San Juan Island, Washington, oviposits on the introduced European weeds field mustard (Brassica campestris) and tumble mustard(Sisymbria altissimum), and the native tall pepper-grass (Lepidium virginicum), family Brassicaceae (Fleckenstein pers. comm. 2002; Miskelly pers. comm. 2004; Pyle pers. comm. 2003). This is consistent with documented use of weedy introduced Brassicaceae by larvae of other subspecies of Euchloe ausonides (Opler 1975). Field mustard grows in more mesic habitat than native rockcress (Arabis) species, and is therefore less susceptible to early dessication.

In Canada, it is likely that the island marble was associated with open grassland successional stages of Garry oak and associated ecosystems. Larval foodplants in British Columbia were likely rockcress (Arabis spp.), perhaps A. hirsuta (Shepard 2000a; 2000b), as well as tall pepper-grass (Lepidium virginicum). Rockcress (Arabis spp.) is the major foodplant for subspecies mayi in the southern interior of BC. However there is no historical record of the larval foodplants or the habitats used.

1.10.5 Spatial requirements

The amount of habitat needed to sustain a viable population is unknown. It is likely that a moderately large habitat patch or close proximity to other patches is required for long term survival. A patch of 50 to 100 hectares of high quality habitat may be required, based on an estimate of a 100 ha area inhabited by the extant population on San Juan Island although further research is required to determine minimum habitat sizes. There are very few habitat patches of this size that remain in British Columbia.

1.10.6 Annual cycle

Eggs hatch from late May to late June and larvae feed until pupation, which occurs between late-June and late-July.

High mortality results if the larval food plants desiccate prior to pupation. The amount of spring and early summer rain affects the date of food plant senescence, and hence larval success in reaching pupation.

1.10.7 Ecological niche

There has been no research on the ecological role of this butterfly. The adults may be minor pollinators of flowers. The larvae are minor herbivores of Brassicaceae family and feed on flowers and fruits. Adults, larvae and pupae may serve as prey for insectivorous birds, small mammals, and predatory insects (Family Braconidae). Eggs, larvae and pupae are likely to also function as hosts for insect parasitoids (van Nouhys and Hanski, 2004).

1.10.8 Biologically limiting factors

There is minimal knowledge of the biologically limiting factors for the island marble.

Flight and dispersal capabilities have not been studied. Observations on the behaviour of subspecies mayi, suggest that individual adults may move significant distances during their lifespan. In California, near the ocean, adults of E. ausonides have been documented moving up to one kilometre (Scott 1975a; 1975b).

The larvae of island marbles are actively feeding during late May and June, the later part of the period in which Btk is scheduled for application in the historic range of the species in British Columbia (Nealis pers. comm. 2003).