Oregon silverspot butterfly
Scientific name: Speyeria zerene hippolyta
Critical Habitat: Designated
Listing Activity: Oregon silverspot butterfly was listed as a threatened species with critical habitat in October 1980. A revised recovery plan was published in 2001.
Historical Status and Current Trends
The historical range of the Oregon silverspot butterfly subspecies extends from the Long Beach Peninsula, Pacific County, Washington, south to Del Norte County, California. All of these populations were restricted to the immediate coast, centered around salt-spray meadows, or within a few miles of the coastline in similar meadow-type habitat.
At the time of listing, the only viable population known was on the Siuslaw National Forest in Tillamook County, Oregon. Additional populations have since been discovered at Cascade Head, Bray Point, and Clatsop Plains in Oregon, on the Long Beach Peninsula in Washington, and in Del Norte County in California.
Description and Life History
The Oregon silverspot butterfly is a medium-sized, orange and brown butterfly with black veins and spots on the dorsal (upper) wing surface, and a yellowish submarginal band and bright metallic silver spots on the ventral (under-side) wing surface. This subspecies is distinguished from other subspecies of silverspot butterflies by a somewhat smaller size and darker coloration at the base of the wings.
These are morphological adaptions for survival in a persistently windy and foggy environment. The forewing length averages about 27 millimeters (1 inch) for males and 29 millimeters (1.1 inch) for females. Hydaspe fritillary ( Speyeria hydaspe), a related species found in adjacent habitats can be distinguished by the cream, rather than silver, colored spots of the ventral wing surface.
The life history of the Oregon silverspot revolves around its obligatory host plant, the early blue violet (Viola adunca). Females oviposit up to 200+ eggs singly amongst the salt-spray meadow vegetation near the violet host plant, usually in late August and early September. Sites with good sun exposure are favored. The eggs hatch in approximately 16 days and the newly hatched larvae wander short distances to find a suitable site for diapause (suspended growth for overwintering).
The larvae end diapause sometime in early spring and begin to feed on the violet leaves. As the larvae grow, they pass through five molts (shed outer covering) before they enter the intermediate stage between larval and adult forms (pupate). Approximately two or more weeks later, the butterflies emerge from their pupal case (eclose). Adult emergence starts in July and extends into September. Shortly thereafter, their wings and other body parts harden and they escape the windy, cool meadows for nearby forests or brush lands.
Mating occurs through August and September. Those individuals (male and female) which are most efficient at basking and maintaining proper body temperature will be able to operate longer and deeper in the windy meadow zone, thus improving their opportunities for successful reproduction.
The Oregon silverspot occupies three types of grassland habitat. One type consists of marine terrace and coastal headland salt-spray meadows (e.g., Cascade Head, Bray Point Rock Creek-Big Creek and portions of Del Norte sites). The second consists of stabilized dunes as found at the Long Beach Peninsula, Clatsop Plains, and the remainder of Del Norte. Both of these habitats are strongly influenced by proximity to the ocean, mild temperatures, high rainfall, and persistent fog. The third habitat type consists of montane grasslands found on Mount Hebo and Fairview Mountains. Conditions at these sites include colder temperatures, significant snow accumulations, less coastal fog, and no salt spray.
The most important feature of the habitat of the Oregon silverspot is the presence of the early blue violet. This plant is normally the only species on which the Oregon silverspot can successfully feed and develop as larva. However, in the laboratory the butterflies will accept other species of violets, and there is evidence that some individuals on Mount Hebo are using another species of violet.
This plant is part of the salt-spray meadow vegetation and is an obligatory component of the butterfly’s habitat. Other features of optimum habitat include moderate grass cover, including red fescue (Festuca rubra) used as a shelter for larvae, and a mixture of herbaceous plants such as California aster (Aster chilensis) used for nectaring by adults. Apparently the more inland meadow sites occupied by related subspecies of silverspots are not accessible to Oregon silverspots. The habitat is similar on Mount Hebo with Viola adunca as the key component. The distribution and composition of the flora may differ slightly, but the habitat functions similarly to the salt-spray meadow. The shallow soil apparently helps to keep this area in the meadow stage.
Although the salt-spray meadow is the nursery area for the butterfly and a key element of this species’ habitat, it is a rather harsh environment for the adults. Upon eclosion (metamorphosis of the pupa into the adult butterfly), the adults generally move out of the meadows into the fringe of conifers or brush where there is shelter for more efficient heat conservation and nectaring flights. The forest shelter may also be used for courtship and mating. Where such sheltered conditions exist, the adults will use various nectar sources, including native and exotic plants, particularly composites such as the native California aster, yarrow (Achillea millefolium), and Indian thistle (Cirsium edule) and some exotics such as false dandelion (Hypochaeris radieata) and tansy ragwort (Senecio jacobaea).
Reasons for Decline
The major limiting factors affecting this species are related primarily to the limitation of suitable habitat. The highly specialized salt-spray meadow habitat within the geographical range for the Oregon silverspot was never common. This early seral community has always had a patchy distribution, occurring only where fire, salt-laden winds, or other natural or man-related occurrences (e.g., grazing, controlled burning) have maintained an open meadow. Evidence suggests that such habitat was more extensive in the past than it is today.
Historical accounts show the butterfly and its habitat as locally common within its range. However, good habitat has steadily been used for residential and business establishments, public parkland development, and parking areas or lawns. Excessive use of the salt-spray meadows by grazing animals or off-road vehicles has directly eliminated habitat. Secondary impacts of people’s activities, introduction of exotic plants, and fire suppression with subsequent succession of meadows to brush and stunted woodland have also contributed to a reduction in suitable habitat.
Habitat destruction is unquestionably the reason for the threatened status of this butterfly today. It should be noted, however, that as colony size is reduced by habitat loss, restricted genetic variability and/or catastrophic events can ultimately cause the extinction of these small populations.
The coastal prairie habitat on which the Oregon silverspot butterfly is dependent will quickly become scrub, brush, or forest land if left unmanaged. Natural processes such as wildfires and wildlife grazing likely functioned to maintain open grasslands in the past. Today the habitat must be actively managed to maintain a grassland structure. Mowing, burning, and the planting of native plants are current habitat management strategies.
An Oregon silverspot butterfly captive-rearing program began in 1999 to raise caterpillars for release into declining population. The Oregon Zoo in Portland, Oregon and the Woodland Park Zoo in Seattle, Washington receive a small number of wild female Oregon silverspot butterflies each year. Each of these females may lay hundreds of eggs which quickly hatch into tiny first instar caterpillars.
The zoos care for the caterpillars throughout their development, overwintering them in their diapause state in cool refregerators, and feeding them violets during the spring and summer until they become pupae. The pupae are then released into the declining populations. These population augmentations or reintroductions are a last resort to prevent further population extinctions. Multiple years of releases are needed to successfully stabilize the declining populations but the augmentation appears to be a promising species recovery tool.