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 this 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 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.
Habitat
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.
Conservation Measures
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.
References
U.S. Fish and Wildlife Service. 2001. Oregon silverspot butterfly
(Speyeria zerene hippolyta) revised recovery plan. U.S. Fish and
Wildlife Service,
Portland, Oregon. 113 pp.
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