Category: Status Reports
Published: November 2005
Author(s): Derek W. Stinson
Native prairies are among the most endangered ecological communities in North America. Western Washington is generally known for its forests; it is less well known that the south Puget Sound area historically had large expanses of prairie and oak savannahs. These prairies and woodland communities developed during a warm dry period from 10,000 to 7,000 years ago on the droughty, gravelly soils deposited by the Vashon Glacier. In the recent past, glacial outwash prairie still existed on at least 150,000 ac, and grassland and oak woodlands occurred in smaller patches throughout the Puget Trough and south to the Columbia River. Local Native American tribes adapted to use the plants and game of these communities and maintained prairie in the area by burning the vegetation every few years during the last 4,000 years. Since settlement by Euro-Americans, the extent of these prairies has steadily declined with their use for agriculture and the cessation of burning that has allowed succession to Douglas-fir forest. Only about 8% of the original prairie still supports grassland vegetation and about 2-3% is still dominated by native prairie vegetation. In addition to prairies on glacial outwash, native grasslands existed on perhaps 10,000 ac of coastal headlands, islands and rocky balds. Some of the wildlife of prairies, though now locally rare, are little different from abundant and widespread forms found across much of eastern Washington and in grassland communities elsewhere. A few of the wildlife species that inhabited these prairies and grasslands have been genetically isolated from their ancestral stocks for a long period of time and have evolved endemic forms found nowhere else. These unique forms have become rare with their habitat, and some are threatened with extinction. This report summarizes what is known about the natural history and status of three species that have their center of abundance in Washington on the prairies of the southern Puget Sound: the Mazama pocket gopher; streaked horned lark; and Taylor's checkerspot butterfly.
Mazama Pocket Gopher
The Mazama pocket gopher (Thomomys mazama) is a regional endemic found only in western Washington, western Oregon and northern California. The subspecific taxonomy of T. mazama is in the process of being revised, but in Washington, T. mazama is likely represented by 3 surviving subspecies: T. m. yelmensis is found on locations scattered on the remnants of prairie in Pierce and Thurston counties; T. m. couchi is found on grassland at a few localities near Shelton in Mason County, including the airport; and T. m. melanops is found on a few alpine meadows in Olympic National Park in Clallam County. Two additional subspecies that occurred around Tacoma (T. m. tacomensis) and in Wahkiakum County (T. m. louiei) appear to be extinct. The Washington population of the Mazama pocket gopher became a candidate for federal listing under the Endangered Species Act in 2002. Mazama pocket gophers are known to persist at 27 sites scattered across the southern Puget Sound grasslands and alpine meadows of the Olympics. These may total in the low thousands, but many are small populations on marginal sites that are unlikely to persist. Pocket gophers play an important a role in ecological communities by altering soil structure and chemistry, affecting plant occurrences, and serving as prey for many predators; their burrows provide a retreat for a wide variety of other species, including the western toad.
With the exception of T. m. melanops and the apparently extinct T. m. louiei, T. mazama is a creature of the south Puget Sound prairie landscape. Most gopher populations are restricted to grassland on remnant and former prairie sites. Mazama pocket gophers are not constrained to live on native vegetation and will eat many introduced grasses and weedy forbs. Soil type seems to affect their distribution, because they are absent from most prairies with particularly rocky soils. Habitat loss to succession, agriculture and development has eliminated most of the prairie vegetation, and habitat continues to be lost to residential development. Existing habitat is being degraded by heavy grazing of pastures and the invasion of Scotch broom and other weedy non-native plants.
Half of the known gopher populations are on private lands, where they are threatened by residential development and may be rapidly dwindling due to degraded habitat and high mortality. Pocket gophers may not persist in residential areas due to persecution by trapping, poisoning, and predation by cats and dogs. The last records of the T. m. tacomensis were of individuals killed by domestic cats. Gravel mining affects gopher habitat on some private lands. Most occupied habitat on public lands is affected by non-conservation uses including military training and recreation. Gopher populations at airports can be affected by development of airport-related facilities and businesses, and management of airport grassland.
The small size and isolation of most remaining populations of Mazama pocket gopher put them at risk of local extinction, and without increased protection, all but T. m. melanops in Olympic National Park could go extinct. Historically, local gopher populations probably exchanged genetic material by individuals occasionally dispersing through intervening oak woodlands and forest; prairie patches where gophers went extinct would eventually be re-colonized. Today, these prairie patches are increasingly surrounded by roads and suburbs that are inhospitable to dispersing gophers. Populations that become extinct are unlikely to be re-colonized without re-introductions.
For these reasons, we recommend that the Mazama pocket gopher be listed as threatened by the State of Washington.
Streaked Horned Lark
The streaked horned lark (Eremophila alpestris strigata) is arguably the most distinct subspecies of horned lark. Its historic breeding range included prairies and open grassland habitats in southwestern British Columbia, western Washington, and western Oregon. The center of abundance of the streaked horned lark in Washington was the prairies of southern Puget Sound, primarily in Pierce and Thurston counties. Streaked horned larks have declined with the loss of prairie habitats to development and succession to forest. With the cessation of burning of the prairies by Native Americans, Douglas-fir has spread over much of the prairie and introduced grasses, weeds, and Scotch broom have degraded much of the remainder. Streaked horned larks may have also been restricted to portions of the prairie where the vegetation was short and sparse due to excessive dryness or repeated burns.
There is little information on historical populations. Streaked horned larks were reported to be a â€�"very abundant summer resident of the gravelly prairies near Fort Steilacoomâ€ in the 1850s (Suckley and Cooper 1860). Bowles (1900) estimated that â€�"fully one hundred pairs must have nestedâ€ on the Tacoma golf links at the turn of the century. Streaked horned lark breeding in Washington is now limited to only 13 known sites: 6 sites in the south Puget Sound area, 4 sites along the outer coast, and 3 sites on islands in the lower Columbia River. The subspecies has also greatly declined in Oregon and may be extinct in British Columbia. The total breeding population is estimated to be 780, with about 330 birds in Washington and about 450 in Oregon. All remaining nesting sites in the south Puget Sound area are on airports or military bases where grassland has been maintained, but where larks are subject to disturbance and human-related mortality, and where their habitat is threatened with development or incompatible use. Horned larks are among the species most frequently killed by collisions with military aircraft. Columbia River sites are affected by management of the islands, including deposition of dredge spoil, and vegetation manipulation to discourage nesting by Caspian terns. Coastal sites are affected by the spread of European beachgrass and disturbance by recreational activities. Most of the streaked horned larks that breed in Washington winter in Oregon where habitat is being lost to development, buried under fresh dredge spoil, or subject to shifting patterns of agriculture.
Genetic data suggest that the streaked horned lark already suffers from reduced genetic diversity and may suffer from inbreeding. Given the small number of breeding locations, low population numbers, and the lack of breeding areas free of human-related mortality and disturbance, the streaked horned lark is likely to go extinct without recovery actions.
For these reasons, the Department recommends that the streaked horned lark be listed as endangered in the State of Washington.
Taylor's checkerspot (Euphydryas editha taylori), a subspecies of Edith's checkerspot, is a medium-sized butterfly with a striking checkered pattern of orange to brick red, black and cream. It was historically found on grassland habitats at over 70 sites from southeastern Vancouver Island, British Columbia through northwestern Oregon, including about 38 known locations in Washington. The subspecies is now restricted to 1 known population in British Columbia, small populations in 2 areas in Oregon, and a small scattering of 10 populations in Washington. Butterfly populations can be extremely variable from year-to-year. Among 5 or 6 populations that appear to have gone extinct over the last 10 years is one population that was estimated at 7,000 in 1997; it declined precipitously and appeared to be extinct by 2001. Most populations in Washington support no more than a few hundred individuals, and several of these are extremely small and may be on the verge of extinction. The subspecies became a candidate for listing under the federal Endangered Species Act in 2001 (USFWS 2001).
Butterfly populations are known to fluctuate dramatically with weather. The critical phases of the life cycle of Edith's checkerspot have often been described as a race by the larvae to develop before their food plants dry out in early summer; larvae that do not mature sufficiently before entering a prolonged diapause which extends through winter, do not survive. Because of this interaction with host plants, local populations sometimes go extinct and the habitat is vacant until being recolonized by dispersing adults. Some populations appear to be dependent on the non-native English plantain or ribwort (Plantago lanceolata), a weedy introduced species. Dependence on this species may negatively affect E. e. taylori population dynamics and lead to more frequent local extinctions.
Butterflies often occur as metapopulations; metapopulations are collections of smaller subpopulations that occupy patches of habitat, and the patches are successively vacant and occupied as local butterfly extinctions are followed by recolonizations. E. editha is a relatively sedentary species and rarely disperses > 5 km. Taylor's checkerspot sites in Washington are located in 4 distinct areas, and may comprise 3 or more metapopulations. Habitat loss has increased isolation of the remaining populations, however, so that many are unlikely to be recolonized when they become extinct. The small size of many populations put them at higher risk of extinction due to fires, disturbance, insecticides, and weather extremes, as well as the potential for reduced survival and reproductive success due to inbreeding.
Several of the largest remaining populations occur on public lands, but most of these lands have uses that can conflict with butterfly conservation, including military training and recreation. Private lands occupied by Taylor's checkerspot are subject to development, agriculture, and gravel extraction that can eliminate habitat. Grassland sites, except where actively maintained, are being degraded by the invasion of Scotch broom, Douglas-fir, and numerous non-native forbs and sod-forming grasses. The remaining populations of Taylor's checkerspot are unlikely to persist without management intervention in the form of habitat restoration and maintenance.
Long-term persistence of isolated populations also requires genetic exchange between subpopulations and recolonization of vacant patches. Maintaining the genetic diversity of populations will require either restoration of many intervening stepping stones of habitat or physical transport of individual butterflies between patches. The subspecies is unlikely to survive without recovery actions. For these reasons we recommend that Taylor's checkerspot be listed as endangered in the State of Washington.
Draft documents are provided for informational purposes only. Drafts may contain factual inaccuracies and may not reflect current WDFW policy.