Screening Model for Determining Likelihood of Site Occupancy by Oregon Spotted Frogs (Rana pretiosa) in Washington State

The Oregon spotted frog (Rana pretiosa; OSF) is a Washington State Endangered species and a Federal Endangered Species Act candidate. Historic abundance and distribution of Oregon spotted frogs within Washington are poorly understood (McAllister and Leonard 1993). However, Oregon spotted frog populations have declined markedly since Euro-American settlement of the coastal Pacific Northwest region, and <10 extant sites are presently known to be occupied in Washington (WDFW unpublished data). Putative reasons for population declines include altered hydrology, wetland loss, predation and competition from exotic fish and amphibians, altered water chemistry, and ultraviolet radiation (Hayes et al. 1997).

Beginning May 2003, the Washington State Department of Transportation (WDOT) funded the Washington Department of Fish and Wildlife to conduct an exhaustive literature review on the Oregon spotted frog. The goal of this review was to synthesize existing information into a comprehensive document describing Oregon spotted frog ecology and habitat associations, and to develop a screen for determining potential suitability of wetland sites in western Washington for supporting populations of Oregon spotted frogs.

The wetland screening model is presented here. This model is explicitly designed to evaluate wetland sites on a case-by-case basis and determine whether they contain habitat characteristics that make them potentially suitable for supporting an established and successfully reproducing population of Oregon spotted frogs. It does not address, nor are ample data available to determine, site suitability for occupancy and survival by dispersing individuals. Because the sample size from which to draw inferences within Washington was prohibitively small (n < 10), we included data from currently occupied wetlands in Oregon and British Columbia, Canada. Since features of occupied sites such as elevation and site size are known to vary with latitude over the species’ geographic range, the model focuses on data relevant to Washington State in instances where range-wide values far exceed those of Washington.

The screening model has a two-tier hierarchy. Each tier contains variables known or believed (based on the literature synthesis and professional expert opinion) to influence site suitability for Oregon spotted frogs. Parameters of variables in Tier 1 are the maximum and minimum observed values for each variable in the tier. Since the region-wide number of known occupied sites was small (n <40) and relevant data often were not available for all of these sites, we expanded parameter bounds by 25% as a buffer to minimize the error of failing to recognize a suitable wetland due to potentially artificially narrow variable bounds.

Tier 1 addresses landscape-level factors known to influence wetland hydrology, floristics, structure, and function, and is quantitative in nature. Tier 1 evaluates candidate wetland suitability based on soils, elevation, U.S.F.W.S. National Wetland Inventory habitat classification, wetland size, U.S.G.S. National Land Cover Descriptions, and the potential influence of connectivity to adjacent wetlands. Suitable wetlands in western Washington are expected to contain: loams, mucks, loamy sands, and/or poorly drained fibrisols, mesisols, organic cryosols, gleysols, or humisols; occur from the upper hydrologic limit of brackish water influence â€" 2,624 ft (800 m); have Palustrine emergent habitat present; be at least 8.9 ac (3.6 ha) in size; and have < 9.8 % area within a 1.6 km radius developed for residential, commercial, industrial, and/or transportation use. Wetlands smaller than 8.9 ac may be considered suitable if connected by surface water and <1 km from an adjacent wetland, with both wetlands (using pooled data) mutually satisfying all Tier 1 criteria.

If a wetland meets all Tier 1 criteria then we recommend using Tier 2 screening. Tier 2 is qualitative in nature and describes habitat characteristics of occupied sites during 3 seasons; breeding, summer, and winter.

Wetlands suitable during breeding season are expected to contain vernal shallows 5 â€" 30 cm in depth; be dominated by native submergent and emergent vegetation, (Typha and/or Phalaris are typically present, but usually not dominant at occupied sites unless domestic livestock grazing also occurs); have >10% bottom substrate covered by submergent, floating, or low emergent vegetation; have low overhead canopy closure by woody-stemmed shrubs and trees; and remain surface-connected to summer habitat until after larvae hatch (usually by April 31) in an average year.

Suitable summer habitats will have perennial lentic pools within 1 km of breeding habitat; be dominated by native vegetation in low emergent, floating, or submerged form; have palustrine forested habitat capable of providing partial or greater amounts of shade; and be surface-water connected to suitable winter habitat during fall.

Suitable winter sites will be <1 km from summer sites; exceed 15 cm depth; have aquatic bed, emergent, or scrub-shrub, and unconsolidated bottom habitat present in areas not scoured by winter floods; and have springs or upwellings present in sites where average winter ice-cap persists for >1-2 weeks.

Wetlands that meet all Tier 1 and 2 criteria are considered potentially suitable Oregon spotted frog habitat, and we recommend on-site Oregon spotted frog surveys prior to activities that may disturb these sites.