Oviposition Ecology of the Oregon Spotted Frog at Beaver Creek, Washington

Wetlands in the floodplain of Beaver Creek, a Thurston County tributary to the Black River, support one of only four known Oregon spotted frog (Rana pretiosa) populations in Washington. To better understand the habitat requirements of this state-listed endangered species, oviposition (egg-laying) sites were located and characterized through measurement of water depth, temperature, and vegetation composition and structure. For comparison, available habitat nearby was similarly characterized.

The winter period of 2000-2001 prior to oviposition was one of the three driest in the past 20 years. As a result, shallow surface waters were limited and most of the surface water available to frogs was in areas that normally hold water through most of the year. In these conditions, we observed unusually high embryonic survival because water levels did not recede and leave eggs exposed to freezing or drying conditions.

Eleven oviposition sites composed of 58 egg masses were found. The two sites with the majority (59%) of the egg masses were both disturbed sites. One had been treated by mechanical removal of all vegetation the previous summer. The other was in the tracks of an off-road vehicle that had driven through the wetland about two years prior. Most oviposition occurred in a narrow range of water depth averaging 5.9 cm (2.3 in). Daytime surface water temperatures at the time of oviposition averaged 16.0° C. ( 60.8° F.). However, the initiation of breeding activity may have been related to gradual warming of the more stable subsurface waters where spotted frogs appear to spend the majority of their time during late winter. Here, water temperatures averaged 7-9° C. ( 45-48° F.) on days when oviposition was initiated.

Oregon spotted frogs selected areas with low vegetation dominated by reed canarygrass or, less frequently, slough sedge. Vegetation structure was either cropped low or pressed flat by vehicle tires or the elements (wind and snow), conditions which allowed full exposure of the water’s surface to the sun. Floating or submerged vegetation did not deter oviposition by spotted frogs but emergent vegetation, which projected over the water’s surface, was avoided.