Degree of Threat: Very high - high
Comments: Habitat loss, degradation, and fragmentation probably have contributed to the decline of this species. Specific factors include altered hydrology, draining, and filling of shallow wetlands (e.g., from dam construction, beaver removal, alteration of drainage patterns, dewatering due to urban and agricultural use of water); excessive livestock grazing (but healthy populations can coexist with moderate, year-round cattle grazing; Watson et al. 2003); vegetation changes resulting from fire supression; habitat changes caused by invasions of exotic plant species such as reed canary grass (McAllister and Leonard 1997); and physiological effects from contaminants and other changes in water chemistry (Hayes et al. 1997, Blaustein et al. 1999, Watson et al. 2003). Inadequate habitat size (most occupied sites are less than 25 hectares) may contribute to ongoing and future population declines (Hayes et al. 1997). The decline of R. pretiosa populations in British Columbia is probably due to habitat losses throughout the frog's historical range in the Fraser River Lowlands (Haycock 2000).
Predation by exotic fishes (e.g., brook trout, centrarchids) and frogs (bullfrogs, northern leopard frogs) may be a threat in some areas (Pearl et al. 2004). However, in at least one location, spotted frogs and non-native bullfrogs have coexisted for several decades (Hayes et al. 2009). Bullfrogs are a threat not only as a predator but also because they may carry a fungal pathogen (Bd, see following) and might transmit it to spotted frogs (Hayes et al. 2009). In central Oregon, presence of non-native fishes in preferred spotted frog overwintering habitat was associated with low numbers of spotted frog egg masses (Pearl et al. 2009).
Batrachochytrium dendrobatidis (Bd), a chytridiomycete fungus pathogenic to amphibians and implicated as the proximate cause of amphibian declines around the world (Berger et al. 1998, Pounds et al. 2006), has been found in Rana pretiosa in Oregon and Washington and may have contributed to declines observed there (Hayes et al. 2009). Other diseases also affect Oregon spotted frogs (see USFWS 2009), but the degree to which these represent signficant threats is poorly known.
Possibly global climate changes are a factor (Hayes and Jennings 1986). At the embryonic stage, UV-B radiation currently does not seem to be contributing to population declines (Blaustein et al. 1999).
The small sizes and isolation of most extant populations makes them vulnerable to extirpation with low probability of natural recolonization.
In Washington, all occupied sites are threatened by development, fluctuating water levels, and/or lack of management of exotic vegetation and predators. In Oregon, all sites are subject to one or more of the following threats: fluctuating water levels, non-native predaceous species, exotic vegetation encroachment, vegetation succession, and livestock grazing. In addition, all sites sampled in Washington and Oregon detected the presence of chytrid fungus. While the risk to an individual site from each of these factors may vary, the cumulative risk of these threats to each site is high.