Short-term occupancy and abundance dynamics of the Oregon spotted frog (Rana pretiosa) across its core range

Abstract

The Oregon spotted frog (Rana pretiosa) occupies only a fraction of its original range and is listed as Threatened under the Endangered Species Act. We surveyed 93 sites in a rotating frame design (2010–13) in the Klamath and Deschutes Basins, Oregon, which encompass most of the species’ core extant range. Oregon spotted frogs are declining in abundance and probability of site occupancy. We did not find an association between the probability that Oregon spotted frogs disappear from a site (local extinction) and any of the variables hypothesized to affect Oregon spotted frog occupancy. This 4-year study provides baseline data, but the 4-year period was too short to draw firm conclusions. Further study is essential to understand how habitat changes and management practices relate to the status and trends of this species. Introduction The Oregon spotted frog (Rana pretiosa) is listed as Threatened under the Endangered Species Act (U.S. Fish and Wildlife Service, 2014) and Vulnerable on the Red List of Threatened Species (International Union for Conservation of Nature, 2013). Our understanding that this species has declined is based on its absence from portions of its range (Pearl and Hayes, 2005), but there is a lack of reliable information on trends in abundance or on the probability of site occupancy. For example, there is currently no information to suggest whether Oregon spotted frogs are still disappearing from occupied sites, if they are colonizing new sites, or if their abundance on average is increasing or decreasing. This information is essential to a basic understanding of the status and trends of this species. The current core extant range of the Oregon spotted frog is from southern British Columbia to southern Oregon. Distribution is disjunct in the northern part of the range and the species is thought to be absent from the Willamette Valley (Jones and others, 2005). The core extant range is mostly in the Deschutes and Klamath Basins of Oregon, with a few additional sites occupied near the headwaters of the Willamette and McKenzie Rivers just west of the divide from the Deschutes Basin drainage (Pearl and others, 2009) and in the northern Oregon Cascades. Hypothesized threats to Oregon spotted frog persistence are invasive species, disease, habitat changes, population isolation, and climate change. Habitat changes may result from changes in beaver (Castor canadensis) activity, management practices that enable encroachment of woody vegetation on historically open wetlands, fire and fuels management, or hydrological manipulations (Pearl and Hayes, 2005). We emphasize that there is little empirical support for any of these hypotheses and, although the species is clearly absent from portions of its historical range, there is little information on current trends.