The Oregon spotted frog (Rana pretiosa) is a highly aquatic frog that has been extirpated from a large portion of its historic range in the Pacific Northwest, and remaining populations are reduced and isolated (Hayes 1997, Pearl and Hayes 2005). Loss and alteration of marsh habitat, predation and competition from exotic fish and bullfrogs, and degraded water quality from agriculture and livestock grazing are implicated in their decline (Hayes 1997, Pearl and Hayes 2005). In 2001, an interagency team translocated a population of frogs from a site that was to be eliminated by the renovation of the dam impounding Wickiup Reservoir, to newly created ponds at Dilman Meadow (121i?? 39' 52" W, 43i?? 41' 58" N), 2.5 km from the original site in central Oregon, USA. We monitored Oregon spotted frog demography and movements at Dilman Meadow for > 4 yr to assess the efficacy of these mitigation efforts, determine metrics for long-term monitoring, and inform future management at the site. More broadly, many aspects of Oregon spotted frog life history are poorly known, so understanding demography and movement patterns is likely to be useful in its conservation. Although wildlife translocations have been attempted extensively as conservation means, few such projects have been sufficiently monitored for demographic rates to understand the causes for the translocation's success or failure (Dodd and Seigel 1991). Our objective here is to document demographic and movement patterns in the population of Oregon spotted frog at Dilman Meadow so that this information will be available to guide management decisions.
To better evaluate amphibian population responses to management actions it is important to consider the contribution of each life history stage and both genders to the balance of reproduction and mortality. Population growth or contraction occurs as a complicated function of the probability of breeding, fecundity, and survival during multiple life history stages and size classes and the transition between these classes. Body size in amphibians is strongly positively linked with the probability of breeding (Semlitsch et al. 1988, Smith 1987), fecundity (Howard 1980, Berven 1981, Berven and Gill 1983), and survival (Altwegg and Reyer 2003, Chelgren et al. 2006). Thus, growth of individuals is an important component of population change. Estimates of demographic rates for one gender are often used to infer population growth rates or population viability (Caswell 2001). However, in anurans such as Ranid frogs, gender is thought to affect survival rate (Wood et al. 1998, Lyapkov et al. 2004), probability of dispersal (Austin et al. 2003, Palo et al. 2004), age at sexual maturation (Lyapkov et al. 2004), and breeding probability (Muths et al. 2006). Moreover, males and females differ in energetic costs associated with breeding (Feder and Burggren 1992) and in growth rate (Lyapkov et al. 2004). Differences in demographic rates between genders will generally affect population growth rate for small populations (Engen et al. 2003, Si??ther et al. 2004, Husby et al. 2006), so it is important to distinguish these differences during monitoring. For example, it has been hypothesized that differences in the frequency at which male and female western toads (Bufo boreas) visit breeding sites have led to differential mortality from Chytridiomycosis, resulting in highly skewed sex ratios and diminished reproductive output (Muths et al. 2003). We examined sex- and size-specific demography at Dilman Meadow with particular focus on a priori hypotheses regarding survival, movement, and growth.
It is generally no longer defensible to use captures or sightings of individuals to estimate demographic rates when numbers are uncorrected for differences in the probability of capture. Instead, capture, survival, and movement probabilities are modeled simultaneously to reduce bias in estimates of demographic rates (Williams et al. 2001). The missin
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Oregon Spotted Frog (Rana pretiosa) movement and demography at Dilman Meadow: implications for future monitoring