The goal of this study was to determine whether Batrachochytrium dendrobatidis is threatening British Columbia's endangered population of Rana pipiens. From this conservation perspective, the most important result in this study was that the prevalence of Bd in R. pipiens increased significantly from 2003 to 2005 and then remained stable over the next two years (Table 3C). Surveyors first noticed frogs with chytridriomycosis-like symptoms in 2000 suggesting that the prevalence of Bd increased from 2000 to 2005 (Doug Adama, personal observation). Although it is difficult to establish causation, we cautiously suggest that Bd caused the 50% decline in the BC population of R. pipiens between 2000 and 2005  but that the prevalence of Bd has stabilized since 2005.
The results of this study are similar to what happened with the Eungella Torrent frog, Taudactylus eungellensis, in Eungella National Park, east-central Queensland, Australia . Between 1985 and 1986, an outbreak of chytridiomycosis is believed to have caused T. eungellensis to all but disappear from the park. Six years later, a mark-recapture study found that populations of T. eungellensis were persisting with stable, endemic infections of Bd. Likewise, in some areas of the Sierra Nevada of California, chytridiomycosis caused the rapid extinction of some local populations of the mountain yellow-legged frog, Rana muscosa, while other populations persisted despite high infection levels . These studies suggest that amphibians can evolve resistance to Bd and may have the ability to coexist with the disease.
Two other results from this study suggest that R. pipiens is coexisting with the disease. Some R. pipiens individuals cleared their Bd infections (Table 4) and the return rates were not significantly different between Bd-infected and uninfected R. pipiens suggesting that they had similar survival. Similar results were found in a mark-recapture study on the Stony Creek frog, Litoria wilcoxii, where 7 individuals cleared their Bd infections and the return rate was not significantly different between Bd-infected and uninfected frogs . While the BC population of R. pipiens may be coexisting with Bd, this does not mean that the population is doing well. Between 2000 and 2005, surveyors have found an average of 10 egg masses per year . Hence R. pipiens remains endangered in British Columbia.
The prevalence of Bd was higher in juveniles and adults than young of the year, (Table 3C; Figure 1), perhaps because the overwintering ponds are infected with Bd and/or sexual transmission between adults in the spring. The results of this study contradict studies on Australian frogs, which found that small frogs were more likely to be infected and carried more intense infections than larger frogs [33, 34]. We were surprised that so few young of the year were infected (Figure 1) because they emerge from the same ponds where highly infected adults mate in the spring. One possibility is that the young of the year lose their infections because they emerge from the breeding ponds during the hottest months of the year (July and August; Figure 3). Alternatively, the low proportion of infected young of the year suggests two other possibilities: (1) R. pipiens tadpoles are rarely infected with Bd or (2) infected R. pipiens tadpoles die before they metamorphose. The susceptibility of amphibian larvae to Bd has important consequences for the dynamics of amphibian populations . In several amphibian species, surveyors have found dead and dying post-metamorphic individuals in the presence of infected but apparently healthy tadpoles [16, 32, 35]. In these species, tadpoles may provide an intraspecific reservoir for the disease, which can drive the adult population to extinction .
As expected, the prevalence of Bd was lower in the summer than in the spring and fall and this was true for all stage classes (Figure 1). This seasonal pattern of lower Bd prevalence in the warmer summer than the cooler spring and fall (Figure 3) has been shown in a number of surveys [15, 27, 37, 38]. Bd is vulnerable to high temperatures and stops growing at an air temperature of 28°C [7, 8]. In addition, in the summer R. pipiens adults spend more time foraging in terrestrial habitats whereas in the spring and fall they enter breeding and overwintering habitat where they are more likely to encounter waterborne Bd zoospores .
This study would have been much improved if we had always used the same tissue-sampling method. We have tried to correct for the tissue-sampling method in our statistical analysis but it is possible that some of our results were biased by the inconsistent tissue-sampling strategy. The bag rinse was equally sensitive at detecting Bd as the toe clip method. Together, these two methods were 3.6 times more sensitive at detecting Bd than the swab method (Table 1). Surveyors in the present study obtained swabs by rubbing the tip of a sterile cotton swab on the abdomen, thighs, groin, and feet 10 to 20 times . In contrast, Kriger et al. swab each frog 70 times and have detected some of the highest prevalences of Bd to date [14, 26, 34, 37]. Hence our low swab success rate may be due to the difference in swabbing effort.