The aim of this study was 1 to investigate Bd prevalence in the two Korean Hylid species, and 2 to study the effect of L. We hypothesized that Bd and the presence of Bullfrogs has a negative effect on the occurrence of D. Due to its restricted range and population decline [ 38 ], the endangered D. Thus, we tested the prediction that Bd prevalence is higher in D. Findings of this study have important implications for the conservation of D.
Our study was comprised two different phases, with separate field work and methodologies. We first studied Bd prevalence for Dryophytes japonicus and D. Secondly, we assessed the relationship between Bd infection and the presence of L.
The observations in this study were approved by the Ministry of Environment of the Republic of Korea permits numbers —03, —05, —6 and —28 , and thus qualifying for ethical assessment for experiments conducted on the endangered D. Field sites were selected from previous studies on the presence of D.
Site selection and sample number of D. Sampling for D. Bd sampling was performed between 15 May and 28 June, , during the peak breeding seasons of these two species [ 44 ]; S1 Table. Description of the range of Dryophytes suweonensis , sites where the species was detected, sites where Lithobates catesbeianus was detected and sites where sampling for Bd sampling was conducted. This map was generated with ArcMap 9. Sampling for Bd infection was conducted at 37 sites throughout the range of Dryophytes suweonensis in Republic of Korea.
To detect Bd infection, individuals were first captured by rapidly placing a hand on the individual, covered with a single-use vinyl bag Clean Wrap, ; Gimhae, Republic of Korea , then kept in separate bags to prevent cross-contamination and finally swabbed on the epidermis. Sampling was conducted at the breeding sites, between 4 pm and 4 am.
Frogs were systematically swabbed five times on each toe of the hind legs, each foot, the inner thighs, and both sides of the abdomen. Since the two Hylid species are difficult to distinguish morphologically [ 45 ], buccal swabs from each individual were collected for species identification through mtDNA CO1 sequencing, following the protocol developed by Jang [ 46 ]. All individuals were released at the point of capture as soon as the site was sampled, i.
Since bullfrogs are a known reservoir for Bd in the Republic of Korea we first swabbed 7 individuals in order to obtain a positive control for PCR. The bullfrogs were obtained from irrigating ditches between rice-paddies in Hwaseong None of the individual caught displayed any sign of clinical disease or abnormality. We obtained two positive reactions from two different individuals out of the seven L.
The samples were then sent to Macrogen Inc. The sequences were manually trimmed in Geneious v9. DNA was extracted following a modified version of the four-step protocol developed by Hyatt [ 47 ]. This reaction consisted of two PCRs that were the same except for the primers. Each PCR contained Samples were then run on a 1.
All samples in this study were considered positive if a band was visible at the same size as the positive control at approximatively bp. If any of the negative controls displayed a band, all PCR results from the run were discarded and all reactions ran again.
Each treefrog sample was run in two independent PCR replicates along with positive known Bd sample and negative controls PCR master-mix and distilled water. A sample was considered positive if both replicates were positive. Two samples for D. All three samples were positive for the third replicate and thus considered positive.
The other variables mentioned. Dryophytes japonicus was not included in this analysis due to its ubiquitous presence at all sites studied. We used preliminary data on the distribution of D. This resulted in the selection of sites with a high probability of D.
All sites selected were below 35 m a. Another common landscape feature for the occurrence of D. Auditory surveys were conducted for both D.
Surveys were conducted during the weeks of highest calling activity, between 15 May and 21 June , and during the time of day when the two species are known to be active, between 6 pm and midnight [ 37 , 38 ].
Each survey was 10 min long and conducted while walking along the longest straight line available in rice paddies. Transect selection was facilitated by the grid structure of modern paddy fields. Multicollinearity among all variables was examined prior to the statistical analysis through the use of Variance Inflation Factors VIF [ 50 ] , instead of bivariate correlations to avoid simple pairwise comparisons of correlations [ 51 ].
Because VIF values were between 1. The fixed factors were bullfrog presence, species, and sex; and the covariates were season, temperature, and precipitation for the three months prior to sampling. Species, sex, and Bd occurrence were binary encoded. Species was either D. Season was defined as the number of days after 5 May , the first day for both treefrog species to produce advertisement calls in We then ran a pair of ANOVAs in order to determine statistical significance for variations in Bd infection between 1 sites with and without bullfrogs, and 2 the two Hylid species.
The data presented relies on the calculation of averages and frequencies to indicate the directionalities in prevalence. Finally, we assessed the impact of L. All analyses were conducted with SPSS v. Bd was detected in both Hylid species, at 31 of 37 sites However, female D. In contrast, sex, season, temperature, and precipitation averaged for the 3 months prior to sampling , were not significant in our model.
For both treefrog species combined, Bd infection was more likely in populations with bullfrogs When considering the sites with D. When sites with D. When considering all treefrogs, Bd prevalence in D.
A Percentage of Korean treefrogs infected by Bd as a function of bullfrog presence. The graph is representative of the whole treefrog population for both species at sites with bullfrogs and at sites without bullfrogs.
B Bd prevalence for each treefrog species in percent. The Bd prevalence was lower in Dryophytes japonicus by a factor of 0. For both figures, the vertical thin bars at the center of the percentage thick bars are standard error bars.
As long as tadpoles and young bullfrogs have enough algae and insects to eat, adult bullfrogs can subsist on the younger frogs. With such a reliable food source, the adult populations can grow well above what would normally be possible, putting additional pressure on the ecosystem. Surhe said this cannibalism, combined with competition for other food resources, gives younger bullfrogs incentive to leap far away from their hungry elders.
And leap they do. By marking and recapturing bullfrogs on and near the Buenos Aires National Wildlife Refuge in Arizona, Suhre has found that the young amphibians can move at least 6 miles 9. To travel from one big pond to the next, the bullfrogs hop between small ponds interspersed throughout the arid landscape covered in grass and mesquite.
Their lack of predators, prolific nature, and incentive to relocate make bullfrogs a difficult invasive species to eradicate. No single method has proved effective in eliminating them, according to Schwalbe. Rotenone and other toxic chemicals can be applied to ponds to effectively kill fish and frog tadpoles.
But bullfrogs have a simple defense to the tactic: They hop out of the water. Schwalbe also noted that such toxins kill indiscriminately and, thus, are a problematic approach for areas with endangered native species. Researchers have had some success controlling bullfrogs at Buenos Aires National Wildlife Refuge, however.
They have drained bullfrog-infested ponds during the dry season, killing bullfrog tadpoles and enabling researchers to capture and dispose of adult bullfrogs that attempt to escape. The drained ponds fill back up when the monsoon rains arrive. Biologists can then reintroduce native leopard frogs. The problem, Suhre said, is that the bullfrogs travel great distances.
Unless eradication programs are done on a large enough scale to encompass whole landscapes, the bullfrogs return. Predatory insects, such as dragonfly nymphs and diving beetles, may help mitigate bullfrog populations, according to Rosen, the biologist who studies bullfrog-prey relationships.
However, to date, the fish found to be most effective at killing bullfrog tadpoles is the largemouth bass, a non-native species in Arizona. Regardless, the biologist said that effective control of bullfrogs may require redesigned ecosystems that incorporate some level of non-native species to control invasive populations of bullfrogs and crayfish.
We present results that sustain a "bullfrog hypothesis" for some native ranid declines, and we present our study as an example of how evidence accumulates to support such a hypothesis. In we began documenting historical localities for wetland herpetofaunas reptiles and amphibians , based on museum records and personal interviews, then revisited these and additional areas to determine current species' status.
Results of this process, plus circumstantial evidence, suggested that the bullfrog was a primary cause for declines of leopard frogs and garter snakes in southern Arizona Schwalbe and Rosen We simultaneously monitored native Chiricahua leopard frogs R.
Bullfrogs: Introduced predators in southwestern wetlands. By: Philip C. Some efforts are being made to physically remove frogs one by one, however some scientists think that we are past the point of being able to eradicate bullfrogs from BC. You can help stop the bullfrog invasion by not buying bullfrogs for ornamental ponds, by never moving a bullfrog from one pond to another, and also by not buying tadpoles to use as fish bait. It will take the teamwork of British Columbians everywhere to ensure that our wonderful native fauna survives the siege of the bullfrog for years to come!
Friday, September 23, When you next visit Science World, keep an eye and your ears out for our uninvited guest in our Science Park pond.
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