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The chytrid fungus Batrachochytrium dendrobatidis (Bd) is notorious amongst current conservation biology challenges, responsible for mass mortality and extinction of amphibian species. World trade in amphibians is implicated in global dissemination. Exports of South African Xenopus laevis have led to establishment of this invasive species on four continents. Bd naturally infects this host in Africa and now occurs in several introduced populations. However, no previous studies have investigated transfer of infection into co-occurring native amphibian faunas. A survey of 27 U.K. institutions maintaining X. laevis for research showed that most laboratories have low-level infection, a risk for native species if animals are released into the wild. RT-PCR assays showed Bd in two introduced U.K. populations of X. laevis, in Wales and Lincolnshire. Laboratory and field studies demonstrated that infection levels increase with stress, especially low temperature. In the U.K., native amphibians may be exposed to intense transmission in spring when they enter ponds to spawn alongside X. laevis that have cold-elevated Bd infections. Exposure to cross-infection has probably been recurrent since the introduction of X. laevis, >20 years in Lincolnshire and 50 years in Wales. These sites provide an important test for assessing the impact of X. laevis on Bd spread. However, RT-PCR assays on 174 native amphibians (Bufo, Rana, Lissotriton and Triturus spp.), sympatric with the Bd-infected introduced populations, showed no foci of self-sustaining Bd transmission associated with X. laevis. The abundance of these native amphibians suggested no significant negative population-level effect after the decades of co-occurrence.
Fig. 1. Frequency distribution of Batrachochytrium dendrobatidis (Bd) infection intensities (genomic equivalents determined by RT-PCR) in Xenopus laevis from U.K. laboratory populations (n = 1795 individuals). X-axis scale gives maximum value for histogram bin ranges.
Fig. 2. Frequency distributions of Batrachochytrium dendrobatidis (Bd) infection intensities (genomic equivalents determined by RT-PCR) in laboratory-raised Xenopus laevis maintained (a) at 10 °C (n = 22), and (b) at 20 °C (n = 35). X-axis scale gives maximum value for histogram bin ranges.
Fig. 3. Frequency distributions of Batrachochytrium dendrobatidis (Bd) infection intensities (genomic equivalents determined by RT-PCR) in samples of Xenopus laevis from the same field population (Croescwtta) in Wales (a) at 8.5 °C in April (n = 55) and (b) at 15 °C in July (n = 97), and (c) after transfer to laboratory maintenance at 20 °C for >6 months (n = 21). X-axis scale gives maximum value for histogram bin ranges.
Fig. 4. Location of habitats of amphibians tested with RT-PCR for infection by Batrachochytrium dendrobatidis in Wales; area centred on 51°27′50″N, 3°34′14″W, south of Bridgend, Glamorgan. Fieldwork periods each typically of 2–4 days in April (a), July (b), August (c) and September (d) 2008; May (e) and June (f) 2010; July (g) and September (h) 2011. Samples of species distinguished by symbol colour with numbers of individuals indicated by symbol size (see key) for native amphibians: frogs (Rana temporaria) in blue, toads (Bufo bufo) red, newts (‘Triturus’ comprising Lissotriton vulgaris, Triturus helveticus and T. cristatus) green; and for introduced Xenopus laevis black in the pond at Croescwtta Farm only.
Fig. 5. Location of habitats of amphibians tested with RT-PCR for infection by Batrachochytrium dendrobatidis in England; area centred on 53°37′30″N, 0°40′14″W, north of Scunthorpe, North Lincolnshire. Fieldwork periods each typically of 3–5 days in May (a), June (b), early July (c) and late July (d) 2008; June (e), July (f), August (g) and September (h) 2009; June (i), July (j) and August (k) 2010. Samples of species distinguished by symbol colour with numbers of individuals indicated by symbol size (see key) for native amphibians: frogs (Rana temporaria) in blue, toads (Bufo bufo) red, newts (‘Triturus’ comprising Lissotriton vulgaris and T. helveticus) green; and for introduced Xenopus laevis black with arrows to specific sites.
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