Using translocations to learn about frog declines and disease

Richard Retallick and Lisa Dwyer

Queensland Parks and Wildlife Service, Atherton, Queensland, Australia

Email: richard.retallick@jcu.edu.au

In this study we have employed an experimental approach to the investigation of "mysterious" frog declines that occurred in the Wet Tropics of north Queensland in recent decades. Rather than exploring correlations in historical information, we aimed to generate new information to facilitate a better understanding of the causes of frog declines. Also, by considering the chytrid fungus in particular, we aimed to cast new light on the interactions between frog declines and chytridiomycosis in the field. In north Queensland, populations of some species of frogs disappeared suddenly from high-altitude rainforest streams, but remained apparently unaffected at low-altitude streams. We exploited that altitudinal pattern of decline by using the Common Mist Frog, Litoria rheocola, a species that underwent catastrophic declines at altitudes above 300 m ASL in the early 1990s. We captured healthy frogs and tadpoles from lowland streams, and translocated them to large enclosures at both upland and lowland streams. We then observed the responses of those animals to the conditions at the two altitudes, and documented changes in their condition, and in their infections of chytrid fungus.

Preliminary translocations of small numbers of frogs and tadpoles served to identify the major husbandry concerns associated with the translocation and retention of animals which have largely-unknown habitat and dietary requirements. The mortality of those frogs and tadpoles in enclosures was high at first, but was effectively reduced with appropriate modifications to food supplements and enclosure design. Subsequently, when larger numbers of frogs and tadpoles were translocated in the summer of 1999/2000, they survived (especially at upland sites) for nearly six months, until that trial was concluded. However, a strongly contrasting pattern emerged from a similar translocation during the winter of 2000; all upland frogs died suddenly, while lowland frogs were much less affected. Thus, the survival of translocated animals appeared to be affected by seasonal conditions, with cool winter conditions promoting higher mortality than during the warmer conditions of summer, and higher mortality of translocated frogs at upland sites than at lowland sites. Despite these marked results, no clear pattern was evident in the disease results, and only some of the animals which were found dead showed signs of chytridiomycosis. Note, however, that disease (pathology, histology, chytrid analysis) results from the two largest and most recent translocations remained unknown in July 2000, thereby preventing a full account in this abstract of the mortality patterns and influence of disease. Work on this project is ongoing.