Aspects of amphibian decline: the influence of stress on water balance; chytrid infection on skin function; plus biosecurity and chytrid pathogen pollution

Abstract

Global amphibian declines have been occurring since the 1960s. Of the suggested causes of decline, environmental stress and disease are the most prominent. Batrachochytrium dendrobatidis (chytrid) is a pathogenic fungus which has recently emerged, been dispersed globally and is suggested to be the proximate cause of amphibian declines in many countries, including New Zealand. However chytrid fungus has not caused population declines in all circumstances indicating its pathogenesis may be dependant on interactions with environmental stressors, or that some amphibians exhibit immunity. It has also been suggested that chytrid affects amphibians by a physical blockage of skin function. I examine how thermal and behavioural stress affects the water balance physiology of Litoria raniformis and then test whether environmental stressors combined with chytrid infection alter both the water balance physiology and metabolic rate of these frogs. The thermal and behavioural stressors used did not affect the water balance physiology of L. raniformis. This result contradicted existing endocrine and tissue isolation literature and suggested that L. raniformis is either tolerant to these stressors or that there is a disparity between endocrine and tissue isolation research and studies of whole amphibians. Additionally, literature on rehydration processes in anuran amphibians suggests that bladder-emptied dehydrated frogs rehydrate their body to maintain osmotic balance before accumulating urine. In contrast I found that L. raniformis accumulates water rapidly in both the body and bladder during the early phases of rehydration. I measured metabolism and water balance in stressed chytrid infected L. raniformis, one week after infection when symptoms of chytridiomycosis were severe and five months after infection when symptoms were absent, but the frogs were still infected. Both metabolic rate and water balance were not altered by chytrid infection, with the exception of rehydration one week after infection. The lack of change in these physiological responses does not support the hypothesis that chytrid affects amphibian skin functioning. Any reduction in skin function was more likely to be attributed to some form of toxin(s). The final component of this research is an examination of causes of mortality of a quarantined population of Litoria aurea, purchased from an exporter to the international pet trade. Examination of dead L. aurea revealed signs of chytridiomycosis and demonstrated that chytrid is potentially being disseminated from New Zealand internationally through the pet trade. Controlling the local and international movement of amphibians out of New Zealand is currently hindered because chytrid does not meet criteria to be declared an 'unwanted organism'. Because chytrid does not fit these criteria, the authority to impose restrictions or preclusion of amphibian export out of New Zealand is not available except through international obligations to inform the importing country, which leaves the imposing of restrictions to their legal and political systems. Additionally, countries with import health standards for amphibians typically have inadequate examination requirements for identifying amphibians carrying chytrid, or other diseases of international significance. These factors combined will continue to allow the international spread of chytrid with consequential risks to global amphibian biodiversity.