In an earlier post, I discussed the enigma of Australia’s disappearing frog species and the discovery that these disappearances were linked with an infectious skin disease called chytridiomycosis. (see post on March 4, 2010, On Australia’s Threatened Frogs, http://maginams.ca/2010/03/04/). Late this afternoon, I attended a seminar at James Cook University titled ‘Amphibian population declines in tropical Australia: lessons learnt.’ Presented by JCU Townsville’s Dr. Ross A. Alford, the seminar explored the past frog disappearances and described recent research projects done by Dr. Alford and his students which have provided new insights into the battle of frog versus chytridiomycosis.
First, a brief history. In 1979, two frog species in the Conondale Range in southeast Queensland disappeared, as did two species in 1985 in the Eungella region in east-central Queensland. Near Paluma in North Queensland, two species seriously declined in 1990. These disappearances and declines led to the speculation that whatever was killing the frogs was linked with elevations above 400 metres. From 1989 to 1991, “populations blinked out all over the wet tropics,” as Dr. Alford said, and in 1993, the last few populations known to have disappeared did so in the far north of Queensland.
The disappearances of the frog populations were consistent with the idea of a wave of epidemic disease, which was in fact revealed to be true with the discovery of the skin disease chytridiomycosis on dead and dying frogs. This disease is caused by the growth of zoospores of the chytrid fungus Batrachochytrium dendrobatidis, which Dr. Alford henceforth referred to as Bd. The number of Bd zoospores increases exponentially on the skin of infected frogs and plateaus after 30 to 40 days. It’s during this plateau phase that the frogs first show signs of the disease, which inhibits normal ion exchange through the frogs’ skin. Two days after first showing signs of the disease, the frogs are dead. Meanwhile, zoospores have been released to infect other frogs.
In addition to censusing frog populations, Dr. Alford tackled the question What’s controlling the outbreaks? Through monitoring the incidence of disease in frogs in various habitats, he and his students determined that the highest numbers of diseased frogs occurred at high elevations during the cooler months of winter. Misty habitats (the Paluma rainforest, for example) facilitate rapid Bd reproduction, and hence, very quick frog deaths.
In general, it appears that a direct inverse correlation exists between temperature and the susceptibility of a frog to the disease. The fungus grows and reproduces more quickly at cooler temperatures, whereas high temperatures kill it. The threshold temperature is apparently 25°C. Below this temperature, the likelihood of a frog being infected is 3.5 to 1. Above the threshold, that ratio decreases. An infected frog exposed to an ambient temperature of 37°C for two days ensures its survival, since Bd can’t withstand that temperature.
As ectotherms, frogs regulate their body temperature by absorbing heat from their surroundings. Dr. Alford suggested that tropical frogs basking on exposed stream stones may increase their body temperature to a level sufficient to mitigate Bd effects. He also suggested that peripheral populations of frogs living in warmer, drier habitats may play a significant role in the conservation of species threatened or endangered by Bd. Interesting information, all.
