Effects of environment and microhabitat on susceptibility to the amphibian chytrid fungus

The emerging disease chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis, has caused declines and extinctions of many amphibian species in many regions.  A broad range of amphibians are highly susceptible to the disease in the field, while many sympatric species are less susceptible.  The prevalence and virulence of the disease in populations is affected by season and by macroenvironmental variables.  Laboratory studies have shown that the growth of the fungus in vitro is strongly affected by environmental temperature and moisture; similar effects occur in experimentally infected animals.  We examined the detailed microenvironmental use of three species of tropical Australian rain forest frogs that ranged from highly affected to unaffected by population declines caused by chytridiomycosis.  We found that the most affected species, Litoria nannotis, spent almost all of its time in microhabitats that should maximize susceptibility to the disease.  Individuals of this species also frequently occurred in aggregations, which should increase transmission rates.  The species with intermediate levels of susceptibility to the disease, Litoria genimaculata, spent substantial periods in the upper canopy, where temperatures are higher and humidity is lower, and often experienced body temperatures outside the optimal growth range for the fungus.  The least susceptible species, Litoria lesueuri, frequently basked in exposed positions, elevating its body temperature into the range lethal to the fungus in vitro.   Our results indicate that variation in microhabitat use may have strong effects on the vulnerability of amphibians to chytridiomycosis.  They also reinforce the fact that sympatric species may experience widely differing thermal and humidity regimes at the same sites, which suggests that modeling the effects of climate change on species distributions may not be a simple matter.