Thermal and hydric relations of Australian tropical rainforest frogs: A long way from constant

Background/Question/Methods: Tropical rainforests are often thought of as having relatively constant environmental conditions.  Amphibians that inhabit them have been regarded as experiencing environments that depend largely on their general habitat; many publications, for example, discuss “stream-dwelling” amphibians as a group that shares a common environment. If this was correct, environmental effects on the ecological interactions of these species should be similar across individuals, species, and time. We examined this hypothesis by tracking individual “stream-dwelling” rainforest frogs of five species (Litoria lesueuri complex, L. lorica, L. nannotis, L. rheocola, and L. serrata) , and collecting information on the thermal and moisture environments each individual experienced.  We did this by using a combination of temperature-sensitive radio transmitters, harmonic direction finding, infrared temperature measurement, and physical models of frogs placed in their retreat and activity sites.

Results/Conclusions:  The temperature and moisture environments experienced by these “stream-dwelling” frogs varied greatly among individuals, species, and seasons.  Within species, their choices of retreat and activity sites strongly affected the temperature and moisture environments that individuals experienced.  Diurnal body temperatures often differed by 10^oC or more among individuals of the same species measured within relatively short time intervals.  Moisture environments selected by individuals also varied substantially within species, with some individuals choosing relatively dry and others relatively wet environments.  Median temperature and moisture environments also differed consistently among species, with some, such as L. lorica, and L. nannotis, usually choosing cooler, less desiccating environments. However individuals of these species sometimes experienced relatively warm body temperatures, moderately dry environments, or both.  The environments experienced at the individual and species levels differed between the winter dry and summer wet seasons.  These differences are potentially very important in determining how these species interact with the amphibian pathogen Batrachochytrium dendrobatidis.  The probability of infection of individuals by this pathogen depended on both individual body temperature and individual moisture environment; it was higher in moister conditions and in individuals that seldom experienced body temperatures outside the pathogen’s optimal range.  Detailed information on the environmental conditions experienced by species and individuals within species can thus be essential for understanding species interactions, even in tropical rainforests.