Dynamics of the amphibian chytrid fungus (Batrachochytrium dendrobatidis) in isolated patches of lowland rainforest

Over the past 20 years, amphibian declines caused by Batrachochytrium dendrobatidis (Bd) have prompted a significant amount of research into the amphibian-chytrid host-parasite relationship. The complexities of the relationship have limited our ability to understand the pathogen, particularly as differences are observed in the impacts of Bd among different amphibian species, and the variation in the impact of the pathogen along an elevation gradient. The significant declines of amphibians at high elevations (400+ m asl) in the Wet Tropics bioregion of north eastern Australia has focused research on endemic species, and the environmental conditions they experience. There is, however, little research on tropical lowland areas. Within tropical lowlands rainforest of the Wet Tropics bioregion, Bd-susceptible amphibians continue to thrive, and although Bd is present in the tropical lowlands, it has not been as devastating to amphibian populations. Much of the lowland tropical rainforest of the Wet Tropics is contiguous with higher elevation areas, providing the potential for Bd at higher elevations to influence lowland populations as infected hosts and Bd zoospores move downstream. However, not all areas of lowland rainforest are contiguous with the uplands. There are isolated rainforest patches, with no connection to high-elevation streams, where Bd is present and experiences a different set of environmental conditions to much of the rest of the Wet Tropics. It is within these lowland areas, which are not contiguous with highland rainforest streams, that I conducted my research. The aims of my study were to investigate the ecology of the common mist frog (Litoria rheocola), and determine the influence of Bd in this system. This was done by a) investigating seasonal variation in the behaviour of uninfected L. rheocola, b) examining behavioural differences between Bd infected and uninfected L. rheocola, and c) investigating the potential reservoir hosts of the amphibian chytrid fungus Bd of various amphibian and non-amphibian species. The common mist frog, L. rheocola, has similar ecological requirements to Bd; both share the need of moisture and relatively cool conditions. I examined seasonal changes in the thermal microenvironment, behaviour, movement, microhabitat selection and body temperatures of uninfected L. rheocola, using harmonic radar tracking. Warm environmental temperatures in my study area limit the suitability of the tropical lowlands for Bd to winter. Movements and microhabitat use of L. rheocola were similar year-round. My results suggested that increases in body temperatures due to seasonal increases in environmental temperatures may protect lowland populations from decline due to Bd. During the winter, a large proportion of L. rheocola in my study area became infected. Comparing the behaviours of infected and uninfected individuals in winter suggested that uninfected individuals maintained higher body temperatures, perched further from the stream, and used microhabitats that minimized the probability of becoming infected. The behaviour of individuals during periods of high infection risk can reduce their probability of infection. In my study areas, prevalence of Bd dropped to zero in L. rheocola in the summer. The range of amphibians that naturally become infected by Bd is large, but Bd has not been recorded in all amphibians, including species common within the distributional range of Bd. By swabbing all the frogs in my study area, I detected Bd in northern dwarf treefrogs (Litoria bicolor) and Australian wood frogs (Hylarana daemeli), in which Bd had not previously been detected. I also detected Bd on a range of stream-associated invertebrates. If Bd can persist on flying invertebrates, it provides a potential method for the spread and dispersal of Bd, an aspect of the pathogen's ecology that remains unknown. Stream-associated invertebrates provide a potential vector that could have aided in the spread of Bd within and between regions and countries. The tropical lowlands of the Wet Tropics provide a refuge for many species of Bd-susceptible amphibians. Their persistence may permit the evolution of increased immunity, allowing recolonisation of upland areas. My study of lowland populations has provided an increased understanding of the dynamics allowing persistence of these frogs.