Postdoctoral Research Scientist, James Cook University
We are using the mating calls of frogs to survey populations and understand environmental impacts
You know how frogs are some of the most threatened animals in the world and one of the main causes of extinctions has been from a fungal disease that has emerged almost everywhere? We have been studying frogs in one remote place where, so far, we have not yet found the fungus. We have swabbed frogs in the wild and museum collections in New Guinea - the world's largest tropical Island, home to 6% of the world's frog species. It appears the island may be free of the amphibian chytrid fungus, providing us with a remarkable opportunity for pre-emptive measures to conserve the frog species. We are using sound recorders to document the incredible diversity of frogs, many of which are unknown to science. Frogs are noisy and each species has its own unique mating call... This allows us to study the distribution and behaviour of the populations. By identifying where species occur and the habitats that they need and mapping the areas where the disease will be most severe, we are starting to figure out which species are vulnerable and need conservation action. We are also working out biosecurity strategies to prevent diseases from arriving, to keep this tropical island a special refuge. We aim to identify the highest risk activities that introduce diseases, like importing equipment or trading frogs for food or pets. Our research is helping to establish the tools necessary to fight the emergence of one of the world’s most destructive pathogens to biodiversity and to understand frogs in this unique part of the world.
Abstract: Ontogenetic changes in disease susceptibility have been demonstrated in many vertebrate taxa, as immature immune systems and limited prior exposure to pathogens can place less developed juveniles at a greater disease risk. By causing the disease chytridiomycosis, Batrachochytrium dendrobatidis (Bd) infection has led to the decline of many amphibian species. Despite increasing knowledge on how Bd varies in its effects among species, little is known on the interaction between susceptibility and development within host species. We compared the ontogenetic susceptibility of post-metamorphic green and golden bell frogs Litoria aurea to chytridiomycosis by simultaneously measuring three host-pathogen responses as indicators of the development of the fungus-infection load, survival rate, and host immunocompetence-following Bd exposure in three life stages (recently metamorphosed juveniles, subadults, adults) over 95 days. Frogs exposed to Bd as recently metamorphosed juveniles acquired higher infection loads and experienced lower immune function and lower survivorship than subadults and adults, indicating an ontogenetic decline in chytridiomycosis susceptibility. By corresponding with an intrinsic developmental maturation in immunocompetence seen in uninfected frogs, we suggest these developmental changes in host susceptibility in L. aurea may be immune mediated. Consequently, the physiological relationship between ontogeny and immunity may affect host population structure and demography through variation in life stage survival, and understanding this can shape management targets for effective amphibian conservation.
Pub.: 30 Mar '16, Pinned: 31 Jul '17
Abstract: A simple diagnosis of the presence or absence of an infection is an uninformative metric when individuals differ considerably in their tolerance to different infection loads or resistance to rates of disease progression. Models that incorporate the relationship between the progression of the infection with the potential alternate outcomes provide a far more powerful predictive tool than diagnosis alone. The global decline of amphibians has been amplified by Batrachochytrium dendrobatidis, a pathogen that can cause the fatal disease chytridiomycosis. We measured the infection load and observed signs of disease in Litoria aurea Receiver operating characteristic curves were used to quantify the dissimilarity between the infection loads of Litoria aurea that showed signs associated with chytridiomycosis and those that did not. Litoria aurea had a 78% probability of developing chytridiomycosis past a threshold of 68 zoospore equivalents (ZE) per swab and chytridiomycosis occurred within a variable range of 0.5-490 ZE. Studies should incorporate a species-specific threshold as a predictor of chytridiomycosis, rather than a binary diagnosis. Measures of susceptibility to chytridiomycosis must account not only for the ability of Batrachochytrium dendrobatidis to increase its abundance on the skin of amphibians but also to determine how each species tolerates these infection loads.
Pub.: 18 May '16, Pinned: 31 Jul '17
Abstract: Michelle P. Stockwell, Deborah S. Bower, John Clulow, Michael J. Mahony - Volume 43(4) Pathogens with reservoir hosts have been responsible for many disease-induced wildlife extinctions. This study investigated whether the green and golden bell frog (Litoria aurea), a threatened species highly susceptible to infection by the amphibian chytrid fungus (Batrachochytrium dendrobatidis), co-occurred with reservoirs hosts and identified four species carrying infection in the absence of signs of disease. The existence of reservoir hosts in this system must therefore be considered in future threat mitigation and conservation efforts.
Pub.: 06 Jul '16, Pinned: 31 Jul '17
Abstract: Bayesian network analyses can integrate complex relationships to examine a range of hypotheses and identify areas that lack associated empirical studies, to prioritise future research. We examined complex relationships in host and pathogen biology to examine disease-driven decline by the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), a pathogen that is reducing amphibian biodiversity globally. We constructed a Bayesian network consisting of a range of behavioural, genetic, physiological, and environmental variables that influence disease, and used them to predict host population trends (the variable 'Population trend' which could be declining or stable). The behaviour of the nodes (the way in which the variables probabilistically responded to changes in states of the parents, which are the nodes or variables that directly influenced them in the graphical model) in our model was consistent with published results, suggesting that the construction of our model reflected the complex relationships characteristic of host-pathogen interactions. We varied the impacts of specific variables in the model, to reveal factors with the most influence on host population trend. Changes to climatic conditions alone did not strongly influence the probability of population decline, suggesting that epidemics in this system do not occur solely because of climate, but instead interacted with other factors such as the capacity of the frog immune system to suppress disease. The effect of the adaptive immune system and disease reservoirs were important to the population trend, but there was little empirical information available for model construction: we suggest research in these areas will aid understanding of chytridiomycosis-induced declines. We include the input of our full model as a base that can be used to understand other systems, and we demonstrate that such analyses are useful tools for reviewing existing literature and identifying links poorly supported by evidence, and for understanding complexities in emerging infectious disease systems. This article is protected by copyright. All rights reserved.
Pub.: 04 May '17, Pinned: 31 Jul '17
Abstract: Surveillance of pathogens can lead to significant advances towards making effective decisions in research and management for species threatened by disease. Batrachochytrium dendrobatidis has been a major contributing factor to the global decline of amphibians. Knowledge of the distribution of B. dendrobatidis can contribute to understanding patterns of species decline and prioritizing action. Therefore, we surveyed four spatially distinct populations of a B. dendrobatidis susceptible species, the green and golden bell frog (Litoria aurea), for evidence of infection in the population. Three mainland populations were infected at a prevalence of 3.5–28.3 %, with median infection loads of 0.28–627.18 genomic equivalents (GE). Conversely, we did not detect infection in an island population 3 km from the mainland; the isolation and infrequent visitation of the island suggests that the pathogen has not arrived. Management actions for B. dendrobatidis and conservation of susceptible frog species are heavily dependent on the presence and absence of the pathogen in the population. Prevention of the accidental introduction of B. dendrobatidis and safe guarding genetic diversity of L. aurea is necessary to preserve unique diversity of the island population, whereas containment and control of the pathogen can be directed towards mainland populations. Knowledge of disease dynamics also provides a context to understand the ecology of remaining populations as variation in the physiology or habitat of the mainland populations have facilitated persistence of these populations alongside B. dendrobatidis. Other islands should be a priority target in disease surveillance, to discover refuges that can assist conservation.
Pub.: 26 Jun '15, Pinned: 31 Jul '17