Host species determines whether infection load increases beyond disease-causing thresholds following exposure to the amphibian chytrid fungus M. P. Stockwell, J. Clulow & M. J. Mahony Environmental Biology and Biotechnology Group, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW, Australia Keywords Amphibian chytrid fungus; Batrachochytrium dendrobatidis; Infection Threshold; Host Species. Correspondence Michelle P. Stockwell, Environmental Biology and Biotechnology Group, School of Environmental and Life Sciences, The University of Newcastle, Callaghan Drive, Callaghan, NSW 2308, Australia. Email: Michelle.Stockwell@ newcastle.edu.au Received 15 September 2010; accepted 7 October 2010 doi:10.1111/j.1469-1795.2010.00407.x Abstract Introduced pathogens are increasingly being implicated in population declines and their effects are difficult to manage. In the absence of methods to eradicate pathogens acting as threatening processes, intervention before population decline is necessary. Such an intervention requires an ability to predict when population declines will occur, and therefore, an understanding of when exposure will lead to infection, disease, death and population decline. This study investigates when pathogen exposure leads to disease for the amphibian chytrid fungus Batrachochy- trium dendrobatidis, which has been implicated as a causal agent in the global amphibian decline. Susceptibility studies were conducted on two anuran species, the green and golden bell frog Litoria aurea and the striped marsh frog Limnody- nastes peronii, when exposed to the fungus as either tadpoles or juveniles. Host species was found to significantly affect the outcome of exposure, with infection loads in L. aurea increasing over time and resulting in significantly lower survival rates than unexposed. By comparison, infection loads in L. peronii remained the same or decreased over time following the initial infection, and survival rates were no different whether exposed to B. dendrobatidis or not. These outcomes were independent of the life stage at exposure. Individuals with higher infection loads were not found to have lower survival rates; rather, an infection load threshold was identified where individuals with infection loads that crossed this threshold had high likelihoods of showing terminal signs of chytridiomycosis. Therefore, host species determined whether infection load crossed this threshold and the crossing of the threshold determined the incidence of disease and survival. The quantifica- tion of infection load thresholds for survival, along with the time it takes to reach them, will enable infection loads in wild populations to be related to the likelihood of disease and is the first step in the understanding and prediction of when exposure will result in population decline. Introduction Pathogens of wildlife were historically believed to have little impact on host populations (Hassell et al., 1982; May, 1988; Scott, 1988; Daszak & Cunningham, 1998) but epidemiolo- gical studies (Keymer, 1981; Anderson & Crombie, 1984; Scott & Anderson, 1984; Scott, 1987) and theoretical mod- elling (Anderson & May, 1978, 1980) conducted throughout the 1970s and 1980s have shown that disease can suppress host population growth rate. In situations where transmis- sion is density dependent, disease can cause cyclical fluctua- tions in population size and can play an important role in the regulation of host population dynamics and community structure (Scott, 1988; Thompkins & Begon, 1999). In recent decades, there has been an increased frequency of emerging infectious diseases of wildlife, which has resulted in rapid population declines and extinctions (McCallum & Dobson, 1995; Daszak, 1999a,b). This emergence has primarily been attributed to anthropogenically induced changes in the ecology of the host and/or pathogen. The loss and fragmen- tation of habitat have resulted in higher host densities and transmission rates, while environmental modifications have increased habitat suitability for pathogen growth and survi- val (Daszak, 1999a,b). The introduction of novel pathogens into naı¨ve host populations through spill over from sympa- tric domesticated animals or the translocation of pathogens into new geographic ranges have also resulted in the emer- gence of infectious diseases (Daszak, 1999a,b). One of the most dramatic disease-induced losses of biodiversity in recent history has been the global amphibian decline. Over 30% of the world’s recognized amphibian species are currently classified by the IUCN as either extinct or threatened with extinction (Stuart et al., 2004; IUCN, 2010). Animal Conservation 13, Suppl. 1 (2010) 62–71 c  2010 The Authors. Animal Conservation c  2010 The Zoological Society of London 62 Animal Conservation. Print ISSN 1367-9430