Lethal Effects of Water Quality on Threatened California Salamanders but Not on Co‐Occurring Hybrid Salamanders

Biological invasions and habitat alteration are often detrimental to native species, but their interactions are difficult to predict. Interbreeding between native and introduced species generates novel genotypes and phenotypes, and human land use alters habitat structure and chemistry. Both invasions and habitat alteration create new biological challenges and opportunities. In the intensively farmed Salinas Valley, California (U.S.A.), threatened California tiger salamanders (Ambystoma californiense) have been replaced by hybrids between California tiger salamander and introduced barred tiger salamanders (Ambystoma tigrinum mavortium). We conducted an enclosure experiment to examine the effects habitat modification and relative frequency of hybrid and native California tiger salamanders have on recruitment of salamanders and their prey, Pacific chorus frogs (Pseudacris regilla). We tested whether recruitment differed among genetic classes of tiger salamanders (hybrid or native) and pond hydroperiod (seasonal or perennial). Roughly 6 weeks into the experiment, 70% (of 378 total) of salamander larvae died in 4 out of 6 ponds. Native salamanders survived (n = 12) in these ponds only if they had metamorphosed prior to the die‐offs. During die‐offs, all larvae of native salamanders died, whereas 56% of hybrid larvae died. We necropsied native and hybrid salamanders, tested water quality, and queried the California Department of Pesticide Regulation database to investigate possible causes of the die‐offs. Salamander die‐offs, changes in the abundance of other community members (invertebrates, algae, and cyanobacteria), shifts in salamander sex ratio, and patterns of pesticide application in adjacent fields suggest that pesticide use may have contributed to die‐offs. That all survivors were hybrids suggests that environmental stress may promote rapid displacement of native genotypes. Efectos Letales de la Calidad del Agua sobre Salamandras de California Amenazadas pero no sobre Salamandras Híbridas Concurrentes     

Effects of Amphibian Chytrid Fungus on Individual Survival Probability in Wild Boreal Toads

Abstract: Chytridiomycosis is linked to the worldwide decline of amphibians, yet little is known about the demographic effects of the disease. We collected capture–recapture data on three populations of boreal toads (Bufo boreas [Bufo = Anaxyrus]) in the Rocky Mountains (U.S.A.). Two of the populations were infected with chytridiomycosis and one was not. We examined the effect of the presence of amphibian chytrid fungus (Batrachochytrium dendrobatidis [Bd]; the agent of chytridiomycosis) on survival probability and population growth rate. Toads that were infected with Bd had lower average annual survival probability than uninfected individuals at sites where Bd was detected, which suggests chytridiomycosis may reduce survival by 31–42% in wild boreal toads. Toads that were negative for Bd at infected sites had survival probabilities comparable to toads at the uninfected site. Evidence that environmental covariates (particularly cold temperatures during the breeding season) influenced toad survival was weak. The number of individuals in diseased populations declined by 5–7%/year over the 6 years of the study, whereas the uninfected population had comparatively stable population growth. Our data suggest that the presence of Bd in these toad populations is not causing rapid population declines. Rather, chytridiomycosis appears to be functioning as a low‐level, chronic disease whereby some infected individuals survive but the overall population effects are still negative. Our results show that some amphibian populations may be coexisting with Bd and highlight the importance of quantitative assessments of survival in diseased animal populations.     

Historical Evidence of Widespread Chytrid Infection in North American Amphibian Populations

Abstract:  Emerging infectious diseases may be contributing to the global decline of amphibian populations. In particular, chytridiomycosis, a cutaneous fungal infection of amphibians caused by the chytrid Batrachochytrium dendrobatidis , gained attention in the 1990s as an apparently new pathogen. This fungus has been implicated as the causative agent of widespread mortalities among wild amphibians in geographically disparate parts of the world. To investigate the global distribution, historical occurrence, and infectiousness of this pathogen, we examined 3371 postmetamorphic and adult amphibians collected between 1895 and 2001 from 25 countries for the presence of chytrid fungi in the epidermis. Cutaneous chytrid infection was diagnosed in 383 of 2931 (13.1%) specimens of 12 common amphibian species from five Canadian provinces and seven American states, including 30 of 69 locations examined in the St. Lawrence River Valley of Québec. Chytrids were observed in 7.0% (46/655) of the adults collected in the 1960s, the earliest cases being two green frogs (  Rana clamitans ) collected in 1961 from Saint-Pierre-de-Wakefield, Québec. In recent studies, morbidity and mortality attributable to chytridiomycosis were not observed in infected amphibians from Québec despite a 17.8% (302/1698) prevalence of chytrid infection during the period 1990–2001. The prevalence of infection during this latter period was not statistically different when compared with the period 1960–1969. Epidermal chytrid infections were not observed in 440 amphibians examined from 23 other countries. In light of the fact that infection by B. dendrobatidis is widely distributed and apparently enzootic in seemingly healthy amphibians from eastern North America, lethal outbreaks of chytridiomycosis among amphibian populations appear to have complex causes and may be the result of underlying predisposing factors.     

Impact and Dynamics of Disease in Species Threatened by the Amphibian Chytrid Fungus, Batrachochytrium dendrobatidis

Abstract:  Estimating disease-associated mortality and transmission processes is difficult in free-ranging wildlife but important for understanding disease impacts and dynamics and for informing management decisions. In a capture–mark–recapture study, we used a PCR-based diagnostic test in combination with multistate models to provide the first estimates of disease-associated mortality and detection, infection, and recovery rates for frogs endemically infected with the chytrid fungus Batrachochytrium dendrobatidis (Bd), which causes the pandemic amphibian disease chytridiomycosis. We found that endemic chytridiomycosis was associated with a substantial reduction (approximately 38%) in apparent monthly survival of the threatened rainforest treefrog Litoria pearsoniana despite a long period of coexistence (approximately 30 years); detection rate was not influenced by disease status; improved recovery and reduced infection rates correlated with decreased prevalence, which occurred when temperatures increased; and incorporating changes in individuals' infection status through time with multistate models increased effect size and support (98.6% vs. 71% of total support) for the presence of disease-associated mortality when compared with a Cormack–Jolly–Seber model in which infection status was restricted to the time of first capture. Our results indicate that amphibian populations can face significant ongoing pressure from chytridiomycosis long after epidemics associated with initial Bd invasions subside, an important consideration for the long-term conservation of many amphibian species worldwide. Our findings also improve confidence in estimates of disease prevalence in wild amphibians and provide a general framework for estimating parameters in epidemiological models for chytridiomycosis, an important step toward better understanding and management of this disease.     

The use of environmental DNA in invasive species surveillance of the Great Lakes commercial bait trade

Over 180 non‐native species have been introduced in the Laurentian Great Lakes region, many posing threats to native species and ecosystem functioning. One potential pathway for introductions is the commercial bait trade; unknowing or unconcerned anglers commonly release unused bait into aquatic systems. Previous surveillance efforts of this pathway relied on visual inspection of bait stocks in retail shops, which can be time and cost prohibitive and requires a trained individual that can rapidly and accurately identify cryptic species. Environmental DNA (eDNA) surveillance, a molecular tool that has been used for surveillance in aquatic environments, can be used to efficiently detect species at low abundances. We collected and analyzed 576 eDNA samples from 525 retail bait shops throughout the Laurentian Great Lake states. We used eDNA techniques to screen samples for multiple aquatic invasive species (AIS) that could be transported in the bait trade, including bighead (Hypophthalmichthys nobilis) and silver carp (H. molitrix), round goby (Neogobius melanostomus), tubenose goby (Proterorhinus marmoratus), Eurasian rudd (Scardinius erythrophthalmus), and goldfish (Carassius auratus). Twenty‐seven samples were positive for at least one target species (4.7% of samples), and all target species were found at least once, except bighead carp. Despite current regulations, the bait trade remains a potential pathway for invasive species introductions in the Great Lakes region. Alterations to existing management strategies regarding the collection, transportation, and use of live bait are warranted, including new and updated regulations, to prevent future introductions of invasive species in the Great Lakes via the bait trade. El Uso del ADN Ambiental en la Vigilancia de Especies Invasoras del Mercado de Carnada Comercial de los Grandes Lagos     

Threat-sensitive generalization of predator recognition by larval amphibians

Despite the importance of acquired predator recognition in mediating predator–prey interactions, we know little about the specific characteristics that prey use to distinguish predators from non-predators. Recent experiments with mammals and fish indicate that some prey lacking innate predator recognition have the ability to display anti-predator responses upon their first encounter with those predators if they are similar to predators that the prey has recently learned to recognize. This phenomenon is referred to as generalization of predator recognition. In this experiment, we documented for the first time that larval amphibians (woodfrog, Rana sylvatica) have the ability to generalize the recognition of known predators to closely related novel predators. Moreover, we demonstrated that this ability is dependent on the level of risk associated with the known predator. When red-bellied newt, Cynops pyrrhogaster (known predator), was paired with simulated low risk, tadpoles displayed fright responses to newts and novel tiger salamanders, Ambystoma tigrinum, but not to novel African clawed frogs, Xenopus laevis. However, when the newt was paired with simulated high risk, tadpoles generalized their responses to both tiger salamanders and African clawed frogs. Larval anurans seem to have a wider generalization frame than other animals.     

Varying responses of northeastern North American amphibians to the chytrid pathogen Batrachochytrium dendrobatidis

Chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd), is widespread among amphibians in northeastern North America. It is unknown, however, whether Bd has the potential to cause extensive amphibian mortalities in northeastern North America as have occurred elsewhere. In the laboratory, we exposed seven common northeastern North American amphibian species to Bd to assess the likelihood of population-level effects from the disease. We exposed larval wood frogs (Lithobates sylvaticus) and postmetamorphic frogs of six other species to two different strains of Bd, a northeastern strain (JEL404) and a strain that caused die-offs of amphibians in Panama (JEL423), under ideal in vitro growth conditions for Bd. Exposed American toads (Anaxyrus americanus) all died; thus, this species may be the most likely to die from Bd-caused disease in the wild. Both Bd strains were associated with mortalities of wood frogs, although half the metamorphs survived. The Bd strain from Panama killed metamorphic green frogs (L. clamitans), whereas the northeastern strain did not, which means novel strains of Bd may lead to death even when local strains may not. No mortality was observed in four species (bullfrogs [L. catesbeianus], northern leopard frogs [L. pipiens], spring peepers [Pseudacris crucifer], and blue-spotted salamanders [Ambystoma laterale]) and in some individuals of green frogs and wood frogs that we exposed. This finding suggests these six species may be Bd vectors. Our results show that systematic exposures of amphibian species to Bd in the laboratory may be a good first step in the identification of species susceptible to Bd-caused declines and in directing regional conservation efforts aimed at susceptible species.     

Reservoir‐host amplification of disease impact in an endangered amphibian

Emerging wildlife pathogens are an increasing threat to biodiversity. One of the most serious wildlife diseases is chytridiomycosis, caused by the fungal pathogen, Batrachochytrium dendrobatidis (Bd), which has been documented in over 500 amphibian species. Amphibians vary greatly in their susceptibility to Bd; some species tolerate infection, whereas others experience rapid mortality. Reservoir hosts—species that carry infection while maintaining high abundance but are rarely killed by disease—can increase extinction risk in highly susceptible, sympatric species. However, whether reservoir hosts amplify Bd in declining amphibian species has not been examined. We investigated the role of reservoir hosts in the decline of the threatened northern corroboree frog (Pseudophryne pengilleyi) in an amphibian community in southeastern Australia. In the laboratory, we characterized the response of a potential reservoir host, the (nondeclining) common eastern froglet (Crinia signifera), to Bd infection. In the field, we conducted frog abundance surveys and Bd sampling for both P. pengilleyi and C. signifera. We built multinomial logistic regression models to test whether Crinia signifera and environmental factors were associated with P. pengilleyi decline. C. signifera was a reservoir host for Bd. In the laboratory, many individuals maintained intense infections (>1000 zoospore equivalents) over 12 weeks without mortality, and 79% of individuals sampled in the wild also carried infections. The presence of C. signifera at a site was strongly associated with increased Bd prevalence in sympatric P. pengilleyi. Consistent with disease amplification by a reservoir host, P. pengilleyi declined at sites with high C. signifera abundance. Our results suggest that when reservoir hosts are present, population declines of susceptible species may continue long after the initial emergence of Bd, highlighting an urgent need to assess extinction risk in remnant populations of other declined amphibian species.     

Selecting for Tolerance against Pathogens and Herbivores to Enhance Success of Reintroduction and Translocation

Abstract: Some species have insufficient defenses against climate change, emerging infectious diseases, and non‐native species because they have not been exposed to these factors over their evolutionary history, and this can decrease their likelihood of persistence. Captive breeding programs are sometimes used to reintroduce individuals back into the wild; however, successful captive breeding and reintroduction can be difficult because species or populations often cannot coexist with non‐native pathogens and herbivores without artificial selection. In captive breeding programs, breeders can select for host defenses that prevent or reduce pathogen or herbivore burden (i.e., resistance) or traits that limit the effects of parasitism or herbivory on host fitness (i.e., tolerance). We propose that selection for host tolerance may enhance the success of reintroduction or translocation because tolerant hosts generally have neutral effects on introduced pathogens and herbivores. The release of resistant hosts would have detrimental effects on their natural enemies, promoting rapid evolution to circumvent the host resistance that may reduce the long‐term probability of persistence of the reintroduced or translocated species. We examined 2 case studies, one on the pathogenic amphibian chytrid fungus ( Batrachochytrium dendrobatidis [Bd]) and the other on the herbivorous cactus moth ( Cactoblastis cactorum) in the United States, where it is not native. In each case study, we provide recommendations for how captive breeders and managers could go about selecting for host tolerance. Selecting for tolerance may offer a promising tool to rescue hosts species from invasive natural enemies as well as new natural enemies associated with climate change‐induced range shifts.     

Killing Tigers to Save Them: Fallacies of the Farming Argument

Abstract: The lucrative, illegal trade in tigers (Panthera tigris) remains a major conservation problem. Tiger farming has been proposed as a potential solution, with farmed tigers substituting for wild tigers. At first glance, this argument's logic seems simple: farming will increase the supply of tigers, prices will fall, and poaching will no longer be profitable. We contend, however, that this supply‐side argument relies on mistaken assumptions. First, tiger markets are imperfect, meaning they are dominated by a few producers who control price. Second, consumers prefer wild tigers to farmed tigers and therefore the two are not pure substitutes. In economic terms, products from wild tigers are luxury goods, commanding a price premium. Third, there is no evidence that farmed tigers can be produced or sold more cheaply than wild tigers. In sum, it is unlikely that farming will drive down the price of wild‐caught tigers or decrease profitability for tiger poachers. Rather, tiger farming is more likely to increase aggregate demand for tiger products and stimulate higher levels of poaching.     

Environmental Determinants of Recent Endemism of Batrachochytrium dendrobatidis Infections in Amphibian Assemblages in the Absence of Disease Outbreaks

The inconsistent distribution of large‐scale infection mediated die‐offs and the subsequent population declines of several animal species, urges us to understand how, when, and why species are affected by disease. It is often unclear when or under what conditions a pathogen constitutes a threat to a host. Often, variation of environmental conditions plays a role. Globally Batrachochytrium dendrobatidis (Bd) causes amphibian declines; however, host responses are inconsistent and this fungus appears equally capable of reaching a state of endemism and subsequent co‐existence with native amphibian assemblages. We sought to identify environmental and temporal factors that facilitate host–pathogen coexistence in northern Europe. To do this, we used molecular diagnostics to examine archived and wild amphibians for infection and general linear mixed models to explore relationships between environmental variables and prevalence of infection in 5 well‐sampled amphibian species. We first detected infection in archived animals collected in 1999, and infection was ubiquitous, but rare, throughout the study period (2008–2010). Prevalence of infection exhibited significant annual fluctuations. Despite extremely rare cases of lethal chytridiomycosis in A. obstetricans, Bd prevalence was uncorrelated with this species’ population growth. Our results suggest context dependent and species‐specific host susceptibility. Thus, we believe recent endemism of Bd coincides with environmentally driven Bd prevalence fluctuations that preclude the build‐up of Bd infection beyond the critical threshold for large‐scale mortality and host population crashes. Determinantes Ambientales del Endemismo Reciente de Infecciones de Batrachochytrium dendrobatidis en Conjuntos de Anfibios en Ausencia de Brotes de Enfermedades Spitzen et al.     

Cover Caption

Cover : Farming of tigers (Panthera tigris) has been proposed as a disincentive to illegal trade that is decreasing the abundance and distribution of tigers in the wild. In this issue, Kirkpatrick and Emerton (pp. 655–659) examine the economic rationale for tiger farming. In theory, the pro. tability of poaching will decrease as availability of tigers increases. Given the consumer preference for wild tigers and price controls by traders, however, farming actually may increase demand for wild tigers and lead to a greater incidence of poaching.     

Captive Breeding, Reintroduction, and the Conservation of Amphibians

Abstract: The global amphibian crisis has resulted in renewed interest in captive breeding as a conservation tool for amphibians. Although captive breeding and reintroduction are controversial management actions, amphibians possess a number of attributes that make them potentially good models for such programs. We reviewed the extent and effectiveness of captive breeding and reintroduction programs for amphibians through an analysis of data from the Global Amphibian Assessment and other sources. Most captive breeding and reintroduction programs for amphibians have focused on threatened species from industrialized countries with relatively low amphibian diversity. Out of 110 species in such programs, 52 were in programs with no plans for reintroduction that had conservation research or conservation education as their main purpose. A further 39 species were in programs that entailed captive breeding and reintroduction or combined captive breeding with relocations of wild animals. Nineteen species were in programs with relocations of wild animals only. Eighteen out of 58 reintroduced species have subsequently bred successfully in the wild, and 13 of these species have established self‐sustaining populations. As with threatened amphibians generally, amphibians in captive breeding or reintroduction programs face multiple threats, with habitat loss being the most important. Nevertheless, only 18 out of 58 reintroduced species faced threats that are all potentially reversible. When selecting species for captive programs, dilemmas may emerge between choosing species that have a good chance of surviving after reintroduction because their threats are reversible and those that are doomed to extinction in the wild as a result of irreversible threats. Captive breeding and reintroduction programs for amphibians require long‐term commitments to ensure success, and different management strategies may be needed for species earmarked for reintroduction and species used for conservation research and education.     

Using a Bayesian network to clarify areas requiring research in a host–pathogen system

Bayesian network analyses can be used to interactively change the strength of effect of variables in a model to explore complex relationships in new ways. In doing so, they allow one to identify influential nodes that are not well studied empirically so that future research can be prioritized. We identified relationships in host and pathogen biology to examine disease‐driven declines of amphibians associated with amphibian chytrid fungus (Batrachochytrium dendrobatidis). We constructed a Bayesian network consisting of behavioral, genetic, physiological, and environmental variables that influence disease and used them to predict host population trends. We varied the impacts of specific variables in the model to reveal factors with the most influence on host population trend. The behavior 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) was consistent with published results. The frog population had a 49% probability of decline when all states were set at their original values, and this probability increased when body temperatures were cold, the immune system was not suppressing infection, and the ambient environment was conducive to growth of B. dendrobatidis. These findings suggest the construction of our model reflected the complex relationships characteristic of host–pathogen interactions. Changes to climatic variables alone did not strongly influence the probability of population decline, which suggests that climate interacts with other factors such as the capacity of the frog immune system to suppress disease. Changes to the adaptive immune system and disease reservoirs had a large effect on the population trend, but there was little empirical information available for model construction. Our model inputs can be used as a base to examine other systems, and our results show that such analyses are useful tools for reviewing existing literature, identifying links poorly supported by evidence, and understanding complexities in emerging infectious‐disease systems.     

Role of Genetic Refuges in the Restoration of Native Gene Pools of Brown Trout

Abstract:  Captive-bred animals derived from native, alien, or hybrid stocks are often released in large numbers in natural settings with the intention of augmenting harvests. In brown trout ( Salmo trutta ), stocking with hatchery-reared non-native fish has been the main management strategy used to maintain or improve depleted wild brown trout populations in Iberian and other Mediterranean regions. This measure has become a serious threat to the conservation of native genetic diversity, mainly due to introgressive hybridization. Aware of this risk, the agency responsible for management of brown trout in the eastern Pyrenees (Spain) created "brown trout genetic refuges" to preserve the integrity of brown trout gene pools in this region. Within refuge areas, the prerefuge status with respect to fishing activities has been maintained, but hatchery releases have been banned completely. We evaluated this management strategy through a comparison of the stocking impact on native populations that accounted for stocking histories before and after refuge designations and fishing activities. In particular we examined the relevant scientific, cultural, and political challenges encountered. Despite agency willingness to change fishery policies to balance exploitation and conservation, acceptance of these new policies by anglers and genetic monitoring of refuge populations should also be considered. To improve management supported by genetic refuges, we suggest focusing on areas where the public is more receptive, considering the situation of local native diversity, and monitoring of adjacent introgressed populations. We recommend the use of directional supportive breeding only when a population really needs to be enhanced. In any case, management strategies should be developed to allow for protection within the context of human use.     

A Case for Using Plethodontid Salamanders for Monitoring Biodiversity and Ecosystem Integrity of North American Forests

Abstract: Terrestrial salamanders of the family Plethodontidae have unique attributes that make them excellent indicators of biodiversity and ecosystem integrity in forested habitats. Their longevity, small territory size, site fidelity, sensitivity to natural and anthropogenic perturbations, tendency to occur in high densities, and low sampling costs mean that counts of plethodontid salamanders provide numerous advantages over counts of other North American forest organisms for indicating environmental change. Furthermore, they are tightly linked physiologically to microclimatic and successional processes that influence the distribution and abundance of numerous other hydrophilic but difficult-to-study forest-dwelling plants and animals. Ecosystem processes such as moisture cycling, food-web dynamics, and succession, with their related structural and microclimatic variability, all affect forest biodiversity and have been shown to affect salamander populations as well. We determined the variability associated with sampling for plethodontid salamanders by estimating the coefficient of variation (CV ) from available time-series data. The median coefficient of variation indicated that variation in counts of individuals among studies was much lower in plethodontids (27%) than in lepidoptera (93%), passerine birds (57%), small mammals (69%), or other amphibians (37–46%), which means plethodontid salamanders provide an important statistical advantage over other species for monitoring long-term forest health.     

The potential of marine live-bait introductions into oceanic islands

The likely introduction of non-indigenous species into the coastal ecosystems of oceanic islands through live-bait utilizations was studied. A store survey identified the species that are traded as live fishing-bait. Only Polychaeta species were identified as imported live-bait on São Miguel Island and all specimens were identified as Perinereis linea. Reponses to surveys of 77 anglers showed that: 44 % harvest from native sources, 44 % buy live-bait and 12 % do both; 56 % of the anglers’ fish on a weekly basis; 31 % dispose of the bait into the sea. A field survey at five sites where fishing rods are routinely utilized was developed to investigate if the non-indigenous polychaetes sold as fishing-bait are already established in the natural environment of the Azores archipelago. Three species of Nereididae were identified: the native Neanthes nubila and P. oliveirae and the cryptogenic P. cultrifera. The escalating utilization of live-bait argues for a change in the habits of fishermen with respect to the discarding of leftover bait, as the higher the volume of discards the higher becomes the probability of non-indigenous species establishment successes, especially on oceanic islands characterized by vulnerable ecosystems with simpler trophic webs.     

Interventions for Reducing Extinction Risk in Chytridiomycosis‐Threatened Amphibians

Wildlife diseases pose an increasing threat to biodiversity and are a major management challenge. A striking example of this threat is the emergence of chytridiomycosis. Despite diagnosis of chytridiomycosis as an important driver of global amphibian declines 15 years ago, researchers have yet to devise effective large‐scale management responses other than biosecurity measures to mitigate disease spread and the establishment of disease‐free captive assurance colonies prior to or during disease outbreaks. We examined the development of management actions that can be implemented after an epidemic in surviving populations. We developed a conceptual framework with clear interventions to guide experimental management and applied research so that further extinctions of amphibian species threatened by chytridiomycosis might be prevented. Within our framework, there are 2 management approaches: reducing Batrachochytrium dendrobatidis (the fungus that causes chytridiomycosis) in the environment or on amphibians and increasing the capacity of populations to persist despite increased mortality from disease. The latter approach emphasizes that mitigation does not necessarily need to focus on reducing disease‐associated mortality. We propose promising management actions that can be implemented and tested based on current knowledge and that include habitat manipulation, antifungal treatments, animal translocation, bioaugmentation, head starting, and selection for resistance. Case studies where these strategies are being implemented will demonstrate their potential to save critically endangered species. Intervenciones para Reducir el Riesgo de Extinción en Anfibios Amenazados por la Quitridiomicosis     

Differential Host Susceptibility to Batrachochytrium dendrobatidis,an Emerging Amphibian Pathogen

Abstract: The amphibian fungal pathogen Batrachochytrium dendrobatidis (Bd) has received considerable attention due to its role in amphibian population declines worldwide. Although many amphibian species appear to be affected by Bd, there is little information on species‐specific differences in susceptibility to this pathogen. We used a comparative experimental approach to examine Bd susceptibility in 6 amphibian species from the United States. We exposed postmetamorphic animals to Bd for 30 days and monitored mortality, feeding rates, and infection levels. In all species tested, Bd‐exposed animals had higher rates of mortality than unexposed (control) animals. However, we found differences in mortality rates among species even though the amount of Bd detected on the different species’ bodies did not differ. Of the species tested, southern toads (Anaxyrus terrestris) and wood frogs (Lithobates sylvaticus) had the highest rates of Bd‐related mortality. Within species, we detected lower levels of Bd on individuals that survived longer and found that the relationship between body size and infection levels differed among species. Our results indicate that, even under identical conditions, amphibian species differ in susceptibility to Bd. This study represents a step toward identifying and understanding species variation in disease susceptibility, which can be used to optimize conservation strategies.     

Ecology of Chytridiomycosis in Rainforest Stream Frog Assemblages of Tropical Queensland

Abstract:  In the wet tropics of Queensland, Australia, eight species of stream-dwelling frogs have experienced population declines. Some declines were associated with an emerging infectious disease of amphibians (chytridiomycosis) caused by the fungus Batrachochytrium dendrobatidis. We examined the spatial and temporal pattern of infection prevalence in a sample of frog populations. Infected adults and tadpoles of all species were found, and infections occurred at every site. Infection prevalence varied among species and was always < 10.0% in adults but ranged from 0.75 to 76.0% in tadpoles. In this system tadpoles and adults of some species may act as disease reservoirs, experiencing avirulent infections, whereas other hosts (declining species) experience virulent infections. Infection prevalence was higher during the cool, dry winter season (May to September) and at high elevations (600–800 m), suggesting regulation by environmental conditions, including temperature and precipitation. We found no relationships between infection prevalence and mean body condition, fluctuating asymmetry of hind limbs, population density, or the presence of metamorphosing tadpoles and juvenile frogs. Although it is not certain whether chytridiomycosis was responsible for past frog population declines in the wet tropics of Queensland, the pathogen is now endemic. Our data indicate that at the landscape level, environmental conditions have strong effects on host-pathogen dynamics. These effects interact with species-specific behavior or immune function and may be important underlying determinants of chytridiomycosis epizootics and emergence.