Ross A. Alford

Pathogenesis of chytridiomycosis, a cause of catastrophic amphibian declines.

Croaking Frogs The global amphibian decline has been attributed, among other causes, to an amphibian skin disease chytridiomycosis caused by the fungus Batrachochytrium dendrobatidis. However, how this pathogen causes mortality has been unclear. Voyles et al. (p. 582) show that this superficial skin infection may lead to cardiac failure owing to changes caused by lowered ion transport through the skin and consequent electrolyte reduction in the blood. A fungal disease that is associated with frog mortality causes changes in electrolyte transport across the skin. The pathogen Batrachochytrium dendrobatidis (Bd), which causes the skin disease chytridiomycosis, is one of the few highly virulent fungi in vertebrates and has been implicated in worldwide amphibian declines. However, the mechanism by which Bd causes death has not been determined. We show that Bd infection is associated with pathophysiological changes that lead to mortality in green tree frogs (Litoria caerulea). In diseased individuals, electrolyte transport across the epidermis was inhibited by >50%, plasma sodium and potassium concentrations were respectively reduced by ~20% and ~50%, and asystolic cardiac arrest resulted in death. Because the skin is critical in maintaining amphibian homeostasis, disruption to cutaneous function may be the mechanism by which Bd produces morbidity and mortality across a wide range of phylogenetically distant amphibian taxa.

Effects of Larval Growth History on Anuran Metamorphosis

Wilbur and Collins (1973) postulated that amphibians should adjust their developmental rates in response to environmental conditions. They suggested that this might be accomplished by the responses of the differentiation rate to the growth rate. We grew tadpoles in eight treatments, of which four initially had low growth rates and four had high growth rates. Three of the initially low treatments were switched to high growth rates at different points in the larval period. Three of the initially high treatments were simultaneously switched to low growth rates. All of the experimentally switched treatments showed responses of mass at metamorphosis consistent with the predictions of the Wilbur-Collins model. Five of the six switched treatments showed responses of larval period that were consistent with the predictions of the model and inconsistent with the predictions of a simpler model proposed by Travis (1984). The response of larval period of the sixth switched treatment was consistent with the predictions of either model. Our experiment demonstrates that differentiation rate responds to food availability throughout the larval period in a manner consistent with the predictions of the Wilbur-Collins model.

Priority Effects in Experimental Pond Communities: Competition between Bufo and Rana

The effect of order to hatching on the outcome of larval competition between two species of frogs breeding in 27 artificial ponds was studied. There were nine different treatments, each replicated three times: all combinations of various introductions of Bufo americanus hatchlings (none added, 500 added on day 0, or 500 added on day 6) and various introductions of Rana sphenocephala hatchlings (none added, 100 added on day 0, or 100 added on day 6). Response variables were the body size at metamorphosis, the length of the larval period of each individual, and the number of survivors of each species in each experimental pond. Bufo individuals and populations did best when alone. Also, they did better when introduced on day 6 rather than day 0. This may have been because the standing crop of food was greater in the communities that were 6 d older. When present with Rana, Bufo did better if added before Rana and worse if added after Rana, as compared to when both species were added at the same time. These results are consistent with a mechanism of size—specific competition. Rana also did best when alone and when introduced late rather than early. Rana did better when added after Bufo and worse when added before Bufo as compared to when both species were introduced at the same time. These results are not consistent with simple size—specific competition. When the species were together, both species did best when Bufo was added early and Rana was added late. These results suggest that optimal oviposition behavior is problematical for female frogs: the time that will be best depends on whether or not another species will be present at the time of hatching.

Experimental exposures of boreal toads (Bufo boreas) to a pathogenic Chytrid fungus (Batrachochytrium dendrobatidis)

One of the major causes of worldwide amphibian declines is a skin infection caused by a pathogenic chytrid fungus (Batrachochytrium dendrobatidis). This study documents the interactions between this pathogen and a susceptible amphibian host, the boreal toad (Bufo boreas). The amount of time following exposure until death is influenced by the dosage of infectious zoospores, duration of exposure, and body size of the toad. The significant relation between dosage and the number of days survived (dose-response curve) supports the hypothesis that the degree of infection must reach a particular threshold of about 107–108 zoosporangia before death results. Variation in air temperature between 12°C and 23°C had no significant effect on survival time. The infection can be transmitted from infected to healthy animals by contact with water containing zoospores; no physical contact between animals is required. These results are correlated with observations on the population biology of boreal toads in which mortalities associated with B.dendrobatidis have been identified.

LIFE-HISTORY TRADE-OFFS INFLUENCE DISEASE IN CHANGING CLIMATES: STRATEGIES OF AN AMPHIBIAN PATHOGEN

Life-history trade-offs allow many animals to maintain reproductive fitness across a range of climatic conditions. When used by parasites and pathogens, these strategies may influence patterns of disease in changing climates. The chytrid fungus, Batrachochytrium dendrobatidis, is linked to global declines of amphibian populations. Short-term growth in culture is maximal at 17 degrees-25 degrees C. This has been used in an argument that global warming, which increases the time that amphibians spend at these temperatures in cloud-covered montane environments, has led to extinctions. Here we show that the amphibian chytrid responds to decreasing temperatures with trade-offs that increase fecundity as maturation rate slows and increase infectivity as growth decreases. At 17 degrees-25 degrees C, infectious zoospores encyst (settle and develop a cell wall) and develop into the zoospore-producing stage (zoosporangium) faster, while at 7 degrees-10 degrees C, greater numbers of zoospores are produced per zoosporangium; these remain infectious for a longer period of time. We modeled the population growth of B. dendrobatidis through time at various temperatures using delayed differential equations and observational data for four parameters: developmental rate of thalli, fecundity, rate of zoospore encystment, and rate of zoospore survival. From the models, it is clear that life-history trade-offs allow B. dendrobatidis to maintain a relatively high long-term growth rate at low temperatures, so that it maintains high fitness across a range of temperatures. When a seven-day cold shock is simulated, the outcome is intermediate between the two constant temperature regimes, and in culture, a sudden drop in temperature induces zoospore release. These trade-offs can be ecologically important for a variety of organisms with complex life histories, including pathogenic microorganisms. The effect of temperature on amphibian mortality will depend on the interaction between fungal growth and host immune function and will be modified by host ecology, behavior, and life history. These results demonstrate that B. dendrobatidis populations can grow at high rates across a broad range of environmental temperatures and help to explain why it is so successful in cold montane environments.

Global amphibian population declines

The decline and disappearance of relatively undisturbed populations of amphibians in several high-altitude regions since the 1970s suggests that they may have suffered a global decline, perhaps with a common cause or causes. Houlahan et al. examined means of trends for 936 amphibian populations and concluded that global declines began in the late 1950s, peaked in the 1960s, and have continued at a reduced rate since. Here we re-analyse their data using a method that accounts for the sampling of different populations over different time periods, and find evidence of a mean global decline in monitored populations only in the 1990s. However it is calculated, the global mean not only masks substantial spatial and temporal variation in population trends and sampling effort, but also fails to distinguish between a global decline with global causes and the cumulative effects of local declines with local causes.

Population trends associated with skin peptide defenses against chytridiomycosis in Australian frogs

Many species of amphibians in the wet tropics of Australia have experienced population declines linked with the emergence of a skin-invasive chytrid fungus, Batrachochytrium dendrobatidis. An innate defense, antimicrobial peptides produced by granular glands in the skin, may protect some species from disease. Here we present evidence that supports this hypothesis. We tested ten synthesized peptides produced by Australian species, and natural peptide mixtures from five Queensland rainforest species. Natural mixtures and most peptides tested in isolation inhibited growth of B. dendrobatidis in vitro. The three most active peptides (caerin 1.9, maculatin 1.1, and caerin 1.1) were found in the secretions of non-declining species (Litoria chloris, L. caerulea, and L. genimaculata). Although the possession of a potent isolated antimicrobial peptide does not guarantee protection from infection, non-declining species (L. lesueuri and L. genimaculata) inhabiting the rainforest of Queensland possess mixtures of peptides that may be more protective than those of the species occurring in the same habitat that have recently experienced population declines associated with chytridiomycosis (L. nannotis, L. rheocola, and Nyctimystes dayi). This study demonstrates that in vitro effectiveness of skin peptides correlates with the degree of decline in the face of an emerging pathogen. Further research is needed to assess whether this non-specific immune defense may be useful in predicting disease susceptibility in other species.

Shelter Microhabitats Determine Body Temperature and Dehydration Rates of a Terrestrial Amphibian (Bufo marinus)

Abstract Selection of diurnal shelter sites varies significantly with season in the cane toad (Bufo marinus), and the aim of this paper is to determine how hydric and thermal conditions of shelter microhabitats changed with season and whether those changes explained seasonal differences in toad behavior. Body temperatures of cane toads were measured by telemetry, and dehydration rates and thermal conditions of shelter microhabitats were measured by using preserved toads as environmental probes. Live toads and preserved toad models were monitored monthly over a 18-month period. Laboratory experiments showed that toad models dehydrated at the same rate as live toads. In the field, dehydration rates varied significantly between seasons and shelter microhabitats, but dehydration rates were always significantly less in shelters compared to a nonshelter control. Daily average body temperature of toads was 16–30°C, and it changed seasonally in proportion to model temperature. Diurnal model temperature was significantly lower in shelters compared to the nonshelter control, but there were significant seasonal differences between shelter sites. It appears that access to suitable diurnal shelter sites is essential for survival of cane toads outside the wet season and that seasonal changes in environmental conditions influence shelter microhabitat selection.

Addition of antifungal skin bacteria to salamanders ameliorates the effects of chytridiomycosis

Chytridiomycosis, caused by the skin fungus Batrachochytrium dendrobatidis (Bd), has caused population declines of many amphibians in remote protected habitats. Progress has been made in understanding the pathogens life cycle, documenting its devastating effects on individual amphibians and on populations, and understanding how and why disease outbreaks occur. No research has directly addressed the critical question of how to prevent declines and extinctions caused by outbreaks of the disease. We have identified a number of bacterial species of amphibian skin that inhibit Bd in vitro. Here, we demonstrate that a species of anti-Bd skin bacteria can be successfully added to skins of salamanders Plethodon cinereus, and that addition of this bacterium reduced the severity of a disease symptom in experimentally infected individuals. This is the first demonstration that manipulating the natural skin microbiota of an amphibian species can alter the pathogens negative effects on infected amphibians and appears to be the first demonstration that an epibiotic manipulation of any wildlife species can lessen the effects of an emerging infectious disease. It suggests that probiotic or bio-augmentation manipulations of cutaneous microbiota could have the potential to reduce susceptibility of amphibians to the disease in nature. This is the first approach suggested that could slow or halt epidemic outbreaks and allow successful reintroductions of amphibian species that have become locally or globally extinct in the wild. Our results also suggest a mechanism for the association of climate change and the likelihood of chytridiomycosis outbreaks via the effects of the former on antifungal bacterial communities.

Comparisons through time and space suggest rapid evolution of dispersal behaviour in an invasive species

During a biological invasion, we expect that the expanding front will increasingly become dominated by individuals with better dispersal abilities. Over many generations, selection at the invasion front thus will favour traits that increase dispersal rates. As a result of this process, cane toads (Bufo marinus) are now spreading through tropical Australia about 5-fold faster than in the early years of toad invasion; but how have toads changed to make this happen? Here we present data from radio-tracking of free-ranging cane toads from three populations (spanning a 15-year period of the toads’ Australian invasion, and across 1800 km). Our data reveal dramatic shifts in behavioural traits (proportion of nights when toads move from their existing retreat-site to a new one, and distance between those successive retreat-sites) associated with the rapid acceleration of toad invasion. Over a maximum period of 70 years (~50 generations), cane toads at the invasion front in Australia apparently have evolved such that populations include a higher proportion of individuals that make long, straight moves.