Are novel or locally adapted pathogens more devastating and why? Resolving opposing hypotheses.

There is a rich literature highlighting that pathogens are generally better adapted to infect local than novel hosts, and a separate seemingly contradictory literature indicating that novel pathogens pose the greatest threat to biodiversity and public health. Here, using Batrachochytrium dendrobatidis, the fungus associated with worldwide amphibian declines, we test the hypothesis that there is enough variance in "novel" (quantified by geographic and phylogenetic distance) host-pathogen outcomes to pose substantial risk of pathogen introductions despite local adaptation being common. Our continental-scale common garden experiment and global-scale meta-analysis demonstrate that local amphibian-fungal interactions result in higher pathogen prevalence, pathogen growth, and host mortality, but novel interactions led to variable consequences with especially virulent host-pathogen combinations still occurring.

Does the thermal mismatch hypothesis predict disease outcomes in different morphs of a terrestrial salamander?

Many aspects of ectotherm physiology are temperature-dependent. The immune system of temperate-dwelling ectothermic host species is no exception and their immune function is often downregulated in cold temperatures. Likewise, species of ectothermic pathogens experience temperature-mediated effects on rates of transmission and/or virulence.

Infection with Batrachochytrium dendrobatidis reduces salamander capacity to mount a cell-mediated immune response.

The vertebrate immune system is a costly defense system that is responsible for preventing and eliminating parasites and pathogens. Theory predicts that hosts experience tradeoffs associated with immune deployment and other physiological functions. Although empirical evidence for immune-physiology tradeoffs are well documented in the literature, fewer studies have examined tradeoffs within the immune system in wild vertebrates.

A meta-analysis reveals temperature, dose, life stage, and taxonomy influence host susceptibility to a fungal parasite.

Complex ecological relationships, such as host-parasite interactions, are often modeled with laboratory experiments. However, some experimental laboratory conditions, such as temperature or infection dose, are regularly chosen based on convenience or convention, and it is unclear how these decisions systematically affect experimental outcomes. Here, we conducted a meta-analysis of 58 laboratory studies that exposed amphibians to the pathogenic fungus Batrachochytrium dendrobatidis (Bd) to understand better how laboratory temperature, host life stage, infection dose, and host species affect host mortality.

Impacts of thermal mismatches on chytrid fungus Batrachochytrium dendrobatidis prevalence are moderated by life stage, body size, elevation and latitude.

Global climate change is increasing the frequency of unpredictable weather conditions; however, it remains unclear how species-level and geographic factors, including body size and latitude, moderate impacts of unusually warm or cool temperatures on disease. Because larger and lower-latitude hosts generally have slower acclimation times than smaller and higher-latitude hosts, we hypothesised that their disease susceptibility increases under 'thermal mismatches' or differences between baseline climate and the temperature during surveying for disease. Here, we examined how thermal mismatches interact with body size, life stage, habitat, latitude, elevation, phylogeny and International Union for Conservation of Nature (IUCN) conservation status to predict infection prevalence of the chytrid fungus Batrachochytrium dendrobatidis (Bd) in a global analysis of 32 291 amphibian hosts.

Influence of substrate orientation on tadpoles' feeding efficiency.

In nature, tadpoles encounter food on substrates oriented at different angles (e.g. vertically along stems, horizontally on the bottom of the pond).

An interaction between climate change and infectious disease drove widespread amphibian declines.

Climate change might drive species declines by altering species interactions, such as host-parasite interactions. However, few studies have combined experiments, field data, and historical climate records to provide evidence that an interaction between climate change and disease caused any host declines. A recently proposed hypothesis, the thermal mismatch hypothesis, could identify host species that are vulnerable to disease under climate change because it predicts that cool- and warm-adapted hosts should be vulnerable to disease at unusually warm and cool temperatures, respectively.

Effects of corticosterone on infection and disease in salamanders exposed to the amphibian fungal pathogen Batrachochytrium dendrobatidis.

Although it is well established that glucocorticoid hormones (GCs) alter immune function and disease resistance in humans and laboratory animal models, fewer studies have linked elevated GCs to altered immune function and disease resistance in wild animals. The chytrid fungal pathogen Batrachochytrium dendrobatidis (Bd) infects amphibians and can cause the disease chytridiomycosis, which is responsible for worldwide amphibian declines. It is hypothesized that long-term exposure to environmental stressors reduces host resistance to Bd by suppressing host immunity via stress-induced release of GCs such as corticosterone (CORT).

The thermal mismatch hypothesis explains host susceptibility to an emerging infectious disease.

Parasites typically have broader thermal limits than hosts, so large performance gaps between pathogens and their cold- and warm-adapted hosts should occur at relatively warm and cold temperatures, respectively. We tested this thermal mismatch hypothesis by quantifying the temperature-dependent susceptibility of cold- and warm-adapted amphibian species to the fungal pathogen Batrachochytrium dendrobatidis (Bd) using laboratory experiments and field prevalence estimates from 15 410 individuals in 598 populations. In both the laboratory and field, we found that the greatest susceptibility of cold- and warm-adapted hosts occurred at relatively warm and cool temperatures, respectively, providing support for the thermal mismatch hypothesis.

Exposure to Corticosterone Affects Host Resistance, but Not Tolerance, to an Emerging Fungal Pathogen.

Host responses to pathogens include defenses that reduce infection burden (i.e., resistance) and traits that reduce the fitness consequences of an infection (i.

Salamanders increase their feeding activity when infected with the pathogenic chytrid fungus Batrachochytrium dendrobatidis.

Immune function is a costly line of defense against parasitism. When infected with a parasite, hosts frequently lose mass due to these costs. However, some infected hosts (e.

Differential effects of temperature on the feeding kinematics of the tadpoles of two sympatric anuran species.

Temperature impacts ectotherm performance by influencing many biochemical and physiological processes. When well adapted to their environment, ectotherms should perform most efficiently at the temperatures they most commonly encounter. In the present study, we tested how differences in temperature affects the feeding kinematics of tadpoles of two anuran species: the benthic tadpole of Rhinella schneideri and the nektonic tadpole of Trachycephalus typhonius.

Amphibians acquire resistance to live and dead fungus overcoming fungal immunosuppression.

Emerging fungal pathogens pose a greater threat to biodiversity than any other parasitic group, causing declines of many taxa, including bats, corals, bees, snakes and amphibians. Currently, there is little evidence that wild animals can acquire resistance to these pathogens. Batrachochytrium dendrobatidis is a pathogenic fungus implicated in the recent global decline of amphibians.

Linking manipulative experiments to field data to test the dilution effect.

The dilution effect, the hypothesis that biodiversity reduces disease risk, has received support in many systems. However, few dilution effect studies have linked mechanistic experiments to field patterns to establish both causality and ecological relevance. We conducted a series of laboratory experiments and tested the dilution effect hypothesis in an amphibian-Batrachochytrium dendrobatidis (Bd) system and tested for consistency between our laboratory experiments and field patterns of amphibian species richness, host identity and Bd prevalence.

Confronting inconsistencies in the amphibian-chytridiomycosis system: implications for disease management.

Chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), is one of the largest threats to wildlife and is putatively linked to the extirpation of numerous amphibians. Despite over a decade of research on Bd, conflicting results from a number of studies make it difficult to forecast where future epizootics will occur and how to manage this pathogen effectively. Here, we emphasize how resolving these conflicts will advance Bd management and amphibian conservation efforts.

Optimal digestion theory does not predict the effect of pathogens on intestinal plasticity.

One prediction of optimal digestion theory is that organisms will increase the relative length of their digestive tracts when food resources become limited. We used theory of optimal digestion to test whether tadpoles can adjust the relative length of their intestines when challenged with the fungal pathogen Batrachochytrium dendrobatidis (Bd). The degree of tadpole mouthpart damage, a symptom of Bd infections that reduces food consumption, was associated positively with the length of tadpole intestines relative to their body size, consistent with optimal digestion theory.

Comparative feeding kinematics of tropical hylid tadpoles.

Anuran larvae, which are otherwise simple in shape, typically have complex keratinized mouthparts (i.e. labial teeth and jaw sheaths) that allow them to graze upon surfaces.

Chytrid fungus Batrachochytrium dendrobatidis has nonamphibian hosts and releases chemicals that cause pathology in the absence of infection.

Batrachochytrium dendrobatidis, a pathogenic chytrid fungus implicated in worldwide amphibian declines, is considered an amphibian specialist. Identification of nonamphibian hosts could help explain the virulence, heterogeneous distribution, variable rates of spread, and persistence of B. dendrobatidis in freshwater ecosystems even after amphibian extirpations.

Selecting for tolerance against pathogens and herbivores to enhance success of reintroduction and translocation.

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.

Dietary protein restriction impairs growth, immunity, and disease resistance in southern leopard frog tadpoles.

The immune system is a necessary, but potentially costly, defense against infectious diseases. When nutrition is limited, immune activity may consume a significant amount of an organism's energy budget. Levels of dietary protein affect immune system function; high levels can enhance disease resistance.

Can differences in host behavior drive patterns of disease prevalence in tadpoles?

Differences in host behavior and resistance to disease can influence the outcome of host-pathogen interactions. We capitalized on the variation in aggregation behavior of Fowler's toads (Anaxyrus [ = Bufo] fowleri) and grey treefrogs (Hyla versicolor) tadpoles and tested for differences in transmission of Batrachochytrium dendrobatidis (Bd) and host-specific fitness consequences (i.e.

Fungal pathogen changes the feeding kinematics of larval anurans.

Pathogens can alter host life-history traits by affecting host feeding activities. In anuran tadpoles, keratinized mouthparts (teeth and jaw sheaths) are essential for feeding. Batrachochytrium dendrobatidis ( Bd ) is a pathogenic fungus of amphibians that can infect these mouthparts and reduce tadpole survival.

How does a change in labial tooth row number affect feeding kinematics and foraging performance of a ranid tadpole (Lithobates sphenocephalus)?

Recent studies have explored feeding kinematics in tadpoles with intact labial teeth; however, it is unknown how missing teeth impacts foraging. We explored the impact of missing labial teeth on the feeding mechanics and foraging performance of Southern leopard frog (Lithobates sphenocephalus [= Rana sphenocephala]) tadpoles by controlling the pattern of labial tooth loss; that is, by surgically removing one row of labial teeth. We then used high-speed (500 frames/second) videography to test the hypothesis that tooth loss reduces the time that tadpoles attach to and graze upon an algal-covered substrate.

Impacts of Batrachochytrium dendrobatidis infection on tadpole foraging performance.

Pathogen-induced modifications in host behavior, including alterations in foraging behavior or foraging efficiency, can compromise host fitness by reducing growth and development. Chytridiomycosis is an infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd), and it has played an important role in the worldwide decline of amphibians. In larval anurans, Bd infections commonly result in reduced developmental rates, however, the mechanism(s) responsible are untested.

Ecological separation in a polymorphic terrestrial salamander.

1. When studying speciation, researchers commonly examine reproductive isolation in recently diverged populations. Polymorphic species provide an opportunity to examine the role of reproductive isolation in populations that may be in the process of divergence.