Research Item (39)
- Mar 2018
Competence, or the propensity of a host to transmit parasites, is partly underlain by host strategies to cope with infection (e.g., resistance and tolerance). Resistance represents the ability of hosts to prevent or clear infections, whereas tolerance captures the ability of individuals to cope with a given parasite burden. Here, we investigated (1) whether one easy-to-measure form of tolerance described well the dynamic relationships between host health and parasite burden, and (2) whether individual resistance and tolerance to West Nile virus (WNV) were predictable from single cytokine measures. We exposed house sparrows (HOSP) to WNV and measured subsequent changes in host performance, viral burden, and cytokine expression. We then used two novel approaches (one complex, one simpler) to estimate tolerance within-individual HOSP using four separate host performance traits. We lastly investigated changes in the expression of pro-inflammatory cytokine interferon-γ (IFN-γ) and anti-inflammatory cytokine interleukin-10 (IL-10). Both approaches to estimating tolerance were equivalent among WNV-infected HOSP; thus, an easy-to-measure tolerance estimation may be successfully applied in field studies. Constitutive expression of IFN-γ and IL-10 were predictive of resistance and tolerance to WNV, implicating these cytokines as viable biomarkers of host competence to WNV.
- Jan 2018
Environmental variation favors the evolution of phenotypic plasticity. For many species, we understand the costs and benefits of different phenotypes, but we lack a broad understanding of how plastic traits evolve across large clades. Using identical experiments conducted across North America, we examined prey responses to predator cues. We quantified five life history traits and the magnitude of their plasticity for 23 amphibian species/populations (spanning three families and five genera) when exposed to no cues, crushed-egg cues, and predatory crayfish cues. Embryonic responses varied considerably among species and phylogenetic signal was common among the traits whereas phylogenetic signal was rare for trait plasticities. Among trait-evolution models, the Ornstein Uhlenbeck (OU) model provided the best fit or was essentially tied with Brownian motion. Using the best fitting model, evolutionary rates for plasticities were higher than traits for three life history traits and lower for two. These data suggest that the evolution of life history traits in amphibian embryos is more constrained by a species’ position in the phylogeny than by life history plasticity. The fact that an OU model of trait evolution was often a good fit to patterns of trait variation may indicate adaptive optima for traits and their plasticities.
- Jul 2017
Glucocorticoid stress hormones, such as corticosterone (CORT), have profound effects on the behaviour and physiology of organisms, and thus have the potential to alter host competence and the contributions of individuals to population- and community-level pathogen dynamics. For example, CORT could alter the rate of contacts among hosts, pathogens and vectors through its widespread effects on host metabolism and activity levels. CORT could also affect the intensity and duration of pathogen shedding and risk of host mortality during infection. We experimentally manipulated songbird CORT, asking how CORT affected behavioural and physiological responses to a standardizedWest Nile virus (WNV) challenge. Although all birds became infected after exposure to the virus, only birdswith elevated CORT had viral loads at or above the infectious threshold. Moreover, though the rate of mortality was faster in birds with elevated CORT compared with controls, most hosts with elevated CORT survived past the day of peak infectiousness. CORT concentrations just prior to inoculation with WNV and anti-inflammatory cytokine concentrations following viral exposure were predictive of individual duration of infectiousness and the ability tomaintain physical performance during infection (i.e. tolerance), revealing putative biomarkers of competence. Collectively, our results suggest that glucocorticoid stress hormones could directly and indirectly mediate the spread of pathogens. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
Variation in host responses to pathogens can have cascading effects on populations and communities when some individuals or groups of individuals display disproportionate vulnerability to infection or differ in their competence to transmit infection. The fungal pathogen, Batrachochytrium dendrobatidis (Bd) has been detected in almost 700 different amphibian species and is implicated in numerous global amphibian population declines. Identifying key hosts in the amphibian-Bd system–those who are at greatest risk or who pose the greatest risk for others–is challenging due in part to many extrinsic environmental factors driving spatiotemporal Bd distribution and context-dependent host responses to Bd in the wild. One way to improve predictive risk models and generate testable mechanistic hypotheses about vulnerability is to complement what we know about the spatial epidemiology of Bd with data collected through comparative experimental studies. We used standardized pathogen challenges to quantify amphibian survival and infection trajectories across 20 post-metamorphic North American species raised from eggs. We then incorporated trait-based models to investigate the predictive power of phylogenetic history, habitat use, and ecological and life history traits in explaining responses to Bd. True frogs (Ranidae) displayed the lowest infection intensities, whereas toads (Bufonidae) generally displayed the greatest levels of mortality after Bd exposure. Affiliation with ephemeral aquatic habitat and breadth of habitat use were strong predictors of vulnerability to and intensity of infection and several other traits including body size, lifespan, age at sexual maturity, and geographic range also appeared in top models explaining host responses to Bd. Several of the species examined are highly understudied with respect to Bd such that this study represents the first experimental susceptibility data. Combining insights gained from experimental studies with observations of landscape-level disease prevalence may help explain current and predict future pathogen dynamics in the Bd system.
Re-fit predictive PGLS models for (A) infection load (IL), (B) log response ratio (LRR) and hazard ratio (HR) with 3 amphibian species removed (i.e., those with high control mortality including L. pipiens, H. versicolor, and A. americanus). (PDF)
- Aug 2016
Stress hormones might represent a key link between individual-level infection outcome, population-level parasite transmission, and zoonotic disease risk. Although the effects of stress on immunity are well known, stress hormones could also affect host–vector interactions via modification of host behaviours or vector-feeding patterns and subsequent reproductive success. Here, we experimentally manipulated songbird stress hormones and examined subsequent feeding preferences, feeding success, and productivity of mosquito vectors in addition to defensive behaviours of hosts. Despite being more defensive, birds with elevated stress hormone concentrations were approximately twice as likely to be fed on by mosquitoes compared to control birds. Moreover, stress hormones altered the relationship between the timing of laying and clutch size in blood-fed mosquitoes. Our results suggest that host stress could affect the transmission dynamics of vector-borne parasites via multiple pathways. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
- Jul 2016
The new fields of ecological immunology and disease ecology have begun to merge, and the classic fields of immunology and epidemiology are beginning to blend with them. This merger is occurring because the integrative study of host–parasite interactions is providing insights into disease in ways that traditional methods have not. With the advent of new tools, mathematical and technological, we could be on the verge of developing a unified theory of infectious disease, one that supersedes the barriers of jargon and tradition. Here we argue that a cornerstone of any such synthesis will be host competence, the propensity of an individual host to generate new infections in other susceptible hosts. In the last few years, the emergence of systems immunology has led to novel insight into how hosts control or eliminate pathogens. Most such efforts have stopped short of considering transmission and the requisite behaviors of infected individuals that mediate it, and few have explicitly incorporated ecological and evolutionary principles. Ultimately though, we expect that the use of a systems immunology perspective will help link suborganismal processes (i.e., health of hosts and selection on genes) to superorganismal outcomes (i.e., community-level disease dynamics and host–parasite coevolution). Recently, physiological regulatory networks (PRNs) were cast as whole-organism regulatory systems that mediate homeostasis and hence link suborganismal processes with the fitness of individuals. Here, we use the PRN construct to develop a roadmap for studying host competence, taking guidance from systems immunology and evolutionary ecology research. We argue that PRN variation underlies heterogeneity in individual host competence and hence host–parasite dynamics.
- Dec 2015
Hosts vary in their ability to transmit new parasite infections (i.e. competence). Although behavior is suggested as a source of individual-level variation, the contribution of host behavior to host-parasite dynamics at the population-level remains largely enigmatic. Here we advocate that behavioral competence be characterized as a syndrome of behaviors that interact to directly or indirectly influence transmission potential. These behaviors can be plastic in response to environmental conditions and/or infection state, and appear linked to immunological traits through shared physiological regulation. By integrating behavioral variation and covariation into a whole-organism view of host competence, disease ecologists might more realistically characterize an individual’s role in host-parasite systems.
- Jul 2015
Contributing to the worldwide biodiversity crisis are emerging infectious diseases, which can lead to extirpations and extinctions of hosts. For example, the infectious fungal pathogen Batrachochytrium dendrobatidis (Bd) is associated with worldwide amphibian population declines and extinctions. Sensitivity to Bd varies with species, season, and life stage. However, there is little information on whether sensitivity to Bd differs among populations, which is essential for understanding Bd-infection dynamics and for formulating conservation strategies. We experimentally investigated intraspecific differences in host sensitivity to Bd across 10 populations of wood frogs (Lithobates sylvaticus) raised from eggs to metamorphosis. We exposed the post-metamorphic wood frogs to Bd and monitored survival for 30 days under controlled laboratory conditions. Populations differed in overall survival and mortality rate. Infection load also differed among populations but was not correlated with population differences in risk of mortality. Such population-level variation in sensitivity to Bd may result in reservoir populations that may be a source for the transmission of Bd to other sensitive populations or species. Alternatively, remnant populations that are less sensitive to Bd could serve as sources for recolonization after epidemic events. © 2015 Society for Conservation Biology.
- Jun 2015
Even apparently similar hosts can respond differently to the same parasites. Some individuals or specific groups of individuals disproportionately affect disease dynamics. Understanding the sources of among-host heterogeneity in the ability to transmit parasites would improve disease management. A major source of host variation might be phenotypic plasticity - the tendency for phenotypes to change across different environments. Plasticity might be as important as, or even more important than, genetic change, especially in light of human modifications of the environment, because it can occur on a more rapid timescale than evolution. We argue that variation in phenotypic plasticity among and within species strongly contributes to epidemiological dynamics when parasites are shared among multiple hosts, which is often the case. Copyright © 2015. Published by Elsevier Ltd.
Carotenoids are considered beneficial nutrients because they provide increased immune capacity. Although carotenoid research has been conducted in many vertebrates, little research has been done in amphibians, a group that is experiencing global population declines from numerous causes, including disease. We raised two amphibian species through metamorphosis on three carotenoid diets to quantify the effects on life-history traits and post-metamorphic susceptibility to a fungal pathogen (Batrachochytrium dendrobatidis; Bd). Increased carotenoids had no effect on survival to metamorphosis in gray treefrogs (Hyla versicolor) but caused lower survival to metamorphosis in wood frogs [Lithobates sylvaticus (Rana sylvatica)]. Increased carotenoids caused both species to experience slower development and growth. When exposed to Bd after metamorphosis, wood frogs experienced high mortality, and the carotenoid diets had no mitigating effects. Gray treefrogs were less susceptible to Bd, which prevented an assessment of whether carotenoids could mitigate the effects of Bd. Moreover, carotenoids had no effect on pathogen load. As one of only a few studies examining the effects of carotenoids on amphibians and the first to examine potential interactions with Bd, our results suggest that carotenoids do not always serve amphibians in the many positive ways that have become the paradigm in other vertebrates.
- Aug 2014
- 99th ESA Annual Convention 2014
Background/Question/Methods Phenotypic plasticity is a ubiquitous phenomenon in nature. Over the past several decades, researchers have provided a tremendous number of insights regarding the ecology and evolution of plasticity of species across a wide range of taxa. However, we know relatively little about how these responses have evolved within a phylogenetic context. Ideally, one would need to conduct identical experiments on a large number of species within a taxonomic group. Doing so would allow assessments of phylogenetic signal for the traits and the trait plasticities of species, which can both constrain the evolution of the traits expressed in different environments. Further, one could examine patterns of plasticity in relation to the habitat breadth , with the expectation that species living across a wider breadth of habitats would evolve greater plasticity. In a large collaborative effort, we addressed these questions by conducting predator-induction experiments on amphibian embryos and predator- and competitor-induction experiments on amphibian tadpoles. Using 20 species of amphibians, we quantified traits that spanned life history, morphology, and behavior. Results/Conclusions The embryo experiments documented large difference in life history traits among species such as time to hatching, mass at hatching, and developmental stage at hatching. After constructing a phylogeny of the amphibians, we found significant phylogenetic signal in these traits. Several species exhibited predator-induced plasticity in their life history traits and this plasticity did not exhibit any phylogenetic signal. The tadpole experiments also documented large differences in relative morphology and activity level. The morphological and behavioral traits exhibited significant phylogenetic signal. Plastic responses were widespread in the tadpoles including predator-induced increases in relative tail depth and decreases in activity. Once again, the plasticity of the morphological traits did not exhibit phylogenetic signal. However, the plasticity of tadpole activity was the one trait that did exhibit phylogenetic signal. This suggests that the plasticity of morphological traits is not constrained from evolving different magnitudes of plasticity whereas activity is somewhat constrained. Finally, our analysis of plasticity versus habitat breadth refuted the prediction that species spanning a wider range of habitats have evolved greater trait plasticity. Collectively, these results suggest that whereas species traits can be phylogenetically constrained, the plasticity of these traits may be quite easily modified over evolutionary time.
- Nov 2013
Many pathogens infect a wide range of host species. However, variation in the outcome of infection often exists amongst hosts and is shaped by intrinsic host traits. For example, contact with pathogens may trigger changes in hosts directed toward preventing, fighting, or tolerating infection. Host responses to infection are dynamic; they change over time and vary depending on health, condition and within the context of the environment.Immunological defences are an important class of responses that mediate host–pathogen dynamics. Here, we examined temporal patterns of immunity in two amphibian species, Pacific tree frogs (Pseudacris regilla) and Cascades frogs (Rana cascadae), exposed to control conditions or experimental inoculation with the emerging infectious fungal pathogen, Batrachochytrium dendrobatidis (Bd). For each species, we compared bacterial killing ability of blood and differential white blood cell counts at four different time-points after pathogen inoculation. We also quantified infection load over time and monitored survival.We detected qualitative and quantitative differences in species responses to Bd. Pseudacris regilla exhibited an increase in infection load over time and 16% of Bd-exposed animals died during the experiment. Tree frogs lacked robust treatment differences in immune responses, but Bd-exposed P. regilla tended to display weaker bacterial killing responses than unexposed control animals. Neutrophil counts did not vary consistently with treatment and lymphocytes tended to be less abundant in Bd-exposed animals at the later sampling time-points.In contrast, Bd-exposed R. cascadae exhibited a decrease in infection load over time and no mortality occurred in the Bd treatment. Bd-exposed Cascades frogs showed stronger bacterial killing responses and an elevated number of neutrophils in blood when compared with control animals, and both responses were upregulated within 48 h of pathogen exposure. Lymphocyte counts did not vary significantly with treatment.Although only statistically significant in Cascades frogs, neutrophil:lymphocyte ratios showed a trend of being elevated in Bd-exposed animals of both species and are indicative of pathogen-induced physiological stress.Our results suggest that variation in systemic immunological responses of two syntopic amphibian species is associated with and may contribute to differential patterns of survival and infection load during exposure to the chytrid fungus. Species variation in immunological responses as soon as 48 h after pathogen exposure suggests that initial host–pathogen interactions may set the stage for subsequent infection and disease progression. Variation in host responses can drive disease dynamics and comparative studies of host responses to pathogens are critical for making predictions about pathogen emergence, spread and persistence.
- Sep 2013
For the past several decades, amphibian populations have been decreasing around the globe at an unprecedented rate. Batrachochytrium dendrobatidis (Bd), the fungal pathogen that causes chytridiomycosis in amphibians, is contributing to amphibian declines. Natural and anthropogenic environmental factors are hypothesized to contribute to these declines by reducing the immunocompetence of amphibian hosts, making them more susceptible to infection. Antimicrobial peptides (AMPs) produced in the granular glands of a frog's skin are thought to be a key defense against Bd infection. These peptides may be a critical immune defense during metamorphosis because many acquired immune functions are suppressed during this time. To test if stressors alter AMP production and survival of frogs exposed to Bd, we exposed wood frog (Lithobates sylvaticus) tadpoles to the presence or absence of dragonfly predator cues crossed with a single exposure to three nominal concentrations of the insecticide malathion (0, 10, or 100 parts per billion [ppb]). We then exposed a subset of post-metamorphic frogs to the presence or absence of Bd zoospores and measured frog survival. Although predator cues and malathion had no effect on survival or size at metamorphosis, predator cues increased the time to metamorphosis by 1.5 days and caused a trend of a 20% decrease in hydrophobic skin peptides. Despite this decrease in peptides determined shortly after metamorphosis, previous exposure to predator cues increased survival in both Bd-exposed and unexposed frogs several weeks after metamorphosis. These results suggest that exposing tadpoles to predator cues confers fitness benefits later in life.
The emerging fungal pathogen, Batrachochytrium dendrobatidis (Bd), has been associated with global amphibian population declines and extinctions. American bullfrogs (Lithobates catesbeianus) are widely reported to be a tolerant host and a carrier of Bd that spreads the pathogen to less tolerant hosts. Here, we examined whether bullfrogs raised from eggs to metamorphosis in outdoor mesocosms were susceptible to Bd. We experimentally exposed metamorphic juveniles to Bd in the laboratory and compared mortality rates of pathogen-exposed animals to controls (non-exposed) in two separate experiments; one using a Bd strain isolated from a Western toad and another using a strain isolated from an American bullfrog. We wanted to examine whether metamorphic bullfrogs were susceptible to either of these strains. We show that bullfrogs were susceptible to one strain of Bd and not the other. In both experiments, infection load detected in the skin decreased over time, suggesting that metamorphic bullfrogs from some populations may be inefficient long-term carriers of Bd.
Species composition within ecological assemblages can drive disease dynamics including pathogen invasion, spread, and persistence. In multi-host pathogen systems, interspecific variation in responses to infection creates important context dependency when predicting the outcome of disease. Here, we examine the responses of three sympatric host species to a single fungal pathogen, Batrachochytrium dendrobatidis, which is associated with worldwide amphibian population declines and extinctions. Using an experimental approach, we show that amphibian species from three different genera display significant differences in patterns of pathgen-induced mortality as well as the magnitude and temporal dynamics of infection load. We exposed amphibians to one of four inoculation dose treatments at both larval and post- metamorphic stages and quantified infection load on day 8 and day 15 post-inoculation. Of the three species examined, only one (the Pacific treefrog; Pseudacris regilla) displayed "dose-dependent" responses; survival was reduced and infection load was elevated as inoculation dose was increased. We observed a reduction in survival but no differences in infection load across pathogen treatments in Cascades frogs (Rana cascadae). Western toads (Anaxyrus boreas) displayed differences in infection load but no differences in survival across pathogen treatments. Within species, responses to the pathogen varied with life history stage, and the most heavily infected species at the larval stage was different from the most heavily infected species at the post-metamorphic stage. Temporal changes in infection load were species and life history stage-specific. We show that variation in susceptibility to this multi-host pathogen is complex when viewed at a fine-scale and may be mediated through intrinsic host traits.
Batrachochytrium dendrobatidis infection load comparisons (ANCOVA or Welch’s t-test) in larval amphibians by species, treatment, and sampling time-point. Abbreviations are used for species: PR = Pseudacris regilla; RC = Rana cascadae; AB = Anaxyrus boreas. Abbreviations for treatments: H = high dose; I = intermediate dose; L = low dose; d = day. NS = non-significant comparisons; MS = marginally significant result (p<0.1). NA indicates that the comparison is not applicable because of low sample size (due to mortality or Bd-negative animals that could not be included in analyses). *Only 1 individual tested positive for infection in the low treatment group so the comparison of interest is high versus intermediate treatment on day 8 of the experiment made with a Welch’s t-test. **Only 1 individual in the low and the intermediate treatment tested positive for infection so no statistical comparisons among treatments were made. ***Rana cascadae was not included in species comparisons because only 1 individual tested positive for infection at this time-point/treatment combination. (DOCX)
Batrachochytrium dendrobatidis infection load comparisons (ANCOVA or Welch’s t-test) in post-metamorphic amphibians by species, treatment, and sampling time-point. Abbreviations are used for species: PR = Pseudacris regilla; RC = Rana cascadae; AB = Anaxyrus boreas. Abbreviations for treatments: H = high dose; I = intermediate dose; L = low dose. NS = non-significant comparisons; MS = marginally significant comparison (p<0.1). NA indicates that the comparison is not applicable because of low sample size (due to mortality or Bd-negative animals that could not be included in analyses). *Only 1PR remained alive by day 15 of the experiment and so was excluded from statistical analyses. **Only 1 AB remained alive by day 15 of the experiment and so was excluded from statistical analyses. ***No among species comparison on day 15 possible because only 1individual alive in PR and AB. (DOCX)
Background/Question/Methods The emergence and spread of infectious diseases has had profound impacts on many ecological systems. Disease is implicated in the loss of biological diversity, including the worldwide decline of amphibian populations. Predicting disease dynamics and ecosystem-level outcomes requires a basic understanding of host-pathogen interactions. While most pathogens can infect multiple host species, there may be considerable variation in the response to infection among different species. Hosts employ a variety of strategies against infection, including resistance (responses that limit parasite burden) or tolerance (responses that limit pathogen-induced damage to host health). Amphibians represent an excellent system for characterizing and categorizing variation in host responses to pathogen infection. The fungal pathogen, Batrachochytrium dendrobatidis (Bd) infects a wide-range of amphibian species and has highly variable effects on host health and survival. We investigated the effects of Bd infection on three different amphibian host species (Pseudacris regilla, Anaxyrus boreas, and Rana cascadae) at two different life history stages (larval and post-metamorphic) and across four pathogen inoculation intensities in standardized laboratory experiments. We monitored host survival throughout the experiment and sampled individuals at day 8 and day 15 of the experiment for quantitative estimates of infection load. Results/Conclusions We found differences in absolute survival, rates of mortality, and infection load across species, life history stages, pathogen inoculation treatments, and sampling time-points. As predicted, survival was reduced and infection load was greater in metamorphic amphibians, as compared with larvae. Among larval amphibians, Anaxyrus boreas displayed the highest infection load, and infection tended to increase over time. In metamorphic amphibians, Pseudacris regilla exhibited the highest infection load, and infection tended to decrease over time. In both larvae and metamorphs, Rana cascadae showed the lowest overall infection load. Interestingly, responses to experimental pathogen inoculation treatments were not necessarily dose dependent. Metamorphic R. cascadae exhibited reduced survival when exposed to higher pathogen inoculation treatments but did not display differences in infection load across treatments. Conversely, A. boreas showed significant variation in infection load but no differences in survival across pathogen inoculation treatments. Only P. regilla exhibited both reduced survival and an increase in infection load as experimental inoculation dose increased. Our results suggest a high degree of variation in responses to a conservation-relevant pathogen and provide evidence that differences in tolerance and resistance to Bd may underlie observed variation in disease susceptibility among amphibian species.
Background/Question/Methods The loss of populations and extinctions of species are occurring at unprecedented rates. As part of this “biodiversity crisis,” amphibians are experiencing population declines and extinctions worldwide. Numerous factors are contributing to these declines, however disease has become one of the most prominent causes being investigated. The fungal pathogen, Batrachochytrium dendrobatidis (Bd), which causes the disease chytridiomycosis, has received extensive attention because of its worldwide distribution and its general ability to infect a diverse array of amphibian hosts. In some systems, population declines putatively caused by Bd occur while nearby populations remain despite the presence of the pathogen. Unfortunately, there is very little information concerning whether different populations of the same host species have differential sensitivity to Bd. We investigated the susceptibility of newly metamorphosed wood frogs (Lithobates sylvatica) from ten different populations in standardized laboratory experiments. Results/Conclusions Within each population, animals exposed to Bd showed reduced survival compared with unexposed control animals. However, survivorship differed significantly across populations suggesting that Bd differentially affects populations. Additional results will be presented regarding how pathogen loads differed across populations as well as how pathogen loads correlated with survival. These results could yield important information on how to control the spread of Bd and reaffirm the importance of population-level conservation to protect species from Bd as well as other stressors.
Infectious diseases are intimately associated with the dynamics of biodiversity. However, the role that infectious disease plays within ecological communities is complex. The complex effects of infectious disease at the scale of communities and ecosystems are driven by the interaction between host and pathogen. Whether or not a given host-pathogen interaction results in progression from infection to disease is largely dependent on the physiological characteristics of the host within the context of the external environment. Here, we highlight the importance of understanding the outcome of infection and disease in the context of host ecophysiology using amphibians as a model system. Amphibians are ideal for such a discussion because many of their populations are experiencing declines and extinctions, with disease as an important factor implicated in many declines and extinctions. Exposure to pathogens and the host's responses to infection can be influenced by many factors related to physiology such as host life history, immunology, endocrinology, resource acquisition, behaviour and changing climates. In our review, we discuss the relationship between disease and biodiversity. We highlight the dynamics of three amphibian host-pathogen systems that induce different effects on hosts and life stages and illustrate the complexity of amphibian-host-parasite systems. We then review links between environmental stress, endocrine-immune interactions, disease and climate change.
- Jul 2011
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.
Population losses and extinctions of species are occurring at unprecedented rates, as exemplified by declines and extinctions of amphibians worldwide. However, studies of amphibian population declines generally do not address the complexity of the phenomenon or its implications for ecological communities, focusing instead on single factors affecting particular amphibian species. We argue that the causes for amphibian population declines are complex; may differ among species, populations, and life stages within a population; and are context dependent with multiple stressors interacting to drive declines. Because amphibians are key components of communities, we emphasize the importance of investigating amphibian declines at the community level. Selection pressures over evolutionary time have molded amphibian life history characteristics, such that they may remain static even in the face of strong, recent human-induced selection pressures.
- Aug 2010
- 95th ESA Annual Convention 2010
Background/Question/Methods Infectious diseases are a growing global concern, particularly when leading to population declines and loss of biodiversity. In amphibians, infectious diseases are among the causes linked to global population declines and extinctions. In particular, the fungal pathogen, Batrachochytrium dendrobatidis (Bd), has received considerable attention due to its apparent role in amphibian population declines around the world. While 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 seven species of anurans (frogs and toads) from the United States. We exposed post-metamorphic animals to Bd for four weeks and monitored mortality, growth, feeding rates, and final infection levels. Results/Conclusions In all species tested, Bd-exposed animals had higher rates of mortality than control (unexposed) animals. Furthermore, we found differences in mortality rates among species even though species did not differ in the amount of Bd detected on their bodies. In general, we detected lower levels of Bd on individuals that survived longer and found that animals with larger initial sizes survived longer and had lower levels of Bd. These baseline data will serve as an important step toward defining species differences in disease susceptibility.
As part of an overall decline in biodiversity, populations of many organisms are declining and species are being lost at unprecedented rates around the world. This includes many populations and species of amphibians. Although numerous factors are affecting amphibian populations, we show potential direct and indirect effects of climate change on amphibians at the individual, population and community level. Shifts in amphibian ranges are predicted. Changes in climate may affect survival, growth, reproduction and dispersal capabilities. Moreover, climate change can alter amphibian habitats including vegetation, soil, and hydrology. Climate change can influence food availability, predator-prey relationships and competitive interactions which can alter community structure. Climate change can also alter pathogen-host dynamics and greatly influence how diseases are manifested. Changes in climate can interact with other stressors such as UV-B radiation and contaminants. The interactions among all these factors are complex and are probably driving some amphibian population declines and extinctions.
- Oct 2007
Summary 1. Phenotypic plasticity may allow an organism to respond to temporally variable opportunities for growth and risks of mortality. However, life-history theory assumes that there are often trade-offs between the benefits afforded by plasticity in one trait and the consequences of that plasticity on other traits that affect fitness. In organisms with a complex life cycle, trade-offs may occur between larval and post-metamorphic traits. 2. Many amphibians metamorphose in temporary ponds, and may accelerate larval development to avoid mortality when a pond desiccates. A younger age at metamorphosis often results in reduced body size, but may also facilitate a trade-off with physiological traits that are linked to fitness in the adult stage. 3. We investigated a potential trade-off between desiccation-driven acceleration of development rate and immune system responsiveness in a species that breeds exclusively in temporary ponds. We exposed Rana sylvatica (wood frog) tadpoles to four possible desiccation regimes and then assayed the cell-mediated immune response to a standardized foreign antigen, phytohaemagglutinin (PHA), injected 3 weeks after metamorphosis. We also quantified total leucocyte numbers from haematological smears to obtain a secondary measure of individual immunological condition. 4. Animals exposed to desiccation had shorter development times, weaker cellular immune system responses to PHA and lower total leucocyte numbers than animals from control groups. Both measures of immune response showed a decrease with increasing severity of the desiccation treatment. 5. It is currently unclear whether the observed depression in immune response is transient or permanent. However, even temporary periods of immune system suppression shortly after meta- morphosis may lead to greater susceptibility to opportunistic pathogens or parasites.
- Apr 2007
Organisms that exploit variable habitats often display phenotypically plastic responses, contingent upon prevailing conditions. Amphibians that metamorphose in ephemeral ponds constitute excellent models for examining plasticity in temporally variable environments. One way in which amphibians cope with variation in the larval environment is through plasticity in the duration and timing of metamorphosis. Facultative acceleration of developmental rate may reduce mortality due to desiccation at the larval stage, but it may also entail long-term costs in overall fitness. Here, we investigate the potential tradeoff between desiccation-driven acceleration of developmental rate and immune system responsiveness in a species that breeds exclusively in temporary ponds. We exposed Rana sylvatica tadpoles to four possible desiccation regimes and then assayed the cell-mediated immune response to a standardized foreign antigen, (phytohemagglutinin-PHA), injected three weeks after metamorphosis. We also quantified total leukocyte numbers from hematological smears to obtain a secondary measure of individual immunological condition. Animals exposed to desiccation treatments had shorter developmental times, weaker cellular immune system responses to PHA, and lower total leukocyte numbers than animals from control groups. Both immune measures showed a decrease in immune responsiveness with increasing severity of the desiccation treatment. It is currently unclear whether the observed depression in immune response is transient or permanent. However, even temporary periods of immune system suppression shortly after metamorphosis may lead to increased opportunistic infection within an environment of ubiquitous pathogens. Although infectious diseases alone are a major factor contributing to global amphibian declines, environmental stressors that increase susceptibility to pathogens may further promote extinction episodes. Desiccation-driven effects on temporary pond breeding amphibians are additionally relevant given projected changes in global and local climate that may impact surface water availability. Master of Science School of Natural Resources and Environment University of Michigan http://deepblue.lib.umich.edu/bitstream/2027.42/50467/1/THESIS.doc