Consequences of an amphibian malformity for development and fitness in complex environments

1. Environmental stressors have both lethal and sublethal effects, such as altered developmental rates and the induction of malformations. Ecological interactions, including predation and competition, often amplify such effects, for instance by inducing behavioural changes that increase susceptibility to the stress. 2. Using experimental mesocosms, we asked whether the density of conspecific competitors and predation risk from larval water beetles (Dytiscus spp.) affect the development of malformations in tadpoles of the wood frog (Rana sylvatica). We also examined whether such malformities increase the susceptibility of tadpoles to predation. 3. The risk of predation decreased the frequency of malformities in both low‐ and high‐density treatments, although this effect was greater at low density. Behavioural observations suggested that reductions in activity by amphibian larvae induced by predators mediated these responses by decreasing cumulative exposure to ultraviolet‐B radiation, the putative stressor causing the observed malformity. These results suggest that predators can reduce negative impacts of stressors by inducing behavioural changes in prey organisms. 4. Malformed individuals were twice as vulnerable to predators as non‐malformed individuals, suggesting that sublethal effects can ultimately cause increased mortality.     

Searching for the physiological mechanism of density dependence: does corticosterone regulate tadpole responses to density?

Density-dependent population regulation is important in many natural populations, and in those populations, high population density is a likely stressor. In amphibians, previous laboratory studies with tadpoles suggest that corticosterone, the main glucocorticoid stress hormone, is one of the key regulators of density-dependent growth. To test this relationship in natural settings, we manipulated wood frog (Rana sylvatica) tadpole density at three levels in outdoor mesocosms and used a capture stress protocol to examine the hormonal stress response. In addition, we used the same capture protocol in six natural ponds (three high density and three low density). In the mesocosms, there was an increase in corticosterone levels in tadpoles following 1 h of confinement at weeks 1, 2, and 5. However, while tadpoles maintained at higher densities were smaller after metamorphosis, density did not alter mean levels of corticosterone obtained during confinement, and baseline levels of corticosterone did not differ between the densities. Similarly, in natural ponds, density did not correlate with baseline corticosterone or mean corticosterone levels obtained during confinement. We suggest that the physiological response to density may vary across the range of natural densities and that the role of corticosterone may be limited to periods of extreme high density, such as during pond-drying events.     

Glucocorticosteroid hormone treatment of larval treefrogs increases infection by Alaria sp. trematode cercariae

In many amphibian species, an apparent increase has occurred in the prevalence of limb deformities caused by parasitic trematodes. We are interested in the role of environmental stressors in increasing these infections in amphibians. One mechanism by which environmental stressors could act to increase disease prevalence is to increase circulating levels of glucocorticosteroid hormones, which are released in response to stressors and can be immunosuppressive. In the present study, we treated gray treefroZg tadpoles (Hyla versicolor) with exogenous corticosterone, which is the main glucocorticosteroid "stress" hormone in amphibians. We then exposed treated tadpoles to Alaria sp. cercariae and scored the number of mesocercariae that successfully infected the tadpoles. In addition, we assayed one function of the immune response by counting the number of circulating eosinophilic granulocytes, which are thought to be important in immune responses to macroparasites. Tadpoles treated with exogenous corticosterone developed higher parasite loads than control tadpoles did, and they had lower numbers of circulating eosinophilic granulocytes. These results provide evidence of glucocorticosteroid-mediated immunosuppression in tadpoles that may help to explain apparent increases in the numbers of trematode-induced deformities in amphibian populations during recent decades.     

A Non-Invasive Stress Assay Shows That Tadpole Populations Infected with Batrachochytrium dendrobatidis Have Elevated Corticosterone Levels

Batrachochytrium dendrobatidis (Bd) is a fungus that causes the disease chytridiomycosis and is associated with widespread amphibian declines. Populations vary in their susceptibility to Bd infections, and the virulence of the infecting lineage can also vary. Both of these factors may manifest as a differential physiological stress response. In addition, variation in disease susceptibility across amphibian populations may be influenced by immunosuppression caused by chronic stress imposed by environmental factors. Here, we use a non-invasive water-borne hormone technique to assess stress levels (corticosterone) of free-living tadpole populations that are infected by Bd. We found that corticosterone release rates were higher in infected populations of two species of tadpoles (Alytes obstetricans and A. muletensis) than in an uninfected population for both species. The relationship between corticosterone and the intensity of infection differed between species, with only the infected A. obstetricans population showing a significant positive correlation. The higher corticosterone release rates found in A. obstetricans may be an outcome of infection by a highly virulent lineage of Bd (BdGPL), whereas A. muletensis is infected with a less virulent lineage (BdCAPE). These results suggest that different lineages of Bd impose different levels of stress on the infected animals, and that this may influence survival. The next step is to determine whether higher corticosterone levels make individuals more susceptible to Bd or if Bd infections drive the higher corticosterone levels.     

Development and disease: how susceptibility to an emerging pathogen changes through anuran development

Ranaviruses have caused die-offs of amphibians across the globe. In North America, these pathogens cause more amphibian mortality events than any other pathogen. Field observations suggest that ranavirus epizootics in amphibian communities are common during metamorphosis, presumably due to changes in immune function. However, few controlled studies have compared the relative susceptibility of amphibians to ranaviruses across life stages. Our objectives were to measure differences in mortality and infection prevalence following exposure to ranavirus at four developmental stages and determine whether the differences were consistent among seven anuran species. Based on previous studies, we hypothesized that susceptibility to ranavirus would be greatest at metamorphosis. Our results did not support this hypothesis, as four of the species were most susceptible to ranavirus during the larval or hatchling stages. The embryo stage had the lowest susceptibility among species probably due to the protective membranous layers of the egg. Our results indicate that generalizations should be made cautiously about patterns of susceptibility to ranaviruses among amphibian developmental stages and species. Further, if early developmental stages of amphibians are susceptible to ranaviruses, the impact of ranavirus epizootic events may be greater than realized due to the greater difficulty of detecting morbid hatchlings and larvae compared to metamorphs.     

First case of ranavirus-associated mass mortality in a natural population of the Huanren frog (Rana huanrenensis) tadpoles in South Korea

Globally, ranavirus is often responsible for the mass mortality of a variety of captive and wild amphibians. In Asia, several mass mortality cases of captive amphibians by ranavirus are known, but one mass mortality case in the wild has been reported in a non-endemic larval bullfrog population in Japan. In order to verify factors involved in mass mortality of Rana huanrenensis tadpoles (> 200 tadpoles) in a mountain stream in South Korea, we investigated possible infections by ranavirus, chytrid fungus, and lethal bacteria by conducting PCR assays of pathogens with specific primers. We found that all R. huanrenensis tadpoles collected (two alive and ten carcasses) showed positive PCR results for two different ranavirus primer sets targeting partial genes of a major capsid protein (MCP). The identified MCP sequence was more closely related to Rana catesbeiana virus JP MCP, isolated from invasive bullfrog tadpoles in Japan. We could not detect any lethal bacteria or chytrid fungus in the specimens. Our finding is the first report in Asia that ranavirus is involved in the mass mortality of endemic wild amphibians.     

Evaluating environmental DNA‐based quantification of ranavirus infection in wood frog populations

A variety of challenges arise when monitoring wildlife populations for disease. Sampling tissues can be invasive to hosts, and obtaining sufficient sample sizes can be expensive and time‐consuming, particularly for rare species and when pathogen prevalence is low. Environmental DNA (eDNA)‐based detection of pathogens is an alternative approach to surveillance for aquatic communities that circumvents many of these issues. Ranaviruses are emerging pathogens of ectothermic vertebrates linked to die‐offs of amphibian populations. Detecting ranavirus infections is critical, but nonlethal methods have the above issues and are prone to false negatives. We report on the feasibility and effectiveness of eDNA‐based ranavirus detection in the field. We compared ranavirus titres in eDNA samples collected from pond water to titres in wood frog (Lithobates sylvaticus; n = 5) tadpoles in sites dominated by this one species (n = 20 pond visits). We examined whether ranavirus DNA can be detected in eDNA from pond water when infections are present in the pond and if viral titres detected in eDNA samples correlate with the prevalence or intensity of ranavirus infections in tadpoles. With three 250 mL water samples, we were able to detect the virus in all visits with infected larvae (0.92 diagnostic sensitivity). Also, we found a strong relationship between the viral eDNA titres and titres in larval tissues. eDNA titres increased prior to observed die‐offs and declined afterwards, and were two orders of magnitude higher in ponds with a die‐off. Our results suggest that eDNA is useful for detecting ranavirus infections in wildlife and aquaculture.     

Ranavirus in wood frogs (Rana sylvatica): potential sources of transmission within and between ponds

Members of the genus Ranavirus (family Iridoviridae) can cause catastrophic mortality of pond-breeding amphibians and are associated with an emerging infectious disease that may be contributing to amphibian declines. We conducted three experiments to examine factors that may affect transmission both within and between local breeding populations of the wood frog (Rana sylvatica). In a laboratory study, when exposed to moribund tadpoles collected during a local ranaviral die-off, uninfected tadpoles died as soon as 4 days after exposure. The onset of death was accelerated when tadpoles were allowed to scavenge on carcasses of infected tadpoles. In a mesocosm experiment that was conducted in outdoor wading pools, die-offs of tadpoles began approximately 19 days after infected tadpoles were added to pools containing uninfected tadpoles. Mass die-offs with greater than 98% mortality occurred in all pools, regardless of the initial tadpole density. In a second mesocosm experiment, the addition of water and bottom sediments that were collected from a pond during a ranaviral die-off did not result in lower tadpole survival or growth relative to controls. Only a small percentage of tadpoles appeared to be sick, and most tadpoles survived until the first individuals began metamorphosing within a pool. However, tests for ranavirus using polymerase chain reaction were positive for most pools that received contaminated sediment, suggesting that some infections were sublethal. Our results indicate that transmission within ponds is enhanced by scavenging and that spread between local ponds could occur via the transport of contaminated sediment by animals or humans.     

Does Facilitation of Faunal Recruitment Benefit Ecosystem Restoration? An Experimental Study of Invertebrate Assemblages in Wetland Mesocosms

We used wetland mesocosms (1) to experimentally assess whether inoculating a restored wetland site with vegetation/sediment plugs from a natural wetland would alter the development of invertebrate communities relative to unaided controls and (2) to determine if stocking of a poor invertebrate colonizer could further modify community development beyond that due to simple inoculation. After filling mesocosms with soil from a drained and cultivated former wetland and restoring comparable hydrology, mesocosms were randomly assigned to one of three treatments: control (a reference for unaided community development), inoculated (received three vegetation/sediment cores from a natural wetland), and stocked + inoculated (received three cores and were stocked with a poorly dispersing invertebrate group—gastropods). All mesocosms were placed 100 m from a natural wetland and allowed to colonize for 82 days. Facilitation of invertebrate colonization led to communities in inoculated and stocked + inoculated treatments that contrasted strongly with those in the unaided control treatment. Control mesocosms had the highest taxa richness but the lowest diversity due to high densities and dominance of Tanytarsini (Diptera: Chironomidae). Community structure in inoculated and stocked + inoculated mesocosms was more similar to that of a nearby natural wetland, with abundance more evenly distributed among taxa, leading to diversity that was higher than in the control treatment. Inoculated and stocked + inoculated communities were dominated by non‐aerial invertebrates, whereas control mesocosms were dominated by aerial invertebrates. These results suggest that facilitation of invertebrate recruitment does indeed alter invertebrate community development and that facilitation may lead to a more natural community structure in less time under conditions simulating wetland restoration.     

Effects of nitrate on the interactions of the tadpoles of two ranids (Rana clamitans and R. catesbeiana)

Nitrogen pollution as a result of agricultural runoff and atmospheric deposition is a major challenge to aquatic ecosystems, and is likely to increase in the future. Nitrogenous pollutants are potential stressors of amphibian larvae through their toxicological impacts on individuals; however, they may also increase primary productivity. We examined how such an anthropogenic stressor could influence the interactions between two potentially competing species of tadpoles (Rana clamitans and R. catesbeiana). In a 42 d mesocosm experiment, R. catesbeiana survival, but not final mass, was reduced by nitrate addition. Rana catesbeiana survival was lower when in competition with R. clamitans than when only experiencing intraspecific competition. For R. clamitans, survival was not affected by nitrate addition or competition type. Rana clamitans in nitrate addition mesocosms were heavier than tadpoles from no nitrate mesocosms, and were heavier in intraspecific than in interspecific mesocosms. Our results suggest that nitrate can influence the performance of amphibian larvae, and that its effects could have complex effects on aquatic ecosystems.     

Exposure to coal combustion residues during metamorphosis elevates corticosterone content and adversely affects oral morphology, growth, and development in Rana sphenocephala

Coal combustion residues (CCRs) are documented to negatively impact oral morphology, growth, and development in larval amphibians. It is currently unclear what physiological mechanisms may mediate these effects. Corticosterone, a glucocorticoid hormone, is a likely mediator because when administered exogenously it, like CCRs, also negatively influences oral morphology, growth, and development in larval amphibians. In an attempt to identify if corticosterone mediates these effects, we raised larval Southern Leopard Frogs, Rana sphenocephala, on either sand or CCR substrate and documented effects of sediment type on whole body corticosterone, oral morphology, and time to and mass at key metamorphic stages. Coal combustion residue treated tadpoles contained significantly more corticosterone than controls throughout metamorphosis. However, significantly more oral abnormalities occurred early in metamorphosis when differences in corticosterone levels between treatments were minimal. Overall, CCR-treated tadpoles took significantly more time to transition between key stages and gained less mass between stages than controls, but these differences between treatments decreased during later stages when corticosterone differences between treatments were greatest. Our results suggest endogenous increase in corticosterone content and its influence on oral morphology, growth and development is more complex than previously thought.     

Effects of an infectious fungus, Batrachochytrium dendrobatidis, on amphibian predator-prey interactions

The effects of parasites and pathogens on host behaviors may be particularly important in predator-prey contexts, since few animal behaviors are more crucial for ensuring immediate survival than the avoidance of lethal predators in nature. We examined the effects of an emerging fungal pathogen of amphibians, Batrachochytrium dendrobatidis, on anti-predator behaviors of tadpoles of four frog species. We also investigated whether amphibian predators consumed infected prey, and whether B. dendrobatidis caused differences in predation rates among prey in laboratory feeding trials. We found differences in anti-predator behaviors among larvae of four amphibian species, and show that infected tadpoles of one species (Anaxyrus boreas) were more active and sought refuge more frequently when exposed to predator chemical cues. Salamander predators consumed infected and uninfected tadpoles of three other prey species at similar rates in feeding trials, and predation risk among prey was unaffected by B. dendrobatidis. Collectively, our results show that even sub-lethal exposure to B. dendrobatidis can alter fundamental anti-predator behaviors in some amphibian prey species, and suggest the unexplored possibility that indiscriminate predation between infected and uninfected prey (i.e., non-selective predation) could increase the prevalence of this widely distributed pathogen in amphibian populations. Because one of the most prominent types of predators in many amphibian systems is salamanders, and because salamanders are susceptible to B. dendrobatidis, our work suggests the importance of considering host susceptibility and behavioral changes that could arise from infection in both predators and prey.     

Widespread occurrence of ranavirus in pond-breeding amphibian populations

Ranaviruses are an emerging threat for many amphibian populations, yet their distribution in amphibian communities and the association of infection with possible stressors and species is not fully understood due to historically sparse surveillance. Agricultural practices that reduce the water quality of amphibian breeding habitats (e.g., cattle access to wetlands) and environmental stressors (e.g., lower temperatures) may contribute to ranavirus emergence. We tested larval amphibians for ranavirus infection across four seasons in farm ponds (n?=?40) located in Tennessee, USA. Cattle at various densities were allowed access to half of the sampled ponds. Ranavirus infections were detected in nine species and in 33 of the sampled ponds (83%), illustrating widespread occurrence of the pathogen. Species within the family Ranidae were the most frequently infected. In 13 of the ponds containing infected individuals, prevalence exceeded 40% during at least one season. Infections were detected in multiple seasons in 20 of the sampled ponds containing infections, suggesting that ranaviruses are relatively persistent in these systems. Cattle had negative effects on water quality (turbidity and ammonia) and there was a positive association between cattle abundance and ranavirus prevalence in the summer. Counter to previous field studies in North America, we found a significant positive association between water temperature and ranavirus prevalence in the fall sampling events. Despite these findings, the influences of cattle and temperature on ranavirus prevalence were not consistent across seasons. As such, the mechanisms driving high ranavirus prevalence across the landscape and over time remain unclear. Given the widespread occurrence of ranaviruses in wild amphibians, we encourage the implementation of surveillance programs to help identify potential drivers of emergence. Sites with high ranavirus prevalence should be monitored annually for outbreaks, and the long-term effects on population size determined.     

A tale of two pesticides: how common insecticides affect aquatic communities

1. Recent ecotoxicology studies show that pesticide exposure can alter community composition, structure and function. Generally, community responses to pesticides are driven by trait‐ and density‐mediated indirect effects resulting from sublethal and lethal effects of pesticide exposure on vulnerable taxa. These effects depend upon the concentration of the pesticide and the frequency of exposure. 2. While more research is needed to understand community‐level responses to pesticide exposure, testing the effects of multitudes of registered chemicals on ecologically relevant communities is overwhelming. Recent reviews suggest that contaminants with similar modes of action should produce comparable community‐level responses because they have similar direct effects and, as a result, similar indirect effects; this hypothesis remains largely untested. 3. We subjected pond communities [containing zooplankton, phytoplankton, periphyton and leopard frog tadpoles (Rana pipiens)] to several applications (single applications of medium or high concentrations or weekly applications of a lower concentration) of two acetylcholine esterase inhibiting insecticides, malathion and carbaryl that have comparable toxicity for aquatic organisms. 4. We found that both insecticides cause comparable trophic cascades that affect zooplankton and phytoplankton abundances; however, their effects on amphibians diverged, especially when exposed to higher concentrations of insecticides. Malathion caused a trophic cascade beginning with a decline in cladocerans followed by increases in phytoplankton. At a medium concentration, this cascade also caused a subsequent decrease in periphyton. Carbaryl caused a similar trophic cascade with the highest application, a weak trophic cascade with the medium application and no cascade with smallest application. Malathion directly reduced tadpole survival at all concentrations. Survivors in the two higher treatments were larger at metamorphosis while survivors in the lowest treatments were smaller and developed slowly. In contrast, carbaryl was not directly toxic to tadpoles, but indirectly reduced survival because slow growth and development prevented some tadpoles from metamorphosing before the mesocosms dried at medium and low applications. 5. These results suggest that these common pesticides, which share the same mode of action, have similar effects on zooplankton and algae, but differences in the strength and timing of their effects on tadpoles reduce the generality of responses at higher trophic levels. Overall, general predictive models of contaminant effects could be improved by incorporating the relative timing of direct and indirect effects of exposure.     

Predation risk in tadpole populations shapes behavioural responses of prey but not strength of trait‐mediated indirect interactions

Prey animals often respond to predators by reducing activity levels. This can produce a trait‐mediated indirect interaction (TMII) between predators and prey resources, whereby reduced foraging by prey in the presence of a predator causes an increase in prey resources. TMIIs play important roles in structuring communities, and it is important to understand factors that determine their strength. One such influence may be behavioural variation in the prey species, with indirect effects of predators being stronger within populations that are more responsive to the presence of a predator. We tested 1) whether the behavioural responsiveness of populations of wood frog tadpoles to predator cues was related to the predation risk in their native ponds, and 2) whether more responsive tadpoles yielded stronger TMIIs. To do this, we 1) measured the activity of tadpoles from 18 populations in mesocosms with and without caged predators, and 2) measured changes in the biomass of periphyton (the tadpoles’ diet) between predator treatments for each population. We found that tadpoles from higher predation risk ponds reduced their time outside refuges more in the presence of predators and tended to move less when visible, suggesting possible local adaptation to predation regimes. Though the presence of predators generally resulted in higher periphyton biomass – a TMII – there was no evidence that the strength of this TMII was affected by variation in tadpole behaviour. Foraging activity and general activity may be decoupled to some extent, enabling high predation risk‐adapted tadpoles to limit the fitness costs of reduced foraging when predators are present.     

Resilience and vulnerability are dose-dependently related to neonatal stressors in mice

Early life experiences have been shown to adjust cognitive abilities, stress reactivity, fear responses and immune activity in adult mammals of many species. However, whereas severe stressors have been generally associated with the emergence of hypothalamic pituitary adreno-cortical (HPA)-mediated pathology, mild neonatal stressful experiences have been traditionally associated with ‘positive’ effects or resilience. External stressors stimulate the HPA axis to induce a corticosterone secretion in mouse dams, which, in turn is directly transmitted to the progeny through lactation. Such corticosteroid transfer may offer a unitary mechanism whereby early low corticosterone exposure may favor resilience in the offspring and high corticosterone increase vulnerability to pathology. In this study we further investigated this hypothesis by evaluating the long-term effects of a neonatal exposure to low (33 mg/l) and high (100 mg/l) doses of corticosterone during the first 10 days of life in outbred CD-1 mice through supplementation in the maternal drinking water. Offspring attentional set-shifting abilities, central neurotrophic regulation and levels of natural auto-antibodies (na-Abs) directed to serotonin (SERT) and dopamine (DAT) transporters were assessed in adulthood. While low levels of neonatal corticosterone improved adult cognitive abilities and increased na-Abs levels directed to SERT, high doses of neonatal corticosterone reduced hippocampal BDNF levels and na-Abs directed to DAT. These findings confirm and extend our previous findings, supporting the view that both adaptive plasticity and pathological outcomes in adulthood may depend on circulating neonatal corticosterone levels and that these effects follow a U-shaped profile.     

Geographic variation in corticosterone response to chronic predator stress in tadpoles

Chronic stress often affects growth and development negatively, and these effects are often mediated via glucocorticoid hormones, which elevate during stress. We investigated latitudinal variation in corticosterone (CORT) response to chronic predator stress in Rana temporaria tadpoles along a 1500‐km latitudinal cline in Sweden tadpoles, in a laboratory experiment. We hypothesized that more time‐constrained high‐latitude populations have evolved a lower CORT response to chronic stress to maintain higher growth under stressful conditions. Southern tadpoles had higher CORT content in response to predators after 1 day of exposure, whereas there was no increase in CORT in the northern populations. Two weeks later, there were no predator‐induced CORT elevations. Artificially elevated CORT levels strongly decreased growth, development and survival in both northern and southern tadpoles. We suggest that the lower CORT response in high‐latitude populations can be connected with avoidance of CORT‐mediated reduction in growth and development, but also discuss other possible explanations.     

Reliability of non-lethal surveillance methods for detecting ranavirus infection

Ranaviruses have been identified as the etiologic agent in many amphibian die-offs across the globe. Polymerase chain reaction (PCR) is commonly used to detect ranavirus infection in amphibian hosts, but the test results may vary between tissue samples obtained by lethal and non-lethal procedures. Testing liver samples for infection is a common lethal sampling technique to estimate ranavirus prevalence because the pathogen often targets this organ and the liver is easy to identify and collect. However, tail clips or swabs may be more practicable for ranavirus surveillance programs compared with collecting and euthanizing animals, especially for uncommon species. Using PCR results from liver samples for comparison, we defined false-positive test results as occurrences when a non-lethal technique indicated positive but the liver sample was negative. Similarly, we defined false-negative test results as occurrences when a non-lethal technique was negative but the liver sample was positive. Using these decision rules, we estimated false-negative and false-positive rates for tail clips and swabs. Our study was conducted in a controlled facility using American bullfrog Lithobates catesbeianus tadpoles; false-positive and false-negative rates were estimated after different periods of time following exposure to ranavirus. False-negative and false-positive rates were 20 and 6%, respectively, for tail samples, and 22 and 12%, respectively, for swabs. False-negative rates were constant over time, but false-positive rates decreased with post-exposure duration. Our results suggest that non-lethal sampling techniques can be useful for ranavirus surveillance, although the prevalence of infection may be underestimated when compared to results obtained with liver samples.     

Grazer diversity affects resistance to multiple stressors in an experimental seagrass ecosystem

When multiple stressors act simultaneously, their effects on ecosystems become more difficult to predict. In the face of multiple stressors, diverse ecosystems may be more stable if species respond differently to stressors or if functionally similar species can compensate for stressor effects on focal species. Many habitats around the globe are threatened by multiple stressors, including highly productive seagrass habitats. For example, in Chesapeake Bay, USA, regional climate change predictions suggest that elevated temperature and freshwater inputs are likely to be increasingly important stressors. Using seagrass mesocosms as a model system, we tested whether species richness of crustacean grazers buffers ecosystem properties against the impacts of elevated temperature and freshwater pulse stressors in a fully factorial experiment. Grazer species responded to pulsed salinity changes differently; abundance of Elasmopus levis responded negatively to freshwater pulses, whereas abundance of Gammarus mucronatus and Erichsonella attenuata responded positively or neutrally. Consistent with the hypothesis that biodiversity provides resistance stability, biomass of epiphytic algae that form the base of the food web was less affected by stressors in species‐rich grazer treatments than in single‐species grazer treatments. Stochastic (among‐replicate) variation of sessile invertebrate biomass within treatments was also reduced in more diverse grazer treatments. Therefore, grazer species richness tended to increase the resistance stability of both major components of the seagrass fouling community, algae and invertebrates, in the face of environmental stressors. Finally, in our model system, multi‐stressor impacts suggested a pattern of antagonism contrary to previous assumptions of synergistic stressor effects. Overall, our results confirm that invertebrate grazer species are functionally diverse in their response to environmental stressors, but are largely functionally redundant in their grazing effects leading to greater resistance stability of certain ecosystem properties in diverse grazer assemblages even when influenced by multiple environmental stressors.     

Daphnia in tadpole mesocosms: trophic links and interactions with Batrachochytrium dendrobatidis

1. Amphibians are in decline, and the disease chytridiomycosis, caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd), has been repeatedly implicated throughout the world. This chytrid reproduces via an infectious, motile zoospore stage that remains viable for weeks in the water column. 2. Daphnia is a keystone zooplankton grazer in intact freshwater ecosystems, whose importance to amphibians may be overlooked. As an efficient grazer, Daphnia can suppress chytrid epidemics by consuming zoospores and may therefore play a role in Bd infection dynamics. Daphnia may also have important effects on tadpoles by mediating the properties of pond food webs. We tested the role of Daphnia in outdoor mesocosms containing the tadpoles of red‐legged frogs (Rana aurora) infected with Bd. We also tested the ability of Daphnia to filter Bd from the water column in laboratory microcosms. 3. In the water of microcosms, Daphnia dramatically decreased the number of Bd genomic equivalents detectable using quantitative PCR. Bd genomic equivalents fell below the limit of detection at very high (>1 Daphnia mL^?1) Daphnia densities. 4. In mesocosms, Daphnia was critical to the development of tadpoles: in the presence of Daphnia, tadpoles were twofold heavier at metamorphosis than in their absence. Daphnia and Bd interacted to affect the tadpole survival: survival was highest in the presence of Daphnia and in the absence of Bd. We were unable to detect an effect of Daphnia on the transmission of Bd in mesocosms. However, Bd transmission among the tadpoles in mesocosms was unexpectedly low, limiting our power to detect an effect of Daphnia on transmission. 5. Tadpole dissection showed that tadpoles also consumed large numbers of Daphnia. Current models of mesocosm food webs that assume no predation by tadpoles on zooplankton therefore probably overlook important features of both natural and experimental systems.