Can predators stabilize host-parasite interactions? Changes in aquatic predator identity alter amphibian responses and parasite abundance across life stages.

The role of parasites can change depending on the food web community. Predators, for instance, can amplify or dilute parasite effects on their hosts. Likewise, exposure to parasites or predators at one life stage can have long-term consequences on individual performance and survival, which can influence population and disease dynamics. To understand how predators affect amphibian parasite infections across life stages, we manipulated exposure of northern leopard frog (Rana pipiens) tadpoles to three predators (crayfish [Orconectes rusticus], bluegill [Lepomis macrochirus], or mosquitofish [Gambusia affinis]) and to trematode parasites (Echinostoma spp.) in mesocosms and followed juveniles in outdoor terrestrial enclosures through overwintering. Parasites and predators both had strong impacts on metamorphosis with bluegill and parasites individually reducing metamorph survival. However, when fish were present, the negative effects of parasites on survival was not apparent, likely because fish altered community composition via increased algal food resources. Bluegill also reduced snail abundance, which could explain reduced abundance of parasites in surviving metamorphs. Bluegill and parasite exposure increased mass at metamorphosis, which increased metamorph jumping, swimming, and feeding performance, suggesting that larger frogs would experience better terrestrial survival. Effects on size at metamorphosis persisted in the terrestrial environment but did not influence overwintering survival. Based on our results, we constructed stage-structured population models to evaluate the lethal and sublethal effects of bluegill and parasites on population dynamics. Our models suggested that positive effects of bluegill and parasites on body size may have greater effects on population growth than the direct effects of mortality. This study illustrates how predators can alter the outcome of parasitic infections and highlights the need for long-term experiments that investigate how changes in host-parasite systems alter population dynamics. We show that some predators reduce parasite effects and have indirect positive effects on surviving individuals potentially increasing host population persistence.

Is overwintering mortality driving enigmatic declines? Evaluating the impacts of trematodes and the amphibian chytrid fungus on an anuran from hatching through overwintering.

Emerging infectious diseases are increasing globally and are an additional challenge to species dealing with native parasites and pathogens. Therefore, understanding the combined effects of infectious agents on hosts is important for species' conservation and population management. Amphibians are hosts to many parasites and pathogens, including endemic trematode flatworms (e.g., Echinostoma spp.) and the novel pathogenic amphibian chytrid fungus (Batrachochytrium dendrobatidis [Bd]). Our study examined how exposure to trematodes during larval development influenced the consequences of Bd pathogen exposure through critical life events. We found that prior exposure to trematode parasites negatively impacted metamorphosis but did not influence the effect of Bd infection on terrestrial growth and survival. Bd infection alone, however, resulted in significant mortality during overwintering-an annual occurrence for most temperate amphibians. The results of our study indicated overwintering mortality from Bd could provide an explanation for enigmatic declines and highlights the importance of examining the long-term consequences of novel parasite exposure.

Larval development and survival of pond-breeding anurans in an agricultural landscape impacted more by phytoplankton than surrounding habitat.

The destruction of freshwater habitat is a major contributor to biodiversity loss in aquatic ecosystems. However, created or restored wetlands could partially mitigate aquatic biodiversity loss by increasing the amount of available habitat across a landscape. We investigated the impact of surrounding terrestrial habitat and water quality variables on suitability for two species of pond-breeding amphibians (bullfrogs [Lithobates catesbeianus] and Blanchard's cricket frogs [Acris blanchardi]) in created permanent wetlands located on an agricultural landscape. We examined tadpole growth and survival in field enclosures placed in ponds surrounded by agricultural, forested, or grassland habitats. We also evaluated the potential for carryover effects of the aquatic environment on terrestrial growth and overwinter survival of cricket frog metamorphs. We found that habitat adjacent to ponds did not predict tadpole growth or survival. Rather, phytoplankton abundance, which showed high variability among ponds within habitat type, was the only predictor of tadpole growth. Cricket frogs emerged larger and earlier from ponds with higher phytoplankton abundance; bullfrogs were also larger and at a more advanced developmental stage in ponds with higher levels of phytoplankton. Overwinter survival of cricket frogs was explained by size at metamorphosis and there were no apparent carryover effects of land use or pond-of-origin on overwinter growth and survival. Our results demonstrate that created ponds in human-dominated landscapes can provide suitable habitat for some anurans, independent of the adjacent terrestrial habitat.

Investigating the Effects of Pesticides on Ramshorn Snails (Planorbella [Helisoma] trivolvis) Infected with Echinostoma spp.

Globally, parasite-induced diseases in humans and wildlife are on the rise, and pesticide pollution may be a contributing factor. Echinostoma spp. trematode parasites are prominent in North America, and they use ramshorn snails (Planorbella [Helisoma] trivolvis) as intermediate hosts. We investigated the impact of chronic exposure to 1 of 5 pesticide treatments (control, or 50 µg/L of atrazine, glyphosate, carbaryl, or malathion) on uninfected and Echinostoma-infected snails for 41 d in the laboratory. We recorded snail mortality, the number of egg masses laid, change in mass, and behavior. Chronic exposure to atrazine, carbaryl, and malathion significantly decreased snail survival, whereas parasite infection status or exposure to glyphosate did not. Pesticide and parasite treatments did not influence growth or behavior, but parasite infection caused complete reproductive failure in snail hosts. Our results indicated that the direct effects of pesticides could threaten snail populations in natural environments and disrupt host-parasite dynamics.  Environ Toxicol Chem 2021;40:2755-2763. © 2021 SETAC.

LETHAL AND SUBLETHAL AMPHIBIAN HOST RESPONSES TO BATRACHOCHYTRIUM DENDROBATIDIS EXPOSURE ARE DETERMINED BY THE ADDITIVE INFLUENCE OF HOST RESOURCE AVAILABILITY.

Host species may differ in their responses to pathogen exposures based on host energy reserves, which could be important for long-term trends in host population growth. Batrachochytrium dendrobatidis (BD) is a pathogen associated with amphibian population declines but also occurs without causing mass mortalities. The impact of BD in populations without associated declines is not well understood, and food abundance could play a role in determining the magnitude of its effects. We exposed American toad (Anaxyrus americanus), northern leopard frog (Lithobates pipiens), and cricket frog (Acris blanchardi) metamorphs to BD under low or high food treatments. Overall, anuran species responded differently to BD exposure and the combined effect of BD exposure and food abundance was additive. American toad survival was lowered by BD exposure and low food availability. Based on these results, we developed a population model for American toads to estimate how reductions in survival could influence population growth. We found that BD could reduce population growth by 14% with high food availability and 21% with low food availability. In contrast, survival of northern leopard frogs was high across all treatments, but their growth was negatively impacted by the additive effects of BD exposure and low food availability. Cricket frog growth and survival were unaffected by BD exposure, suggesting that this species is not sensitive to the effects of this pathogen in terms of growth and survival across environments of different quality in the time period examined. Our results showed that low food availability additively increased the species-specific lethal and sublethal impacts of BD on hosts, which could have implications for long-term host population dynamics.

Effects of larval atrazine exposure in mesocosms on Blanchard's cricket frogs (Acris blanchardi) reared through overwintering and to reproductive age.

We exposed Blanchard's cricket frog (Acris blanchardi) tadpoles to atrazine in simulated aquatic communities (outdoor mesocosms) at nominal concentrations of 0, 1, 10, 100, and 200 µg/L and tracked the effects of exposure to spring emergence in the laboratory, as well as to reproductive age in outdoor, terrestrial enclosures. We tested hypotheses that 1) atrazine addition increases the prevalence and intensity of testicular ova (TO) among phenotypic males at metamorphosis and after overwintering, 2) atrazine reduces maturation of ova after overwintering among phenotypic females, and 3) atrazine alters mass, time, and survival to metamorphosis, as well as growth and survival across terrestrial life stages. Atrazine addition increased probability of TO presence at metamorphosis, but only when treatments were pooled and compared to the control, where background atrazine was detected. Atrazine did not influence the intensity of TO among metamorphs. We observed TO among males at spring emergence and at reproductive age regardless of exposure concentration. We found no evidence for effects of exposure on gonadal maturation among females after overwintering. Exposure to 200 µg/L reduced survival to metamorphosis, but atrazine did not affect mass at metamorphosis, time to metamorphosis, or survival or mass after overwintering. We demonstrate that atrazine addition can increase TO prevalence relative to background rates at metamorphosis and that TO are also present among phenotypic males after overwintering. We suggest that this non-model species is sensitive to effects of larval EDC exposures on gonadal development and morphology and that further work with cricket frogs is warranted.

High juvenile mortality in amphibians during overwintering related to fungal pathogen exposure.

The result of pathogen exposures may depend upon trade-offs in energetic demands for immune responses against host growth and survival. Environmental conditions may influence these trade-offs by affecting host size, or trade-offs may change across seasons, altering impacts of pathogens. We exposed northern leopard frog Lithobates pipiens tadpoles to different larval environments (low leaf litter, high density of conspecifics, atrazine, caged fish, or controls) that influenced size at metamorphosis. Subsequently, we exposed metamorphs to Batrachochytrium dendrobatidis (Bd), a fungal pathogen, just after metamorphosis and/or prior to overwintering 12 wk later. Bd exposure dramatically reduced survival during overwintering, with the strongest effects when hosts were exposed at both time points. Larval environments resulted in differences in host size. Those exposed to caged fish were 2.5 times larger than the smallest (those exposed to high density of conspecifics), but larval environment did not influence Bd effects on growth and survival. The largest frogs exposed to caged fish had greater survival through overwintering, but in the absence of Bd. We built stage-structured models to evaluate if overwinter mortality from Bd is capable of having effects on host populations. Our models suggest that Bd exposure after metamorphosis or before overwintering can reduce population growth rates. Our study demonstrates that hosts suffer little effects of Bd exposures following metamorphosis and that small body size did not hamper growth and survival. Instead, we provide evidence that winter mortality from Bd exposure is capable of reducing population sizes, providing a plausible mechanism for amphibian declines in temperate regions.

An amphibian with a contracting range is not more vulnerable to pesticides in outdoor experimental communities than common species.

In areas with heavy pesticide use, it is easy to attribute population declines to environmental contamination. The Blanchard's cricket frogs (Acris blanchardi) is an amphibian experiencing declines and range contractions across its distribution in the Midwest Corn Belt (USA). Experimental studies suggest that cricket frogs are sensitive to pesticides, but there are few studies examining this species' susceptibility to contaminants in realistic environments or comparing relative impacts with other anuran species. I reared 3 summer breeding anurans in outdoor mesocosms posthatching through metamorphosis to examine the effects of 2 insecticides (imidacloprid and carbaryl) and 1 herbicide (glyphosate with polyoxyethylene tallow amine) on larval development and metamorphosis. Cricket frogs were positively affected by insecticide exposure, likely a result of changes in the food web that increased food abundance. However, metamorphosis of green frogs (Lithobates clamitans) and gray tree frogs (Hyla chrysoscelis) appeared unaffected by pesticide exposure. The results of the present study suggest that the impacts of pesticides alone are unlikely to have population-level impacts for the anurans examined. Environ Toxicol Chem 2018;37:2699-2704. © 2018 SETAC.

Atrazine feminizes sex ratio in Blanchard's cricket frogs (Acris blanchardi) at concentrations as low as 0.1 µg/L.

We exposed Blanchard's cricket frogs (Acris blanchardi) to ecologically relevant concentrations (0, 0.1, 1, and 10 µg/L) of a commercial formulation of atrazine throughout the larval period to determine effects on survival, somatic growth and development (time to metamorphosis and mass at metamorphosis), and gonadal development (sex ratio at metamorphosis and the prevalence of testicular ova in phenotypic males). We tested the following hypotheses: 1) atrazine feminizes the sex ratio, 2) atrazine increases the proportion of phenotypic males with testicular ova, and 3) atrazine differentially affects somatic growth (mass at metamorphosis) and development (time to metamorphosis) for males and females. Although the control sex ratio was male-biased, exposure to 0.1 and 10 µg/L atrazine feminized sex ratios, because these treatments produced 51 and 55% fewer males than the control, respectively. We did not observe testicular ova. Atrazine did not impact survival or metamorphosis, and we did not detect sexually dimorphic impacts on time to metamorphosis or mass at metamorphosis. However, males metamorphosed 2.3 d later than females, regardless of treatment. Sex biases in timing of metamorphosis are underexplored in anurans, but if prevalent, could have important implications for theory surrounding the impact of environmental factors on metamorphosis. Our data suggest that cricket frog sex ratios are sensitive to environmentally relevant concentrations of atrazine and that feminization in the field is likely. Environ Toxicol Chem 2018;37:427-435. © 2017 SETAC.

Host size influences the effects of four isolates of an amphibian chytrid fungus.

Understanding factors that influence host-pathogen interactions is key to predicting outbreaks in natural systems experiencing environmental change. Many amphibian population declines have been attributed to an amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd). While this fungus is widespread, not all Bd-positive populations have been associated with declines, which could be attributed to differences in pathogen virulence or host susceptibility. In a laboratory experiment, we examined the effects of Bd isolate origin, two from areas with Bd-associated amphibian population declines (El Copé, Panama, and California, USA) and two from areas without Bd-related population declines (Ohio and Maine, USA), on the terrestrial growth and survival of American toad (Anaxyrus americanus) metamorphs reared in larval environments with low or high intraspecific density. We predicted that (1) Bd isolates from areas experiencing declines would have greater negative effects than Bd isolates from areas without declines, and (2) across all isolates, growth and survival of smaller toads from high-density larval conditions would be reduced by Bd exposure compared to larger toads from low-density larval conditions. Our results showed that terrestrial survival was reduced for smaller toads exposed to Bd with variation in the response to different isolates, suggesting that smaller size increased susceptibility to Bd. Toads exposed to Bd gained less mass, which varied by isolate. Bd isolates from areas with population declines, however, did not have more negative effects than isolates from areas without recorded declines. Most strikingly, our study supports that host condition, measured by size, can be indicative of the negative effects of Bd exposure. Further, Bd isolates' impact may vary in ways not predictable from place of origin or occurrence of disease-related population declines. This research suggests that amphibian populations outside of areas experiencing Bd-associated declines could be impacted by this pathogen and that the size of individuals could influence the magnitude of Bd's impact.

Short-term atrazine exposure at breeding has no impact on Blanchard's cricket frog (Acris blanchardi) reproductive success.

Studies of endocrine-disrupting contaminants have focused on early-life exposures, but later exposures could impact fitness. We exposed adult frogs (Acris blanchardi) at reproduction to ecologically relevant atrazine concentrations (0, 1, or 10 µg/L) in outdoor arenas. We measured likelihood of breeding and number of resulting tadpoles. Atrazine impacted neither the probability of breeding nor the number of tadpoles produced, suggesting anuran reproductive success may not be impacted by short-term exposure to low concentrations. Environ Toxicol Chem 2017;36:3284-3288. © 2017 SETAC.

Variation in malathion sensitivity among populations of Blanchard's cricket frogs (Acris blanchardi) and implications for risk assessment.

Intraspecific variability in contaminant sensitivity could undermine risk assessments for nontarget organisms such as amphibians. To test how amphibian populations vary in tolerance to anticipated lethal and sublethal exposures to a pesticide, we exposed Blanchard's cricket frogs (Acris blanchardi) from 3 populations across a broad portion of their range to the insecticide malathion. Exposure in mesocosms to a nominal concentration of 1 mg/L (measured concentrations at 1 h and 24 h postaddition of 0.160 mg/L and 0.062 mg/L, respectively), a realistic direct-overspray scenario, reduced survival to metamorphosis by 43% relative to controls and revealed variation in tolerance among populations. Survival ranged from 74% for the most tolerant population to 18% for the least tolerant population, a 4.1-fold difference. Mass at metamorphosis and time to metamorphosis were unaffected. Although malathion reduced zooplankton abundance, it did not alter food resources (periphyton or phytoplankton relative abundance), or a suite of water-quality variables (pH, temperature, and dissolved oxygen). A 96-h time-to-death assay designed to isolate direct, lethal effects also revealed variation in tolerance among populations. Time to death (mean ± standard error) ranged from 2.4 ± 0.18 h for the least tolerant population to 17.8 ± 4.72 h for the most tolerant population, a 7.4-fold difference. However, relative sensitivities of populations differed in the mesocosm and laboratory studies, which differed in exposure concentrations, suggesting that populations tolerant of high concentrations can be more sensitive to lower concentrations. We suggest that direct overspray could reduce larval survival in the field for this species. Studies assessing the role of contaminants in declines or extrapolating to untested populations, especially across large geographical regions, should quantify the range of intraspecific variation. Risk assessors could address intraspecific variability directly by using an intraspecific uncertainty factor. Environ Toxicol Chem 2017;36:1917-1923. © 2016 SETAC.

The effects of the amphibian chytrid fungus, insecticide exposure, and temperature on larval anuran development and survival.

Chytridiomycosis, a disease caused by Batrachochytrium dendrobatidis (Bd), has been implicated as a cause of amphibian declines. Susceptibility may be influenced by environmental factors that suppress the immune response. The authors conducted a laboratory study to examine the effect of temperature, insecticide exposure, and Bd exposure during larval anuran development. The authors examined the consequences of exposure to Bd, an insecticide (carbaryl or malathion), and static or fluctuating temperature (15 °C, 20 °C, 25 °C, or 15 °C to 25 °C 72-h flux) on larval development through metamorphosis of the Pacific treefrog (Pseudacris regilla). High and fluctuating temperature had negative effects on survival in the presence of Bd. Insecticides inhibited the effects of Bd; time to tail resorption of Pacific treefrogs decreased when tadpoles were exposed to carbaryl. The present study indicates that abiotic factors may play a role in the host-pathogen interactions in this system.

Effects of amphibian chytrid fungus exposure on American toads in the presence of an insecticide.

Abiotic factors such as pesticides may alter the impact of a pathogen on hosts, which could have implications for host-pathogen interactions and may explain variation in disease outbreaks in nature. In the present laboratory experiment, American toad (Anaxyrus americanus) metamorphs were exposed to the amphibian chytrid fungal pathogen Batrachochytrium dendrobatidis (Bd) and environmentally relevant concentrations of the insecticide malathion to determine whether malathion altered the effects of Bd exposure on growth and survival of toad metamorphs. Exposure to Bd significantly decreased survival over the 51 d of the experiment, suggesting that Bd could reduce recruitment into the terrestrial life stage when exposure occurs at metamorphosis. Malathion did not impact survival, but a 12-h exposure at metamorphosis significantly reduced terrestrial growth. Toads that were exposed to both Bd and malathion showed a nonsignificant trend toward the smallest growth compared with other treatments. The present study suggests that Bd may pose a threat to American toads even though population declines have not been observed for this species; in addition, the presence of both the insecticide malathion and Bd could reduce terrestrial growth, which could have implications for lifetime fitness and suggests that environmental factors could play a role in pathogen impacts in nature.

Effects of pesticide exposure and the amphibian chytrid fungus on gray treefrog (Hyla chrysoscelis) metamorphosis.

Pesticides are detectable in most aquatic habitats and have the potential to alter host-pathogen interactions. The amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), has been associated with amphibian declines around the world. However, Bd-associated declines are more prominent in some areas, despite nearly global distribution of Bd, suggesting other factors contribute to disease outbreaks. In a laboratory study, the authors examined the effects of 6 different isolates of Bd in the presence or absence of a pesticide (the insecticide carbaryl or the fungicide copper sulfate) to recently hatched Cope's gray treefrog (Hyla chrysoscelis) tadpoles reared through metamorphosis. The authors found the presence or absence of pesticides differentially altered the mass at metamorphosis of tadpoles exposed to different Bd isolates, suggesting that isolate could influence metamorphosis but not in ways expected based on origin of the isolate. Pesticide exposure had the strongest impact on metamorphosis of all treatment combinations. Whereas copper sulfate exposure reduced the length of the larval period, carbaryl exposure had apparent positive effects by increasing mass at metamorphosis and lengthening larval period, which adds to evidence that carbaryl can stimulate development in counterintuitive ways. The present study provides limited support to the hypothesis that pesticides can alter the response of tadpoles to isolates of Bd and that the insecticide carbaryl can alter developmental decisions.

The effects of 24-h exposure to carbaryl or atrazine on the locomotor performance and overwinter growth and survival of juvenile spotted salamanders (Ambystoma maculatum).

Understanding the effects of pesticide exposure on organisms throughout their life cycle is critical to predict population-level effects. For many taxa, including amphibians, juveniles are the main dispersal stage and are disproportionally important to population persistence when compared with other life stages. In the present study, we examined the effects of a single 24-h exposure to the insecticide carbaryl or the herbicide atrazine on locomotor performance (endurance, distance traveled, speed, and fatigue) in the laboratory and terrestrial growth and survival through overwintering in field enclosures using recent metamorphs of spotted salamanders (Ambystoma maculatum). We found that neither atrazine nor carbaryl impacted endurance, but fatigue increased with carbaryl exposure, which could leave salamanders less able to escape repeated attacks by predators. Terrestrial growth and overwinter survival were not affected by short-term exposure to carbaryl or atrazine, suggesting that when individuals can overcome acute effects, no long-term consequences result for the endpoints measured.

Characterization of gene expression endpoints during postembryonic development of the northern green frog (Rana clamitans melanota).

Postembryonic development of a larval tadpole into a juvenile frog involves the coordinated action of thyroid hormone (TH) across a diversity of tissues. Changes in the frog transcriptome represent a highly sensitive endpoint in the detection of developmental progression, and for the identification of environmental chemical contaminants that possess endocrine disruptive properties. Unfortunately, in contrast with their vital role as sentinels of environmental change, few gene expression tools currently exist for the majority of native North American frog species. We have isolated seven expressed gene sequences from the Northern green frog (Rana clamitans melanota) that encode proteins associated with TH-mediated postembryonic development and global stress response, and established a quantitative real-time polymerase chain reaction (qPCR) assay. We also obtained three additional species-specific gene sequences that functioned in the normalization of the expression data. Alterations in mRNA abundance profiles were identified in up to eight tissues during R. clamitans postembryonic development, and following exogenous administration of TH to premetamorphic tadpoles. Our results characterize tissue distribution and sensitivity to TH of select mRNA of a common North American frog species and support the potential use of this qPCR assay in identification of the presence of chemical agents in aquatic environments that modulate TH action.

Specific time of exposure during tadpole development influences biological effects of the insecticide carbaryl in green frogs (Lithobates clamitans).

The orchestration of anuran metamorphosis is initiated and integrated by thyroid hormones, which change dynamically during larval development and which may represent a target of disruption by environmental contaminants. Studies have found that some anurans experience increased rates of development when exposed to the insecticide carbaryl later in larval development, suggesting that this insecticide could affect thyroid hormone-associated biological pathways. However, the time in development when tadpoles are sensitive to insecticide exposure has not been clearly defined nor has the mechanism been tested. In two separate studies, we exposed recently hatched green frog (Lithobates clamitans) tadpoles to a single, three day carbaryl exposure in the laboratory at either 2, 4, 8, or 16 weeks post-hatching. We examined the impact of carbaryl exposure on mRNA abundance patterns in the brains of frogs following metamorphosis months after a single three day exposure (experiment 1) and in tadpole tails three days after exposure (experiment 2) using cDNA microarrays and quantitative real time polymerase chain reaction (QPCR) analyses. For tadpoles reared through metamorphosis, we measured tadpole growth and development, as well as time to, mass at, and survival to metamorphosis. Although carbaryl did not significantly impact tadpole development, metamorphosis, or survival, clear exposure-related alterations in both tail and brain transcript levels were evident when tadpoles were exposed to carbaryl, particularly in tadpoles exposed at weeks 8 and 16 post-hatching, indicating both short-term and long-term alterations in mRNA expression. These results indicate that carbaryl can have long-lasting effects on brain development when exposure occurs at sensitive developmental stages, which may have implications for animal fitness and function later in the life cycle.

Effects of terrestrial buffer zones on amphibians on golf courses.

A major cause of amphibian declines worldwide is habitat destruction or alteration. Public green spaces, such as golf courses and parks, could serve as safe havens to curb the effects of habitat loss if managed in ways to bolster local amphibian communities. We reared larval Blanchard's cricket frogs (Acris blanchardi) and green frogs (Rana clamitans) in golf course ponds with and without 1 m terrestrial buffer zones, and released marked cricket frog metamorphs at the golf course ponds they were reared in. Larval survival of both species was affected by the presence of a buffer zone, with increased survival for cricket frogs and decreased survival for green frogs when reared in ponds with buffer zones. No marked cricket frog juveniles were recovered at any golf course pond in the following year, suggesting that most animals died or migrated. In a separate study, we released cricket frogs in a terrestrial pen and allowed them to choose between mown and unmown grass. Cricket frogs had a greater probability of using unmown versus mown grass. Our results suggest that incorporating buffer zones around ponds can offer suitable habitat for some amphibian species and can improve the quality of the aquatic environment for some sensitive local amphibians.

Impact of an insecticide changes with amount of leaf litter input: implications for amphibian populations.

Changes in percentage of forest cover can influence nutrient levels in aquatic systems and change abiotic conditions that may influence species. The authors examined how increasing amounts of leaf litter influenced toxicity of the insecticide carbaryl using larval green frogs (Rana clamitans) in outdoor mesocosm ponds. Insecticides can have direct negative effects on individual physiology and behavior and indirect effects on the food web, which can result in trophic cascades. They predicted that direct effects of the insecticide would dominate when nutrients were low (resulting in negative impacts on amphibian development and survival), whereas indirect effects could offset direct effects when nutrients were more abundant through a trophic cascade that leads to more food for tadpoles (resulting in positive effects on amphibian development and survival). The authors found support for this hypothesis: first, total green frog survival was greatest with increased leaf litter input in the presence of carbaryl. Additionally, most green frogs that reached metamorphosis were from ponds with high leaf litter input and carbaryl. Second, the impact of carbaryl on developmental stage and tadpole mass varied depending on the amount of leaf litter present. With high amounts of leaf litter, carbaryl had a positive impact on development and growth; in contrast, with low amounts of leaf litter, carbaryl had negative or no effects on tadpole development and mass. The present study suggests that differences in nutrient levels between ponds exposed to pesticides could play a role in amphibian population dynamics.