Impacts of cattle on amphibian larvae and the aquatic environment

• 1 Agricultural practices such as cattle farming may have direct or indirect negative effects on larval amphibians by decreasing water quality through deposition of nitrogenous waste, causing eutrophication, and grazing shoreline vegetation that contributes to detrital cover and food.
• 2 We sampled amphibian larvae on the Cumberland Plateau, Tennessee, U.S.A., twice per week, water quality twice per month and algal and detrital biomass once per month at seven wetlands (three cattle‐access and four non‐access) from March to August 2005 and 2006.
• 3 In general, species richness and diversity of amphibian larvae were greater in wetlands without cattle. Mean relative abundance of green frog (Rana clamitans) and American bullfrog (Rana catesbeiana) tadpoles was greater in non‐access wetlands. Body size of some ranid larvae was larger in cattle‐access wetlands but this trend did not exist for juveniles or adults. Dissolved oxygen was lower, while specific conductivity and turbidity were higher in cattle‐access wetlands. Mean biomass of detritus was lower in cattle‐access wetlands compared to non‐access wetlands; no differences were detected in algal biomass.
• 4 Given the negative impacts of cattle on water quality, detrital biomass, larval amphibian species richness and relative abundance of some amphibian species, we recommend that farmers consider excluding these livestock from aquatic environments.

     

Development of arginine vasotocin innervation in two species of anuran amphibian:Rana catesbeiana andRana sylvatica

Arginine vasotocin (AVT) is a neurotransmitter in the amphibian central nervous system and is released from the neurohypophysis in the regulation of hydromineral balance and other homeostatic functions. Many amphibians experience drastic changes in habitat with respect to water availability during their transformation from aquatic larvae to terrestrial adults. To examine whether metamorphosis is accompanied by a reorganization of central vasotocinergic neurons, the developmental organization of vasotocin neurons and nerve fibers was studied with immunocytochemistry in the brains of bullfrogs (Rana catesbeiana) and woodfrogs (R. sylvatica). In bullfrogs, early limb-bud-stage tadpoles had AVT-immunoreactive neurons and nerve fibers in the lateral septal nucleus, amygdala, preoptic hypothalamus, suprachiasmatic nucleus, and posterodorsal tegmentum. Woodfrog larvae showed similar patterns of hypothalamic AVT immunoreactivity, although neuronal staining in the amygdala did not appear until metamorphic climax, and never appeared in septal neurons or in the posterodorsal tegmentum. Whereas the highly terrestrialR. sylvatica adults must adapt to an adult habitat with prolonged periods of dehydration,R. catesbeiana adults remain semiaquatic and, as such, need not develop extreme mechanisms for water retention. Nonetheless, vasotocinergic pathways showed developmental similarities in the two species. The early appearance of AVT innervation in bothRana suggests that AVT has neuroregulatory functions well before metamorphosis.     

Higher temperature variability increases the impact of Batrachochytrium dendrobatidis and shifts interspecific interactions in tadpole mesocosms

The emergence of amphibian chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (Bd) has led to the decline and extinction of numerous amphibian species. Multiple studies have observed links between climatic factors and amphibian declines apparently caused by Bd. Using outdoor experimental mesocosms, we tested the response of red‐legged frog (Rana aurora) tadpoles to increased variation in temperature, a component of climate linked to amphibian declines, and Bd exposure. We included tadpoles of a sympatric competitor species, Pacific chorus frog (Pseudacris regilla), in a fully factorial design to test the effects of Bd and temperature on interspecific interactions. We found that higher variation in temperature had numerous effects in mesocosms, including interacting with Bd presence to decrease the condition of R. aurora, shifting the relative performance of competing P. regilla and R. aurora, and accelerating the development of P. regilla relative to R. aurora. Our results demonstrate that increased variation in temperature can affect amphibians in multiple ways that will be contingent on ecological context, including the presence of Bd and competing species.     

Amphibian survival,growth and development in response to mineral nitrogen exposure and predator cues in the field: an experimental approach

Mineral nitrogen (N) has been suggested as a potential factor causing declines in amphibian populations, especially in agricultural landscapes; however, there is a question as to whether it remains in the water column long enough to be toxic. We explored the hypothesis that mineral N can cause both lethal and sublethal toxic effects in amphibian embryos and larvae in a manipulative field experiment. We sampled 12 ponds, fertilizing half with ammonium nitrate fertilizer early in the spring, and measured hatching, survival, development, growth, and the incidence of deformities in native populations of wood frog (Rana sylvatica) and eastern tiger salamander (Ambystoma tigrinum tigrinum) embryos and larvae held in in situ enclosures. We found that higher ammonium concentrations negatively affect R. sylvatica more strongly than A. tigrinum. R. sylvatica tended to have lower survival as embryos and young tadpoles, slowed embryonic development, and an increased proportion of hatchlings with deformities at experimentally elevated ammonium. A. tigrinum did not experience significantly reduced survival, but their larval development was slowed in response to elevated ammonium and the abundance of large invertebrate predators. Variable species susceptibility, such as that shown by R sylvatica and A. tigrinum, could have large indirect effects on aquatic community structure through modification of competitive or predator-prey relationships. Ammonium and nitrate + nitrite concentrations were not correlated with other measures that might have affected amphibians, such as pH, pond area, depth, or vegetation. Our results highlight the potential importance of elevated ammonium on the growth, development and survival of amphibians, especially those that breed in surface waters receiving anthropogenic N inputs. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The choice of oviposition site and the effects of density and oviposition timing on survivorship in<Emphasis Type="Italic"> Rana japonica</Emphasis>

In species that lack parental care and whose larvae are restricted to a given space throughout development, choice of egg-laying or reproductive sites often greatly influences fitness. Furthermore, reproductive success of individuals may be affected not only by conditions of the breeding area but also by the time when they start to breed, because intraspecific variation in the timing of breeding may result in asymmetric competition between larvae. We addressed these issues in an experimental and field study of the Japanese brown frog, Rana japonica. We tested whether the survival success of larvae decreased with an increase in the number of deposited egg masses and with a greater delay in oviposition. We found that the number of egg masses and timing of oviposition together significantly predicted larval survival per clutch. Moreover, we observed the natural oviposition of R. japonica to examine the prediction that if the density of larvae and the timing of oviposition affect survivorship of larvae, R. japonica will avoid depositing eggs in pools in which egg masses have already been deposited. We found that for small pools only, R. japonica tended to favor oviposition at unoccupied pools (those lacking egg masses and larvae) in the spawning season. For large pools, however, adults favored those already occupied by egg masses in the spawning period. The density of egg masses and tadpoles or the timing of oviposition may have less effect on spawning or the survival of tadpoles in large pools than in small pools.     

Invasive Plant and Experimental Venue Affect Tadpole Performance

Introductions of non-native predators and competitors appear to contribute to worldwide amphibian declines; however, potential negative impacts of invasive plants on habitat quality and amphibian populations have not been examined. Loss of diversity and alterations in ecosystem function associated with plant invasions may disrupt food webs, potentially leading to further declines of already threatened amphibian populations. We used a combination of small bins, mesocosms, and field experiments to examine the impacts of Eurasian purple loosestrife (Lythrum salicaria) replacing native cattails (Typha latifolia) in North American freshwater wetlands on survival, developmental rate, and diet (freshwater algae) of American toad (Bufo americanus) tadpoles. Tadpoles developed slower in L. salicaria compared to tadpoles developing in T. latifolia. This effect was consistent across experimental venues, although mesocosms showed this effect only in the second year of our study. Survival and development rates were always more variable in purple loosestrife than in cattail. In bins, tadpoles showed significantly reduced survival when raised in purple loosestrife extract and addition of leaf litter exacerbated this negative effect. Tadpole survival rates in mesocosms and field cages were not significantly different between plant species, most likely an effect of high variability among replicates. We suspect a combination of direct toxicity of high tannin concentrations in L. salicaria leaves and their indirect negative impacts on aquatic food webs are responsible for these results. Tadpole gut analyses revealed differences in algal communities among venues and between L. salicaria and T. latifolia suggesting that alterations in tadpole food quality and quantity contribute to the observed reduced tadpole performance. The replacement of native wetland plant species by L. salicaria does not represent a simple exchange of ecological equivalents and the function of invaded habitats for native species has clearly changed. While we were investigating only a single amphibian species, our results suggest that the impact of L. salicaria on ecosystem processes and aquatic food webs may be more general and likely to negatively affect other wetland species. The threats non-indigenous plants represent for amphibian populations and food webs may be underestimated, and warrant further investigation.     

Impacts of hydroperiod on growth and survival of larval amphibians in temporary ponds of Central Pennsylvania,USA

The effects of variable hydroperiod (three levels) and initial density of amphibians (two levels) on survival, growth rate, and time to and mass at metamorphosis were studied for wood frogs (Rana sylvatica), Jefferson salamanders (Ambystoma jeffersonianum), and spotted salamanders (A. maculatum). Experiments were carried out in 260-1 mesocosms set up outdoors in a forest. These pond simulations were designed to mimic conditions that occur in palustrine temporary wetlands in central Pennsylvania. No animals reached metamorphosis in the short hydroperiod (56 days). However a greater proportion (66%) of tadpoles of R. sylvatica survived to the end of the 56-day, treatment than the 84- or 158-day treatments (29 and 14%, respectively), from which all survivors metamorphosed. In contrast, neither of the salamanders metamorphosed by 84 days; survival to metamorphosis at 158 days was 15% for A. jeffersonianum and 10% for A. maculatum. Average instantaneous growth rates for A. jeffersonianum decreased with each increase in hydroperiod. Growth of R. sylvatica was greater in the 56-day hydroperiod than in hydroperiods of 84 or 158 days. Initial amphibian density had no effect on growth or survival of any species. It appears that salamander larvae were predatory on tadpoles, since survival of R. sylvatica was negatively correlated with survival of A. jeffersonianum in 84-day treatments and with growth of A. maculatum in 158-day treatments.     

An examination of multiple factors affecting community structure in an aquatic amphibian community

Summary The potential effects of multiple factors structuring certain larval amphibian communities were studied using a pen experiment in a natural pond. Potential factors (predation and competition from other species) were allowed to act in a stepwise fashion such that their relative importance could be evaluated. Based on a previous study, it was hypothesized that predation by Ambystoma salamander larvae on other larval amphibian species would be the most important factor. Survival of Ambystoma jeffersonianum salamander larvae and Rana sylvatica tadpoles was significantly depressed only by Ambystoma opacum predation. Survival of Ambystoma maculatum salamander larvae was significantly greater in the absence of both A. opacum and A. jeffersonianum predators. The virtual elimination of Hyla chrysoscelis larvae in all treatments also can be largely attributed to Ambystoma predation. Thus, Ambystoma predation was the dominant factor determining larval survival of four amphibian prey species in the experimental communities.     

Evaluating a predator–prey interaction in the field: the interaction between beetle larvae (predator) and tadpoles (prey)

The larval amphibian community of temporary pond ecosystems has served as a model for studies in community ecology, with a majority of this work being conducted in mesocosms. Recent research has suggested that mesocosms may overestimate ecological effects; therefore, experimental studies conducted under field conditions are required to gauge the results of mesocosm studies. To assess a species interaction under more natural conditions, we conducted a series of field experiments examining the predator–prey interaction between beetle larvae ( Dytiscus sp.; predator) and larval wood frogs Rana sylvatica (prey) in central Pennsylvania, USA. Quantitative sampling of woodland ponds indicated that beetle larvae of the genus Dytiscus were the most common predator of tadpoles. In a field enclosure experiment, dytiscids were effective predators of tadpoles in the pond environment. Moreover, tadpoles avoided areas in a pond containing caged dytiscids, demonstrating that tadpoles recognize the chemical stimuli of predators in complex environments. The results of this study are consistent with data from prior laboratory and mesocosm studies and suggest that these venues can produce reliable interpretations of predator–prey dynamics in this community.     

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.     

No apparent effects of soil inoculum on green frog (Lithobates clamitans Latreille) tadpole performance

Invasions by nonnative plant species are transforming plant communities across the globe. An important challenge for ecologists is to understand how animals will respond to these changes. One way that plant invasions could affect aquatic animals is by changing the rate at which soil communities decompose litter, which could alter the flow of energy and nutrients from plant litter to aquatic communities. In this study, we measured larval amphibian responses to soil conditioned by either introduced or native genotypes of Phragmites australis L. (common reed) in northeastern North America. We collected soil from adjacent stands of introduced and native P. australis at three sites in central New York and inoculated outdoor aquatic mesocosms with soil extracts. Mesocosms contained six Lithobates clamitans Latreille (green frog) tadpoles and either low- or high-quality native P. australis americanus litter. We found that litter decomposition differed based on soil inoculum, and we observed a significant interaction between litter quality and soil inoculum; higher-quality litter tended to decompose faster when exposed to inocula from introduced P. australis, while lower-quality litter tended to decompose faster when exposed to inocula from native P. australis americanus. Tadpoles raised with high-quality litter developed faster and achieved greater body size, but soil inocula had no apparent effect on tadpoles. Our results suggest that plant invasions may alter microbial communities, causing subtle changes in litter decomposition rates, but these changes do not appear strong enough to influence larvae of a widespread amphibian.     

Multiple predator effects on size-dependent behavior and mortality of two species of anuran larvae

This study examined the effects of multiple predators on size‐specific behavior and mortality of two species of anuran larvae. Particularly, we focused on how trait changes in predators and prey may be transmitted to other species in the food web. In laboratory experiments, we examined the effects of bluegill sunfish, Lepomis macrochirus, and the odonate larva Anax junius on behavior and mortality of tadpoles of the bullfrog, Rana catesbeiana, and the green frog R. clamitans. Experiments were conducted with predators alone and together to assess effects on behavior and mortality of the tadpoles. The experiments were replicated on five size classes of the tadpoles to evaluate how responses varied with body size.
Predation rates by Anax were higher on bullfrogs than on green frogs, and both bullfrogs and green frogs suffered greater mortality from Anax than from bluegill. Bluegill only consumed green frogs. Predation rates by both predators decreased with increasing tadpole size and decreased in the non‐lethal (caged) presence of the other predator. Both anuran larvae decreased activity when exposed to predators. Bullfrogs, however, decreased activity more in the presence of Anax than in the presence of bluegill, whereas green frogs decreased activity similarly in the presence of both predators. The largest size class of green frogs, but not of bullfrogs, exhibited spatial avoidance of bluegill. These responses were directly related to the risk posed by the different predators to each anuran species. Anax activity (speed and move frequency) also was higher when alone than in the non‐lethal presence of bluegill. We observed decreased predation rate of each predator in the non‐lethal presence of the other, apparently caused by two different mechanisms. Bluegill decreased Anax mortality on tadpoles by restricting the Anax activity. In contrast, Anax decreased bluegill mortality on tadpoles by reducing tadpole activity. We discuss how the activity and spatial responses of the tadpoles interact with palatability and body size to create different mortality patterns in the prey species and the implications of these results to direct and indirect interactions in this system.     

Synergistic effects of the invasive Chinese tallow (Triadica sebifera) and climate change on aquatic amphibian survival

Changes in climate and the introduction of invasive species are two major stressors to amphibians, although little is known about the interaction between these two factors with regard to impacts on amphibians. We focused our study on an invasive tree species, the Chinese tallow (Triadica sebifera), that annually sheds its leaves and produces leaf litter that is known to negatively impact aquatic amphibian survival. The purpose of our research was to determine whether the timing of leaf fall from Chinese tallow and the timing of amphibian breeding (determined by weather) influence survival of amphibian larvae. We simulated a range of winter weather scenarios, ranging from cold to warm, by altering the relative timing of when leaf litter and amphibian larvae were introduced into aquatic mesocosms. Our results indicate that amphibian larvae survival was greatly affected by the length of time Chinese tallow leaf litter decomposes in water prior to the introduction of the larvae. Larvae in treatments simulating warm winters (early amphibian breeding) were introduced to the mesocosms early in the aquatic decomposition process of the leaf litter and had significantly lower survival compared with cold winters (late amphibian breeding), likely due to significantly lower dissolved oxygen levels. Shifts to earlier breeding phenology, linked to warming climate, have already been observed in many amphibian taxa, and with most climate models predicting a significant warming trend over the next century, the trend toward earlier breeding should continue if not increase. Our results strongly suggest that a warming climate can interact with the effects of invasive plant species, in ways we have not previously considered, to reduce the survival of an already declining group of organisms.     

First Evidence of <Emphasis Type="Italic">Batrachochytrium dendrobatidis</Emphasis> in China: Discovery of Chytridiomycosis in Introduced American Bullfrogs and Native Amphibians in the Yunnan Province,China

Although the chytrid fungus Batrachochytrium dendrobatidis (Bd), the etiological agent of amphibian chytridiomycosis, has been implicated in mass mortality and population declines on several continents around the world, there have been no reports on the presence of Bd infections in amphibians in China. We employed quantitative PCR and histological techniques to investigate the presence of Bd in introduced North American bullfrogs (Rana catesbeiana) (referred to hereafter as bullfrog) and native amphibians in bullfrog-invaded areas of the Yunnan Province, China. A total of 259 samples at five wild sites were collected between June and September in 2007 and 2008, including bullfrogs and four native amphibian species (Rana pleuraden, Rana chaochiaoensis, Odorrana andersonii, and Bombina maxima). In addition, 37 samples of adult bullfrogs were obtained from a food market. Bd infections were discovered in bullfrogs and three native amphibian species from all of the surveyed sites. Of the 39 Bd-positive samples, 35 were from wild-caught bullfrog tadpoles, postmetamorphic bullfrogs, R. pleuraden, R. chaochiaoensis, and O. andersonii, and four were from adult bullfrogs from the market. Our results provide the first evidence of the presence of Bd in Chinese amphibians, suggesting that native amphibian diversity in China is at risk from Bd. There is an urgent need to monitor the distribution of Bd in amphibians in China and understand the susceptibility of native amphibian species to chytridiomycosis. Strict regulations on the transportation of bullfrogs and the breeding of bullfrogs in markets and farms should be drafted in order to stop the spread of Bd by bullfrogs.     

Effects of nutrient supplementation on host‐pathogen dynamics of the amphibian chytrid fungus: a community approach

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Predators and trematode parasites jointly affect larval anuran functional traits and corticosterone levels

Non‐consumptive predator effects may have dramatic consequences for host–parasite interactions by influencing the ability of prey items to avoid, resist, or tolerate infection. Both predators and parasites can affect host traits, such as growth rates and behavior, and these effects may in part be mediated through shared physiological pathways (e.g. the glucocorticoid stress hormone, corticosterone [CORT]). Here, we examined the effects of trematode parasites (Digena: Echinostomatidae) and predator (larval odonate) exposure on larvae of two amphibian species (Rana sylvatica and R. clamitans) in laboratory experiments. First, we measured behavior and CORT responses of tadpoles exposed to predator chemical cue in combination with parasite cue or under direct exposure to parasites. We then measured the combined effects of predator cue and parasite infection on survival and traits. Evidence for effects of parasite cue in our study was equivocal, but we found novel interactive effects of parasites and predators on larval frogs. Parasites and predators had antagonistic effects on CORT, behavior, and morphology, and negative synergistic effects on development. In addition, parasite infection and predator cues additively reduced activity levels of both species and growth in wood frogs. Negative effects of parasite infection on survival and traits were dose‐dependent for both species, although wood frogs generally experienced stronger effects of infection than green frogs. Our results emphasize the importance of considering effects of parasites as well as predators, since both can have strong effects on survival and the combination can have both additive and non‐additive effects on key traits. These effects likely have important implications for amphibian population dynamics, community structure, and conservation.     

Density of an intraguild predator mediates feeding group size, intraguild egg predation, and intra- and interspecific competition

Intraguild predation (IGP) is common in most communities, but many aspects of density-dependent interactions of IG predators with IG prey are poorly resolved. Here, we examine how the density of an IG predator can affect feeding group size, IG egg predation, and the growth responses of IG prey. We used laboratory feeding trials and outdoor mesocosm experiments to study interactions between a social intraguild predator (larvae of the wood frog; Rana sylvatica) and its prey (spotted salamander; Ambystoma maculatum). Larvae of R. sylvatica could potentially affect A. maculatum by consuming shared larval food resources or by consuming eggs and hatchlings. However, successful egg predation requires group feeding by schooling tadpoles. We established from five to 1,190 hatchlings of R. sylvatica in mesocosms, then added either 20 A. maculatum hatchlings to study interspecific competition, or a single egg mass to examine IGP. Crowding strongly suppressed the growth of R. sylvatica, and IGP was restricted to the egg stage. In the larval competition experiment, growth of A. maculatum was inversely proportional to R. sylvatica density. In the predation experiment, embryonic mortality of A. maculatum was directly proportional to the initial density of R. sylvatica and the mean number of tadpoles foraging on egg masses. IGP on eggs reduced A. maculatum hatchling density, which accelerated larval growth. Surprisingly, the density of R. sylvatica had no overall effect on A. maculatum growth because release from intraspecific competition via egg predation was balanced by increased interspecific competition. Our results demonstrate that the density of a social IG predator can strongly influence the nature and intensity of interactions with a second guild member by simultaneously altering the intensity of IGP and intra- and interspecific competition.L . A. Burley and A. T. Moyer contributed equally to this work.     

The effects of multiple stressors on wetland communities: pesticides,pathogens and competing amphibians

1. Anthropogenic effects have propelled us into what many have described as the sixth mass extinction, and amphibians are among the most affected groups. The causes of global amphibian population declines and extinctions are varied, complex and context‐dependent and may involve multiple stressors. However, experimental studies examining multiple factors contributing to amphibian population declines are rare. 2. Using outdoor mesocosms containing zooplankton, phytoplankton, periphyton and tadpoles, we conducted a 2 × 2 × 3 factorial experiment that examined the separate and combined effects of an insecticide and the fungal pathogen Batrachochytrium dendrobatidis (Bd) on three different assemblages of larval pacific treefrogs (Pseudacris regilla) and Cascades frogs (Rana cascadae). 3. Larval amphibian growth and development were affected by carbaryl and the amphibian assemblage treatment, but only minimally by Bd. Carbaryl delayed metamorphosis in both amphibian species and increased the growth rate of P. regilla. Carbaryl also reduced cladoceran abundance, which, in turn, had positive effects on phytoplankton abundance but no effect on periphyton biomass. Substituting 20 intraspecific competitors with 20 interspecific competitors decreased the larval period but not the growth rate of P. regilla. In contrast, substituting 20 intraspecific competitors with 20 interspecific competitors had no effect on R. cascadae. Results of real‐time quantitative polymerase chain reaction (qPCR) analysis confirmed infection of Bd‐exposed animals, but exposure to Bd had no effects on either species in univariate analyses, although it had significant or nearly significant effects in several multivariate analyses. In short, we found no interactive effects among the treatments on amphibian growth and development. 4. We encourage future research on the interactive effects of pesticides and pathogens on amphibian communities.     

Pond tadpoles with generalized morphology: is it time to reconsider their functional roles in aquatic communities?

With rare exceptions, anuran larvae have traditionally been considered to occupy lower trophic levels in aquatic communities where they function as microphagous suspension feeders. This view is being challenged by studies showing that tadpoles with generalized morphology often function as macrophagous predators. Here, we review the literature concerning macrophagy by tadpoles and provide two additional examples involving generalized tadpoles. In the first, we demonstrate with laboratory and field experiments that wood frog (Rana sylvatica) tadpoles are major predators of macroinvertebrates in ponds. In the second, we show that green frog (R. clamitans) tadpoles can cause catastrophic reproductive failure of the wood frog via egg predation. These results and data from other studies challenge the assumption that generalized tadpoles function as filter-feeding omnivores, and question the general applicability of community organization models which assume that predation risk increases with pond permanence. We suggest that predation risk is greater in temporary ponds than in more permanent ponds for many organisms that are vulnerable to predation by tadpoles. This being so, a conditional model based upon interactions that are species-specific, life-stage-specific, and context-dependent may better explain community organization along hydrological gradients than models which assume that temporary ponds have few or no predators. Received: 30 November 1998 / Accepted: 2 May 1999     

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.