Effects of induced variation in anuran larval development on postmetamorphic energy reserves and locomotion

Anuran larvae exhibit high levels of phenotypic plasticity in growth and developmental rates in response to variation in temperature and food availability. We tested the hypothesis that alteration of developmental pathways during the aquatic larval stage should affect the postmetamorphic performance of the Iberian painted frog (Discoglossus galganoi). We exposed tadpoles to different temperatures and food types (animal- vs. plant-based diets) to induce variation in the length of the larval period and body size at metamorphosis. In this species, larval period varied with temperature but was unaffected by diet composition. In contrast, size at metamorphosis was shaped by the interaction between food quality and temperature; tadpoles fed on an animal-based diet became bulkier metamorphs than those fed on plant-based food at high (22°C) but not at low (12°C) temperature. Body condition of newly metamorphosed frogs was unrelated to the temperature or food type experienced during the premetamorphic stage. Frogs maintained at high temperature during the larval period showed reduced jumping ability, especially when fed on the plant-based diet. However, when considering size-independent jumping ability, cold-reared individuals exhibited the lowest performance, and herbivores reared at 17°C the highest. Cold-reared (12°C) frogs accumulated larger amounts of energy reserves than individuals raised at 17°C or 22°C. This was still the case after correction for differences in body mass, thus indicating some size-independent effect of developmental temperature. Despite the higher lipid content of the carnivorous diet, the differences in energy reserves between herbivores and carnivores were relatively weak and associated with differences in body size. These results suggest that the consequences of environmental variation in the larval habitat can extend to the terrestrial phase and influence juvenile growth and survival.     

Evolutionary reduction of developmental plasticity in desert spadefoot toads

Organisms vary their rates of growth and development in response to environmental inputs. Such developmental plasticity may be adaptive and positively correlate with environmental heterogeneity. However, the evolution of developmental plasticity among closely related taxa is not well understood. To determine the evolutionary pattern of plasticity, we compared plasticity in time to and size at metamorphosis in response to water desiccation in tadpoles among spadefoot species that differ in breeding pond and larval period durations. Like most tadpoles, spadefoot tadpoles possess the remarkable ability to accelerate development in response to pond drying to avoid desiccation. Here, we hypothesize that desert spadefoot tadpoles have evolved reduced plasticity to avoid desiccation in ephemeral desert pools compared to their nondesert relatives that breed in long-duration ponds. We recorded time to and size at metamorphosis following experimental manipulation of water levels and found that desert-adapted species had much less plasticity in larval period and size at metamorphosis than nondesert species, which retain the hypothetical ancestral state of plasticity. Furthermore, we observed a correlation between degree of plasticity and fat body content that may provide mechanistic insights into the evolution of developmental plasticity in amphibians.     

Influence of density and salinity on larval development of salt‐adapted and salt‐naïve frog populations

Environmental change and habitat fragmentation will affect population densities for many species. For those species that have locally adapted to persist in changed or stressful habitats, it is uncertain how density dependence will affect adaptive responses. Anurans (frogs and toads) are typically freshwater organisms, but some coastal populations of green treefrogs (Hyla cinerea) have adapted to brackish, coastal wetlands. Tadpoles from coastal populations metamorphose sooner and demonstrate faster growth rates than inland populations when reared solitarily. Although saltwater exposure has adaptively reduced the duration of the larval period for coastal populations, increases in densities during larval development typically increase time to metamorphosis and reduce rates of growth and survival. We test how combined stressors of density and salinity affect larval development between salt‐adapted (“coastal”) and nonsalt‐adapted (“inland”) populations by measuring various developmental and metamorphic phenotypes. We found that increased tadpole density strongly affected coastal and inland tadpole populations similarly. In high‐density treatments, both coastal and inland populations had reduced growth rates, greater exponential decay of growth, a smaller size at metamorphosis, took longer to reach metamorphosis, and had lower survivorship at metamorphosis. Salinity only exaggerated the effects of density on the time to reach metamorphosis and exponential decay of growth. Location of origin affected length at metamorphosis, with coastal tadpoles metamorphosing slightly longer than inland tadpoles across densities and salinities. These findings confirm that density has a strong and central influence on larval development even across divergent populations and habitat types and may mitigate the expression (and therefore detection) of locally adapted phenotypes.     

Energy systems concepts and self-organization: a rebuttal

Tadpoles in small, ephemeral pools whose duration and food content are unpredictable can potentially encounter substantial variation in diet composition and availability. We compared the effects of 10 days of food deprivation occurring early, midway and late in ontogeny on the metamorphic size and bioenergetic properties of Hyla chrysoscelis tadpoles. Tadpoles fed throughout ontogeny were controls. Metamorphs from tadpoles starved early and midway in ontogeny had the same snout-vent length and dry mass as controls, but the time to metamorphosis was extended by 8 and 19% respectively. Metamorphs of tadpoles starved late in development attained 85% of the length and 55% of the mass of controls, metamorphosed at the same time as controls, and suffered mortality 15 times greater than other treatments, perhaps because they were near the absolute minimum necessary level of energy reserves. There were no significant differences in percent organic matter, percent tissue water, condition index, and protein or glycogen concentrations between any experimental and control treatments. If food deprivation occurred early in development, the tadpoles caught up to the size of controls, but an extended developmental time would increase the risk of predation or habitat loss. If food reductions occur late in development, perhaps magnified by pond desiccation, tadpoles are stimulated to metamorphose at the same time as controls but at a smaller size. The bioenergetic composition of tadpoles at metamorphosis is unaffected by time of food deprivation.     

Interacting effects of predation risk and food availability on larval anuran behaviour and development

Age and size at metamorphosis are two important fitness components in species with complex life cycles. In anurans, metamorphic traits show remarkable phenotypic plasticity, especially in response to changes in growth conditions. It is also possible that the perception of risk directly determines changes in larval period and the size of metamorphs. This study examines how the perception of predation risk affects the timing of and size at metamorphosis in common frogs (Rana temporaria). I raised tadpoles at two risk levels (fish-conditioned water or unconditioned water) crossed with the availability or lack of food at night (all tadpoles had food available in the day). Tadpoles reacted to chemical cues from predatory fish by decreasing activity. A novel behavioural result was a predation×food interaction effect on refuge use, which also accounted for most of the predator main effect: predation risk only caused increased refuge use in the night-starved treatment. Despite these behavioural modifications, the perception of predation risk did not affect growth rate and mass at metamorphosis in a simple way: the effects of food regime on growth and size at metamorphosis were dependent on the level of predation risk as revealed by significant predation×food interaction effects. Tadpoles who had food withheld at night metamorphosed at the smallest size, suggesting a negative relationship between size at metamorphosis and refuge use. Tadpoles raised in fish-conditioned water had longer larval periods than those in unconditioned water, but these differences were significant only if food was available at night. These results conflict with the hypotheses that tadpoles should reduce their larval period or growth rates (and hence metamorphose at a smaller size) as the risk of predation increases. In contrast to predation risk, food availability strongly affected the length of the larval period: night-starved tadpoles metamorphosed relatively early with or without fish stimulus. Thus, early metamorphosis resulted from periods of low food availability, but not from a heightened ”perceived risk” of predation. This example counters the hypothesis of acceleration of the developmental rate (which shortens the time to metamorphosis) as a mechanism to escape a risky environment. Received: 18 August 1999 / Accepted: 10 January 2000     

Relatedness and competitive asymmetry – the growth and development of common frog tadpoles

Individuals can compete either through direct interference or uptake of limiting resources. If competing individuals are able to recognize their relatives, relatedness of competitors may evoke kin selection, which favours relatively even resource share among relatives. Resource competition is often size-symmetric, i.e. proportional to an individual's biomass, while interference competition is asymmetric giving large individuals a disproportionate advantage over small individuals. Kin-selection is predicted to reduce the intensity of direct interference and competitive asymmetry, leading to increased mean and decreased variation in individual size. We tested these predictions by investigating the effects of relatedness on age and size at metamorphosis in the common frog Rana temporaria tadpoles in a laboratory experiment. We reared related (full- and half-sibs) and unrelated tadpoles of different sizes (small, large, small and large together) at two densities until metamorphosis. Relatedness had little effect on mean growth, but it reduced size variation, as measured with coefficient of variation. Furthermore, there was a significant interaction between relatedness and density in size at metamorphosis, so that relatives always grew better in lower density, but growth was less affected by density among unrelated individuals. This indicates that the effects of relatedness on tadpole performance may be context dependent. Initial size differences in the mixed size treatment evened out during the course of the experiment, and initially small tadpoles were able to compensate the early growth losses, although it took longer for them to reach metamorphosis. We conclude that although relatedness may have rather small effects on the growth and development of R. temporaria tadpoles, it increases the symmetry of resource share decreasing between-individual variation in size at metamorphosis.     

Effect of changes in resource level on age and size at metamorphosis in Hyla squirella

Recent experiments suggest that timing of metamorphosis is fixed during development in some anurans, insects, and freshwater invertebrates. Yet, these experiments do not exclude a growth rate optimization model for the timing of metamorphosis. I manipulated food resources available to larvae of squirrel treefrogs (Hyla squirella) to determine if there is a loss of plasticity in duration of larval period during development and to critically test growth rate models for the timing of metamorphosis. Size-specific resource levels for individual tadpoles were switched from low to high or high to low at three developmental stages spaced throughout larval development. The effects of changes in resource availability on larval period and mass at metamorphosis were measured. Switching food levels after late limb bud development did not significantly affect larval period in comparison to constant food level treatments. Therefore, developmental rate in H. squirella is better described by a fixed developmental rate model, rather than a growth rate optimization model. The timing of fixation of developmental rate in H. squirella is similar to that found in other anuran species, suggesting a taxonomically widespread developmental constraint on the plasticity of larval period duration. Mass at metamorphosis was not significantly affected by the timing of changes in food levels; the amount of food available later in development determined the size at metamorphosis. Larval period and mass at metamorphosis were negatively correlated in only one of two experiments, which contrasts with the common assumption of a phenotypic trade-off between decreased larval period and increased mass at metamorphosis. Received: 19 August 1996 / Accepted: 20 June 1997     

Mechanisms and Consequences of Developmental Acceleration in Tadpoles Responding to Pond Drying

Many amphibian species exploit temporary or even ephemeral aquatic habitats for reproduction by maximising larval growth under benign conditions but accelerating development to rapidly undergo metamorphosis when at risk of desiccation from pond drying. Here we determine mechanisms enabling developmental acceleration in response to decreased water levels in western spadefoot toad tadpoles (Pelobates cultripes), a species with long larval periods and large size at metamorphosis but with a high degree of developmental plasticity. We found that P. cultripes tadpoles can shorten their larval period by an average of 30% in response to reduced water levels. We show that such developmental acceleration was achieved via increased endogenous levels of corticosterone and thyroid hormone, which act synergistically to achieve metamorphosis, and also by increased expression of the thyroid hormone receptor TRΒ, which increases tissue sensitivity and responsivity to thyroid hormone. However, developmental acceleration had morphological and physiological consequences. In addition to resulting in smaller juveniles with proportionately shorter limbs, tadpoles exposed to decreased water levels incurred oxidative stress, indicated by increased activity of the antioxidant enzymes catalase, superoxide dismutase, and gluthatione peroxidase. Such increases were apparently sufficient to neutralise the oxidative damage caused by presumed increased metabolic activity. Thus, developmental acceleration allows spadefoot toad tadpoles to evade drying ponds, but it comes at the expense of reduced size at metamorphosis and increased oxidative stress.     

Plasticity in age and size at metamorphosis in Rana temporaria - comparison of high and low latitude populations

Effects of different combinations of stressors (viz. temperature, food level) on growth, developmental and survival rates of Rana temporaria tadpoles from two geographically widely (∼ 1500 km) separated populations were studied in a common garden experiment. In both populations, low temperature and low food level lead to towered growth rates and delayed metamorphosis, whereas high temperature and high food level had the opposite effect. Tadpoles from north metamorphosed earlier and exhibited higher growth rates than tadpoles from south, suggesting local adaptation to shorter growth period and cooler ambient temperature in north. Size at metamorphosis did not differ between the two populations, but when the differences in metamorphic age were accounted for, then the tadpoles from north were larger than those from south. These results suggest considerable adaptive genetic differentiation in growth rates, size and timing of metamorphosis between northern and southern R. temporaria populations. In both populations, high food levels tended to reduce tadpole survival rates and there was a negative correlation between growth and survival rates across different treatments in both populations. In general, tadpoles from north experienced high mortality rates in high food level - low temperature treatments, whereas southern tadpoles experienced high mortality in high food level-high temperature treatments. This suggest that there may be genetic differences among different populations as how they would be influenced by high nutrient loads, such as brought along for example by fertilization of forest or agricultural soils.     

High temperatures influence sexual development differentially in male and female tadpoles of the Indian skipper frog, <Emphasis Type="Italic">Euphlyctis cyanophlyctis</Emphasis>

Although sex determination in amphibians is believed to be a genetic process, environmental factors such as temperature are known to influence the sex differentiation and development. Extremely low and high temperatures influence gonadal development and sex ratio in amphibians but the mechanism of action is not known. In the present study, effect of different temperatures on gonadal development, sex ratio and metamorphosis was studied in the Indian skipper frog, Euphlyctis cyanophlyctis. The embryos of Gosner stage 7 were exposed to 20, 22, 24, 26, 28, 30 and 32°C up to tadpole stage 42. The embryos (stage 7) were also exposed to 20 and 32°C up to tadpole stage 25 (non-feeding stages). Tadpoles of stage 25 were reared at 20 and 32°C up to stage 42 (feeding stages). The results show that exposure to higher temperatures (28, 30 and 32°C) during stages 7–42 produced male-biased sex ratio. Rearing of tadpoles at 32°C during stages 25–42 produced male-biased sex ratio, while exposure during stages 7–25 did not affect sex ratio. Embryos and tadpoles exposed to lower temperatures (20 and 22°C) died during the early stages. High temperatures stimulated testis development, and disturbed ovary development. Exposure to high temperatures resulted in the early metamorphosis of tadpoles with reduced body size. These results demonstrated that high temperatures influence gonadal development differently in male and female tadpoles, leading to male-biased sex ratio. These results suggest that high temperature probably acts through stress hormones and favours the small-sized sex.     

Influence of density and predators on metamorphic size in Rhinella schneideri tadpoles raised in mesocosm conditions

Amphibians exhibit extreme plasticity in the timing of metamorphosis, and several species respond to water availability, accelerating metamorphosis when their ponds dry. We analyzed the plasticity of the developmental response to water volume in Rhinella schneideri tadpoles. We raised tadpoles in mesocosm. Covariation between body size at metamorphosis and timing of development was positive. Nevertheless, the first approximately 53% of the metamorphoses finishing the cycle required between 34 and 56 days, and the covariation between body size at metamorphosis and timing of development was negative. For these tadpoles, the larval density and the presence of predators did not significantly affect their mass to metamorphosis. Nevertheless, predators affected time to metamorphosis. For the remainder of the tadpoles that reached metamorphosis at > 56 days, the relationship between body size at metamorphosis and timing of development was positive. For these tadpoles, larval density was important for mass at metamorphosis and presence of predators was also important for time to metamorphosis. Two dominant features were observed: (i) approximately 53% of metamorphs had morphological features similar to individuals developing in desiccating ponds, and (ii) the other individuals had morphological characteristics comparable to metamorphs developing in an unchanging environment.     

Maternal effects on offspring growth and development depend on environmental quality in the frogBombina orientalis

Summary Differences in maternal investment and initial offspring size can have important consequences for offspring growth and development. To examine the effects of initial size variability in the frogBombina orientalis, we reared larvae (N=360) in one of two treatments representing different levels of environmental quality. We used snout-vent length at the feeding stage (stage 25, Gosner 1960) as a measure of maternal investment. In a “low quality” treatment, larvae were reared with two conspecific tadpoles and food was limited, whereas in a “high quality” treatment, larvae were reared individually and were fed ad libitum. Among tadpoles reared in the low quality treatment, individuals that were initially small had smaller body sizes through metamorphosis and longer larval periods than individuals that were initially large. Among tadpoles reared in the high quality treatment, initial size had only a weak influence on later larval size, and did not significantly affect metamorphic size of the duration of the larval period. This interaction between maternal investment and rearing conditions suggests that production of initially small offspring could be advantageous if these offspring develop in relatively benign environments, but disadvantageous if environments are more severe. These findings are discussed in light of previous studies that have demonstrated such interactions in organisms with complex life cycles.     

Effects of temperature and food quality on life-history parameters in Parameletus chelifer and P. minor (Ephemeroptera): a laboratory study

SUMMARY. 1. Growth rate of Parameletus minor was greatest between 10.8 and 19.8°C, survival rate peaked at 5.9°C, developmental time was shortest at 14.6°C, and adult size and fecundity reached maximum values between 5.9 and 10.8°C. Growth rate of P. chelifer was greatest between 14.6 and 19.8°C, survival rate peaked at 5.9°C, and developmental time was shortest at 14.6°C. A large adult size was found at 10.8°C, and highest fecundity between 10.8 and 14.6°C.
2. Food quality significantly affected growth rate, developmental time, adult size and fecundity of both species. Both P. chelifer and P minor attained highest growth rate, largest adult size and highest fecundity when the C/N ratio of food was 5.95. Developmental time was shortest at a C/N ratio between 5.95 and 12.8.
3. Nymphs of P. chelifer had a higher temperature 'optimum' for growth than nymphs of P. minor . Growth rate of nymphs of P. chelifer reared on detritus from a seasonal stream (C/N ratio 12.8) was about 3 times that of nymphs reared on detritus from a river margin (C/N ratio 20.9). The corresponding growth rate difference for nymphs of P. minor was only about 1.5.
4. When all life-history parameters are taken into consideration, P. chelifer had a higher temperature 'optimum' than P. minor .     

How do changes in parental investment influence development in echinoid echinoderms?

SUMMARY Understanding the relationship between egg size, development time, and juvenile size is critical to explaining patterns of life-history evolution in marine invertebrates. Currently there is conflicting information about the effects of changes in egg size on the life histories of echinoid echinoderms. We sought to resolve this conflict by manipulating egg size and food level during the development of two planktotrophic echinoid echinoderms: the green sea urchin, Strongylocentrotus droebachiensis and the sand dollar, Echinarachnius parma . Based on comparative datasets, we predicted that decreasing food availability and egg size would increase development time and reduce juvenile size. To test our prediction, blastomere separations were performed in both species at the two-cell stage to reduce egg volume by 50%, producing whole- and half-size larvae that were reared to metamorphosis under high or low food levels. Upon settlement, age at metamorphosis, juvenile size, spine number, and spine length were measured. As predicted, reducing egg size and food availability significantly increased age at metamorphosis and reduced juvenile quality. Along with previous egg size manipulations in other echinoids, this study suggests that the relationship between egg size, development time, and juvenile size is strongly dependent upon the initial size of the egg.     

Life history consequences of feeding versus non-feeding in a facultatively non-feeding toad larva

Summary Bufo periglenes, a toad endemic to montane Costa Rica, produces an unusually small clutch of large, yolk-rich eggs. The toads breed in small ephemeral pools that are unpredictable in duration and may be low in food availability. Two congeners, Bufo coniferus and Bufo marinus, occur nearby, breed in more permanent bodies of water that offer more food, and exhibit the typical toad pattern of large clutches of small eggs. Tadpoles of all three species feed on detritus and suspended organic material. By raising tadpoles of the three species individually with and without food I investigated the relationship between egg size (yolk provision) and tadpole survival. All of the unfed B. coniferus and B. marinus tadpoles grew little and died soon after developing to the hindlimb bud stage. On the other hand, all of the unfed B. periglenes tadpoles metamorphosed successfully, demonstrating that the tadpoles are facultatively non-feeding; developmental time from hatching to metamorphosis was significantly shorter for unfed tadpoles than for fed tadpoles, but fed individuals were significantly larger at transformation. Faster developmental rate and larger body size at transformation are both advantageous for frogs and toads, but cannot be attained simultaneously. Large egg size may afford flexibility in unpredictable environments. In pools where food is available, tadpoles presumably eat, take longer to metamorphose, but are larger at transformation than tadpoles developing in nutrient-poor sites. Small body size at transformation (a consequence of not eating) has potential costs, but the large quantity of yolk provided by a large egg enhances the probability of metamorphosis in food-limited environments.     

POPULATION STRUCTURE AND COMPETITION AMONG KIN IN THE CHORUS FROG (PSEUDACRIS TRISERIATA)

In natural populations on Isle Royale, tadpoles of the chorus frog live in small pools on the shore of Lake Superior. Hatchling densities are high and sufficient to cause competitive impact on survivorship, growth, and development. The temporal and spatial pattern of egg laying indicates that tadpoles in many of the pools belong to single sibships. I calculated average coefficients of relationship among tadpoles under the assumption that eggs laid together are the products of the same breeding pair; the coefficients indicate that relationship among competing larvae averages approximately 0.35, and varies widely among larval subpopulations, from less than 0.1 to about 0.5. Two growth experiments were carried out in pens to test whether growth trajectories and larval characteristics at metamorphosis are influenced by relationship among competing tadpoles. In both experiments, initial density was crossed with average relationship; relationship was controlled by varying the number of sibships per pen from one to four. The same sibships were used in both experiments, but one experiment had lower initial densities and less water volume per pen than the other. In both experiments, density reduced growth, developmental rate, size at metamorphosis, survivorship to the onset of metamorphosis, and the proportion of survivors which actually attained metamorphosis by the end of the experiment. Kin effects occurred only in the experiment carried out in small pens at high initial densities: in this experiment, pure sib populations grew faster, and a higher proportion attained metamorphosis. However, there were no kin effects on larval period or body size at metamorphosis. The chorus frog appears to have a population structure conducive to kin-group selection. Furthermore, high variance in the average coefficient of relationship among pools should favor kin recognition and kin-specific interference behavior. The growth experiments suggest that the tadpoles respond to the genetic relationship of competitors, with significant effects on the distribution of fitness at metamorphosis among members of the group.     

Immunocytochemistry of luteinizing hormone-releasing hormone during spontaneous and thyroxine-induced metamorphosis of bullfrogs

Double-bridge peroxidase-antiperoxidase immunocytochemistry was used to compare the developmental appearance of immunoreactive LH-RH (ir-LH-RH) in brains of bullfrog (Rana catesbeiana) tadpoles during either spontaneous or thyroxine-induced metamorphosis. During spontaneous metamorphosis, ir-LH-RH was localized in fibers of the external layer of the median eminence (ME) of stage XIII-XXV animals, while immunoreactive perikarya and other immunostained brain structures were absent. The extent and intensity of ME immunostaining increased concomitantly with measured ME morphological development. Tadpoles induced with thyroxine to metamorphic stages XIX-XXI exhibited ME structural development and neurohypophysial neurosecretory staining similar to spontaneously metamorphosed individuals of equal stages. However, comparable ME ir-LH-RH immunostaining and gonadal size were both less developed in thyroxine-treated animals, although increased relative to non-metamorphic vehicle-injected controls. These results indicate that the hypothalamic LH-RH system changes concurrently with ME structural development during spontaneous metamorphosis. Reduced ME ir-LH-RH staining and gonadal size in thyroxine-treated animals suggest that during prometamorphosis, factors other than thyroxine alone may coordinate the normal maturation of the hypothalamo-pituitary-gonadal axis of the bullfrog.     

Changes in reproductive strategy in the ruffe during a period of establishment in a new habitat

The pattern of maturation, body size and fecundity was examined in a population of ruffe ( Gymnocephalus cernuus L.) three times during a period of rapid growth, and eventual stabilization, following its introduction to a new habitat. When the ruffe were less common, maturing ruffe were relatively large and immature ruffe relatively small, compared with when the ruffe were abundant. Intermediate ruffe population size showed a maturation pattern intermediate between these two extremes. It is suggested that this pattern of maturation is a response of the ruffe population to changing growth opportunity induced by changing intraspecific competition. This fluctuating maturation pattern is interpreted in terms of a threshold-dependent maturation trigger, operating on the rate of accumulation of energy and a trade-off between somatic growth and gonad development. When the ruffe population was large, high intraspecific competition resulted in low opportunity for growth; only fish with the highest rate of food acquisition were able to mature in a given year–the investment in gonadal tissue reducing somatic growth. When the ruffe population was low, the high rate of energy acquisition in the population resulted in the triggering of maturation, even at small size, only the very smallest fish remaining immature. High growth opportunity allowed maturing fish to develop gonad and maintain somatic growth. The pattern of size related fecundity also changed over the three periods. When growth opportunity was low, size related fecundity was greater than when opportunity for growth was high. This suggests that maturing females faced with poor growth conditions compensated by increasing egg number for a given body size either by decreasing egg size or by increasing total investment in ovarian tissue.     

Interactions between freshwater snails and tadpoles: competition and facilitation

Summary Freshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an indirect mutualistic mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.     

Interacting effects of predation risk and resource level on escape speed of amphibian larvae along a latitudinal gradient

Fast‐growing genotypes living in time‐constrained environments are often more prone to predation, suggesting that growth‐predation risk trade‐offs are important factors maintaining variation in growth along climatic gradients. However, the mechanisms underlying how fast growth increases predation‐mediated mortality are not well understood. Here, we investigated if slow‐growing, low‐latitude individuals have faster escape swimming speed than fast‐growing high‐latitude individuals using common frog (Rana temporaria) tadpoles from eight populations collected along a 1500 km latitudinal gradient. We measured escape speed in terms of burst and endurance speeds in tadpoles raised in the laboratory at two food levels and in the presence and absence of a predator (Aeshna dragonfly larvae). We did not find any latitudinal trend in escape speed performance. In low food treatments, burst speed was higher in tadpoles reared with predators but did not differ between high‐food treatments. Endurance speed, on the contrary, was lower in high‐food tadpoles reared with predators and did not differ between treatments at low food levels. Tadpoles reared with predators showed inducible morphology (increased relative body size and tail depth), which had positive effects on speed endurance at low but not at high food levels. Burst speed was positively affected by tail length and tail muscle size in the absence of predators. Our results suggest that escape speed does not trade‐off with fast growth along the latitudinal gradient in R. temporaria tadpoles. Instead, escape speed is a plastic trait and strongly influenced by the interaction between resource level and predation risk.