Breeding site characteristics regulating life history traits of the brown tree frog, Litoria ewingii

Past research has determined the habitat requirements of amphibian species predominantly from presence/absence studies. This study tested the hypothesis that relationships between breeding site habitat components, life history traits and fitness may provide a higher resolution of biological data relating to the habitat requirements of amphibian species. We tested this novel approach by using Litoria ewingii as our model species. We correlated larval and metamorph life history traits with habitat variables at 28 small to medium sized ponds within a commercially logged forest in southern Tasmania, Australia. To avoid larval mortality due to pond desiccation, L. ewingii laid eggs and metamorphosed earlier in smaller ponds. Snout vent length at metamorphosis increased with elevation and metamorphosis was earlier in less shaded ponds. Breeding ponds that maximised the fitness of L. ewingii were higher elevation ponds with reduced shading, steeper bank slopes and reduced pond isolation. The findings of the study equip land managers with a greater ecological understanding of ecosystem function in relation to specific species. The methodological approach has broad application to conservation biology where an awareness of the specific habitat requirements of amphibians is critical to successful ecosystem management.     

Plethodontid salamanders do not conform to "general rules" for ectotherm life histories: insights from allocation models about why simple models do not make accurate predictions

Recent theoretical models hold that temperature imposes unalterable physiological effects on ectotherm growth and development such that their life histories are dictated by local biophysical environments. Models relying on this premise have been offered to explain many life history phenotypes including threshold traits such as age/size at metamorphosis/maturity. Because threshold traits are thought to influence adult fitness components by affecting performance of an individual in its new habitat, they are evolutionarily important components of complex life cycles. Consequently, the ecological and genetical basis of variation in such traits has been the focus of a large research program by evolutionary biologists, and amphibians have been model systems for these studies in the last three decades. Smith‐Gill and Berven proposed a physiological model to explain commonly observed clinal patterns of variation in metamorphosis, and it appears to account successfully for patterns observed in a few species of pond‐breeding frogs and salamanders (Rana and Ambystoma). However, six species of stream‐breeding salamanders (family Plethodontidae: Desmognathus, Pseudotriton, Eurycea) contradict both the phenotypic patterns found in nature for pond‐breeding species as well as the predictions of this model. Four of the six species of plethodontids have significantly larger metamorphs at lower, warmer elevations (or more southerly latitudes) rather than at higher, cooler sites; two species show no clinal pattern. In light of these results, we critically examine the assumptions of and support for SGB. We propose alternative hypotheses to explain patterns of variation in metamorphic traits along thermal gradients, focusing on a) differences between pond‐breeding Amphibia and plethodontids in basic biology and larval habitats, b) gradients in other biotic and abiotic factors, and c) other effects of temperature on organismal function. Finally, we discuss our results in the context of current models of how ectotherm life histories are affected by temperature.     

Local divergence of thermal reaction norms among amphibian populations is affected by pond temperature variation

Although temperature variation is known to cause large‐scale adaptive divergence, its potential role as a selective factor over microgeographic scales is less well‐understood. Here, we investigated how variation in breeding pond temperature affects divergence in multiple physiological (thermal performance curve and critical thermal maximum [CT_max]) and life‐history (thermal developmental reaction norms) traits in a network of Rana arvalis populations. The results supported adaptive responses to face two main constraints limiting the evolution of thermal adaptation. First, we found support for the faster–slower model, indicating an adaptive response to compensate for the thermodynamic constraint of low temperatures in colder environments. Second, we found evidence for the generalist–specialist trade‐off with populations from colder and less thermally variable environments exhibiting a specialist phenotype performing at higher rates but over a narrower range of temperatures. By contrast, the local optimal temperature for locomotor performance and CT_max did not match either mean or maximum pond temperatures. These results highlight the complexity of the adaptive multiple‐trait thermal responses in natural populations, and the role of local thermal variation as a selective force driving diversity in life‐history and physiological traits in the presence of gene flow.     

Variation of life‐history traits of the Asian corn borer,Ostrinia furnacalis in relation to temperature and geographical latitude

Life‐history traits from four geographical populations (tropical Ledong population [LD], subtropical Guangzhou [GZ] and Yongxiu populations, and temperate Langfang population [LF]) of the Asian corn borer, Ostrinia furnacalis were investigated at a wide range of temperatures (20–32°C). The larval and pupal times were significantly decreased with increasing rearing temperature, and growth rate was positively correlated with temperature. The relationship between body weight and rearing temperature in O. furnacalis did not follow the temperature–size rule (TSR); all populations exhibited the highest pupal and adult weights at high temperatures or intermediate temperatures. However, development time, growth rate, and body weight did not show a constant latitudinal gradient. Across all populations at each temperature, female were significantly bigger than males, showing a female‐biased sexual size dimorphism (SSD). Contrary to Rensch's rule, the SSD tended to increase with rising temperature. The subtropical GZ population exhibited the largest degree of dimorphism while the temperate LF exhibited the smallest. Male pupae lose significantly more weight at metamorphosis compared to females. The proportionate weight losses of different populations were significantly different. Adult longevity was significantly decreased with increasing temperature. Between sexes, all populations exhibit a rather female‐biased adult longevity. Finally, we discuss the adaptive significance of higher temperature‐inducing high body weight in the moth's life history and why the moth exhibits the reverse TSR.     

The genetic basis of altitudinal variation in the wood frog Rana sylvatica II. An experimental analysis of larval development

Summary The variation in larval developmental patterns in the wood frog, Rana sylvatica, along an elevation gradient of 1,000 m was experimentally studied. Larval populations at high elevation ponds had lower growth rates, developmental rates and were larger at all stages (including metamorphic climax) than larval populations developing in low elevation ponds. There was considerable variation among ponds within each elevation in both the length of the larval period and size at metamorphic climax. Reciprocal transplant experiments and controlled laboratory experiments revealed that most of the observed variation between high and low elevation populations could be explained by the effects of temperature induction during ontogeny. Significant genetic differences in growth rates and non-genetic maternal effects on developmental rates between larvae of mountain origin and lowland origin were also demonstrated. Selection in both environments has acted to minimize the prevailing environmental effect of pond temperature on developmental rates, but has accentuated the prevailing environmental effects on larval body size. As a consequence mountain larvae were capable of completing metamorphosis sooner and at a larger size in all environments than lowland larvae.     

Quantitative genetic analysis of larval life history traits in two alpine populations of Rana temporaria

We estimated genetic and maternal variance components of larval life history characters in alpine populations of Rana temporaria (the common frog) using a full-sib/half-sib breeding design. We studied trait plasticity by raising tadpoles at 14 or 20°C in the laboratory. Larval period and metamorphic mass were greater at 14°C. Larval period did not differ between populations, but high elevation metamorphs were larger than low elevation metamorphs. Significant additive variation for larval period was detected in the low altitude population. No significant additive variation was detected for mass at metamorphosis (MM), which instead displayed significant maternal effects. Plasticity in metamorphic mass of froglets was greater in the high altitude population. The plastic response of larval period to temperature did not differ between the populations. Evolution of metamorphic mass is likely constrained by lack of additive genetic variation. In contrast, significant heritability for larval period suggests this trait may evolve in response to environmental change. These results differ from other studies on R. temporaria, suggesting that populations of this broadly distributed species present substantial geographic variation in the genetic architecture and plasticity of tadpole life history traits.     

Respiration and acid-base balance in the salamander,Ambystoma tigrinum: Influence of temperature acclimation and metamorphosis

Summary Effects of temperature acclimation (5 or 25 °C for 2–4 weeks) and metamorphosis on oxygen uptake, acid-base balance and blood-O₂ affinity have been investigated inAmbystoma tigrinum. The results differ from previous studies in three ways. (1) The transition from gilled to gill-less adults had no effect on the O₂ affinity of blood. (2) Cold acclimation increased blood O₂ affinity in neotenes and had no effect in adults. (3) O₂ uptake increased, rather than decreased, after acclimation to a higher temperature. The results resemble previous studies also in three ways. (1) O₂ uptake increased with the transition from gilled-neotenes to gill-less adults as did the dependence on air-breathing. (2) Metamorphosis resulted in CO₂ retention and a fall in arterial pH. (3) The temperature coefficient of blood pH was about –0.014dpH/dT in vivo and in vitro. The physiological significance of the results is discussed with respect to the natural history, modes of breathing, and dependance on aerial respiration ofAmbystoma tigrinum.     

Genie Control of Axolotl Metamorphosis

SYNOPSIS. Neoteny in the Mexican axolotl, Ambystoma mexicanum,is caused by homozygosity for a single recessive gene. The dominantallele causing physical metamorphosis is found in the closelyrelated species, Ambystoma tigrinum, with which it can hybridize.Despite the failure of axolotls to undergo physical metamorphosis,they do undergo a cryptic metamorphosis. A larval-to-adult hemoglobinform change, serum protein changes and other physiological eventsusually associated with amphibian metamorphosis occur duringearly larval life at ages comparable to the age at which Ambystomatigrinum undergoes both the cryptic and external metamorphicevents. Axolotl cryptic metamorphosis can be induced precociouslyby immersion of the larvae in low concentrations of thyroxine;physical metamorphosis can be induced with higher thyroxineconcentrations. The site of action of the gene responsible foraxolotl neoteny has not been identified. A change in the sensitivityof external metamorphic processes to thyroxine, or reduced hormonalstimulation by the pituitary or hypothalamus may be responsible.A comparison of these functions in Ambystoma tigrinum and theaxolotl may identify the lesion.     

Influence of seasonal time constraints on growth and development of common frog tadpoles: a photoperiod experiment

Organisms living in seasonal environments are often limited by the time available to complete their development. Especially individuals in northern populations may face severe time constraints in their need of completing development before the end of the growth season. Larval amphibians have been widely used in studies of phenotypic plasticity. However, their responses to changes in photoperiod, the main seasonal cue in many organisms, are unknown. In a laboratory experiment, we studied whether common frog (Rana temporaria) tadpoles originating from two populations (separated latitudinally by 1600 km) adjust their growth and development according to the progress of the season by using photoperiodic cues, and whether these responses are temperature dependent. We hypothesised that if frogs use photoperiod as a cue, they should increase growth and development rates as a response to photoperiodic treatments mimicking progressing season. Although our predictions were not verified in either of the populations, photoperiod manipulations had effects on larval life history in both populations. When exposed to progressing season treatments and high temperature, tadpoles from the southern population ceased feeding, which led to delayed metamorphosis and increased mortality. In the northern population, age at metamorphosis was unaffected by the photoperiod treatments, but growth rate until metamorphosis and metamorphic size were slightly larger in the treatments with shorter (increasing or decreasing) day length. Irrespective of photoperiod treatments, growth and development rates, size at metamorphosis and food consumption were higher in the northern as compared to the southern population. These results indicate that in contrast to several insect species, the critical life history decisions in amphibian larvae may not be strongly influenced by photoperiodic cues, but different populations seem to differ in this respect. However, the strong temperature×photoperiod interactions in several traits in the southern population suggest that the role of photoperiodic cues may be affected by other environmental factors, although the ecological significance of these differences remains unclear.     

Effects of temperature on survival, development, growth and feeding of larvae of Yellowtail clownfish Amphiprion clarkii (Pisces: Perciformes)

To understand the physiological and ecological responses of marine fishes to the change of water temperature, newly-hatched larvae of Yellowtail clownfish Amphiprion clarkii were reared in captivity at water temperatures of 23, 26 and 29 °C till they completed the metamorphosis to juvenile phase, and larval survival, development, growth and feeding were evaluated during the experimental period. The results showed that water temperature influenced the physiological performance of larvae of A. clarkii significantly. The survival and growth rates of larvae of A. clarkii increased significantly with the increase of water temperature from 23 to 29 °C (P < 0.05). Water temperature also influenced larval development of A. clarkii significantly and larvae reared at 23 °C took longer time for post-larval development and metamorphosis compared to 26 and 29 °C (P < 0.05). Total length and body weight for post-larval development and metamorphosis decreased with the increase of water temperature from 23 to 29 °C (P < 0.05). Q₁₀ in developmental rate was higher than in daily growth rate at the same rearing temperature, indicating that at water temperature had greater influence on larval development than on growth. Water temperature also influenced larval feeding of A. clarkii significantly with feed ration (FR) and feed conversion efficiency (FCE) increased with the increase of water temperature from 23 to 29 °C (P < 0.05). There was a positive correlation between FR and specific growth rate (SGR) (P < 0.05) but not between FCE and SGR (P > 0.05), indicating that FR influenced growth rate significantly in larvae of A. clarkii. This study demonstrated that the physiological responses of larvae of A. clarkii to the change of water temperature and confirmed that water temperature influenced larval survival, development, growth and feeding significantly. This study suggests that the decline of larval survival and growth rates, extension of pelagic larval duration and reduction of larval feeding at lower temperature have ecological impacts on larval dispersal and metamorphosis, juvenile settlement and population replenishment in A. clarkii in the wild.     

Growth,developmental and stress responses of larvae of the clouded sulphur butterfly Colias eriphyle to repeated exposure to high,sub‐lethal temperatures

The optimal temperature at which an organism grows and develops is commonly correlated with latitude and elevation; however, the maximum temperature for physiological performance often is not. This makes performance at temperatures between the optimum and the maximum of particular interest. Temperature can influence long‐term performance (growth and development), as well as short‐term performance (heat shock protein) responses differentially. In the present study, two populations of the clouded sulphur butterfly Colias eriphyle Edwards that differ in elevation, thermal regime and optimal and maximum temperatures are studied to quantify their responses to repeated, sub‐lethal heat treatments early in development (second instar). Heat treatments accelerate development during the second to fourth instars in both populations initially, although this effect disappears by pupation. Heat treatment decreases pupal mass in the lower elevation population, suggesting that repeated exposure to high temperatures early in development may reduce final size and fecundity in this population. Heat shock protein gene (hsp70) expression levels in the lower elevation (1633 m a.s.l.) population are highest 24 h after the start of the heat treatment and then the fall to pre‐exposure levels by 36–72 h, suggesting a rapid response to stressful temperatures. By contrast, heat treatment has no significant effect on pupal mass in the higher elevation (2347 m a.s.l.) population. This population has higher levels of hsp70 expression overall but constant expression levels, suggesting that the temperature treatments used are insufficient to elicit a heat stress response. Overall, the effects of repeated exposure to sub‐lethal high temperatures early in development on growth, final size and gene expression differ between populations that differ in thermal sensitivity.     

Moving south: effects of water temperatures on the larval development of invasive cane toads (Rhinella marina) in cool‐temperate Australia

The distributional limits of many ectothermic species are set by thermal tolerances of early‐developmental stages in the life history; embryos and larvae often are less able to buffer environmental variation than are conspecific adults. In pond‐breeding amphibians, for example, cold water may constrain viability of eggs and larvae, even if adults can find suitable thermal conditions in terrestrial niches. Invasive species provide robust model systems for exploring these questions, because we can quantify thermal challenges at the expanding range edge (from field surveys) and larval responses to thermal conditions (in the laboratory). Our studies on invasive cane toads (Rhinella marina) at the southern (cool‐climate) edge of their expanding range in Australia show that available ponds often average around 20°C during the breeding period, 10°C lower than in many areas of the toads’ native range, or in the Australian tropics. Our laboratory experiments showed that cane toad eggs and larvae cannot develop successfully at 16°C, but hatching success and larval survival rates were higher at 20°C than in warmer conditions. Lower temperatures slowed growth rates, increasing the duration of tadpole life, but also increased metamorph body mass. Water temperature also influenced metamorph body shape (high temperatures reduced relative limb length, head width, and body mass) and locomotor performance (increased speed from intermediate temperatures, longer hops from high temperatures). In combination with previous studies, our data suggest that lower water temperatures may enhance rather than reduce recruitment of cane toads, at least in areas where pond temperatures reach or exceed 20°C. That condition is fulfilled over a wide area of southern Australia, suggesting that the continuing expansion of this invasive species is unlikely to be curtailed by the impacts of relatively low water temperatures on the viability of early life‐history stages.     

Expression of alternative developmental pathways in the cabbage butterfly,Pieris melete and their differences in life history traits

The seasonal life cycle of the cabbage butterfly, Pieris melete is complicated because there are three options for pupal development: summer diapause, winter diapause, and nondiapause. In the present study, we tested the influence of temperature, day length, and seasonality on the expression of alternative developmental pathways and compared the differences in life history traits between diapausing and directly developing individuals under laboratory and field conditions. The expression of developmental pathway strongly depended on temperature, day length, and seasonality. Low temperatures induced almost all individuals to enter diapause regardless of day length; relatively high temperatures combined with intermediate and longer day lengths resulted in most individuals developing without diapause in the laboratory. The field data revealed that the degree of phenotypic plasticity in relation to developmental pathway was much higher in autumn than in spring. Directly developing individuals showed shorter development times and higher growth rates than did diapausing individuals. The pupal and adult weights for both diapausing and directly developing individuals gradually decreased as rearing temperature increased, with the diapausing individuals being slightly heavier than the directly developing individuals at each temperature. Female body weight was slightly lower than male body weight. The proportional weight losses from pupa to adult were almost the same in diapausing individuals and in directly developing individuals, suggesting that diapause did not affect weight loss at metamorphosis. Our results highlight the importance of the expression of alternative developmental pathways, which not only synchronizes this butterfly's development and reproduction with the growth seasons of the host plants but also exhibits the bet‐hedging tactic against unpredictable risks due to a dynamic environment.     

The relationship between habitat permanence and larval development in California spadefoot toads: field and laboratory comparisons of developmental plasticity

We evaluated differences in larval habitats and life history of three species of spadefoot toads, then compared their life histories in a common garden study. Our field work defined the selective regime encountered by each species. Our Great Basin spadefoot (Spea intermontana) bred asynchronously in permanent streams and springs where there was no risk of larval mortality due to drying. The water chemistry remained fairly stable throughout the larval period. The western spadefoot toad, Sp. hammondii, bred fairly synchronously following heavy spring rains in temporary pools that remained filled an average of 81 d. Fifteen % of the breeding pools dried completely on or before the day the first larvae metamorphosed. The desert spadefoot toad, Scaphiopus couchii, bred synchronously after heavy summer showers in very short duration pools; 62% of the breeding pools dried completely on or before the day the first larvae metamorphosed. The concentration of ammonium nitrogen and CaCO₃ increased markedly as the Sp. hammondii and S. couchii pools dried. S. couchii attained metamorphosis at a much earlier age and smaller size than the other two species. S. couchii also showed little variation in the age at metamorphosis but considerable variation in the size at metamorphosis, while the other two species varied in both age and size. The results identify some variables that could serve as cues of pool drying and demonstrate an association between breeding pool duration, breeding synchrony, development rate, and larval development. Our laboratory study yields information about the genetic basis of the differences in development and controlled comparisons of phenotypic plasticity. We manipulated food supply to study the plastic response of age and size at metamorphosis and hence construct the reaction norm for these variables as a function of growth rate. The growth rates ranged from below to above those observed in natural populations. As in the field, in the lab S. couchii attained metamorphosis at an earlier age and smaller size than the other two species. All three species had a similarly shaped reaction norm for size(y‐axis) and age (x‐axis) at metamorphosis, which was a concave upward curve. A consequence of this shape is that age at metamorphosis changes more readily at low levels of food availability and size at metamorphosis changes more readily at high levels of food availability. If we restrict our observations to just those growth rates that are seen in nature, then S. couchii has almost no variation in the age at metamorphosis but considerable variation in size at metamorphosis, while the other two species vary in both age and size at metamorphosis. All three species increased in size at metamorphosis with increased food levels. Our comparative reaction norm approach thus demonstrates that S. couchii has adapted to ephemeral environments by shifting its growth rate reaction norm so that age at metamorphosis is uniformly fast and is not associated with growth rate. The realized variation is concentrated in size rather than age at metamorphosis.     

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.     

Temperature adaptation in a geographically widespread zooplankter,Daphnia magna

Evidence for temperature adaptation in Daphnia magna was inferred from variation in the shape of temperature reaction norms for somatic growth rate, a fitness‐related trait. Ex‐ephippial clones from eight populations across Europe were grown under standardized conditions after preacclimation at five temperatures (17–29 °C). Significant variation for grand mean growth rates occurred both within populations (among clones) and between populations. Genetic variation for reaction norm shape was found within populations, with temperature‐dependent trade‐offs in clone relative fitness. However, the population average responses to temperature were similar, following approximately parallel reaction norms. The among‐population variation is not evidence for temperature adaptation. Lack of temperature adaptation at the population level may be a feature of intermittent populations where environmentally terminated diapause can entrain the planktonic stage of the life‐history within a similar range of temperatures.     

Rapid evolution of thermal plasticity in mountain lake Daphnia populations

Populations at risk of extinction due to climate change may be rescued by adaptive evolution or plasticity. Selective agents, such as introduced predators, may enhance or constrain plastic or adaptive responses to temperature. We tested responses of Daphnia to temperature by collecting populations from lakes across an elevational gradient in the presence and absence of fish predators (long‐term selection). We subsequently grew these populations at two elevations in field mesocosms over two years (short‐term selection), followed by a common‐garden experiment at two temperatures in the lab to measure life‐history traits. Both long‐term and short‐term selection affected traits, suggesting that genetic variation of plasticity within populations enabled individuals to rapidly evolve plasticity in response to high temperature. We found that short‐term selection by high temperature increased plasticity for growth rate in all populations. Fecundity was higher in populations from fishless lakes and body size showed greater plasticity in populations from warm lakes (long‐term selection). Neither body size nor fecundity were affected by short‐term thermal selection. These results demonstrate that plasticity is an important component of the life‐history response of Daphnia, and that genetic variation within populations enabled rapid evolution of plasticity in response to selection by temperature.     

Phenotypic plasticity in a complex world: interactive effects of food and temperature on fitness components of a seed beetle

Most studies of phenotypic plasticity investigate the effects of an individual environmental factor on organism phenotypes. However, organisms exist in an ecologically complex world where multiple environmental factors can interact to affect growth, development and life histories. Here, using a multifactorial experimental design, we examine the separate and interactive effects of two environmental factors, rearing host species (Vigna radiata, Vigna angularis and Vigna unguiculata) and temperature (20, 25, 30 and 35°C), on growth and life history traits in two populations [Burkina Faso (BF) and South India (SI)] of the seed beetle, Callosobruchus maculatus. The two study populations of beetles responded differently to both rearing host and temperature. We also found a significant interaction between rearing host and temperature for body size, growth rate and female lifetime fecundity but not larval development time or larval survivorship. The interaction was most apparent for growth rate; the variance in growth rate among hosts increased with increasing temperature. However, the details of host differences differed between our two study populations; the degree to which V. unguiculata was a better host than V. angularis or V. radiata increased at higher temperatures for BF beetles, whereas the degree to which V. unguiculata was the worst host increased at higher temperatures for SI beetles. We also found that the heritabilities of body mass, growth rate and fecundity were similar among rearing hosts and temperatures, and that the cross-temperature genetic correlation was not affected by rearing host, suggesting that genetic architecture is generally stable across rearing conditions. The most important finding of our study is that multiple environmental factors can interact to affect organism growth, but the degree of interaction, and thus the degree of complexity of phenotypic plasticity, varies among traits and between populations.     

Life history comparisons of Daphnia obtusa from temporary ponds, cultured with a low-quality food

Temporary pond Daphnia flourish on relatively poor-quality food, suggesting adaptation to stringent temporary pond conditions. We conducted laboratory life history experiments on populations of Daphnia obtusa from a shaded woodland temporary pond (short hydroperiod, dystrophic) and an open farmland temporary pond (long hydroperiod, eutrophic), and compared a suite of physical/chemical conditions in the ponds to evaluate habitat conditions. We hypothesized that the shaded woodland pond population would be more fit in terms of life history variables for individuals (age and size at first reproduction, mean brood size, mean number of neonates) and populations (generation time, net reproductive rate R ₀, and intrinsic reproductive rate r) given a standard, low-quality food (trout chow and yeast). Life history traits of woodland pond animals were mixed, relative to farm pond animals, and consistent with bet-hedging for an unpredictable habitat. Values of life history traits rivaled or exceeded those of other studies using phytoplankton as food, and were influenced by the pond water used for our study. Life histories clearly differ among local and regional temporary pond Daphnia obtusa populations, and should be valuable for examining the relative influences of local selection and metapopulation dynamics on population structure.     

Life cycle polyphenism as a factor affecting ecological divergence within <Emphasis Type="Italic">Notophthalmus viridescens</Emphasis>

Polyphenism, which allows a single genotype to express multiple discrete phenotypes in response to environmental cues, is an adaptive trait in heterogeneous environments. Pond hydroperiod is an important ecological parameter affecting amphibian life history, and variation in local pond hydrology has been hypothesized to play a role in species divergence via changes in polyphenism. The eastern newt (Notophthalmus viridescens) expresses life cycle polyphenism. Larvae develop along three possible pathways: metamorphosis to aquatic lunged adult via a terrestrial juvenile (eft) stage, metamorphosis directly to an aquatic lunged adult, or maturation directly to an aquatic gilled adult without metamorphosis (i.e., paedomorphosis). Subspecies of N. viridescens vary in their polyphenic patterns, suggesting possible adaptation to different environments. However, no studies have experimentally tested how genetic and environmental components contribute to the observed differences among subspecies and whether such differences may facilitate divergence. We tested whether adaptation to local pond hydrology via polyphenic changes existed among subspecies by rearing larvae of three subspecies (N. v. dorsalis, N. v. louisianensis, and N. v. viridescens) along three hydroperiod regimes (short, long, and constant) in outdoor artificial ponds. We found that larval N. v. viridescens obligately metamorphosed to efts under all hydroperiods, whereas N. v. dorsalis and N. v. louisianensis exhibited plasticity: larvae metamorphosed to efts under drying conditions but metamorphosed directly to aquatic adults or became paedomorphic in constant water. Also, N. v. viridescens metamorphosed to efts faster and at a smaller body size than the other two subspecies. These data suggest that subspecies of N. viridescens are adapted to different pond hydroperiods, supporting the potential for polyphenism to facilitate divergence. Canalizing selection for certain alternative phenotypes within a single species in which other populations remain plastic may play an important role in the initiation of ecological divergence.