PARATHYROIDS AND CALCIUM METABOLISM

The experiments reported in this article are in full agreement with the facts known about the action of Ca and Mg salts in tetanic animals. In the concentrations used here both Ca lactate and Mg lactate suppressed the muscular convulsions in the tetanic salamander larvæ. The Mg lactate, however, appears to be more effective than the Ca lactate. At any rate the suppression of the tetanic convulsions does not seem to be a specific action of the calcium. The most important result seems to be the fact that the salts used, though they prevented the muscular convulsions, did not prevent the other symptoms of tetany which in the salamander larvæ are very definite and constant. The permanent spasmodic contractions and the paralysis of the muscles developed in spite of the presence of the Ca and Mg, Furthermore, the muscular contractions and the paralysis developed even in such thymus-fed animals in which the convulsions had been suppressed completely; this was the case in one of the animals of the Mg series. From the experiments of Biedl and others it is likely that the tetanic convulsions are due to lesions of the central nervous system, since convulsions of a leg can be prevented by isolating it from the central nervous system by cutting the nerves which connect the muscles with the central nervous system. Evidently these lesions of the central nervous system are the chief factor in tetany, while the convulsions of the muscles are only an effect. In the larvæ of salamanders these lesions find a definite expression in the permanent paralysis of almost the entire muscular system. In the writer''s opinion, MacCallum''s hypothesis that the tetany toxin has a special affinity for Ca, thereby diminishing the Ca content of the organism, cannot be disproved at present. But the present experiments seem to prove, first, that the tetany-producing substance causes permanent lesions of the nervous system, which lead to permanent spasmodic contractions and paralysis of the muscle even in the absence of tetanic convulsions, and second, that these cannot be prevented by either Ca or Mg. For the most part they result in an early death of the animals no matter whether or not Ca or Mg has been applied. In connection with this fact we wish to mention Biedl''s claim that no one has yet succeeded in prolonging the life of parathyroidectomized animals by the application of Ca. From MacCallum''s paper, on account of the lack of controls, it cannot be seen whether his parathyroidectomized dogs lived longer with Ca treatment than without. That in spontaneous tetany Ca treatment may effect a cure, as is evident from the report by Howland and Marriott, does not prove that in this case Ca has inhibited tetany as a disease. In spontaneous tetany the period of the action of the tetany-producing substance may be a very short one and the mere prevention of the tetanic convulsions may keep the patient alive until normal function of the glands involved has been restored. The pathological changes which the central nervous system undergoes in this short period may not be severe enough to endanger the life of the patient after the cessation of the action of the tetany toxin. In the light of the facts presented our experiments lead to the following conclusions: 1. The thymus gland excretes a tetany-producing substance which in the normal animal is antagonized in an unknown way by the parathyroids. 2. In animals devoid of parathyroids (salamander larvæ, parathyroidectomized mammals) this substance may, according to MacCallum, reduce the Ca content of the organism; but by far the most dangerous and important quality of this substance is its highly injurious effect upon the central nervous system, which causes permanent spasmodic contractions of the muscles and paralysis of almost the entire muscular system. 3. It is possible to prevent the muscular contractions by introducing Ca salts into the body, though this can be done more effectively by means of Mg salts. 4. No substance, however, has been found so far to antagonize the tetany toxin and to prevent the development of the lesions of the central nervous system caused by the tetany toxin. 5. This explains why in spite of the application of Ca or Mg and in spite of the suppression by these substances of the tetanic convulsions the other symptoms of tetany develop and frequently lead to the death of the animal. 6. Accordingly the most important function of the parathyroids is to prevent the tetany toxin, by antagonizing it, from coming into contact with the central nervous system.     

EXPERIMENTAL PRODUCTION OF GIGANTISM BY FEEDING THE ANTERIOR LOBE OF THE HYPOPHYSIS

1. Metamorphosed salamanders of the species Ambystoma opacum and Ambystoma tigrinum were fed on a pure diet of the anterior lobe of the hypophysis of cattle; the controls were fed on an abundant diet of earthworms. 2. The rate of growth of the animals fed on the anterior lobe of the hypophysis was greatly increased over the rate of growth of normal animals. 3. Growth of the animals fed on anterior lobe did not cease after they had reached the normal "maximum" size of the species, and experimental giants were produced. 4. The largest animal of the species Ambystoma opacum fed on anterior lobe of the hypophysis was 19 mm. larger than the largest normal animal of this species known to the writer; the largest animal of the species Ambystoma tigrinum fed on anterior lobe is at present about 28 mm. larger than the largest normal animal of the eastern race of this species known to the writer.     

RELATION BETWEEN THYROID GLAND,METAMORPHOSIS, AND GROWTH

1. Two substances are involved in amphibian metamorphosis as studied in Ambystoma opacum: first, iodine, which is taken up by the food, and second, an excretor substance, which is evolved during the processes of growth and serves to induce the excretory function of the thyroid gland. 2. This explains why in larvæ, whose metamorphosis is inhibited by lack of iodine, growth is checked at the time when metamorphosis should occur; for at this time the excretor substance commences to act and this results, if iodine is absent, in the excretion by the thyroid of toxic substances which cause the breakdown of proteins and consequently a decrease in size of the larvæ. 3. Larvæ whose metamorphosis is inhibited by extirpation of the thyroid or by the hereditary lack of a thyroid (as is the case in Typhlomolge) can grow normally, since in them the action of the excretor substance cannot result in the excretion by the thyroid of a toxic growth-inhibiting substance. 4. At low temperature less excretor substance is produced than at high temperature during an equal rate of growth; therefore larvæ kept at low temperature reach a larger size than larvæ kept at high temperature, before they metamorphose.     

NATURE OF THE RETARDING INFLUENCE OF THE THYMUS UPON AMPHIBIAN METAMORPHOSIS

1. Though thymus-fed salamander larvæ often metamorphose normally, thymus feeding sometimes retards and in rare cases inhibits metamorphosis completely. 2. The addition of normal food to a thymus diet abolishes the inhibitory effect of the thymus. 3. Addition of a small quantity of iodothyrin leads rapidly to precocious metamorphosis of thymus-fed larvæ. 4. The inhibitory effect of the thymus is not due to a specific inhibiting substance in the thymus, but to the absence from the thymus of a substance required to develop the thyroid to the secretory state.     

Evolution mediates the effects of apex predation on aquatic food webs

Ecological and evolutionary mechanisms are increasingly thought to shape local community dynamics. Here, I evaluate if the local adaptation of a meso-predator to an apex predator alters local food webs. The marbled salamander (Ambystoma opacum) is an apex predator that consumes both the spotted salamander (Ambystoma maculatum) and shared zooplankton prey. Common garden experiments reveal that spotted salamander populations which co-occur with marbled salamanders forage more intensely than those that face other predator species. These foraging differences, in turn, alter the diversity, abundance and composition of zooplankton communities in common garden experiments and natural ponds. Locally adapted spotted salamanders exacerbate prey biomass declines associated with apex predation, but dampen the top-down effects of apex predation on prey diversity. Countergradient selection on foraging explains why locally adapted spotted salamanders exacerbate prey biomass declines. The two salamander species prefer different prey species, which explains why adapted spotted salamanders buffer changes in prey composition owing to apex predation. Results suggest that local adaptation can strongly mediate effects from apex predation on local food webs. Community ecologists might often need to consider the evolutionary history of populations to understand local diversity patterns, food web dynamics, resource gradients and their responses to disturbance.     

Predator size and phenology shape prey survival in temporary ponds

Theoretical efforts suggest that the relative sizes of predators and their prey can shape community dynamics, the structure of food webs, and the evolution of life histories. However, much of this work has assumed static predator and prey body sizes. The timing of recruitment and the growth patterns of both predator and prey have the potential to modify the strength of predator–prey interactions. In this study, I examined how predator size dynamics in 40 temporary ponds over a 3-year period affected the survival of spotted salamander (Ambystoma maculatum) larvae. Across communities, gape-limited predator richness, but not size, was correlated with habitat duration (pond permanence). Within communities, mean gape-limited predator size diminished as the growing season progressed. This size reduction occurred because prey individuals grew into a body size refuge and because the largest of the predators left ponds by mid-season. Elevated gape-limited predation risk across time and space was predicted by the occurrence of two large predatory salamanders: marbled salamander larvae (Ambystoma opacum) and red-spotted newt adults (Notophthalmus viridescens). The presence of the largest gape-limited predator, A. opacum, predicted A. maculatum larval survival in the field. The distribution of large predatory salamanders among ponds and across time is expected to lead to differing community dynamics and to generate divergent natural selection on early growth and body size in A. maculatum. In general, a dynamic perspective on predator size often will be necessary to understand the ecology and evolution of species interactions. This will be especially true in frequently disturbed or seasonal habitats where phenology and ontogeny interact to determine body size asymmetries. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Does breeding ecology alter selection on developmental and life history traits? A case study in two Ambystomatid salamanders

Terrestrial breeding with extended incubation has evolved repeatedly in fishes and amphibians but raises challenges for embryos, which must conserve sufficient yolk to continue development until water is available. One means of avoiding premature yolk consumption is for embryos to slow their development such that yolk is consumed more slowly. However slower development is associated with smaller adult body sizes and lower survivorship, particularly in amphibians occupying ephemeral habitats. The variable breeding strategies of Ambystomatid salamanders provide an excellent system in which to examine whether variation in breeding ecology is associated with divergence in embryonic development. Phylogenies of Ambystoma strongly support aquatic egg-laying and winter breeding as ancestral. Early breeding has evolved in three species and terrestrial breeding in two of the three. We propose that early, terrestrial breeding has altered selection on embryonic development. In particular, we propose that the prolonged incubation typically associated with early, terrestrial breeding has selected for longer development times. We compared embryonic development of the terrestrial breeding Ambystoma opacum (Gravenhorst 1807) with the co-occuring, aquatic breeding Ambystoma maculatum (Shaw 1802) under a range of laboratory conditions over two breeding seasons. We found that A. opacum embryos took longer to develop and hatched at a later stage, consistent with the hypothesis that early, terrestrial breeding favors embryos that extend development.     

Interaction of an insecticide with larval density in pond-breeding salamanders (Ambystoma)

1. Amphibian populations residing in or near agricultural areas are often susceptible to pesticide contamination. Recent evidence suggests that the effects of pesticides on amphibians often exceed those estimated in laboratory toxicity tests because other environmental factors (e.g. predators, resource abundance) can influence pesticide toxicity. 2. To examine the effects of an insecticide (carbaryl) on two species of Ambystoma salamanders experiencing the natural stress of competition, we manipulated chemical concentration (control, 3.5 and 7.0 mg L^?1) and larval density (low and high). We determined the effect of treatments on snout‐vent length (SVL), growth rate, lipid reserves, time to metamorphosis, per cent survival and per cent metamorphosis. 3. Carbaryl negatively affected all response variables of Ambystoma maculatum significantly, and significantly reduced survival and metamorphosis of A. opacum. Increased density significantly influenced SVL, lipid reserves, growth rate and metamorphosis of A. maculatum. 4. The effects of carbaryl and increased density on per cent metamorphosis were nearly additive, but were generally less than additive on other variables. 5. The negative effects of chemical contamination on salamanders were likely because of pesticide‐induced reductions of food resources, as zooplankton abundance decreased by as much as 97% following carbaryl application. 6. Our study demonstrates the importance of the interactive effects that chemical contamination and natural environmental factors have on salamanders.     

Microgeographic adaptations of spotted salamander morphological defenses in response to a predaceous salamander and beetle

Spatial heterogeneity in the selection imposed by different predator species could promote the adaptive diversification of local prey populations. However, high gene flow might swamp local adaptations at limited spatial scales or generalized phenotypic plasticity might evolve in place of local diversification. Spotted salamander larvae Ambystoma maculatum face strongly varying risks from gape‐limited marbled salamander larvae Ambystoma opacum and gape‐unconstrained diving beetle larvae Dytiscus spp. across natural landscapes. To evaluate if A. maculatum adapts to these predation risk across micro‐geographic scales, I measured selection gradients in response to the two focal predators and then assayed the defensive morphologies of ten populations in a common garden experiment. I found that A. opacum induced selection on A. maculatum for larger tailfins and bodies whereas beetles induced selection for larger tail muscles and smaller bodies. In accordance with the local adaptation hypothesis, A. maculatum populations inhabiting ponds with high beetle densities grew larger tail muscles relative to other populations when raised in a common environment. However, populations exposed to strong A. opacum selection did not evolve larger tailfins as predicted. High gene flow or morphological plasticity could explain the absence of this morphological response to A. opacum. Overall, results suggest that populations can sometimes evolve adaptive traits in response to locally variable selection regimes even across the very limited distances that separate populations in this study. If prey populations often differ in their defenses against local predators, then this variation could affect the outcome of species interactions in local communities.     

Effects of Timber Management on Pond-Breeding Salamanders

Abstract: Pond-breeding salamanders spend most of their lives in forested habitat surrounding the vernal pools where they breed. Timber harvesting has been demonstrated to have negative impacts on salamander populations due to changes in soil temperature, soil compaction, and general degradation of habitat. However, little is known about how long it takes for harvested forest habitat to once again become suitable for salamanders. Questions also remain as to whether salamanders will use an area that has been harvested in recent years if an older intact forest area is available. We used drift fences and pitfall traps to capture adult spotted salamanders (Ambystoma maculatum) and marbled salamanders (A. opacum) migrating to 3 vernal ponds during their breeding seasons. The study area contained tracts of forest that were clear-cut 11–12 years prior to the study. All 3 ponds were surrounded by areas of clear-cut and intact forest and drift fences were placed in both habitat types. Similar numbers of spotted salamanders entered the ponds from clear-cut and intact forest areas. The number of marbled salamanders migrating to the ponds did not differ between areas of clear-cut and intact forest. These results suggest that clear-cut habitats may become suitable for adult pond-breeding salamanders after a relatively short regeneration period.     

Spatial and temporal niche partitioning in grassland ants

We examined whether the species composition of a community influences the persistence of larval Ambystoma maculatum in assemblages composed of two larger intraguild predators (A. opacum and A. jeffersonianum) and an alternative prey species (tadpoles of Rana sylvatica). We predicted a priori that A. maculatum would have higher survival in more diverse communities containing alternative species of prey and top predators (A. opacum), the latter of which may lower the abundance of intermediate predators (A. jeffersonianum) via intraguild predation. In a factorial experiment, we manipulated the presence of larval A. opacum, A. jeffersonianum, and R. sylvatica in replicated artificial ponds containing larval A. maculatum. The presence of all three species significantly depressed biomass production in A. maculatum: biomass was highest in ponds lacking the other species and was zero in ponds initially containing all four species. Tadpoles severely reduced the growth of filamentous algae in the ponds. This, in turn, may have affected the abundance of some herbivorous prey of larval salamanders, although this possibility was not tested. The presence of congeneric predators severely restricted the presence in the water column of larval A. maculatum, which otherwise exhibited significant diel patterns of activity in the absence of predators. Together, the presence of tadpoles and a predator-mediated reduction in activity patterns may have limited foraging opportunities for A. maculatum, thus exacerbating the direct impact of predation on survival in this species. These results suggest that diverse assemblages consisting of these particular species may actually inhibit, rather than promote, inclusion of A. maculatum in some communities of pond-breeding amphibians.     

Differential vulnerability to predation and refuge use in competing larval salamanders

The aquatic larvae of two species of salamanders coexist as a result of differences in their competitive abilities: Ambystoma talpoideum is a superior aggressor, whereas A. maculatum is a superior forager. I examined the behavioral mechanisms that permit these species to coexist with their predatory congener, A. opacum. I asked whether the two prey species differ in their vulnerability to predation and in their use of structural and spatial refugia when under the risk of predation; such inter-specific variation may allow predation to contribute indirectly to prey coexistence. Larval A. maculatum (the superior forager) was more vulnerable to predation by A. opacum than was A. talpoideum, and only the latter species significantly increased its use of structural refugia (leaf litter) in the presence of the predator. In pond enclosures, both species of prey exhibited diel patterns of microhabitat use; significantly more larvae occupied shallow regions of enclosures during the day and migrated to deeper water (a spatial refugium) at night. However, when considered separately, neither (1) the presence of a predatory larval A. opacum nor (2) an increased density of intra- and interspecific competitors significantly altered this habitat shift for either prey species. Rather, diel microhabitat usage in A. talpoideum was significantly affected by an interaction between predator presence and competitor density. My results demonstrate the importance of refugia to coexistence in this predator-prey assemblage. Furthermore, predation by A. opacum may mediate prey competition; that is, preferential consumption of A. maculatum may reduce the competitive impact of this superior forager on A. talpoideum, thus enhancing their coexistence.     

STRONG SELECTION BARRIERS EXPLAIN MICROGEOGRAPHIC ADAPTATION IN WILD SALAMANDER POPULATIONS

Microgeographic adaptation occurs when populations evolve divergent fitness advantages across the spatial scales at which focal organisms regularly disperse. Although an increasing number of studies find evidence for microgeographic adaptation, the underlying causes often remain unknown. Adaptive divergence requires some combination of limited gene flow and strong divergent natural selection among populations. In this study, we estimated the relative influence of selection, gene flow, and the spatial arrangement of populations in shaping patterns of adaptive divergence in natural populations of the spotted salamander (Ambystoma maculatum). Within the study region, A. maculatum co‐occur with the predatory marbled salamander (Ambystoma opacum) in some ponds, and past studies have established a link between predation risk and adaptive trait variation in A. maculatum. Using 14 microsatellite loci, we found a significant pattern of genetic divergence among A. maculatum populations corresponding to levels of A. opacum predation risk. Additionally, A. maculatum foraging rate was strongly associated with predation risk, genetic divergence, and the spatial relationship of ponds on the landscape. Our results indicate the sorting of adaptive genotypes by selection regime and strongly suggest that substantial selective barriers operate against gene flow. This outcome suggests that microgeographic adaptation in A. maculatum is possible because strong antagonistic selection quickly eliminates maladapted phenotypes despite ongoing and substantial immigration. Increasing evidence for microgeographic adaptation suggests a strong role for selective barriers in counteracting the homogenizing influence of gene flow.     

Multiscale resistant kernel surfaces derived from inferred gene flow: An application with vernal pool breeding salamanders

The importance of assessing spatial data at multiple scales when modelling species–environment relationships has been highlighted by several empirical studies. However, no landscape genetics studies have optimized landscape resistance surfaces by evaluating relevant spatial predictors at multiple spatial scales. Here, we model multiscale/layer landscape resistance surfaces to estimate resistance to inferred gene flow for two vernal pool breeding salamander species, spotted (Ambystoma maculatum) and marbled (A. opacum) salamanders. Multiscale resistance surface models outperformed spatial layers modelled at their original spatial scale. A resistance surface with forest land cover at a 500‐m Gaussian kernel bandwidth and normalized vegetation index at a 100‐m Gaussian kernel bandwidth was the top optimized resistance surface for A. maculatum, while a resistance surface with traffic rate and topographic curvature, both at a 500‐m Gaussian kernel bandwidth, was the top optimized resistance surface for A. opacum. Species‐specific resistant kernels were fit at all vernal pools in our study area with the optimized multiscale/layer resistance surface controlling kernel spread. Vernal pools were then evaluated and scored based on surrounding upland habitat (local score) and connectivity with other vernal pools on the landscape, with resistant kernels driving vernal pool connectivity scores. As expected, vernal pools that scored highest were in areas within forested habitats and with high vernal pool densities and low species‐specific landscape resistance. Our findings highlight the success of using a novel analytical approach in a multiscale framework with applications beyond vernal pool amphibian conservation.     

Can road mortality limit populations of pool-breeding amphibians?

We integrated road maps, traffic volume data, and pool locations in a modeling study to estimate the potential effects of road mortality on populations of pool-breeding spotted salamanders (Ambystoma maculatum Shaw). Population projections based on spotted salamander life tables imply that an annual risk of road mortality for adults of >10% can lead to local population extirpation; mitigation efforts (tunnels, road closures, and other measures) should seek to reduce road mortality rates to below this threshold. For central and western Massachusetts, we estimated that salamanders would be exposed to at least this threshold level of risk at 22–73% of populations (assuming a 100 vs. 500 m migration distance, respectively). We conclude that road mortality can be a significant source of additive mortality for individual spotted salamanders in many parts of the species’ range. Efforts to prevent such mortality by transportation planners are likely warranted strictly on a biological basis in areas with road densities >2.5 km per km² of landscape and traffic volumes >250 vehicles/lane/day within the migration range of a breeding population of spotted salamanders.     

Will pre‐adaptation buffer the impacts of climate change on novel species interactions?

Species are expected to alter their ranges as climates change. Climate‐induced range expansions of predators could threaten evolutionarily naïve prey populations, leading to high mortality at the invasion front. If prey can apply existing defenses against local predators to novel predation threats induced by climate change, mortality threats will be less than expected. Here, we examine if spotted salamander larvae Ambystoma maculatum from populations that coexist with native red‐spotted newts Notophthalmus viridescens survive better when exposed to a novel predator, the marbled salamander Ambystoma opacum. We show that regional mean winter temperatures warmed 2.0°C over 116 yr in the region, and that A. opacum survival increases in ponds with higher winter temperatures. Hence as winters continue to warm, this apex predator will likely colonize ponds north of their current range limit. Next, we performed common garden experiments to determine if local adaptations to native N. viridescens and exposure to A. opacum or N. viridescens kairomones (predator chemical cues) altered A. maculatum survival in predation trials. We did not find evidence for local adaptation to N. viridescens. However, A. maculatum from high‐N. viridescens ponds that were reared with A. opacum kairomones suffered significantly higher mortality from the native predator N. viridescens. This outcome suggests an unanticipated interaction between local adaptation and plastic responses to novel kairomones from a potentially range‐expanding predator. Current projections of biodiversity losses from climate change generally ignore the potential for eco‐evolutionary interactions between native and range‐expanding species and thus could be inaccurate.     

The effects of conspecifics on burrow selection in juvenile spotted salamanders (<Emphasis Type="Italic">Ambystoma maculatum)</Emphasis>

Spotted salamanders (Ambystoma maculatum) are pond-breeding amphibians that disperse into terrestrial habitat from natal wetlands after undergoing metamorphosis, relying on small-mammal burrows and coarse woody debris for refugia. The effect of conspecifics on burrow use in juvenile salamanders is poorly understood. Determining how the presence of conspecifics influences the settlement decisions of juvenile salamanders can increase our understanding of amphibian dispersal and our ability to predict population dynamics. We conducted behavioral laboratory trials using 58 recently metamorphosed salamanders to examine how salamanders selected burrows in the presence of conspecifics. Salamanders were more likely to settle in a burrow that was occupied by a conspecific versus an unoccupied burrow. Our results indicate that juvenile salamanders may show conspecific attraction and/or trailing behavior during the dispersal phase.     

Mortality Rates Differ Among Amphibian Populations Exposed to Three Strains of a Lethal Ranavirus

Infectious diseases are a growing threat to biodiversity, in many cases because of synergistic effects with habitat loss, environmental contamination, and climate change. Emergence of pathogens as new threats to host populations can also arise when novel combinations of hosts and pathogens are unintentionally brought together, for example, via commercial trade or wildlife relocations and reintroductions. Chytrid fungus (Batrachochytrium dendrobatidis) and amphibian ranaviruses (family Iridoviridae) are pathogens implicated in global amphibian declines. The emergence of disease associated with these pathogens appears to be at least partly related to recent translocations over large geographic distances. We experimentally examined the outcomes of novel combinations of host populations and pathogen strains using the amphibian ranavirus Ambystoma tigrinum virus (ATV) and barred tiger salamanders (Ambystoma mavortium, formerly considered part of the Ambystoma tigrinum complex). One salamander population was highly resistant to lethal infections by all ATV strains, including its own strain, and mortality rates differed among ATV strains according to salamander population. Mortality rates in novel pairings of salamander population and ATV strain were not predictable based on knowledge of mortality rates when salamander populations were exposed to their own ATV strain. The underlying cause(s) for the differences in mortality rates are unknown, but local selection pressures on salamanders, viruses, or both, across the range of this widespread host–pathogen system are a plausible hypothesis. Our study highlights the need to minimize translocations of amphibian ranaviruses, even among conspecifc host populations, and the importance of considering intraspecific variation in endeavors to manage wildlife diseases.     

The origin of tiger salamander (Ambystoma tigrinum) populations in California, Oregon, and Nevada: introductions or relicts?

Whether intentionally or accidentally introduced, exotic species have the capacity to dramatically disrupt native communities. In central California, tiger salamanders (Ambystoma tigrinum) have been introduced as a by-product of the sport fishing bait industry. Some of these introductions are relatively well known and have resulted in the formation of hybrids with the imperiled native California tiger salamander (A. californiense). Other populations of A. tigrinum, particularly in the northern and eastern parts of the state, remain poorly characterized and are present in regions where relictual amphibian populations of other species have persisted, suggesting that these might be relictual, native A. tigrinum. We used genetic sequence data to determine the provenance of all known extralimital A. tigrinum populations in California and adjacent Oregon and Nevada through comparison with reference samples from the native range of A. tigrinum. Our results suggest that A. tigrinum have been introduced in Northern California, Southern California and the Sierra Nevada, originating from multiple sources across the Great Plains of the US. Furthermore, two populations near the California-Oregon border are most closely related to A. tigrinum populations from Washington and Oregon and may represent native tiger salamander lineages.     

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.