Spatial variation in breeding phenology at small spatial scales: A stochastic effect of population size

Spatial variation in phenology can occur at small spatial scales over which individuals can disperse or forage within one generation. Previous studies have assumed that variations in phenological peaks are caused by differences in abiotic environmental characteristics. However, environments should generally be similar among local habitats over small spatial scales. When the local population size is small, the phenological peak of the local population should be strongly affected by the variation in timing expressed by individuals. If a regional population consists of small local subpopulations (e.g., a metapopulation), the stochastic processes regulated by population sizes may explain the spatial variation in phenology. In this study, we quantitatively evaluated the extent of the spatial and annual variations in the breeding phenology of the forest green tree frog, Rhacophorus arboreus habiting a small area (<10 km²). The spatial variation in phenological peaks among 25 breeding sites was large over 6 years. This spatial variation was not explained by differences in air temperature or water depth. Randomization tests revealed that a large portion of the spatial variation could be explained by differences in population size, without considering site-specific factors. Annual variations in phenological peaks tended to be greater for smaller populations. These results imply that the stochastic process might have caused the spatial and annual variations in the phenological peaks of R. arboreus observed in the study region. Understanding spatiotemporal variation in phenology determined by stochastic process would be important to better predict interspecific interactions and (meta)population dynamics at small spatial scales.     

Warmer winters reduce frog fecundity and shift breeding phenology,which consequently alters larval development and metamorphic timing

One widely documented phenological response to climate change is the earlier occurrence of spring‐breeding events. While such climate change‐driven shifts in phenology are common, their consequences for individuals and populations have rarely been investigated. I addressed this gap in our knowledge by using a multi‐year observational study of six wood frog (Rana sylvatica) populations near the southern edge of their range. I tested first if winter temperature or precipitation affected the date of breeding and female fecundity, and second if timing of breeding affected subsequent larval development rate, mass at metamorphosis, date of metamorphosis, and survival. Warmer winters were associated with earlier breeding but reduced female fecundity. Winter precipitation did not affect breeding date, but was positively associated with female fecundity. There was no association between earlier breeding and larval survival or mass at metamorphosis, but earlier breeding was associated with delayed larval development. The delay in larval development was explained through a counterintuitive correlation between breeding date and temperature during larval development. Warmer winters led to earlier breeding, which in turn was associated with cooler post‐breeding temperatures that slowed larval development. The delay in larval development did not fully compensate for the earlier breeding, such that for every 2 days earlier that breeding took place, the average date of metamorphosis was 1 day earlier. Other studies have found that earlier metamorphosis is associated with increased postmetamorphic growth and survival, suggesting that earlier breeding has beneficial effects on wood frog populations.     

Local selection modifies phenotypic divergence among Rana temporaria populations in the presence of gene flow

In ectotherms, variation in life history traits among populations is common and suggests local adaptation. However, geographic variation itself is not a proof for local adaptation, as genetic drift and gene flow may also shape patterns of quantitative variation. We studied local and regional variation in means and phenotypic plasticity of larval life history traits in the common frog Rana temporaria using six populations from central Sweden, breeding in either open‐canopy or partially closed‐canopy ponds. To separate local adaptation from genetic drift, we compared differentiation in quantitative genetic traits (Q_ST) obtained from a common garden experiment with differentiation in presumably neutral microsatellite markers (F_ST). We found that R. temporaria populations differ in means and plasticities of life history traits in different temperatures at local, and in F_ST at regional scale. Comparisons of differentiation in quantitative traits and in molecular markers suggested that natural selection was responsible for the divergence in growth and development rates as well as in temperature‐induced plasticity, indicating local adaptation. However, at low temperature, the role of genetic drift could not be separated from selection. Phenotypes were correlated with forest canopy closure, but not with geographical or genetic distance. These results indicate that local adaptation can evolve in the presence of ongoing gene flow among the populations, and that natural selection is strong in this system.     

Frog survival and population viability in an agricultural landscape with a drying climate

Amphibians are the most threatened class of vertebrate in the world. Although a number of causes of the amphibian decline phenomenon are emerging, there is a need for robust demographic data to be able to monitor current and future threats such as climate change. Despite this, few studies on amphibians have the life-history data available to undertake these analyses and fewer still have looked at the challenges to population viability posed by fragmentation—a feature inherent in agricultural landscapes where the matrix is highly modified. Our aim was to investigate the population viability of a large burrowing frog in an agricultural landscape. Specifically, we aimed to investigate the future persistence of populations under a range of scenarios including populations connected by various levels of dispersal and reduced rainfall. We used the life-history parameters of Heleioporus albopunctatus, a frog species widely distributed in the extensively cleared agricultural regions of south-western Australia. We investigated the viability of 24 partially connected populations under a range of scenarios using the program Vortex Version 10.1.6.0. Metapopulations were consistently more robust to extinction than isolated local populations. Both meta- and local populations were more susceptible to increases in age-specific mortality rates than to variation in the estimated ability of H. albopunctatus to disperse between breeding ponds, the survival rate of dispersers, or the frequency of drought. Our results reinforce the importance of metapopulations for survival in fragmented landscapes and point to the need to manage amphibian breeding ponds across landscapes to ensure high survival rates, particularly for juveniles.     

Fine‐grained adaptive divergence in an amphibian: genetic basis of phenotypic divergence and the role of nonrandom gene flow in restricting effective migration among wetlands

Adaptive ecological differentiation among sympatric populations is promoted by environmental heterogeneity, strong local selection and restricted gene flow. High gene flow, on the other hand, is expected to homogenize genetic variation among populations and therefore prevent local adaptation. Understanding how local adaptation can persist at the spatial scale at which gene flow occurs has remained an elusive goal, especially for wild vertebrate populations. Here, we explore the roles of natural selection and nonrandom gene flow (isolation by breeding time and habitat choice) in restricting effective migration among local populations and promoting generalized genetic barriers to neutral gene flow. We examined these processes in a network of 17 breeding ponds of the moor frog Rana arvalis, by combining environmental field data, a common garden experiment and data on variation in neutral microsatellite loci and in a thyroid hormone receptor (TRβ) gene putatively under selection. We illustrate the connection between genotype, phenotype and habitat variation and demonstrate that the strong differences in larval life history traits observed in the common garden experiment can result from adaptation to local pond characteristics. Remarkably, we found that haplotype variation in the TRβ gene contributes to variation in larval development time and growth rate, indicating that polymorphism in the TRβ gene is linked with the phenotypic variation among the environments. Genetic distance in neutral markers was correlated with differences in breeding time and environmental differences among the ponds, but not with geographical distance. These results demonstrate that while our study area did not exceed the scale of gene flow, ecological barriers constrained gene flow among contrasting habitats. Our results highlight the roles of strong selection and nonrandom gene flow created by phenological variation and, possibly, habitat preferences, which together maintain genetic and phenotypic divergence at a fine‐grained spatial scale.     

Earlier breeding,lower success: does the spatial scale of climatic conditions matter in a migratory passerine bird?

Following over 20 years of research on the climatic effects on biodiversity we now have strong evidence that climate change affects phenology, fitness, and distribution ranges of different taxa, including birds. Bird phenology likely responds to changes in local weather. It is also affected by climatic year‐to‐year variations on larger scales. Although such scale‐related effects are common in ecology, most studies analyzing the effects of climate change were accomplished using climatic information on a single spatial scale. In this study, we aimed at determining the scale‐dependent sensitivity of breeding phenology and success to climate change in a migratory passerine bird, the barn swallow (Hirundo rustica). For both annual broods, we investigated effects of local weather (local scale) and the North Atlantic Oscillation (NAO, large scale) on the timing of breeding and breeding success. Consistent with previous studies in migratory birds we found that barn swallows in Eastern Germany bred progressively earlier. At the same time, they showed reduced breeding success over time in response to recent climatic changes. Responses to climatic variation were observed on both local and large climatic scales, but they differed with respect to the ecological process considered. Specifically, we found that the timing of breeding was primarily influenced by large‐scale NAO variations and to a lesser extent by local weather on the breeding grounds. Conversely, climatic conditions on the local scale affected breeding success, exclusively. The observed decrease in breeding success over years is likely a consequence of scale‐related mismatches between climatic conditions during different breeding phases. This provides further evidence that a species' response of earlier breeding may not be enough to cope with climate change. Our results emphasize the importance of considering the response of ecological processes along different climatic scales in order to better understand the complexity of climate change effects on biodiversity.     

Getting the timing right: antler growth phenology and sexual selection in a wild red deer population

There has been growing interest in the determinants of the annual timing of biological phenomena, or phenology, in wild populations, but research on vertebrate taxa has primarily focused on the phenology of reproduction. We present here analyses of the phenology of the annual growth of a secondary sexual characteristic, antlers in red deer (Cervus elaphus) males. The long-term individual-based data from a wild population of red deer on the Isle of Rum, Scotland allow us to consider ecological factors influencing variation in the phenology of growth of antlers, and the implications of variation in antler growth phenology with respect to the phenotype of antler grown (antler mass) and annual breeding success. The phenology of antler growth was influenced by local environmental conditions: higher population density delayed both the start date (during spring) and the relative end date (in late summer) of antler growth, and warmer temperatures in the September and April prior to growth advanced start and end dates, respectively. Furthermore, there was variation between individuals in this phenotypic plasticity of start date, although not in that of end date of growth. The phenology of antler growth impacted on the morphology of antlers grown, with individuals who started and ended growth earliest having the heaviest antlers. The timing of antler growth phenology was associated with breeding success in the following mating season, independently of the mass of antlers grown: an earlier start of antler growth was associated with siring a higher number of the calves born the following spring. Our results suggest that the phenology of traits that are not directly correlated with offspring survival may also regularly show correlations with fitness.     

Microgeographic morphological variation across larval wood frog populations associated with environment despite gene flow

Gene flow has historically been thought to constrain local adaptation; yet, recent research suggests that populations can diverge despite exchanging genes. Here I use a common garden experiment to assess the combined effects of gene flow and natural selection on morphological variation of 16 wood frog (Rana sylvatica) populations, a species known to experience divergent selection pressures in open‐ and closed‐canopy ponds across relatively small geographic scales. Wood frog tadpoles from different ponds showed significant morphological variation associated with canopy type with a trade‐off between tail length and body depth consistent with previous research. In contrast, neutral genetic differentiation of nine microsatellite loci as measured by Jost's D was not associated with canopy type, indicating no pattern of isolation by environment. Genetic structure analyses indicated some substructure across the 16 ponds (K = 4); however, three out of four assigned clusters included both open‐ and closed‐canopy ponds. Together, these results suggest that morphological divergence among these wood frog populations is occurring despite gene flow and that selection within these environments is strong. Furthermore, morphological variation among ponds differed across two sampling periods during larval development, demonstrating the importance of evaluating phenotypic divergence over multiple time periods and at a time relevant to the processes being studied.     

Leaf phenology mediates provenance differences in herbivore populations on valley oaks in a common garden

1. Plants from different populations often display a variation in herbivore resistance. However, it is rarely understood what plant traits mediate such differences. 2. It was tested how leaf phenology affects herbivore populations in a 15‐year‐old common garden of valley oaks (Quercus lobata Née) with different populations and maternal parents from throughout the Q. lobata range. 3. The abundance of leaf miners (Stigmella sp. Shrank) and leaf phenology of oaks in the common garden was measured. 4. Leaf miner abundance varied among provenance locations (population), but not among maternal parents within populations. Leaf phenology varied by provenance location and maternal parent, and trees that leafed out earlier accrued higher leaf‐miner abundance. Path analysis indicated that leaf phenology was the likely driver of provenance and parental differences in resistance to leaf miners. 5. Understanding population differences is particularly important when considering transport of genotypes for ornamental or restoration purposes. The present study suggests that similarity in leaf phenology may be one factor that could be used to find genotypes with a similar herbivore resistance to local genotypes.     

Microgeographical variation in thermal preference by an amphibian

Ectotherms use behaviour to buffer effects of temperature on growth, development and survival. While behavioural thermoregulation is widely reported, localized adaptation of thermal preference is poorly documented. Larval amphibians live in wetlands ranging from entirely open to heavily shaded by vegetation. We hypothesized that populations undergo localized selection leading to countergradient patterns of thermal preference behaviour. Specifically, we predicted that wood frog (Rana sylvatica) larvae from closed canopy ponds would be more strongly temperature selective and would prefer higher temperatures than conspecifics from populations found in open canopy ponds. In a study of six breeding ponds in north‐eastern Connecticut, USA, these predictions were upheld. The countergradient, microgeographical variation in thermal preference documented here implies that wood frog populations may have diverged rapidly in the face of contrasting selection pressures. Rapid, behaviourally mediated responses to changing thermal environments have important implications for understanding population responses to climate change.     

Quo vadis amphibia? Global warming and breeding phenology in frogs,toads and salamanders

As the earth is getting warmer, many animals and plants have shifted their timing of breeding towards earlier dates. However, there is substantial variation between populations in phenological shifts that typically goes unexplained. Identification of the different location and species characteristics that drive such variable responses to global warming is crucial if we are to make predictions for how projected climate change scenarios will play out on local and global scales. Here we conducted a phylogenetically controlled meta‐analysis of breeding phenology across frogs, toads and salamanders to examine the extent of variation in amphibian breeding phenology in response to global climate change. We show that there is strong geographic variation in response to global climate change, with species at higher latitudes exhibiting a more pronounced shift to earlier breeding than those at lower latitudes. Our analyses suggest that this latitude effect is a result of both the increased temperature (but not precipitation) at higher latitudes as well as a greater responsiveness by northern populations of amphibians to this change in temperature. We suggest that these effects should reinforce any direct effect of increasing warming at higher latitudes on breeding phenology. In contrast, we found very little contribution from other location factors or species traits. There was no evidence for a phylogenetic signal on advancing breeding phenology or responsiveness to temperature, suggesting that the amphibians that have been studied to date respond similarly to global warming.     

Phenological delay despite warming in wood frog Rana sylvatica reproductive timing: a 20-year study

Across all taxa, amphibians exhibit some of the strongest phenological shifts in response to climate change. As climates warm, amphibians and other animals are expected to breed earlier in response to temperature cues. However, if species use fixed cues such as daylight, their breeding timing might remain fixed, potentially creating disconnects between their life history and environmental conditions. Wood frogs Rana sylvatica are a cold-adapted species that reproduce in early spring, immediately after breeding ponds are free of ice. We used long-term surveys of wood frog oviposition timing in 64 breeding ponds over 20 yr to show that, despite experiencing a warming of 0.29°C per decade in annual temperature, wood frog breeding phenology has shifted later by 2.8 d since 2000 (1.4 d per decade; 4.8 d per °C). This counterintuitive pattern is likely the result of changes in the timing of snowpack accumulation and melting. Finally, we used relationships between climate and oviposition between 2000 and 2018 to hindcast oviposition dates from climate records to model longer-term trends since 1980. Our study indicates that species can respond to fine-grained seasonal climate heterogeneity within years that is not apparent or counterintuitive when related to annual trends across years.     

Use of intraspecific variation in thermal responses for estimating an elevational cline in the timing of cold hardening in a sub‐boreal conifer

To avoid winter frost damage, evergreen coniferous species develop cold hardiness with suitable phenology for the local climate regime. Along the elevational gradient, a genetic cline in autumn phenology is often recognised among coniferous populations, but further quantification of evolutionary adaptation related to the local environment and its responsible signals generating the phenological variation are poorly understood. We evaluated the timing of cold hardening among populations of Abies sachalinensis, based on time series freezing tests using trees derived from four seed source populations × three planting sites. Furthermore, we constructed a model to estimate the development of hardening from field temperatures and the intraspecific variations occurring during this process. An elevational cline was detected such that high‐elevation populations developed cold hardiness earlier than low‐elevation populations, representing significant genetic control. Because development occurred earlier at high‐elevation planting sites, the genetic trend across elevation overlapped with the environmental trend. Based on the trade‐off between later hardening to lengthen the active growth period and earlier hardening to avoid frost damage, this genetic cline would be adaptive to the local climate. Our modelling approach estimated intraspecific variation in two model components: the threshold temperature, which was the criterion for determining whether the trees accumulated the thermal value, and the chilling requirement for trees to achieve adequate cold hardiness. A higher threshold temperature and a lower chilling requirement could be responsible for the earlier phenology of the high‐elevation population. These thermal responses may be one of the important factors driving the elevation‐dependent adaptation of A. sachalinensis.     

Spatial and temporal variation in breeding parameters of two south-temperate populations of House Wrens

Studies of variation in breeding parameters are often based on temporal analyses of a single population. However, to differentiate between the effects of regional and local factors, neighboring populations with limited interpopulational dispersal need to be compared. We studied two nearby (< 5 km apart) populations of House Wrens (Troglodytes aedon bonariae) at two ranches (Los Zorzales, 10 years; La Esperanza, 13 years) in south-temperate Argentina to assess the possible effects of regional and local factors on breeding phenology. For each breeding season, we recorded laying dates, clutch sizes, and length of the breeding season, and estimated the reproductive synchrony of first and second breeding attempts. We examined how these breeding parameters were affected by weather, population density, and rates of nest failure. With favorable temperatures during the pre-reproductive period (September–October), wrens in both populations initiated first breeding attempts earlier. However, ordinal laying dates were also affected by local factors, with wrens at Los Zorzales initiating breeding attempts earlier than those at La Esperanza. We found a spatial correlation in clutch sizes between populations for the 2007–2012 breeding seasons, but clutch sizes of first and second nesting attempts showed low variability. Reproductive synchrony of first nesting attempts varied among years, suggesting an effect of regional factors. However, we detected no synchronization between populations and were unable to identify environmental variables that explained the temporal variation. Ordinal laying dates of second clutches were strongly correlated with the ordinal laying dates of first clutches. We also found that the length of breeding seasons was longer when daily nest mortality rates were lower. Although environmental factors seemed to affect the decision of when to start breeding, pairs with successful first nesting attempts were more likely to initiate second nests, thus affecting the length of the breeding season. The spatial variation and temporal variation of the breeding parameters of House Wrens in our study provide evidence of marked plasticity in their breeding decisions and allowed us to identify local and regional environmental factors related to this variation.     

Limited influence of local and landscape factors on finescale gene flow in two pond‐breeding amphibians

Dispersal and gene flow within animal populations are influenced by the composition and configuration of the landscape. In this study, we evaluated hypotheses about the impact of natural and anthropogenic factors on genetic differentiation in two amphibian species, the spotted salamander (Ambystoma maculatum) and the wood frog (Lithobates sylvaticus) in a commercial forest in central Maine. We conducted this analysis at two scales: a local level, focused on factors measured at each breeding pond, and a landscape level, focused on factors measured between ponds. We investigated the effects of a number of environmental factors in six categories including Productivity, Physical, Land Composition, Land Configuration, Isolation and Location. Embryos were sampled from 56 spotted salamander breeding ponds and 39 wood frog breeding ponds. We used a hierarchical Bayesian approach in the program GESTE at each breeding pond and a random forest algorithm in conjunction with a network analysis between the ponds. We found overall high genetic connectivity across distances up to 17 km for both species and a limited effect of natural and anthropogenic factors on gene flow. We found the null models best explained patterns of genetic differentiation at a local level and found several factors at the landscape level that weakly influenced gene flow. This research indicates multiscale investigations that incorporate local and landscape factors are valuable for understanding patterns of gene flow. Our findings suggest that dispersal rates in this system are high enough to minimize genetic structuring and that current forestry practices do not significantly impede dispersal.     

Habitat specialization and adaptive phenotypic divergence of anuran populations

We tested for adaptive population structure in the frog Rana temporaria by rearing tadpoles from 23 populations in a common garden experiment, with and without larval dragonfly predators. The goal was to compare tadpole phenotypes with the habitats of their source ponds. The choice of traits and habitat variables was guided by prior information about phenotypic function. There were large differences among populations in life history, behaviour, morphological shape, and the predator-induced plasticities in most of these. Body size and behaviour were correlated with predation risk in the source pond, in agreement with adaptive population divergence. Tadpoles from large sunny ponds were morphologically distinct from those inhabiting small woodland ponds, although here an adaptive explanation was unclear. There was no evidence that plasticity evolves in populations exposed to more variable environments. Much among-population variation in phenotype and plasticity was not associated with habitat, perhaps reflecting rapid changes in wetland habitats.     

Restoring ponds for amphibians: a success story

Large-scale restoration of quality habitats is often considered essential for the recovery of threatened pond-breeding amphibians but only a few successful cases are documented, so far. We describe a landscape-scale restoration project targeted at two declining species—the crested newt (Triturus cristatus Laur.) and the common spadefoot toad (Pelobates fuscus Wagler)—in six protected areas in southern and southeastern Estonia. The ponds were restored or created in clusters to (i) increase the density and number of breeding sites at local and landscape levels; (ii) provide adjacent ponds with differing depths, hydroperiods and littoral zones and (iii) restore an array of wetlands connected to appropriate terrestrial habitat. In only 3 years, where 22 of the 405 existing ponds (5%) were restored and 208 new ponds (51%) created, the number of ponds occupied by the common spadefoot toad increased 6.5 times. Concerning the crested newt and the moor frog (Rana arvalis Nilsson), the increase was 2.3 and 2.5 times, respectively. The target species had breeding attempts in most of the colonised ponds—even more frequently than common species. Also, the amphibian species richness was higher in the restored than in the untreated ponds. The crested newt preferably colonised ponds that had some submerged vegetation and were surrounded by forest or a mosaic of forest and open habitats. The common spadefoot toad favoured ponds having clear and transparent water. Our study reveals that habitat restoration for threatened pond-breeding amphibians can rapidly increase their numbers if the restoration is implemented at the landscape scale, taking into account the habitat requirements of target species and the ecological connectivity of populations. When the remnant populations are strong enough, translocation of individuals may not be necessary. Guest editors: B. Oertli, R. Cereghino, A. Hull & R. Miracle Pond Conservation: From Science to Practice. 3rd Conference of the European Pond Conservation Network, Valencia, Spain, 14–16 May 2008     

Arrival and onset of breeding of three passerine birds in eastern Finland tracks climatic variation and phenology of insects

Anthropogenic climate change poses a challenge to the annual cycles of migratory birds. It has become urgent to understand whether migratory birds are able to advance their spring phenology when the climate is warming and whether they are able to adjust these phenological phases to the spring phenology in their breeding areas. In this work, we studied long‐term trends in first arrival and onset of breeding for three passerine birds in eastern Finland; the pied flycatcher Ficedula hypoleuca, the common redstart Phoenicurus phoenicurus and the great tit Parus major. The pied flycatcher and the common redstart are long‐distance migrants while the great tit is a partial migrant in Finland. We asked what environmental variables best explain the first arrival or onset of breeding, if there is evidence of ‘thermal delay’ (long‐term increase in the accumulated temperatures) at arrival or onset of breeding and if the interannual variation in the onset of breeding correlates with variation in spring phenology of local insects. We found that the pied flycatcher and the common redstart had advanced their first arrival (explained by increased temperatures at the migration route), but we found no long‐term change in the onset of breeding (explained by local temperatures). Also, the onset of breeding of the great tit is tracking local temperatures. We found no or only weak evidence of thermal delay at arrival or onset of breeding for any of the species. The onsets of breeding for the pied flycatcher and the great tit are also closely tracking the spring phenology of the local insects. The stable or increasing population sizes of all three species in Finland could be a result from their ability to effectively track climatic and environmental variation.     

Tracking ice phenology by migratory waterbirds: settling phenology and breeding success of species with divergent population trends

Dependence on climate‐driven environmental cues in the initiation of life cycle stages is a critical attribute when assessing vulnerability of species to climate change impacts. This study focused on spring ice phenology as a cue to the settling of migratory waterbirds, asking whether there is an asynchrony between ice phenology and settling phenology that could affect breeding success of six species with divergent population trends. In the 37 study lakes in southeastern Finland, the ice‐out date not only varied considerably between years, but became progressively earlier during the study period, 1991–2018. Settling phenology of all species tracked inter‐annual variation in ice phenology. However, the degree of asynchrony between ice phenology and settling phenology varied between species, allowing discrimination between early and late settlers. Considerable inter‐annual variation also occurred within species, but in only one species did the degree of asynchrony correlate with the ice‐out date: for the horned grebe Podiceps auritus an earlier ice‐out date meant greater asynchrony between settling phenology and ice phenology. The degree of asynchrony between settling phenology and ice phenology did not affect breeding success in any species. However, ice phenology per se affected breeding success of horned grebes: earlier ice‐out was associated with lower annual breeding success. Breeding numbers of horned grebe showed a long‐term decline. Results suggest that short‐distance migratory birds are able to respond to climate change‐driven phenological changes in their breeding environments, and that this ability may not depend on the relative timing of breeding.     

Genetic diversity and fitness in peripheral and central populations of the European tree frog Hyla arborea

Genetic diversity is expected to decrease in small and isolated populations as a consequence of founder effects, bottlenecks, inbreeding and genetic drift. In this study we analyse temporal and spatial effects on genetic variation and progeny viability of the European tree frog (Hyla arborea) at two scales. First, the Swedish distribution has been isolated from the continental distribution for more than 8000 thousand years, and secondly, within Sweden, recent habitat alterations that have taken place during this century have increased isolation between local populations. Genetic variation and progeny survival in relation to isolation was studied within the entire Swedish distribution of the tree frog. Allozyme electrophoresis analysis of froglets, sampled across the Swedish distribution, revealed a low overall genetic variation (1.06 alleles/locus) at the protein level in comparison with continental populations (1.54-1.68 alleles/locus). However, egg hatchability (97%) and early larval survival (95%) were not lower than in other parts of the tree frog distribution or in other anuran species. Within the Swedish distribution, early larval survival was lower in isolated breeding ponds than in more central ones. However, no differences in genetic variation were found in relation to isolation. Polymorphism was detected only at a single locus, and was restricted geographically to the eastern part of the Swedish distribution. Bottlenecks due to climatic changes and fragmentation of suitable habitat (primarily natural pastures with ponds) are suggested as possible causes of the low genetic diversity of the Swedish tree frog population.