Range Margins Research Articles

Long-term stress level in a small mammal species undergoing range expansion

Abstract Rapid climate change is currently altering species distribution ranges. Evaluating the long-term stress level in wild species undergoing range expansion may help better understanding how species cope with the changing environment. Here, we focused on the white-footed mouse (Peromyscus leucopus), a widespread small mammal species in North-America whose distribution range is rapidly shifting northward. We evaluated long-term stress level in several populations of P. leucopus in Quebec (Canada), from the northern edge of the species distribution to more core populations in Southern Quebec. We first tested the hypothesis that populations at the range margin are under higher stress than more established populations in the southern region of our study area. We then compared four measures of long-term stress level to evaluate the congruence between these commonly used methods. We did not detect any significant geographical trend in stress level across our study populations of P. leucopus. Most notably, we found no clear congruence between the four measures of stress level we used, and conclude that these four commonly used methods are not equivalent, thereby not comparable across studies.

Crustal Structure of the UAE‐Oman Mountain Range and Arabian Rifted Passive Margin: New Constraints From Active and Passive Seismic Methods

AbstractThe Semail ophiolite, a thick thrust sheet of Late Cretaceous oceanic crust and upper mantle, was obducted onto the previously rifted Arabian continental margin in the Late Cretaceous, and now forms part of the United Arab Emirates (UAE)‐Oman mountain belt. A deep foreland basin along the west and SW margin of the mountains developed during the obduction process, as a result of flexure due to loading of the ophiolite and underlying thrust sheets. The nature of the crust beneath the deep sedimentary basins that flank the mountain belt, and the extent to which the Arabian continental crust has thickened due to the obduction process are outstanding questions. We use a combination of active‐ and passive‐source seismic data to constrain the stratigraphy, velocity structure and crustal thickness beneath the UAE‐Oman mountains and its bounding basins. Depth‐migrated multichannel seismic reflection profile data are integrated in the modeling of traveltimes from long offset reflections and refractions, which are used to resolve the crustal thickness and velocity structure along two E‐W onshore/offshore transects in the UAE. Additionally, we apply the virtual deep seismic sounding method to distant earthquake data recorded along the two transects to image crustal thickness variations. Active seismic methods define the Semail ophiolite as a high‐velocity body dipping to the east at 40°–45°. The new crustal thickness model presented in this work provides evidence that a crustal root is present beneath the Semail ophiolite, suggesting that folding and thrusting during the obduction process may have thickened the pre‐existing crust by 16 km.

Genomic consequences of human-mediated translocations in margin populations of an endangered amphibian.

Due to their isolated and often fragmented nature, range margin populations are especially vulnerable to rapid environmental change. To maintain genetic diversity and adaptive potential, gene flow from disjunct populations might therefore be crucial to their survival. Translocations are often proposed as a mitigation strategy to increase genetic diversity in threatened populations. However, this also includes the risk of losing locally adapted alleles through genetic swamping. Human‐mediated translocations of southern lineage specimens into northern German populations of the endangered European fire‐bellied toad (Bombina bombina) provide an unexpected experimental set‐up to test the genetic consequences of an intraspecific introgression from central population individuals into populations at the species range margin. Here, we utilize complete mitochondrial genomes and transcriptome nuclear data to reveal the full genetic extent of this translocation and the consequences it may have for these populations. We uncover signs of introgression in four out of the five northern populations investigated, including a number of introgressed alleles ubiquitous in all recipient populations, suggesting a possible adaptive advantage. Introgressed alleles dominate at the MTCH2 locus, associated with obesity/fat tissue in humans, and the DSP locus, essential for the proper development of epidermal skin in amphibians. Furthermore, we found loci where local alleles were retained in the introgressed populations, suggesting their relevance for local adaptation. Finally, comparisons of genetic diversity between introgressed and nonintrogressed northern German populations revealed an increase in genetic diversity in all German individuals belonging to introgressed populations, supporting the idea of a beneficial transfer of genetic variation from Austria into North Germany.

Improving predictions of range expansion for invasive species using joint species distribution models and surrogate co‐occurring species

AbstractAimsSpecies distribution models (SDMs) are often used to forecast potential distributions of important invasive or rare species. However, situations where models could be the most valuable ecologically or economically, such as for predicting invasion risk, often pose the greatest challenges to SDM building. These challenges include non‐equilibrium range expansion, low or uneven prevalence, and projecting distributions into environments that are non‐analogous to the environments used for model building.LocationMinnesota, USA.TaxonCardamine impatiens (Narrowleaf Bittercress), Celastrus orbiculatus (Oriental Bittersweet), and Humulus japonicus (Japanese Hops).MethodsWe took a novel approach to build robust species distribution models of invasive species using occurrence‐environment correlations between invasive species and co‐occurring native community members. The correlations were obtained from a joint species distribution model (JSDM) of a densely sampled database of 10,336 MN plant communities from across the state of Minnesota, USA. Positively and negatively associated native species were incorporated into the model as surrogate presences and pseudoabsences (weighted by their environmental correlations) along with invasive species occurrences records (surrogate SDMs).ResultsSurrogate models performed better than traditional SDMs in predicting occurrences along the northern invasion margin (outside the training area). Both types of models had similarly high cross‐validation metrics in the area of training. Surrogate models also predicted greater range expansion beyond the current geographic range.Main conclusionsThese results demonstrate that modelers can take advantage of detailed community data to develop SDMs that leverage surrogate native species as phytometers of environments beyond the current area of occupancy. The additional information in surrogate models generates highly effective predictions of invasive species along expanding range margins.

Calibrated uncertainty estimation for interpretable proton computed tomography image correction using Bayesian deep learning

Integrated-type proton computed tomography (pCT) measures proton stopping power ratio (SPR) images for proton therapy treatment planning, but its image quality is degraded due to noise and scatter. Although several correction methods have been proposed, techniques that include estimation of uncertainty are limited. This study proposes a novel uncertainty-aware pCT image correction method using a Bayesian convolutional neural network (BCNN). A DenseNet-based BCNN was constructed to predict both a corrected SPR image and its uncertainty from a noisy SPR image. A total 432 noisy SPR images of 6 non-anthropomorphic and 3 head phantoms were collected with Monte Carlo simulations, while true noise-free images were calculated with known geometric and chemical components. Heteroscedastic loss and deep ensemble techniques were performed to estimate aleatoric and epistemic uncertainties by training 25 unique BCNN models. 200-epoch end-to-end training was performed for each model independently. Feasibility of the predicted uncertainty was demonstrated after applying two post-hoc calibrations and calculating spot-specific path length uncertainty distribution. For evaluation, accuracy of head SPR images and water-equivalent thickness (WET) corrected by the trained BCNN models was compared with a conventional method and non-Bayesian CNN model. BCNN-corrected SPR images represent noise-free images with high accuracy. Mean absolute error in test data was improved from 0.263 for uncorrected images to 0.0538 for BCNN-corrected images. Moreover, the calibrated uncertainty represents accurate confidence levels, and the BCNN-corrected calibrated WET was more accurate than non-Bayesian CNN with high statistical significance. Computation time for calculating one image and its uncertainties with 25 BCNN models is 0.7 s with a consumer grade GPU. Our model is able to predict accurate pCT images as well as two types of uncertainty. These uncertainties will be useful to identify potential cause of SPR errors and develop a spot-specific range margin criterion, toward elaboration of uncertainty-guided proton therapy.

Abandonment of traditional land use and climate change threaten the survival of an endangered relict butterfly species

Abstract The decline of butterflies exceeds those of many other animal taxa due to their high sensitivity to habitat alterations driven by land‐use change. Moreover, cold‐adapted species frequently suffer severe range retractions due to rising temperatures at their trailing‐edge range margins. In this study, we aim to identify drivers of occupancy of the post‐glacial relict species Lycaena helle at three spatial scales – (i) landscape, (ii) habitat, and (iii) microhabitat – in one of its last refuges in central Europe. In our study in the Eifel low mountain range (western Germany), the occurrence of L. helle was mainly driven by the (i) isolation, (ii) size, and (iii) quality of habitat patches. Lycaena helle formed metapopulations that were dependent on networks of interconnected but often small habitat patches. Habitat quality within the semi‐natural grasslands was determined by (i) macro‐ and mesoclimate, (ii) host‐plant abundance, and (iii) vegetation structure, which was interrelated with microclimate. Lycaena helle preferred moist, nutrient‐poor grasslands in deep, narrow valleys at the highest elevations of the study area, which were characterised by (i) cold winters, (ii) high abundance of the host plant, and (iii) short and sparse swards providing a warm microclimate in summer. According to these findings, abandonment of traditional land use and climate change are considered the most severe threats for long‐term survival of the species. Hence, conservation measures should aim at maintaining and restoring networks of large and well‐connected habitat patches of high quality, preferably in cold‐air depressions within mountain systems.

Illuminating the intrinsic and extrinsic drivers of ecological stability across scales

AbstractOur knowledge of ecological stability is built on assumptions of scale. These assumptions limit our ability to reach a generalizable and mechanistic understanding of stability under global environmental change. Moving towards a multiscale approach—across space, time and environment—will allow us to better understand the intrinsic (e.g., demographic) and extrinsic (environmental) drivers of ecological stability. In this perspective, we review multiple sources of variation responsible for shaping ecological dynamics, and how scale affects our observation of these dynamics through its confounding effect on drivers of variation in ecosystems. We discuss the effect of temporal scale when combining empirical dynamic modeling with high‐resolution population time series to consider the time‐varying nature of multispecies interaction networks, highlighting interspecific interactions as an intrinsic driver of community dynamics. Next, we examine energy landscape analysis as a method for inferring stability and transience during community assembly and its interaction with spatial scale, emphasizing the intrinsic role of compositional variability in assembly dynamics. We then examine population dynamics at species' range margins and show how considering the interaction between spatial and temporal environmental heterogeneity, an extrinsic driver of population dynamics, can facilitate a nuanced understanding of population expansions, range shifts, and species invasions. Finally, we discuss broadly how the sources of intrinsic and extrinsic variation interact with each other and with spatiotemporal scale to shape ecological dynamics. Better recognition of the scale‐dependent nature of the relationship between drivers of variation and ecological dynamics will be invaluable to illuminate the dynamics influencing ecological stability across scales.

Climate extreme variables generated using monthly time‐series data improve predicted distributions of plant species

Extreme weather can have significant impacts on plant species demography; however, most studies have focused on responses to a single or small number of extreme events. Long‐term patterns in climate extremes, and how they have shaped contemporary distributions, have rarely been considered or tested. BIOCLIM variables that are commonly used in correlative species distribution modelling studies cannot be used to quantify climate extremes, as they are generated using long‐term averages and therefore do not describe year‐to‐year, temporal variability. We evaluated the response of 37 plant species to base climate (long‐term means, equivalent to BIOCLIM variables), variability (standard deviations) and extremes of varying return intervals (defined using quantiles) based on historical observations. These variables were generated using fine‐grain (approx. 250 m), time‐series temperature and precipitation data for the hottest, coldest and driest months over 39 years. Extremes provided significant additive improvements in model performance compared to base climate alone and were more consistent than variability across all species. Models that included extremes frequently showed notably different mapped predictions relative to those using base climate alone, despite often small differences in statistical performance as measured as a summary across sites. These differences in spatial patterns were most pronounced at the predicted range margins, and reflect the influence of coastal proximity, continentality, topography and orographic barriers on climate extremes. Species occupying hotter and drier locations that are exposed to severe maximum temperature extremes were associated with better predictive performance when modelled using extremes. Understanding how plant species have historically responded to climate extremes may provide valuable insights into our understanding of contemporary distributions and help to make more accurate predictions under a changing climate.

Plant–soil interactions limit lifetime fitness outside a native plant’s geographic range margin

Plant species' distributions are often thought to overwhelmingly reflect their climatic niches. However, climate represents only a fraction of the n-dimensional environment to which plant populations adapt, and studies are increasingly uncovering strong effects of nonclimatic factors on species' distributions. We used a manipulative, factorial field experiment to quantify the effects of soil environment and precipitation (the putatively overriding climatic factor) on plant lifetime fitness outside the geographic range boundary of a native California annual plant. We grew plants outside the range edge in large mesocosms filled with soil from either within or outside the range, and plants were subjected to either a low (ambient) or high (supplemental) spring precipitation treatment. Soil environment had large effects on plant lifetime fitness that were similar in magnitude to the effects of precipitation. Moreover, mean fitness of plants grown with within-range soil in the low precipitation treatment approximated that of plants grown with beyond-range soil in the high precipitation treatment. The positive effects of within-range soil persisted in the second, wetter year of the experiment, though the magnitude of the soil effect was smaller than in the first, drier year. These results are the first we know of to quantify the effects of edaphic variation on plant lifetime fitness outside a geographic range limit and highlight the need to include factors other than climate in models of species' distributions.

The genetic diversity ofPaeonia anomala(Paeoniaceae), as indicated by nuclear ITS and plastid ycf1 molecular markers

Variability of nuclear ITS rDNA and plastid ycf1 gene was studied to elucidate genetic diversity ofPaeonia anomala(Paeoniaceae), which possesses the most extensive range among all species of the genus. Sixteen populations were sampled across the entire distribution range of the species. The principal coordinate analysis suggested the existence of three ycf1 haplotype groups distributed in distinct geographical locations. For ITS region, no variability throughout the range was found; only samples ofP. anomalafrom its north-western range margin (the Kola Peninsula) are characterized by unique nucleotide substitutions.

Drivers of large‐scale spatial demographic variation in a perennial plant

AbstractTo understand how the environment drives spatial variation in population dynamics, we need to assess the effects of a large number of potential drivers on vital rates (survival, growth, and reproduction) and explore these relationships over large geographical areas and broad environmental gradients. In this study, we examined the effects of a wide variety of abiotic and biotic environmental factors on the demography of the forest understory herb Actaea spicata between 2017 and 2019 at 40 sites across Sweden, including the northern range margin of its distribution. We assessed the effects of potential environmental drivers on vital rates using generalized linear mixed models (GLMMs) and then quantified the impact of each important driver on population growth rate (λ) using integral projection models (IPMs). Population dynamics of A. spicata were mostly driven by environmental factors affecting survival and growth, such as air humidity, soil depth, and forest tree species composition, and thus, those drivers jointly determined the realized niche of the species. Soil pH had a strong effect on the flowering probability, while the effect on λ was relatively small. In addition to identifying specific drivers for A. spicata’s population dynamics, our study illustrates the impact that spatial variation in environmental conditions can have on λ. Assessing the effects of a broad range of potential drivers, as done in this study, is important not only to quantify the relative importance of different drivers for population dynamics but also to understand species distributions and abundance patterns.

Fitness effects of mutation in natural populations of Arabidopsis thaliana reveal a complex influence of local adaptation.

Little is empirically known about the contribution of mutations to fitness in natural environments. However, Fisher's Geometric Model (FGM) provides a conceptual foundation to consider the influence of the environment on mutational effects. To quantify mutational properties in the field, we established eight sets of MA lines (7-10 generations) derived from eight founders collected from natural populations of Arabidopsis thaliana from French and Swedish sites, representing the range margins of the species in Europe. We reciprocally planted the MA lines and their founders at French and Swedish sites, allowing us to test predictions of FGM under naturally occurring environmental conditions. The performance of the MA lines relative to each other and to their respective founders confirmed some and contradicted other predictions of the FGM: the contribution of mutation to fitness variance increased when the genotype was in an environment where its fitness was low, that is, in the away environment, but mutations were more likely to be beneficial when the genotype was in its home environment. Consequently, environmental context plays a large role in the contribution of mutations to the evolutionary process and local adaptation does not guarantee that a genotype is at or close to its optimum.

Drought sensitivity of Empetrum nigrum shrub growth at the species\u2019 southern lowland distribution range margin

The ongoing warming of the Earth’s atmosphere is projected to cause a northward shift of species’ distributions, as they track their climatic optimum. In the rapidly warming Arctic, this has already led to an increase of shrubs in tundra ecosystems. While this northern expansion of woody biomass has been studied relatively extensively over the last decade, little research has been devoted to shrub growth responses at the southern margins of Northern Hemisphere shrubs. Here, we studied shoot length growth, its responses to climate over the period 2010–2017, and differences in leaf C and N content of the evergreen dwarf shrub Empetrum nigrum, as well as the vegetation composition and soil parameters at four sites located along a gradient of increasing dune age on the island Spiekeroog, northern Germany. The sites are located in the tri-national UNESCO world heritage site, the Wadden Sea. E. nigrum has a predominantly circum-arctic-boreal distribution and its southern distribution margin in European lowlands runs through northern Germany, where it is retreating northwards. We found a negative response to autumn (surface) temperatures and previous summer surface temperatures and/or a positive response to summer precipitation of E. nigrum growth, except at the oldest dune with the strongest E. nigrum dominance. Growth rates and plant species diversity declined with dune age. Our results suggest that E. nigrum growth is drought sensitive at its European southern range margin. We hypothesize that this sensitivity may form the basis for its northward retreat, which is supported by recent observations of E. nigrum dieback in Germany after the extreme drought in 2018 and model projections.

Microbes, mutualism, and range margins: testing the fitness consequences of soil microbial communities across and beyond a native plant's range.

Interactions between plants and soil fungi and bacteria are ubiquitous and have large effects on individual plant fitness. However, the degree to which spatial variation in soil microbial communities modulates plant species' distributions remains largely untested. Using the California native plant Clarkia xantiana ssp. xantiana we paired glasshouse and field reciprocal transplants of plant populations and soils to test whether plant-microbe interactions affect the plant's geographic range limit and whether there is local adaptation between plants and soil microbe communities. In the field and glasshouse, one of the two range interior inocula had a positive effect on plant fitness. In the field, this benefit was especially pronounced at the range edge and beyond, suggesting possible mutualist limitation. In the glasshouse, soil inocula from beyond-range tended to increase plant growth, suggesting microbial enemy release beyond the range margin. Amplicon sequencing revealed stark variation in microbial communities across the range boundary. Plants dispersing beyond their range limit are likely to encounter novel microbial communities. In C.x.xantiana, our results suggest that range expansion may be facilitated by fewer pathogens, but could also be hindered by a lack of mutualists. Both negative and positive plant-microbe interactions will likely affect contemporary range shifts.

An experimental test of the Allee effect range limitation hypothesis.

Understanding how climate change impacts trailing‐edge populations requires information about how abiotic and biotic factors limit their distributions. Theory indicates that socially mediated Allee effects can limit species distributions by suppressing growth rates of peripheral populations when social information is scarce.The goal of our research was to determine if socially mediated Allee effects limit the distribution of Canada warbler Cardellina canadensis at the trailing‐edge of the geographic range.Using 4 years of observational data from 71 sites and experimental data at 10 sites, we tested two predictions of the socially mediated range limitation hypothesis: (a) local growth rates should be positively correlated with local density and (b) the addition of social cues immediately outside the trailing‐edge range boundary would result in colonization of formerly unoccupied habitat and increased growth rates. During the third breeding season, social cues were experimentally added at 10 formerly unoccupied sites within and beyond the species’ local range margin to determine if the addition of social information could increase density and effectively expand the species’ range.No experimental sites were colonized after adding social cues and no evidence of Allee effects was found. Rather, temperature, precipitation and negative density dependence strongly influenced population growth rates.Although theoretical models indicate that the presence of socially mediated Allee effects at species range boundaries could increase the rate of climate‐induced range shifts and local extinctions, empirical results from the first test of this hypothesis suggest that Allee effects play a minimal role in limiting species’ distributions.

Genetic and demographic trends from rear to leading edge are explained by climate and forest cover in a cold‐adapted ectotherm

AbstractAimDetermining whether altitudinal shifts in species distributions leave molecular footprints on wild populations along their range margins from rear to leading edge.LocationSouth‐west France.MethodsWe compared the demographic and genetic variation in 42 wild populations of the Western oviparous subclade B2 of a cold‐adapted lizard (Zootoca vivipara louislantzi). These populations can be divided into four ecological units across altitudinal clines in South‐west France (rear edge: <100 m, admixture zone: 100–500 m, continuous range: 500–1,300 m and leading edge: >1,300 m above sea level).ResultsWithin the rear edge were found the highest levels of inbreeding, genetic differentiation and evidence of interrupted gene flow compared to central or colonizing areas. Within the leading edge, altitudinal range expansion occurred over the last centuries and populations showed relatively low genetic diversity. These demographic and genetic trends were better explained by inhospitable (warm and dry) climate conditions and forest cover.Main conclusionsThis empirical evidence illustrates that molecular footprints of climate conditions and habitat quality on wild population trends can be perceived after recent events, which should be of particular importance to accurately understand and anticipate human‐induced global change on wild species and ecosystems.

Ecological Factors Generally Not Altitude Related Played Main Roles in Driving Potential Adaptive Evolution at Elevational Range Margin Populations of Taiwan Incense Cedar (Calocedrus formosana).

Population diversification can be shaped by a combination of environmental factors as well as geographic isolation interacting with gene flow. We surveyed genetic variation of 243 samples from 12 populations of Calocedrus formosana using amplified fragment length polymorphism (AFLP) and scored a total of 437 AFLP fragments using 11 selective amplification primer pairs. The AFLP variation was used to assess the role of gene flow on the pattern of genetic diversity and to test environments in driving population adaptive evolution. This study found the relatively lower level of genetic diversity and the higher level of population differentiation in C. formosana compared with those estimated in previous studies of conifers including Cunninghamia konishii, Keteleeria davidiana var. formosana, and Taiwania cryptomerioides occurring in Taiwan. BAYESCAN detected 26 FST outlier loci that were found to be associated strongly with various environmental variables using multiple univariate logistic regression, latent factor mixed model, and Bayesian logistic regression. We found several environmentally dependent adaptive loci with high frequencies in low- or high-elevation populations, suggesting their involvement in local adaptation. Ecological factors, including relative humidity and sunshine hours, that are generally not altitude related could have been the most important selective drivers for population divergent evolution in C. formosana. The present study provides fundamental information in relation to adaptive evolution and can be useful for assisted migration program of C. formosana in the future conservation of this species.

Resident vegetation modifies climate-driven elevational shift of a mountain sedge

Mountain plant species are changing their ranges in response to global warming. However, these shifts vary tremendously in rate, extent and direction. The reasons for this variation are yet poorly understood. A process potentially important for mountain plant re-distribution is a competition between colonizing species and the resident vegetation. Here, we focus on the impact of this process using the recent elevational shift of the sedge Carex humilis in the northern Italian Alps as a model system. We repeated and extended historical sampling (conducted in 1976) of the species in the study region. We used the historical distribution data and historical climatic maps to parameterize a species distribution model (SDM) and projected the potential distribution of the species under current conditions. We compared the historical and the current re-survey for the species in terms of the cover of important potential competitor species as well as in terms of the productivity of the resident vegetation indicated by the Normalized Difference Vegetation Index (NDVI). We found that Carex humilis has shifted its leading range margin upward rapidly (51.2 m per decade) but left many sites that have become climatically suitable since 1976 according to the SDM uncolonized. These suitable but uncolonized sites show significantly higher coverage of all dwarf shrub species and higher NDVI than the sites occupied by the sedge. These results suggest that resistance of the resident vegetation against colonization of migrating species can indeed play an important role in controlling the re-distribution of mountain plants under climate change.

Elevation alters outcome of competition between resident and range-shifting species.

Species' geographic range shifts toward higher latitudes and elevations are among the most frequently reported consequences of climate change. However, the role of species interactions in setting range margins remains poorly understood. We used cage experiments in ponds to test competing hypotheses about the role of abiotic and biotic mechanisms for structuring range boundaries of an upslope range-shifting caddisfly Limnephilus picturatus. We found that competition with a ubiquitous species Limnephilus externus significantly decreased L. picturatus survival and emergence at subalpine elevations supporting the notion that species interactions play a critical role in determining upslope range limits. However, without competitors, L. picturatus survival was greater at high-elevation than low-elevation sites. This was contrary to decreases in body mass (a proxy for fecundity) with elevation regardless of the presence of competitors. We ultimately show that species interactions can be important for setting upslope range margins. Yet, our results also highlight the complications in defining what may be abiotically stressful for this species and the importance of considering multiple demographic variables. Understanding how species ranges will respond in a changing climate will require quantifying species interactions and how they are influenced by the abiotic context in which they play out.

What Do We Really Know About Adaptation at Range Edges?

Recent theory and empirical evidence have provided new insights regarding how evolutionary forces interact to shape adaptation at stable and transient range margins. Predictions regarding trait divergence at leading edges are frequently supported. However, declines in fitness at and beyond edges show that trait divergence has sometimes been insufficient to maintain high fitness, so identifying constraints to adaptation at range edges remains a key challenge. Indirect evidence suggests that range expansion may be limited by adaptive genetic variation, but direct estimates of genetic constraints at and beyond range edges are still scarce. Sequence data suggest increased genetic load in edge populations in several systems, but its causes and fitness consequences are usually poorly understood. The balance between maladaptive and positive effects of gene flow on fitness at range edges deserves further study. It is becoming increasingly clear that characterizations about degree of adaptation based solely on geographical peripherality are unsupported.