Abstract Changes in habitat structure across species’ distributions may contribute to the generation and maintenance of range limits, but few studies have evaluated this by directly measuring habitat availability across relevant spatial scales. Here, we test the predictions that coarse-scale and patch-level habitat availability decline towards and beyond the northern range limit of Pacific coastal dune endemic Camissoniopsis cheiranthifolia . We used aerial imagery and geographic information system (GIS) tools to measure the coarse-scale availability of coastal dune habitat in California and Oregon. The availability of finer-scale habitat patches specifically suitable for C. cheiranthifolia was measured in a 2-generation field survey of > 4,200 5m x 5m plots randomly distributed across 1100 km of coastal dune habitat transcending the species northern range limit. At each plot, we estimated the proportion of area that contained suitable habitat as well as recorded occupancy by C. cheiranthifolia . As an alternative approach to visually estimating habitat suitability, we recorded plant community composition at each plot to predict beyond-range habitat suitability using a random forest model. Contrary to our predictions, we found that coastal dune habitat, measured coarsely from aerial imagery, was more abundant and continuous towards and beyond the northern range limit. At the fine scale, however, the proportion of plots with suitable habitat (patch suitability) and the proportion of habitat within plots that was suitable (patch size) declined across the range limit. Moreover, patches were more isolated from one another and, in one survey year, less temporally stable towards and beyond the range limit. Finally, occupancy by C. cheiranthifolia was less likely in smaller, more isolated, and temporally unstable patches, providing mechanistic insight to the previously observed decline in occupancy towards the range limit. Synthesis : Taken together, our results suggest that fine-scale habitat patch configuration changes in ways that likely impede patch colonization, thereby reducing occupancy and limiting the species’ northern distribution. Thus, consideration of geographic variation in patch and landscape structure, rather than only coarse-scale habitat availability, may be essential for understanding the processes that limit species ranges.

Background and Aims Urbanization can alter the interplay of evolutionary processes such as natural selection and genetic drift, but these effects will depend on the biology and history of a species. To explore the influences of drift and selection in an urban context, we investigated patterns of flower colour variation among stands of the introduced ornamental mustard Hesperis matronalis along an urban-rural gradient in eastern Ontario, Canada.Methods We surveyed 136 naturalized stands of H. matronalis over three generations, and for each stand estimated the diversity of the three colour morphs (white, pink, purple), the number of reproductive plants and the degree of urbanization based on night sky brightness.Key Results Flower colour morph diversity increased with both stand size and urbanization, which is consistent with effects of genetic drift during colonization combined with multiple introductions of this horticultural plant in urban areas. However, the frequency and fixation of the purple morph systematically increased towards the rural end of the gradient. Although lifetime seed production did not vary among morphs, pre-dispersal seed predation by a recent adventive weevil was higher in the purple morph, particularly in rural areas. Estimated seed production in the absence of predation possibly suggests a previous fitness advantage for the purple and pink morphs in rural areas and for the white morph in urban areas.Conclusions Random variation in flower colour diversity may be influenced by stochastic processes and colonization history, while systematic variation in colour morph frequencies may reflect past fitness differences among morphs that have been recently erased by seed predation.

Introduction Proper selection of genetic material is critical for restoring populations, with local seed often selected to maximize local adaptation. But if local populations are small, inbred, or maladapted, then including genotypes from various populations may enhance population growth and long-term adaptation.Objectives Here, we report a novel test of whether planting locally sourced or genetically mixed populations results in higher fitness over single and multiple generations using Camissoniopsis cheiranthifolia, a species widely used in Pacific coastal dune restoration.Methods We tested for benefits of planting local versus nonlocal seed using two reciprocal transplant experiments in unmanipulated dune habitat. In the second experiment, we also compared the fitness of descendants from unmanipulated populations to those from populations where genotypes from genetically differentiated populations were transplanted approximately 10 generations previously.Results Neither transplant experiment revealed any fitness advantage to local genotypes. Moreover, the fitness of transplants did not decrease with increasing geographic or climatic distance between home and planting site, as would be expected under local adaptation. Fitness did not differ between tenth-generation descendants from transplant sites versus plants from unmanipulated sites when planted at home or elsewhere. Naturally occurring plants at transplant sites sometimes produced fewer fruits but were not less dense.Conclusions Our results do not support the assumption that local genotypes are best, or that mixing genotypes increases or decreases fitness over multiple generations. The option to use nonlocal seed may alleviate logistic constraints on seed source for restoration plantings.

Premise of research. Understanding the factors that influence pathogen prevalence is essential to mitigating the negative consequences of disease. Infection prevalence should be influenced by the abundance and distribution of the pathogen's hosts, yet tests of this general expectation from natural populations are few. Furthermore, human activity is profoundly altering species distributions, which may have consequences for the pathogens that are associated with them. Methodology. We investigated whether urbanization influences infection prevalence by, in part, affecting host population size and density in a study that surveyed the occurrence of turnip mosaic virus (TuMV) infection across 132 populations of the invasive mustard plant Hesperis matronalis in Ontario, Canada, along an urban-rural gradient. We scored TuMV infection by the appearance of flower color breaking for a total of similar to 38,000 plants across three generations. Pivotal results. Overall, 39% of populations included at least one infected individual, and 10% of individuals were infected within these populations. As predicted, the probability of population infection increased with human activity, even after controlling the positive effect of population size. Larger populations in areas of high human activity were also more likely to remain infected across generations. The effect of human activity on the infection frequency within populations was less consistent. Within populations, the probability of individuals being infected increased with local density of conspecifics, yet mean density did not influence population infection or infection frequency within infected populations. Conclusions. Our results highlight how urbanization can influence the prevalence of infection by an economically important plant virus.

The metapopulation hypothesis for range limits proposes that geographic variation in a species' extinction from and/or colonisation of habitat can generate an abrupt range limit. We tested whether this contributes to the northern range limit of coastal dune plant Camissoniopsis cheiranthifolia by quantifying suitable habitat area and the rates of extinction and colonisation across 3485 plots throughout the northern half of the species' range. Colonisation of previously unoccupied plots increased with suitable habitat area and abundance in nearby plots and consequently declined towards the range limit. Extinction was more frequent from plots with less habitat and lower initial abundance but did not increase significantly towards the limit. Incorporating spatial variation in estimated rates of colonisation and extinction in a metapopulation model predicted a decline in plot occupancy towards the limit that closely matched the observed decline in occupancy. Thus, variation in metapopulation dynamics may contribute to this species' range limit.

Over the last century, the Eurasian clonal aquatic plant Butomus umbellatus has invaded along the Canada-US border. While there appear to be three invasive diploid genotypes, previous genetic analyses have found almost no genotypic variation within or among introduced populations. Diploids from adventive populations produce numerous viable seed and asexual bulbils however, genetic evidence suggests a lack of successful seed recruitment. Here, we investigate two potential factors limiting sexual reproduction: (1) a climatic mismatch between the native and introduced ranges that impedes germination and seedling establishment in adventive populations; and (2) that rampant clonal spread renders sexual progeny highly inbred, inhibiting recruitment via inbreeding depression. Widespread genotypic uniformity was confirmed because progeny from experimental crosses both within populations, between populations and between the putatively different clonal genotypes were not superior to those from self-fertilization. In growth chambers, seedling germination was fastest at 30 and 35 °C and highest at 30 °C, a temperature potentially experienced by populations in the native region where North American diploids likely originated, but much warmer than what adventive populations likely experience. Progeny produced through sexual recombination grew slower and produced fewer bulbils than the naturalized clonal genotypes from which they were derived under two water depths. Our results suggest that suboptimal conditions for germination combined with inbreeding depression expressed by sexual progeny contribute to the lack of sex in adventive populations. Management efforts should target bulbil production, limit the spread of B. umbellatus to warmer regions and prevent other diploid genotypes from becoming naturalized in North America.

The enemy release hypothesis underpins classical (or importation) biocontrol as a management technique for invasive species. Classical biocontrol has had resounding success when prospective control agents have been subject to appropriate screening before release. Occasionally, however, natural enemies have been reunited with their hosts accidentally. Such adventive agents may provide effective control but have also avoided the careful screening characteristic of modern importation biocontrol programmes. We were studying the invasive mustard, Hesperis matronalis L. (Dame's rocket; Brassicaceae: Hesperidae), when we discovered rampant seed predation by an unknown seed predator. Using DNA barcoding, we identified this seed predator as Ceutorhynchus inaffectatus Gyllenhal (Coleoptera: Curculionidae), a recently (2018) detected species in North America. Comparing potential and realised seed production, we found that seed predation by C. inaffectatus strongly reduces H. matronalis fecundity, and that this effect was not moderated by infection with turnip mosaic virus (TuMV), a commercially important pathogen hosted by H. matronalis and transmitted by polyphagous aphid species. C. inaffectatus is expected to be highly host-specific, and the absence of native Hesperidae species in North America suggests the potential for C. inaffectatus as a classical, but adventive, biocontrol agent of H. matronalis. We suggest population genetic research to identify the origin of C. inaffectatus, and host specificity testing before any intentional redistribution of this species for H. matronalis biocontrol. More generally, this system acts as a model for biocontrol prospects with adventive insect herbivore species.

Specialised species interactions are likely vulnerable to urbanisation because fragmented habitat patches within cities are often smaller and more isolated than natural habitat. As a result, semi-natural habitat and key resources are sometimes deliberately maintained in cities to support biodiversity (e.g., pollinator gardens), but the effectiveness of these efforts is often unclear. We studied four specialised herbivores (monarch butterflies, milkweed leaf miner flies, milkweed aphids and milkweed weevils) of the ruderal plant common milkweed (Asclepias syriaca) to test the predictions that (1) herbivore occupancy of milkweed stands and stems declines towards urban areas and (2) milkweed maintained in urban gardens ameliorates this potentially negative effect of urbanisation. We surveyed 1848 stems in 119 common milkweed stands across an urban-rural gradient in Ontario, Canada, and fit occupancy of stands and individual stems to mixed-effects models to estimate the effects of urbanisation, stand size and stand type (maintained vs. unmaintained) on milkweed occupancy by the four herbivores. The effects of urbanisation, stand size and type varied among herbivore species. Unexpectedly, stand and stem occupancy by monarchs and leaf miners, and stem occupancy by aphids (but neither for weevils) increased towards urban areas, and the presence of milkweed maintained in urban gardens was largely responsible for this. The presence of maintained common milkweed stands may boost occupancy by several specialist herbivores in urban areas. We recommend encouraging urban residents and municipal organisations to plant and maintain common milkweed and other critical host plants to provide habitat for these specialist herbivores.

Adaptation to new habitats might facilitate species' range shifts in response to climate change. In 2005, we transplanted experimental populations of coastal dune plant Camissoniopsis cheiranthifolia into 4 sites within and 1 site beyond its poleward range limit. Beyond-range transplants had high fitness but often delayed reproduction. To test for adaptation associated with experimental range expansion, we transplanted descendants from beyond- and within-range populations after 10 generations in situ into 2 sites within the range, 1 at the range edge, and 2 sites beyond the range. We expected to detect adaptation to beyond-range conditions due to substantial genetic variation within experimental populations and environmental variation among sites. However, individuals from beyond-range experimental populations were not fitter than those from within the range when planted at either beyond-range site, indicating no adaptation to the beyond-range site or beyond-range environments in general. Beyond-range descendants also did not suffer lower fitness within the range. Although reproduction was again delayed beyond the range, late reproduction was not favored more strongly beyond than within the range, and beyond-range descendants did not delay reproduction more than within-range descendants. Persistence in beyond-range environments may not require adaptation, which could allow a rapid response to climate change.

Evolution of self-fertilization may be initiated by a historical population bottleneck, which should diagnostically reduce lineage-wide genetic variation. However, selfing can also strongly reduce genetic variation after it evolves. Distinguishing process from pattern is less problematic if mating system divergence is recent and geographically simple. Dramatically reduced diversity is associated with the transition from outcrossing to selfing in the Pacific coastal endemic Abronia umbellata that includes large-flowered, self-incompatible populations (var. umbellata) south of San Francisco Bay and small-flowered, autogamous populations (var. breviflora) to the north. Compared to umbellata, synonymous nucleotide diversity across 10 single-copy nuclear genes was reduced by 94% within individual populations and 90% across the whole selfing breviflora lineage, which contained no unique polymorphisms. The geographic pattern of genetic variation is consistent with a single origin of selfing that occurred recently (7-28 kya). These results are best explained by a historical bottleneck, but the two most northerly umbellata populations also contained little variation and clustered with selfing populations, suggesting that substantial diversity loss preceded the origin of selfing. A bottleneck may have set the stage for the eventual evolution of selfing by purging genetic load that prevents the spread of selfing.

For many plants, asexual, clonal reproduction appears to be the predominant form of reproduction at their geographic range edge, but how long clonal populations persist and their role in range dynamics are largely unknown. If asexuality enables well-adapted genotypes to persist, this may allow species to expand beyond their sexual niche. Under this scenario, genomic signatures of long-term asexuality should be detectable in range-edge populations. We investigated this hypothesis in the wetland plant, Decodon verticillatus (Lythraceae), which reproduces predominantly through clonal reproduction at its northern range limit. We assembled the transcriptome de novo, identified single nucleotide polymorphisms (SNPs), and compared patterns of genetic variation in sexual and asexual populations at and approaching the range limit. Asexual genotypes exhibited several signatures of long-term asexuality including higher heterozygosity, fewer unique homozygous SNPs (doubletons), and a breakdown of isolation by distance. They also tended to include more deleterious non-synonymous and radical amino acid altering mutations. However, the frequency of unique heterozygous SNPs (singletons) did not differ between sexual and asexual genotypes, and average genetic differentiation was unexpectedly higher among sexual than among asexual genotypes. Yet, overall, our results are consistent with the hypothesis that asexual reproduction enabled D. verticillatus to expand its range further north than would have been possible under sexual reproduction alone. Understanding the factors that influence range dynamics is becoming increasingly important to better anticipate the capacity of species to adapt and shift their ranges in response to anthropogenic environmental changes and to prioritize range-edge populations for conservation.

Understanding the causes and limits of population divergence in phenotypic traits is a fundamental aim of evolutionary biology, with the potential to yield predictions of adaptation to environmental change. Reciprocal transplant experiments and the evaluation of optimality models suggest that local adaptation is common but not universal, and some studies suggest that trait divergence is highly constrained by genetic variances and covariances of complex phenotypes. We analyze a large database of population divergence in plants and evaluate whether evolutionary divergence scales positively with standing genetic variation within populations (evolvability), as expected if genetic constraints are evolutionarily important. We further evaluate differences in divergence and evolvability-divergence relationships between reproductive and vegetative traits and between selfing, mixed-mating, and outcrossing species, as these factors are expected to influence both patterns of selection and evolutionary potentials. Evolutionary divergence scaled positively with evolvability. Furthermore, trait divergence was greater for vegetative traits than for floral (reproductive) traits, but largely independent of the mating system. Jointly, these factors explained ~40% of the variance in evolutionary divergence. The consistency of the evolvability-divergence relationships across diverse species suggests substantial predictability of trait divergence. The results are also consistent with genetic constraints playing a role in evolutionary divergence.

Whether geographically peripheral populations are worth conserving has been hotly debated yet remains unresolved. This is especially relevant in high-latitude countries where, within their political jurisdictions, many species reach their range limits as peripheral isolates and require conservation attention, even if they are common elsewhere. In Canada, ~ 77% of “at-risk” plant species are at their northern range limit in southern Canada but more common south of the Canada-USA border. Peripheral populations might contain little genetic variation, suffer low fitness and be prone to extinction, or they might be adapted to extreme range-edge environments and thus well-poised to participate in range shifts during climate change. Abronia umbellata is endemic to coastal dunes from Baja California, Mexico to Oregon, USA but also occurs as disjunct populations designated “at-risk” in Washington, USA and British Columbia, Canada. Based on sequence variation at nine single-copy nuclear genes assayed for 94 individuals from 25 populations across the species range, these disjunct populations were very similar to range edge populations 350 and 650 km to the south in Oregon, and likely arose through recent, long-distance dispersal or fragmentation of a recently expanded range. In contrast, southern-edge populations in Baja, though not disjunct, were genetically unique and unexpectedly diverse, may currently be in decline yet receive no conservation protection. In this case, the conservation significance of range edge populations depends on which edge, and the unprotected southern edge populations seem a higher priority than those benefitting from special status at the northern range limit.

Premise In plants, meristic traits, such as petal and sepal numbers, are usually considered invariant within taxa, yet certain species consistently exhibit great variability in these traits. The factors contributing to “atypical” counts are not well-known, published hypotheses include relaxation of pollinator selection, inbreeding, and hybridization, among others. The sand verbenas, Abronia (Nyctaginaceae), usually have five perianth lobes (‘petals’), yet certain taxa exhibit marked departures from this norm. Methods Here we integrate an analysis of images from community science data (iNaturalist) and common garden experiments to evaluate a comprehensive set of adaptive and nonadaptive explanations for the production of these ‘atypical’ flowers across an evolutionary transition from xenogamy (outcrossing) to autogamy (selfing) in the coastal sand verbena Abronia umbellata . Key results The shift to autogamy in this lineage correlated with a higher frequency of atypical flowers from ~7% to ~20% and a significant reduction in mean petal number per inflorescence. Autogamous success did not change with petal number, and neither hybridization or up to three generations of inbreeding consistently increased production of atypical flowers or decreased mean petal number, all in contrast to previously-published hypotheses. In contrast, intra-inflorescence, inter-plant (intra-population), inter-population, and inter-variety comparisons demonstrated a correlation of reduced floral size with reduced petal number, suggesting correlated evolution due to a well-established relation between organ number and meristem size. Conclusions The reduction in petal number was probably a consequence of selection for smaller flowers associated with increased selfing. While we could not completely eliminate several alternative hypotheses, including a long-term history of inbreeding or relaxed selection on petal number constancy, those are less likely to explain the observed changes, though they may have contributed to the trend. In general, we develop a framework of hypotheses for evolutionary investigations of meristic variation in floral organs.

Colonization along ubiquitous gradients of growing season length should require adaptation of phenological traits, driven by natural selection. Although phenology often varies with season length and genetic differentiation in phenological traits sometimes seems adaptive, few studies test whether natural selection is responsible for these patterns. The annual plant Rhinanthus minor is genetically differentiated for phenology across a 1000-m elevational gradient of growing season length in the Canadian Rocky Mountains. We estimated phenotypic selection on five phenological traits for three generations of naturally occurring individuals at 12 sites (n = 10,112), and two generations of genetically and phenotypically more variable transplanted populations at nine of these sites (n = 24,611). Selection was weak for most traits, but consistently favored early flowering across the gradient rather than only under short seasons. There was no evidence that apparent selection favoring early reproduction arose from failure to consider all components of fitness, or variation in other correlated phenological traits. Instead, selection for earlier flowering may be balanced by selection for strong cogradient phenological plasticity that indirectly favors later flowering. However, this probably does not explain the consistency of selection on flowering time across this steep, elevational gradient of growing season length.

Ecological experiments usually infer long-term processes from short-term data, and the analysis of geographic range limits is a good example. Species' geographic ranges may be limited by low fitness due to niche constraints, a hypothesis most directly tested by comparing the fitness of populations transplanted within and beyond the range. Such studies often fail to find beyond-range fitness declines strong enough to conclude that geographic range limits are solely imposed by niche limits. However, almost all studies only follow transplants for a single generation, which will underestimate the importance of niche limitation because critical but infrequent range-limiting events may be missed and methodological issues may artificially boost the fitness of beyond-range transplants. Here, we present the first multi-generation beyond-range transplant experiment that involves adequate replication and proper experimental controls. In 2005, experimental populations of the coastal dune plant Camissoniopsis cheiranthifolia were planted at four sites within and one site beyond the northern limit. Fitness of initial transplants was high beyond the limit, suggesting that the range was limited by dispersal and not niche constraints. To better address the niche-limitation hypothesis, we quantified density and fitness of descendant C. cheiranthifolia populations 12-14 years (~10 generations) after transplant. Average annual fruit production and density of reproductive individuals were as high beyond the range as at four comparable experimental populations and eight natural populations within the range, and the beyond-range population had more than tripled in size since it was planted. This provides unprecedented support for the conclusion that northern range limit of C. cheiranthifolia results from something other than niche limitation, likely involving constraints on local dispersal.

High-latitude countries tend to contain the polar range-edge of many species that are nationally rare but globally common. This can focus national conservation efforts toward range-edge populations, whose conservation needs and value are disputed. Using plants in Canada as a case study, we ask whether national species-conservation rankings prioritize range-edge populations, and whether conservation priority is matched by habitat protection and research effort. We found that > 75% of federally protected plants only occur in Canada peripherally, at the northernmost 20% or less of their total range, and that the most imperilled taxa had the smallest percentage of their range in Canada (endangered plants: median = 1.0%). Occurring peripherally in Canada was associated with higher threat even after accounting for range area, potentially because range-edge taxa experienced 85% higher human population densities in their Canadian range than non-peripheral taxa. High conservation priority was not matched by habitat protection, as more imperilled and more peripheral taxa had smaller fractions of their Canadian range in protected areas. Finally, peer-reviewed research on plants at-risk in Canada was low. Only 42% of plants considered at-risk in Canada had been studied in Canada, and only 11% of species with broad distributions outside Canada had been studied in the context of their wider geographic range-information that is critical to establishing their relative conservation value. Our results illustrate that plant conservation in Canada is fundamentally linked to conserving range-edge populations, yet edge populations themselves are understudied, a research gap we must close to improve evidence-based conservation.

Evolutionary biologists have long trained their sights on adaptation, focusing on the power of natural selection to produce relative fitness advantages while often ignoring changes in absolute fitness. Ecologists generally have taken a different tack, focusing on changes in abundance and ranges that reflect absolute fitness while often ignoring relative fitness. Uniting these perspectives, we articulate various causes of relative and absolute maladaptation and review numerous examples of their occurrence. This review indicates that maladaptation is reasonably common from both perspectives, yet often in contrasting ways. That is, maladaptation can appear strong from a relative fitness perspective, yet populations can be growing in abundance. Conversely, resident individuals can appear locally adapted (relative to nonresident individuals) yet be declining in abundance. Understanding and interpreting these disconnects between relative and absolute maladaptation, as well as the cases of agreement, is increasingly critical in the face of accelerating human-mediated environmental change. We therefore present a framework for studying maladaptation, focusing in particular on the relationship between absolute and relative fitness, thereby drawing together evolutionary and ecological perspectives. The unification of these ecological and evolutionary perspectives has the potential to bring together previously disjunct research areas while addressing key conceptual issues and specific practical problems.

According to theory, edge populations may be poised to expand species' ranges if they are locally adapted to extreme conditions, or ill-suited to colonise beyond-range habitat if their offspring are genetically and competitively inferior. We tested these contrasting predictions by transplanting low-, mid-, and high-elevation (edge) populations of an annual plant throughout and above its elevational distribution. Seed from poor-quality edge habitat (one of two transects) had inferior emergence, but edge seeds also had adaptive phenology (both transects). High-elevation plants flowered earlier, required less heat accumulation to mature seed, and so achieved higher lifetime fitness at and above the range edge. Experimental warming improved fitness above the range, but eliminated the advantage of local cold-edge populations, supporting recent models in which cold-adapted edge populations do not facilitate warming-induced range shifts. The highest above-range fitness was achieved by a 'super edge phenotype' from a neighbouring mountain, suggesting key adaptations exist regionally even if absent from local edge populations.

Species are commonly distributed along latitudinal and elevational gradients of growing season length to which they might respond via phenotypic plasticity and/or adaptive genetic differentiation. However, the relative contribution of these processes and whether plasticity, if it occurs, facilitates expansion along season-length gradients remain unclear, but are important for predicting species fates during anthropogenic change. We quantified phenological trait variation in the montane annual Rhinanthus minor for three generations at 12 sites across 900 m of elevation in the Canadian Rocky Mountains and conducted a reciprocal transplant experiment for two generations among nine sites. We compared clines and interannual variation of phenological traits between natural and transplanted individuals. Season length declined by c. 37% along our elevational gradient and, as expected, plants emerged, reached first flower and made their first seed in c. 41% fewer growing degree days under shorter growing seasons. Although reciprocal transplants revealed modest genetic differentiation across elevation, trait clines primarily were due to striking co-gradient plasticity that paralleled genetic differentiation. Co-gradient plasticity likely evolved in response to considerable interannual variation in season length across our elevational transect, and should prepare R. minor to make adaptive changes to phenology in response to ongoing climate change predicted for montane environments.