Adaptive plasticity and niche expansion in an invasive thistle

Among-population variation in drought responses is consistent across life stages but not between native and non-native ranges

Understanding how widespread species adapt to variation in abiotic conditions across their ranges is fundamental to ecology. Insight may come from studying how among-population variation (APV) in the common garden corresponds with the environmental conditions of source populations. However, there are no such studies comparing native vs non-native populations across multiple life stages. We examined APV in the performance and functional traits of 59 Conyza canadensis populations, in response to drought, across large aridity gradients in the native (North America) and non-native (Eurasia) ranges in three experiments. Our treatment (dry vs wet) was applied at the recruitment, juvenile, and adult life stages. We found contrasting patterns of APV in drought responses between the two ranges. In the native range, plant performance was less reduced by drought in populations from xeric than mesic habitats, but such relationship was not apparent for non-native populations. These range-specific patterns were consistent across the life stages. The weak adaptive responses of non-native populations indicate that they can become highly abundant even without complete local adaptation to abiotic environments and suggest that long-established invaders may still be evolving to the abiotic environment. These findings may explain lag times in invasions and raise concern about future expansions.

Climate biogeography of Arabidopsis thaliana: linking distribution models and individual variation

AIM

Patterns of individual variation are key to testing hypotheses about the mechanisms underlying biogeographic patterns. If species distributions are determined by environmental constraints, then populations near range margins may have reduced performance and be adapted to harsher environments. Model organisms are potentially important systems for biogeographical studies, given the available range-wide natural history collections, and the importance of providing biogeographical context to their genetic and phenotypic diversity.

LOCATION

Global.

TAXON

Arabidopsis thaliana ("Arabidopsis").

METHODS

We fit occurrence records to climate data, and then projected the distribution of Arabidopsis under last glacial maximum, current, and future climates. We confronted model predictions with individual performance measured on 2,194 herbarium specimens, and we asked whether predicted suitability was associated with life-history and genomic variation measured on ~900 natural accessions.

RESULTS

The most important climate variables constraining the Arabidopsis distribution were winter cold in northern and high elevation regions and summer heat in southern regions. Herbarium specimens from regions with lower habitat suitability in both northern and southern regions were smaller, supporting the hypothesis that the distribution of Arabidopsis is constrained by climate-associated factors. Climate anomalies partly explained interannual variation in herbarium specimen size, but these did not closely correspond to local limiting factors identified in the distribution model. Late-flowering genotypes were absent from the lowest suitability regions, suggesting slower life histories are only viable closer to the center of the realized niche. We identified glacial refugia farther north than previously recognized, as well as refugia concordant with previous population genetic findings. Lower latitude populations, known to be genetically distinct, are most threatened by future climate change. The recently colonized range of Arabidopsis was well-predicted by our native-range model applied to certain regions but not others, suggesting it has colonized novel climates.

MAIN CONCLUSIONS

Integration of distribution models with performance data from vast natural history collections is a route forward for testing biogeographical hypotheses about species distributions and their relationship with evolutionary fitness across large scales.

Adaptive responses to living in stressful habitats: Do invasive and native plant populations use different strategies?

Plants inhabit stressful environments characterized by a variety of stressors, including mine sites, mountains, deserts, and high latitudes. Populations from stressful and reference (non-stressful) sites often have performance differences. However, while invasive and native species may respond differently to stressful environments, there is limited understanding of the patterns in reaction norms of populations from these sites. Here, we use phylogenetically controlled meta-analysis to assess the performance of populations under stress and non-stress conditions. We ask whether stress populations of natives and invasives differ in the magnitude of lowered performance under non-stress conditions and if they vary in the degree of performance advantage under stress. We also assessed whether these distinctions differ with stress intensity. Our findings revealed that natives not only have greater adaptive advantages but also more performance reductions than invasives. Populations from very stressful sites had more efficient adaptations, and performance costs increased with stress intensity in natives only. Overall, the results support the notion that adaptation is frequently costless. Reproductive output was most closely associated with adaptive costs and benefits. Our study characterized the adaptive strategies used by invasive and native plants under stressful conditions, thereby providing important insights into the limitations of adaptation to extreme sites.

Beyond reaction norms: the temporal dynamics of phenotypic plasticity

Phenotypic plasticity can allow organisms to cope with environmental changes. Although reaction norms are commonly used to quantify plasticity along gradients of environmental conditions, they often miss the temporal dynamics of phenotypic change, especially the speed at which it occurs. Here, we argue that studying the rate of phenotypic plasticity is a crucial step to quantify and understand its adaptiveness. Iteratively measuring plastic traits allows us to describe the actual dynamics of phenotypic changes and avoid quantifying reaction norms at times that do not truly reflect the organism's capacity for plasticity. Integrating the temporal component in how we describe, quantify, and conceptualise phenotypic plasticity can change our understanding of its diversity, evolution, and consequences.

Indications for rapid evolution of trait means and thermal plasticity in range-expanding populations of a butterfly

Currently, poleward range expansions are observed in many taxa, often in response to anthropogenic climate change. At the expanding front, populations likely face cooler and more variable temperature conditions, imposing thermal selection. This may result in changes in trait means or plasticity, the relative contribution of which is not well understood. We, here, investigate evolutionary change in range-expanding populations of the butterfly Pieris mannii, by comparing populations from the core and the newly established northern range under laboratory conditions. We observed both changes in trait means and in thermal reaction norms. Range-expanding populations showed a more rapid development, potentially indicative of counter-gradient variation and an increased cold tolerance compared with core populations. Genotype-environment interactions prevailed in all associated traits, such that the above differences were restricted to cooler environmental conditions. In range-expanding populations, plasticity was decreased in developmental traits enabling relatively rapid growth even under cooler conditions but increased in cold tolerance arguably promoting higher activity under thermally challenging conditions. Notably, these changes must have occurred within a time period of ca. 10 years only. Our results suggest, in line with contemporary theory, that the evolution of plasticity may play a hitherto underestimated role for adaptation to climatic variation. However, rather than generally increased or decreased levels of plasticity, our results indicate fine-tuned, trait-specific evolutionary responses to increase fitness in novel environments.

Increased ploidy of Butomus umbellatus in introduced populations is not associated with higher phenotypic plasticity to N and P

Separate introductions or post-introduction evolution may lead to multiple invader genotypes or cytotypes that differ in growth rates, biomass or chemical profile responses (phenotype) to a range of environments. If the invader has high trait plasticity to a range of resource levels, then sediment N or P enrichment may enhance invasiveness. However, the ways in which ploidy, plasticity, and available N or P interact are unknown for most species despite the potential to explain spread and impacts by invaders with multiple introduced lineages. We conducted a common garden experiment with four triploid and six diploid populations of Butomus umbellatus, collected from across its invasive range in the USA. Plants were grown under different N or P nutrient levels (4, 40, 200, 400 mg L^-1 N; 0.4, 4, 40 mg L^-1 P) and we measured reaction norms for biomass, clonal reproduction and tissue chemistry. Contrary to our expectation, triploid B. umbellatus plants were less plastic to variation in N or P than diploid B. umbellatus in most measured traits. Diploid plants produced 172 % more reproductive biomass and 57 % more total biomass across levels of N, and 158 % more reproductive biomass and 33 % more total biomass across P than triploid plants. Triploid plants had lower shoot:root ratios and produced 30 % and 150 % more root biomass than diploid plants in response to increases in N and P, respectively. Tissue chemistry differed between cytotypes but plasticity was similar; N was 8 % higher and C:N ratio was 30 % lower in triploid than diploid plants across levels of N and plant parts, and N was 22 % higher and C:N ratio 27 % lower across levels of P and plant parts. Our results highlight differences in nutrient response between cytotypes of a widespread invader, and we call for additional field studies to better understand the interaction of nutrients and ploidy during invasion.

Phenotypic plasticity in chemical defence of butterflies allows usage of diverse host plants

Host plant specialization is a major force driving ecological niche partitioning and diversification in insect herbivores. The cyanogenic defences of Passiflora plants keep most herbivores at bay, but not the larvae of Heliconius butterflies, which can both sequester and biosynthesize cyanogenic compounds. Here, we demonstrate that both Heliconius cydno chioneus and H. melpomene rosina have remarkable plasticity in their chemical defences. When feeding on Passiflora species with cyanogenic compounds that they can readily sequester, both species downregulate the biosynthesis of these compounds. By contrast, when fed on Passiflora plants that do not contain cyanogenic glucosides that can be sequestered, both species increase biosynthesis. This biochemical plasticity comes at a fitness cost for the more specialist H. m. rosina, as adult size and weight for this species negatively correlate with biosynthesis levels, but not for the more generalist H. c. chioneus. By contrast, H. m rosina has increased performance when sequestration is possible on its specialized host plant. In summary, phenotypic plasticity in biochemical responses to different host plants offers these butterflies the ability to widen their range of potential hosts within the Passiflora genus, while maintaining their chemical defences.

Genomic Analyses of Phenotypic Differences Between Native and Invasive Populations of Diffuse Knapweed (Centaurea diffusa)

Invasive species represent excellent opportunities to study the evolutionary potential of traits important to success in novel environments. Although some ecologically important traits have been identified in invasive species, little is typically known about the genetic mechanisms that underlie invasion success in non-model species. Here, we use a genome-wide association (GWAS) approach to identify the genetic basis of trait variation in the non-model, invasive, diffuse knapweed [Centaurea diffusa Lam. (Asteraceae)]. To assist with this analysis, we have assembled the first draft genome reference and fully annotated plastome assembly for this species, and one of the first from this large, weedy, genus, which is of major ecological and economic importance. We collected phenotype data from 372 individuals from four native and four invasive populations of C. diffusa grown in a common environment. Using these individuals, we produced reduced-representation genotype-by-sequencing (GBS) libraries and identified 7,058 SNPs. We identify two SNPs associated with leaf width in these populations, a trait which significantly varies between native and invasive populations. In this rosette forming species, increased leaf width is a major component of increased biomass, a common trait in invasive plants correlated with increased fitness. Finally, we use annotations from Arabidopsis thaliana to identify 98 candidate genes that are near the associated SNPs and highlight several good candidates for leaf width variation.

Genetically Based Trait Differentiation but Lack of Trade-offs between Stress Tolerance and Performance in Introduced Canada Thistle

Trade-offs between performance and tolerance of abiotic and biotic stress have been proposed to explain both the success of invasive species and frequently observed size differences between native and introduced populations. Canada thistle seeds collected from across the introduced North American and the native European range were grown in benign and stressful conditions (nutrient stress, shading, simulated herbivory, drought, and mowing), to evaluate whether native and introduced individuals differ in performance or stress tolerance. An additional experiment assessed the strength of maternal effects by comparing plants derived from field-collected seeds with those derived from clones grown in the glasshouse. Introduced populations tended to be larger in size, but no trade-off of stress tolerance with performance was detected; introduced populations had either superior performance or equivalent trait values and survivorship in the treatment common gardens. We also detected evidence of parallel latitudinal clines of some traits in both the native and introduced ranges and associations with climate variables in some treatments, consistent with recent climate adaptation within the introduced range. Our results are consistent with rapid adaptation of introduced populations, but, contrary to predictions, the evolution of invasive traits did not come at the cost of reduced stress tolerance.

Competitive Interactions of Flowering Rush (Butomus umbellatus L.) Cytotypes in Submersed and Emergent Experimental Aquatic Plant Communities

The ability to invade communities in a variety of habitats (e.g., along a depth gradient) may facilitate establishment and spread of invasive plants, but how multiple lineages of a species perform under varying conditions is understudied. A series of greenhouse common garden experiments were conducted in which six diploid and four triploid populations of the aquatic invasive plant Butomus umbellatus L. (Butomaceae) were grown in submersed or emergent conditions, in monoculture or in a multispecies community, to compare establishment and productivity of cytotypes under competition. Diploid biomass overall was 12 times higher than triploids in the submersed experiment and three times higher in the emergent experiment. Diploid shoot:root ratio was double that of triploid plants in submersed conditions overall, and double in emergent conditions in monoculture. Relative interaction intensities (RII) indicated that triploid plants were sixteen times more negatively impacted by competition under submersed conditions but diploid plants were twice as impacted under emergent conditions. Recipient communities were similarly negatively impacted by B. umbellatus cytotypes. This study supports the idea that diploid and triploid B. umbellatus plants are equally capable of invading emergent communities, but that diploid plants may be better adapted for invading in submersed habitats. However, consistently lower shoot:root ratios in both monoculture and in communities suggests that triploid plants may be better-adapted competitors in the long term due to increased resource allocation to roots. This represents the first examination into the role of cytotype and habitat on competitive interactions of B. umbellatus.

Rapid growth and defence evolution following multiple introductions

Rapid adaptation can aid invasive populations in their competitive success. Resource allocation trade-off hypotheses predict higher resource availability or the lack of natural enemies in introduced ranges allow for increased growth and reproduction, thus contributing to invasive success. Evidence for such hypotheses is however equivocal and tests among multiple ranges over productivity gradients are required to provide a better understanding of the general applicability of these theories.Using common gardens, we investigated the adaptive divergence of various constitutive and inducible defence-related traits between the native North American and introduced European and Australian ranges, while controlling for divergence due to latitudinal trait clines, individual resource budgets, and population differentiation, using >11,000 SNPs.Rapid, repeated clinal adaptation in defence-related traits was apparent despite distinct demographic histories. We also identified divergence among ranges in some defence-related traits, although differences in energy budgets among ranges may explain some, but not all, defence-related trait divergence. We do not identify a general reduction in defence in concert with an increase in growth among the multiple introduced ranges as predicted trade-off hypotheses. Synthesis: The rapid spread of invasive species is affected by a multitude of factors, likely including adaptation to climate and escape from natural enemies. Unravelling the mechanisms underlying invasives' success enhances understanding of eco-evolutionary theory and is essential to inform management strategies in the face of ongoing climate change.

OPEN RESEARCH BADGES

This article has been awarded Open Materials, Open Data, Preregistered Research Designs Badges. All materials and data are publicly accessible via the Open Science Framework at https://doi.org/10.6084/m9.figshare.8028875.v1, https://github.com/lotteanna/defence_adaptation,https://doi.org/10.1101/435271.

Trait divergence, not plasticity, determines the success of a newly invasive plant

BACKGROUND AND AIMS

Phenotypic plasticity and genetic differentiation both play important roles in the establishment and spread of species after extra-range dispersal; however, the adaptive potential of plasticity and genetic divergence in successful invasions remains unclear.

METHODS

We measured six anatomical traits associated with drought tolerance in contrasting water environments for individuals from the invasive and native range of the bunchgrass Brachypodium sylvaticum. To represent sources contributing to admixed genotypes in the invasive range accurately, we used unique alleles to determine probabilities of genetic contribution, and utilized these as weights in our analyses. The adaptive values of plasticity and genetic differentiation were assessed using regression.

KEY RESULTS

No plasticity was found in response to water availability for any of the measured traits. Bulliform cell area and three traits related to xylem morphology displayed genetic differentiation between invasive and native ranges, indicating a shift in the invasive range towards drought-tolerant phenotypes. Genetic divergence was not consistently in the direction indicated by selection, suggesting that responses are limited by trade-offs with other traits or physical constraints.

CONCLUSIONS

Our results indicate that invasive adaptation is the consequence of post-introduction selection leading to genetic differentiation. Selection, rather than plasticity, is driving B. sylvaticum success in its invaded range.

Exotic plant species are locally adapted but not to high ultraviolet-B radiation: a reciprocal multispecies experiment

Ultraviolet (UV) radiation intensities differ among global regions, with significantly higher levels in the southern hemisphere. UV-B may act as an environmental filter during plant invasions, which might particularly apply to plant species from Europe introduced to New Zealand. Just like for any other abiotic or biotic filter, successful invaders can cope with novel environmental conditions via plastic responses and/or through rapid adaptation by natural selection in the exotic range. We conducted a multispecies experiment with herbaceous plants in two common gardens located in the species' native and exotic ranges, in Germany and New Zealand, respectively. We used plants of German and New Zealand origin of eight species to test for adaptation to higher UV-B radiation in their new range. In each common garden, all plants were exposed to three radiation treatments: (1) ambient sunlight, (2) exclusion of UV-B while transmitting ambient UV-A, and (3) combined exclusion of UV-B and UV-A. Linear mixed-effect models revealed significant effects of UV-B on growth and leaf traits and an indication for UV-B-induced biomass reduction in both common gardens pointing to an impact of natural, ambient UV radiation intensities experienced by plants in the northern and in the southern hemisphere. In both common gardens, the respective local plants (i.e., German origins in Germany, New Zealand origins in New Zealand) displayed enhanced productivity and aboveground biomass allocation, thus providing evidence for recent evolutionary processes in the exotic range. Genetic differentiation between different origins in consequence of divergent local selection pressures was found for specific leaf area. This differentiation particularly hints at different selective forces in both ranges while only little evidence was found for an immediate selective effect of high UV-B intensities in the exotic range. However, reaction norm slopes across ranges revealed higher plasticity of exotic individuals in functional leaf traits that might allow for a more sensitive regulation of photoprotection measures in response to UV-B. During the colonization, New Zealand populations might have been selected for the observed higher phenotypic plasticity and a consequently increased ability to successfully spread in the exotic range.

Traits influence detection of exotic plant species in tropical forests

Detecting exotic plant species is essential for invasive species management. By accounting for factors likely to affect species’ detection rates (e.g. survey conditions, observer experience), detectability models can help choose search methods and allocate search effort. Integrating information on species’ traits can refine detectability models, and might be particularly valuable if these traits can help improve estimates of detectability where data on particular species are rare. Analysing data collected during line transect distance sampling surveys in Indonesia, we used a multi-species hierarchical distance sampling model to evaluate how plant height, leaf size, leaf shape, and survey location influenced plant species detectability in secondary tropical rainforests. Detectability of the exotic plant species increased with plant height and leaf size. Detectability varied among the different survey locations. We failed to detect a clear effect of leaf shape on detectability. This study indicates that information on traits might improve predictions about exotic species detection, which can then be used to optimise the allocation of search effort for efficient species management. The innovation of the study lies in the multi-species distance sampling model, where the distance-detection function depends on leaf traits and height. The method can be applied elsewhere, including for different traits that may be relevant in other contexts. Trait-based multispecies distance sampling can be a practical approach for sampling exotic shrubs, herbs, or grasses species in the understorey of tropical forests.

Combining niche shift and population genetic analyses predicts rapid phenotypic evolution during invasion

The rapid evolution of non-native species can facilitate invasion success, but recent reviews indicate that such microevolution rarely yields expansion of the climatic niche in the introduced habitats. However, because some invasions originate from a geographically restricted portion of the native species range and its climatic niche, it is possible that the frequency, direction, and magnitude of phenotypic evolution during invasion have been underestimated. We explored the utility of niche shift analyses in the red seaweed Gracilaria vermiculophylla, which expanded its range from the northeastern coastline of Japan to North America, Europe, and northwestern Africa within the last 100 years. A genetically informed climatic niche shift analysis indicates that native source populations occur in colder and highly seasonal habitats, while most non-native populations typically occur in warmer, less seasonal habitats. This climatic niche expansion predicts that non-native populations evolved greater tolerance for elevated heat conditions relative to native source populations. We assayed 935 field-collected and 325 common-garden thalli from 40 locations, and as predicted, non-native populations had greater tolerance for ecologically relevant extreme heat (40°C) than did Japanese source populations. Non-native populations also had greater tolerance for cold and low-salinity stresses relative to source populations. The importance of local adaptation to warm temperatures during invasion was reinforced by evolution of parallel clines: Populations from warmer, lower-latitude estuaries had greater heat tolerance than did populations from colder, higher-latitude estuaries in both Japan and eastern North America. We conclude that rapid evolution plays an important role in facilitating the invasion success of this and perhaps other non-native marine species. Genetically informed ecological niche analyses readily generate clear predictions of phenotypic shifts during invasions and may help to resolve debate over the frequency of niche conservatism versus rapid adaptation during invasion.

Leveraging contemporary species introductions to test phylogenetic hypotheses of trait evolution

Plant trait evolution is a topic of interest across disciplines and scales. Phylogenetic studies are powerful for generating hypotheses about the mechanisms that have shaped plant traits and their evolution. Introduced plants are a rich source of data on contemporary trait evolution. Introductions could provide especially useful tests of a variety of evolutionary hypotheses because the environments selecting on evolving traits are still present. We review phylogenetic and contemporary studies of trait evolution and identify areas of overlap and areas for further integration. Emerging tools which can promote integration include broadly focused repositories of trait data, and comparative models of trait evolution that consider both intra and interspecific variation.