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As a vector of human health concern, blacklegged ticks ( Ixodes scapularis ) transmit pathogens that cause tick-borne diseases (TBDs), including Lyme disease (LD). Several hot spots of elevated LD risk have emerged across Canada as I. scapularis expands its range.

Ticks in the familyIxodidaeare important vectors of zoonoses including Lyme disease (LD), which is caused by spirochete bacteria from theBorreliella(Borrelia)burgdorferisensu lato (Bbsl) complex. The blacklegged tick (Ixodes scapularis) continues to expand across Canada, creating hotspots of elevated LD risk at the leading edge of its expansion range. Current efforts to understand the risk of pathogen transmission associated withI. scapularisin Canada focus primarily on targeted screens, while variation in the tick microbiome remains poorly understood. Using multi-omics consisting of 16S metabarcoding and ribosome-depleted, whole-shotgun RNA transcriptome sequencing, we examined the microbial communities associated with adultI. scapularis(N = 32), sampled from four tissue types (whole tick, salivary glands, midgut, and viscera) and three geographical locations within a LD hotspot near Kingston, Ontario, Canada. The communities consisted of both endosymbiotic and known or potentially pathogenic microbes, including RNA viruses, bacteria, and aBabesiasp. intracellular parasite. We show that β-diversity is significantly higher between individual tick salivary gland and midgut bacterial communities, compared to whole ticks; while linear discriminant analysis (LDA) effect size (LEfSe) determined that the three potentially pathogenic bacteria detected by V4 16S rDNA sequencing also differed among dissected tissues only, including aBorreliafrom theBbslcomplex,Borrelia miyamotoi, andAnaplasma phagocytophilum. Importantly, we find co-infection ofI. scapularisby multiple microbes, in contrast to diagnostic protocols for LD, which typically focus on infection from a single pathogen of interest (B. burgdorferisensu stricto).IMPORTANCEA vector of human health concern, blacklegged ticks,Ixodes scapularis, transmit pathogens that cause tick-borne diseases (TBDs), including Lyme disease (LD). Several hotspots of elevated LD risk have emerged across Canada asI. scapularisexpands its range. Focusing on a hotspot in southeastern Ontario, we used high-throughput sequencing on whole ticks and dissected salivary glands and midguts. Compared to whole ticks, analysis of salivary glands and midguts revealed greater β-diversity among microbiomes that are less dominated byRickettsiaendosymbiont bacteria and enriched for pathogenic bacteria including aBbsl-associatedBorrelia,Borrelia miyamotoi, andAnaplasma phagocytophilum. We also find evidence of co-infection ofI. scapularisin this region by multiple microbes. Overall, our study highlights the challenges and opportunities associated with the surveillance of the microbiome ofI. scapularisfor pathogen detection using metabarcoding and metatranscriptome approaches.

The extent to which evolution can rescue a species from extinction, or facilitate range expansion, depends critically on the rate, duration, and geographical extent of the evolutionary response to natural selection. Adaptive evolution can occur quickly, but the duration and geographical extent of contemporary evolution in natural systems remain poorly studied. This is particularly true for species with large geographical ranges and for timescales that lie between “long-term” field experiments and the fossil record. Here, we introduce the Virtual Common Garden (VCG) to investigate phenotypic evolution in natural history collections while controlling for phenotypic plasticity in response to local growing conditions. Reconstructing 150 y of evolution in Lythrum salicaria (purple loosestrife) as it invaded North America, we analyze phenology measurements of 3,429 herbarium records, reconstruct growing conditions from more than 12 million local temperature records, and validate predictions across three common gardens spanning 10° of latitude. We find that phenological clines have evolved repeatedly throughout the range, during the first century of evolution. Thereafter, the rate of microevolution stalls, recapitulating macroevolutionary stasis observed in the fossil record. Our study demonstrates that preserved specimens are a critical resource for investigating limits to evolution in natural populations. Our results show how natural selection and trade-offs measured in field studies predict adaptive divergence observable in herbarium specimens over 15 decades at a continental scale.

Global emergence of vector-borne and zoonotic diseases presents a rapidly growing 'wicked' problem. We outline the need for a transdisciplinary research program that is grounded in ecological and evolutionary theory but integrates fundamentally with research perspectives spanning the health, social, and natural sciences.

Species’ ranges are limited by both ecological and evolutionary constraints. While there is a growing appreciation that ecological constraints include interactions among species, like competition, we know relatively little about how interactions contribute to evolutionary constraints at species' niche and range limits. Building on concepts from community ecology and evolutionary biology, we review how biotic interactions can influence adaptation at range limits by impeding the demographic conditions that facilitate evolution (which we term a ‘demographic pathway to adaptation’), and/or by imposing evolutionary trade-offs with the abiotic environment (a ‘trade-offs pathway’). While theory for the former is well-developed, theory for the trade-offs pathway is not, and empirical evidence is scarce for both. Therefore, we develop a model to illustrate how fitness trade-offs along biotic and abiotic gradients could affect the potential for range expansion and niche evolution following ecological release. The model shows that which genotypes are favoured at species' range edges can depend strongly on the biotic context and the nature of fitness trade-offs. Experiments that characterize trade-offs and properly account for biotic context are needed to predict which species will expand their niche or range in response to environmental change., Philosophical Transactions of the Royal Society B: Biological Sciences, 377 (1848), ISSN:0962-8436, ISSN:1471-2970, ISSN:0080-4622

BackgroundAntibody responses to SARS-CoV-2 can be observed as early as 14 days post-infection, but little is known about the stability of antibody levels over time. Here we evaluate the long-term stability of anti-SARS-CoV-2 IgG antibodies following infection with SARS-CoV-2 in 402 adult donors.MethodsWe performed a multi-center study carried out at Plasma Donor Centers in the city of Heidelberg (Plasmazentrum Heidelberg, Germany) and Munich (Plasmazentrum München, Germany). We present anti-S/N and anti-N IgG antibody levels in prospective serum samples collected up to 403 days post recovery from SARS-CoV-2 infected individuals.ResultsThe cohort includes 402 adult donors (185 female, 217 male; 17 - 68 years of age) where anti-SARS-CoV-2 IgG levels were measured in plasma samples collected between 18- and 403-days post SARS-CoV-2 infection. A linear mixed effects model demonstrated IgG decay rates that decrease over time (χ2=176.8, p2=10.0, pin vitroneutralization capacity in 91% of our cohort.ConclusionLong-term antibody levels and age-specific antibody decay rates suggest the potential need for age-specific vaccine booster guidelines to ensure long term vaccine protection against SARS-CoV-2 infection.

BackgroundRespiratory viruses are transmitted and acquired via the nasal mucosa, and thereby may influence the nasal metabolome composed of biochemical products produced by both host cells and microbes. Studies of the nasal metabolome demonstrate virus-specific changes that sometimes correlate with viral load and disease severity. Here, we evaluate the nasopharyngeal metabolome of COVID-19 infected individuals and report several small molecules that may be used as potential therapeutic targets. Specimens were tested by qRT-PCR with target primers for three viruses: Influenza A (INFA), respiratory syncytial virus (RSV), and SARS-CoV-2, along with asymptomatic controls. The nasopharyngeal metabolome was characterized using an LC-MS/MS-based small-molecule screening kit capable of quantifying 141 analytes. A machine learning model identified 28 discriminating analytes and correctly categorized patients with a viral infection with an accuracy of 96% (R2=0.771, Q2=0.72). A second model identified 5 analytes to differentiate COVID19-infected patients from those with INFA or RSV with an accuracy of 85% (R2=0.442, Q2=0.301). Specifically, LysoPCaC18:2 concentration was significantly increased in COVID19 patients (P< 0.0001), whereas beta-hydroxybutyric acid, Met SO, succinic acid, and carnosine concentrations were significantly decreased (P< 0.0001). This study demonstrates that COVID19 infection results in a unique NP metabolomic signature with carnosine and LysoPCaC18:2 as potential therapeutic targets.Significance StatementEfforts to elucidate how SARS-CoV-2 interacts with the host has become a global priority. To identify biomarkers for potential therapeutic interventions, we used a targeted metabolomics approach evaluating metabolite profiles in the nasal mucosa of COVID-19 patients and compared metabolite profiles to those of other respiratory viruses (influenza A, RSV). We identified a COVID-19-specific signature characterized by changes to LysoPCaC18:2, beta-hydroxybutyric acid, Met SO, succinic acid, and carnosine. Carnosine is a promising potential target against SARS-CoV-2 as it has been shown to interfere with binding of SARS-CoV-2 to the ACE2 receptor. This study provides compelling evidence for the use of metabolomics as an avenue for the identification of novel drug targets for viral respiratory infections in the nasopharynx.

Genome-wide variation in SARS-CoV-2 reveals evolution and transmission dynamics which are critical considerations for disease control and prevention decisions. Here, we review estimates of the genome-wide viral mutation rates, summarize current COVID-19 case load in the province of Ontario, Canada (5 January 2021), and analyze published SARS-CoV-2 genomes from Ontario (collected prior to 24 November 2020) to test for more infectious genetic variants or lineages., Genome-wide variation in SARS-CoV-2 reveals evolution and transmission dynamics which are critical considerations for disease control and prevention decisions. Here, we review estimates of the genome-wide viral mutation rates, summarize current COVID-19 case load in the province of Ontario, Canada (5 January 2021), and analyze published SARS-CoV-2 genomes from Ontario (collected prior to 24 November 2020) to test for more infectious genetic variants or lineages. The reported mutation rate (∼10−6 nucleotide [nt]−1 cycle−1) for SARS-CoV-2 is typical for coronaviruses. Analysis of published SARS-CoV-2 genomes revealed that the G614 spike protein mutation has dominated infections in Ontario and that SARS-CoV-2 lineages present in Ontario have not differed significantly in their rate of spread. These results suggest that the SARS-CoV-2 population circulating in Ontario has not changed significantly to date. However, ongoing genome monitoring is essential for identification of new variants and lineages that may contribute to increased viral transmission.

Book Review

The extent to which evolution can rescue a species from extinction, or facilitate range expansion, depends critically on the rate, duration, and geographical extent of the evolutionary response to natural selection. While field experiments have demonstrated that adaptive evolution can occur quickly, our understanding of the duration and geographical extent of contemporary evolution in natural systems remains limited. This is particularly true for species with large geographical ranges and for timescales that lie between ‘long-term’ field experiments and the fossil record. Here, we introduce the Virtual Common Garden (VCG) to estimate genetic differences among phenotypes observed in natural history collections. Reconstructing 150 years of evolution in Lythrum salicaria (purple loosestrife) as it invaded across North America, we analyze phenology measurements of 3,429 herbarium records, reconstruct growing conditions from more than 12 million local temperature records, and validate predictions across three common gardens spanning 10 degrees of latitude. We find that phenology evolves rapidly and repeatedly along parallel climatic gradients during the first century of evolution. However, the rate of microevolution stalls thereafter, recapitulating macroevolutionary stasis observed in the fossil record. Our study demonstrates why preserved specimens are a critical resource for understanding limits to evolution in natural. Our results show predictability of evolution emerging at a continental scale across 15 decades of rapid, adaptive evolution. Significance Adaptive evolution can help species to persist in new environments. The fossil record contains many examples of phenotypic stasis punctuated by rapid evolution, with distinct lineages converging on similar phenotypes over geological timescales. In contrast, the spatio-temporal dynamics of evolution over ecological timescales are largely unknown. Here, we use a computational approach to reconstruct 15 decades of evolution in an invasive plant as it spread across North America. Flowering phenology evolves in parallel throughout the range but stalls after about a century. This punctuated, convergent evolution recapitulates long-term dynamics in the fossil record, suggesting constraints on adaptation that are not evident for the first hundred years.

Soil feedback is thought to be an important contributor to the success of invasive plants. Despite evidence that invasive plants change soil microbial diversity, the functional roles of microbes impacted by invasion are still unclear. This knowledge is a critical component of our understanding of ecological mechanisms of plant invasion. Mounting evidence suggests Alliaria petiolata can suppress arbuscular mycorrhizal fungi (AMF) to disrupt native plant communities in controlled laboratory and greenhouse experiments, though it is less clear if allelochemicals persist under natural field conditions. Alternatively, invasive plants may accumulate pathogens that are more harmful to competitors as predicted by the Enemy of my Enemy Hypothesis (EEH). We examined changes in functional groups of soil bacteria and fungi associated with ten naturally occurring populations of A. petiolata using amplicon sequences (16S and ITS rRNA). To relate soil microbial communities to impacts on co-occurring plants, we measured root infections and AMF colonization. We found no changes in the diversity and abundance of AMF in plants co-occurring with A. petiolata, suggesting that mycorrhizal suppression in the field may not be as critical to the invasion of A. petiolata as implied by more controlled experiments. Instead, we found changes in pathogen community composition and marginal evidence of increase in root lesions of plants growing with A. petiolata, lending support to the EEH. In addition to these impacts on plant health, changes in ectomycorrhiza, and other nutrient cycling microbes may be important forces underlying the invasion of A. petiolata and its impact on ecosystem function.

The emergence and global spread of SARS-CoV-2 has had profound social and economic consequences and has shed light on the importance of continued and additional investment in global health and infectious disease surveillance. Identifying changes in viral genomes provides key insights into viral diversity, how viruses spread within populations, and viral strategies for evasion of host immune systems. Here we report twenty-five SARS-CoV-2 genome sequences collected from some of the first COVID-19 cases in eastern Ontario, Canada (March 18-30, 2020). The reported genomes belong to the S-clade (n=2) and G-clade (n=23) of SARS-CoV-2 and contain 45 polymorphic sites including one shared missense and three unique synonymous variants in the gene encoding the spike protein. A phylogenetic analysis enabled the tracing of viral origin and potential transmission into and within Canada. There may be as many as sixteen unique infection events represented in these samples, including at least three that were likely introduced from Europe and seven from the USA. In addition, four separate genomes are each shared by multiple patients, suggesting a common origin or community spread even during this early stage of infection. These results demonstrate how molecular epidemiology and evolutionary phylogenetics can help local health units track origins and vectors of spread for emerging diseases like SARS-CoV-2. Earlier detection and screening in this way could improve the effectiveness of regional public health interventions to prevent future pandemics.

Plants and animals have evolved a variety of strategies to limit the negative fitness consequences of natural enemies (i.e. herbivores, predators, parasites and pathogens). Demographic bottlenecks occurring during the invasion process reduce the number of co-introduced natural enemies, providing opportunities to study rapid evolution in environments with different or reduced enemy loads. Enemy release theory provides a set of hypotheses and predictions about the role of natural enemies in the proliferation of invasive species. This body of theory includes the Enemy Release Hypothesis (ERH) and the related Evolution of Increased Competitive Ability Hypothesis (EICA), but there is often confusion about these hypotheses and the data needed to test them. We introduce a simple, general model of enemy release to identify and clarify some of the key assumptions and predictions implicit in enemy release theory and its impacts on invasion. Although introduced populations likely benefit from a reduction in the direct fitness impacts of natural enemies in the early stages of invasion, an evolutionary shift in resource allocation from defence to growth and reproduction is much less likely and depends on a delicate balance between the fitness costs and benefits of defence and the fitness impacts of natural enemies in both the native and introduced ranges. Even when the abundance of natural enemies is lower in the introduced range, the majority of scenarios do not favour evolution of less defended genotypes that are more competitive or more fecund, contrary to predictions of EICA. Perhaps surprisingly, we find that the level of damage by natural enemies in field surveys is not generally a good parameter for testing enemy release theory. Instead, common garden experiments characterizing fitness reaction norms of multiple genotypes from the native and introduced range are crucial to estimate the historic rate of adaptive evolution or predict it into the future. Incorporating spatial autocorrelation and methods from population genetics can further improve our understanding of the role of enemy release and evolution in the proliferation of invasive species.

Repeatable experiments with accurate data collection and reproducible analyses are fundamental to the scientific method but may be difficult to achieve in practice. Several flexible, open-source tools developed for the R and Python coding environments aid the reproducibility of data wrangling and analysis in scientific research. In contrast, analogous tools are generally lacking for earlier stages, such as systematic labelling and processing of field samples with hierarchical structure (e.g. time points of individuals from multiple lines or populations) or curating heterogenous data collected by different researchers over several years. Such tools are critical for modern research given trends toward globally distributed collaborators using higher-throughput technologies. As a step toward improving repeatability of methods for the collection of biological samples, and curation of biological data, we introduce the R package baRcodeR and the PyTrackDat pipeline in Python. The baRcodeR package provides tools for generating biologically informative, hierarchical labels with digitally encoded 2D barcodes that can be printed and scanned using low-cost commercial hardware. The PyTrackDat pipeline integrates with baRcodeR output to build a web interface for sample management and tracking along with data collection and curation. We briefly describe the application of principles from baRcodeR and PyTrackDat in three large research projects, which demonstrate their value to (i) help document sampling methods, (ii) facilitate collaboration and (iii) reduce opportunities for human errors and omissions that could otherwise propagate through downstream data analysis to compromise biological inference.

Biological invasions are 'natural' experiments that can improve our understanding of contemporary evolution. We evaluate evidence for population differentiation, natural selection and adaptive evolution of invading plants and animals at two nested spatial scales: (i) among introduced populations (ii) between native and introduced genotypes. Evolution during invasion is frequently inferred, but rarely confirmed as adaptive. In common garden studies, quantitative trait differentiation is only marginally lower (~3.5%) among introduced relative to native populations, despite genetic bottlenecks and shorter timescales (i.e. millennia vs. decades). However, differentiation between genotypes from the native vs. introduced range is less clear and confounded by nonrandom geographic sampling; simulations suggest this causes a high false-positive discovery rate (>50%) in geographically structured populations. Selection differentials (¦s¦) are stronger in introduced than in native species, although selection gradients (¦β¦) are not, consistent with introduced species experiencing weaker genetic constraints. This could facilitate rapid adaptation, but evidence is limited. For example, rapid phenotypic evolution often manifests as geographical clines, but simulations demonstrate that nonadaptive trait clines can evolve frequently during colonization (~two-thirds of simulations). Additionally, QST-FST studies may often misrepresent the strength and form of natural selection acting during invasion. Instead, classic approaches in evolutionary ecology (e.g. selection analysis, reciprocal transplant, artificial selection) are necessary to determine the frequency of adaptive evolution during invasion and its influence on establishment, spread and impact of invasive species. These studies are rare but crucial for managing biological invasions in the context of global change.

Biological invasions exert multiple pervasive effects on ecosystems, potentially disrupting species interactions and global ecological processes. Our ability to successfully predict and manage the ecosystem-level impacts of biological invasions is strongly dependent on our capacity to empirically characterize complex biological interactions and their spatiotemporal dynamics. In this chapter, we argue that the comprehensive integration of multiple complementary tools within the explicit context of ecological networks is essential for providing mechanistic insight into invasion processes and their impact across organizational levels. We provide an overview of traditional (stable isotopes, populations genetics) and emerging (metabarcoding, citizen science) techniques and methods, and their practical implementation in the context of biological invasions. We also present several currently available models and machine-learning approaches that could be used for predicting novel or undocumented interactions, thus allowing a more robust and cost-effective forecast of network and ecosystem stability. Finally, we discuss the importance of methodological advancements on the emergence of scientific and societal challenges for investigating local and global species histories with several skill sets.

Anthropogenic climate change has already altered the timing of major life-history transitions, such as the initiation of reproduction. Both phenotypic plasticity and adaptive evolution can underlie rapid phenological shifts in response to climate change, but their relative contributions are poorly understood. Here, we combine a continuous 38 year field survey with quantitative genetic field experiments to assess adaptation in the context of climate change. We focused on Boechera stricta (Brassicaeae), a mustard native to the US Rocky Mountains. Flowering phenology advanced significantly from 1973 to 2011, and was strongly associated with warmer temperatures and earlier snowmelt dates. Strong directional selection favoured earlier flowering in contemporary environments (2010–2011). Climate change could drive this directional selection, and promote even earlier flowering as temperatures continue to increase. Our quantitative genetic analyses predict a response to selection of 0.2 to 0.5 days acceleration in flowering per generation, which could account for more than 20 per cent of the phenological change observed in the long-term dataset. However, the strength of directional selection and the predicted evolutionary response are likely much greater now than even 30 years ago because of rapidly changing climatic conditions. We predict that adaptation will likely be necessary for long-term in situ persistence in the context of climate change.

Divergent natural selection promotes local adaptation and can lead to reproductive isolation of populations in contrasting environments; however, the genetic basis of local adaptation remains largely unresolved in natural populations. Local adaptation might result from antagonistic pleiotropy, where alternate alleles are favoured in distinct habitats, and polymorphism is maintained by selection. Alternatively, under conditional neutrality some alleles may be favoured in one environment but neutral at other locations. Antagonistic pleiotropy maintains genetic variation across the landscape; however, there is a systematic bias against discovery of antagonistic pleiotropy because the fitness benefits of local alleles need to be significant in at least two environments. Here, we develop a generally applicable method to investigate polygenic local adaptation and identify loci that are the targets of selection. This approach evaluates allele frequency changes after selection at loci across the genome to distinguish antagonistic pleiotropy from conditional neutrality and deleterious variation. We investigate local adaptation at the qualitative trait loci (QTL) level in field experiments, in which we expose 177 F(6) recombinant inbred lines and parental lines of Boechera stricta (Brassicaceae) to their parental environments over two seasons. We demonstrate polygenic selection for native alleles in both environments, with 2.8% of the genome exhibiting antagonistic pleiotropy and 8% displaying conditional neutrality. Our study strongly supports antagonistic pleiotropy at one large-effect flowering phenology QTL (nFT): native homozygotes had significantly greater probabilities of flowering than foreign homozygotes in both parental environments. Such large-scale field studies are essential to elucidate the genetic basis of adaptation in natural populations.

Evolution during biological invasion may occur over contemporary timescales, but the rate of evolutionary change may be inhibited by a lack of standing genetic variation for ecologically relevant traits and by fitness trade-offs among them. The extent to which these genetic constraints limit the evolution of local adaptation during biological invasion has rarely been examined. To investigate genetic constraints on life-history traits, we measured standing genetic variance and covariance in 20 populations of the invasive plant purple loosestrife (Lythrum salicaria) sampled along a latitudinal climatic gradient in eastern North America and grown under uniform conditions in a glasshouse. Genetic variances within and among populations were significant for all traits; however, strong intercorrelations among measurements of seedling growth rate, time to reproductive maturity and adult size suggested that fitness trade-offs have constrained population divergence. Evidence to support this hypothesis was obtained from the genetic variance-covariance matrix (G) and the matrix of (co)variance among population means (D), which were 79.8% (95% C.I. 77.7-82.9%) similar. These results suggest that population divergence during invasive spread of L. salicaria in eastern North America has been constrained by strong genetic correlations among life-history traits, despite large amounts of standing genetic variation for individual traits.

Theory suggests that the contemporary evolution of local adaptation may increase the rate of biological invasion, yet natural selection has rarely been measured in invasive species. A recently published model predicted that latitudinal variation in the strength of stabilizing selection on two correlated traits—flowering time and size—can result in local adaptation during plant invasion but implicitly assumed that population × environment interactions are weak. We tested for stabilizing selection and the effect of growing environment on 13 populations of the invasive plant Lythrum salicaria sampled along a latitudinal gradient. Plants were grown under uniform glasshouse conditions and in a 3-year common garden field experiment at midlatitude. Consistent with model assumptions, the ranking of population means was highly correlated across the 3 years, and with glasshouse measurements, despite significant interannual differences in flowering time and vegetative size. As predicted, the first principal componen...

Colonization may favor self‐compatibility (SC) in invasive plants, a process consistent with Baker’s law. We investigated this hypothesis in invasive eastern North American populations of tristylous Lythrum salicaria L. (purple loosestrife) by controlled self‐ and cross‐pollinations of 124 plants sampled from 12 populations grown under uniform glasshouse conditions. We evaluated whether populations at the northern front of the invasion exhibited higher levels of SC than southern populations, which are closer to source populations for the North American invasion. We also sought evidence for morph‐specific differences in the strength of trimorphic incompatibility. We used the ASTER statistical program to jointly model three measures of SC: (1) the probability of producing at least one seed following self‐pollination, (2) the number of fruits produced by self‐pollination, and (3) the average number of seeds per fruit resulting from self‐pollination. Just over one‐quarter of plants set at least one fruit afte...

Common garden studies are increasingly used to identify differences in phenotypic traits between native and introduced genotypes, often ignoring sources of among-population variation within each range. We re-analyzed data from 32 common garden studies of 28 plant species that tested for rapid evolution associated with biological invasion. Our goals were: (i) to identify patterns of phenotypic trait variation among populations within native and introduced ranges, and (ii) to explore the consequences of this variation for how differences between the ranges are interpreted. We combined life history and physiologic traits into a single principal component (PCALL) and also compared subsets of traits related to size, reproduction, and defense (PCSIZE, PCREP, and PCDEF, respectively). On average, introduced populations exhibited increased growth and reproduction compared to native conspecifics when latitude was not included in statistical models. However, significant correlations between PC-scores and latitude were detected in both the native and introduced ranges, indicating population differentiation along latitudinal gradients. When latitude was explicitly incorporated into statistical models as a covariate, it reduced the magnitude and reversed the direction of the effect for PCALL and PCSIZE. These results indicate that unrecognized geographic clines in phenotypic traits can confound inferences about the causes of evolutionary change in invasive plants.

Invasions biologists have frequently debated whether the definition of invasive should include ecological and economic impacts. More recent criticisms posit that objective definitions are impossible in any absolute sense, while subjectivity is desirable for its flexibility and motivational qualities. We argue that such criticisms underestimate the extent of subjectivity already present in invasion biology. Ecological questions may be methodological if they relate directly to other ecological theories and models, or motivational if they focus on issues important to society as a whole. Motivational questions are important for engaging scientists, improving public understanding of science, and often have applied benefits. In contrast, methodological questions are constrained by established scientific theories, and are therefore more efficient for the development of scientific knowledge. Contrary to recent critiques, we suggest that greater objectivity is both achievable and desirable for the discipline of invasion biology and ecology generally.

Recent biological invasions provide opportunities to investigate microevolution during contemporary timescales. The tempo and scope of local adaptation will be determined by the intensity of natural selection and the amounts and kinds of genetic variation within populations. In flowering plants, genetic diversity is strongly affected by interactions between reproductive systems and stochastic forces associated with immigration history and range expansion. Here, we explore the significance of reproductive system diversity for contemporary evolution during plant invasion. We focus in particular on how reproductive modes influence the genetic consequences of long-distance colonization and determine the likelihood of adaptive responses during invasion. In many clonal invaders, strong founder effects and restrictions on sexual reproduction limit opportunities for local adaptation. In contrast, adaptive changes to life-history traits should be a general expectation in both outbreeding and inbreeding species. We provide evidence that evolutionary modifications to reproductive systems promote the colonizing ability of invading populations and that reproductive timing is an important target of selection during range expansion. Knowledge of the likelihood and speed at which local adaptation evolves in invasive plants will be particularly important for management practices when evolutionary changes enhance ecological opportunities and invasive spread.

The Eurasian spiny waterflea ( Bythotrephes longimanus ) is a predacious zooplankter that has increased its range in Europe and is rapidly invading inland water-bodies throughout North America’s Great Lakes region. To examine the genetics of these invasions, we isolated five microsatellite DNA loci with between 5 and 19 alleles per locus. We sampled three populations where B. longimanus has been historically present (Switzerland, Italy, and Finland) as well as an introduced European population (the Netherlands) and three North American populations (Lakes Erie, Superior, Shebandowan). Consistent with a bottleneck during colonization (i.e. founder effect), average heterozygosities of the four European populations ranged from 0.310 to 0.599, and were higher than that of three North American populations (0.151–0.220). Pairwise F ST estimates among North American populations (0.002–0.063) were not significantly different from zero and were much lower than among European populations (0.208–0.474). This is consistent with a scenario of high gene flow among North American populations relative to that of European ones. Contrary to an invasion bottleneck, however, Erie and Superior populations contained similar numbers of rare alleles as European populations. Assignment tests identified several migrant genotypes in all introduced populations (the Netherlands, Erie, Superior, Shebandowan), but rarely in native ones (Switzerland, Italy and Finland). A large number of genotypes from North America were assigned to our Italian population suggesting a second, previously unidentified, invasion source somewhere in the region of northern Italy. Together, our results support an invasion bottleneck for North American populations that has been largely offset by gene flow from multiple native sources, as well as gene flow among introduced populations.

Invade and Adapt The mechanisms by which plant and animal species spread into new habitats have become an increasing focus of ecological research, particularly in the context of climate change and species invasions. Colautti and Barrett (p. 364 ) examined the ecological consequence of local adaptation evolving rapidly along a 1000-kilometer climatic gradient in purple loosestrife (Lythrum salicaria) , one of the most notorious invasive plant species in North America. These invasive populations have evolved to become locally adapted within 50 to 100 years with important ecological consequences—increasing reproductive output by more than an order of magnitude.

A recent trend in invasion ecology relates the success of non-indigenous species (NIS) to reduced control by enemies such as pathogens, parasites and predators (i.e. the enemy release hypothesis, ERH). Despite the demonstrated importance of enemies to host population dynamics, studies of the ERH are split - biogeographical analyses primarily show a reduction in the diversity of enemies in the introduced range compared with the native range, while community studies imply that NIS are no less affected by enemies than native species in the invaded community. A broad review of the invasion literature implies at least eight non-exclusive explanations for this enigma. In addition, we argue that the ERH has often been accepted uncritically wherever (i) NIS often appear larger, more fecund, or somehow 'better' than either congeners in the introduced region, or conspecifics in the native range; and (ii) known enemies are conspicuously absent from the introduced range. However, all NIS, regardless of their abundance or impact, will lose natural enemies at a biogeographical scale. Given the complexity of processes that underlie biological invasions, we argue against a simple relationship between enemy 'release' and the vigour, abundance or impact of NIS.

The use of simple terms to articulate ecological concepts can confuse ideological debates and undermine management efforts. This problem is particularly acute in studies of nonindigenous species, which alternatively have been called ‘exotic’, ‘introduced’, ‘invasive’ and ‘naturalised’, among others. Attempts to redefine commonly used terminology have proven difficult because authors are often partial to particular definitions. In an attempt to form a consensus on invasion terminology, we synthesize an invasional framework based on current models that break the invasion process into a series of consecutive, obligatory stages. Unlike previous efforts, we propose a neutral terminology based on this framework. This ‘stagebased’ terminology can be used to supplement terms with ambiguous meanings (e.g. invasive, introduced, naturalized, weedy, etc.), and thereby improve clarity of future studies. This approach is based on the concept of ‘propagule pressure’ and has the additional benefit of identifying factors affecting the success of species at each stage. Under this framework, invasions can be more objectively understood as biogeographical, rather than taxonomic, phenomena; and author preferences in the use of existing terminology can be addressed. An example of this recommended protocol might be: ‘We examined distribution data to contrast the characteristics of invasive species (stages IVa and V) and noninvasive species (stages III and IVb)’.

Since completion of the St. Lawrence Seaway in 1959, at least 43 nonindigenous species (NIS) of animals and protists have established in the Laurentian Great Lakes, of which ~67% were attributed to discharge of ballast water from commercial ships. Twenty-three NIS were first discovered in four "hotspot" areas with a high representation of NIS, most notably the Lake Huron – Lake Erie corridor. Despite implementation of the voluntary (1989, Canada) and mandatory (1993, U.S.A.) ballast water exchange (BWE) regulations, NIS were discovered at a higher rate during the 1990s than in the preceding three decades. Here we integrate knowledge of species' invasion histories, shipping traffic patterns, and physicochemical factors that constrain species' survivorship during ballast-mediated transfer to assess the risk of future introductions to the Great Lakes. Our risk-assessment model identified 26 high-risk species that are likely to survive intercontinental transfer in ballast tanks. Of these, 10 species have already invaded the Great Lakes. An additional 37 lower-risk species, of which six have already invaded, show some but not all attributes needed for successful introduction under current BWE management. Our model indicates that the Great Lakes remain vulnerable to ship-mediated NIS invasions.

Warmer and drier climates have shifted phenologies of many species. However, the magnitude and direction of phenological shifts vary widely among taxa, and it is often unclear when shifts are adaptive or how they affect long-term viability. Here, we model evolution of flowering phenology based on our long-term research of two species exhibiting opposite shifts in floral phenology: Lythrum salicaria , which is invasive in North America, and the sparse Rocky Mountain native Boechera stricta . Genetic constraints are similar in both species, but differences in the timing of environmental conditions that favour growth lead to opposite phenological shifts under climate change. As temperatures increase, selection is predicted to favour earlier flowering in native B. stricta while reducing population viability, even if populations adapt rapidly to changing environmental conditions. By contrast, warming is predicted to favour delayed flowering in both native and introduced L. salicaria populations while increasing long-term viability. Relaxed selection from natural enemies in invasive L. salicaria is predicted to have little effect on flowering time but a large effect on reproductive fitness. Our approach highlights the importance of understanding ecological and genetic constraints to predict the ecological consequences of evolutionary responses to climate change on contemporary timescales. This article is part of the themed issue ‘Human influences on evolution, and the ecological and societal consequences’.

Invasive and endangered species reflect opposite ends of a spectrum of ecological success, yet they experience many similar eco-evolutionary challenges including demographic bottlenecks, hybridization and novel environments. Despite these similarities, important differences exist. Demographic bottlenecks are more transient in invasive species, which (i) maintains ecologically relevant genetic variation, (ii) reduces mutation load, and (iii) increases the efficiency of natural selection relative to genetic drift. Endangered species are less likely to benefit from admixture, which offsets mutation load but also reduces fitness when populations are locally adapted. Invading species generally experience more benign environments with fewer natural enemies, which increases fitness directly and also indirectly by masking inbreeding depression. Adaptive phenotypic plasticity can maintain fitness in novel environments but is more likely to evolve in invasive species encountering variable habitats and to be compromised by demographic factors in endangered species. Placed in an eco-evolutionary context, these differences affect the breadth of the ecological niche, which arises as an emergent property of antagonistic selection and genetic constraints. Comparative studies of invasions and extinctions that apply an eco-evolutionary perspective could provide new insights into the environmental and genetic basis of ecological success in novel environments and improve efforts to preserve global biodiversity. This article is part of the themed issue ‘Human influences on evolution, and the ecological and societal consequences’.

Recent advances in molecular genetics combined with field manipulations are yielding new insight into the origin, evolutionary fate, and genetic architecture of phenotypic variation in natural plant populations, with two surprising implications for the evolution of plant genomes. First, genetic loci exhibiting antagonistic pleiotropy across natural environments appear rare relative to loci that are adaptive in one or more environments and neutral elsewhere. These ‘conditionally neutral’ alleles should sweep to fixation when they arise, yet genome comparisons find little evidence for such selective sweeps. Second, genes under biotic selection tend to be of larger effect than genes under abiotic selection. Recent theory suggests this may be a consequence of high gene flow among populations under selection for local adaptation.

Evolution during biological invasion may occur over contemporary timescales, but the rate of evolutionary change may be inhibited by a lack of standing genetic variation for ecologically relevant traits and by fitness trade-offs among them. The extent to which these genetic constraints limit the evolution of local adaptation during biological invasion has rarely been examined. To investigate genetic constraints on life-history traits, we measured standing genetic variance and covariance in 20 populations of the invasive plant purple loosestrife (Lythrum salicaria) sampled along a latitudinal climatic gradient in eastern North America and grown under uniform conditions in a glasshouse. Genetic variances within and among populations were significant for all traits; however, strong intercorrelations among measurements of seedling growth rate, time to reproductive maturity and adult size suggested that fitness trade-offs have constrained population divergence. Evidence to support this hypothesis was obtained from the genetic variance-covariance matrix (G) and the matrix of (co)variance among population means (D), which were 79.8% (95% C.I. 77.7-82.9%) similar. These results suggest that population divergence during invasive spread of L. salicaria in eastern North America has been constrained by strong genetic correlations among life-history traits, despite large amounts of standing genetic variation for individual traits.

Biological invasions by nonindigenous species (NIS) can have adverse effects on economically important goods and services, and sometimes result in an 'invisible tax' on natural resources (e.g. reduced yield). The combined economic costs of NIS may be significant, with implications for environmental policy and resource management; yet economic impact assessments are rare at a national scale. Impacts of nuisance NIS may be direct (e.g. loss of hardwood trees) or indirect (e.g. alteration of ecosystem services provided by growing hardwoods). Moreover, costs associated with these effects may be accrued to resources and services with clear 'market' values (e.g. crop production) and to those with more ambiguous, 'non-market' values (e.g. aesthetic value of intact forest). We characterised and projected economic costs associated with nuisance NIS in Canada, through a combination of case-studies and an empirical model derived from 21 identified effects of 16 NIS. Despite a severe dearth of available data, characterised costs associated with ten NIS in Canadian fisheries, agriculture and forestry totalled $187 million Canadian (CDN) per year. These costs were dwarfed by the 'invisible tax' projected for sixteen nuisance NIS found in Canada, which was estimated at between $13.3 and $34.5 billion CDN per year. Canada remains highly vulnerable to new nuisance NIS, but available manpower and financial resources appear insufficient to deal with this problem. © Springer 2006.

I have presented here an optimistic view of the CM model and its potential as a unifying framework for both ecological theory and management practices. However, this model is only in an early stage of development, and there is still much work to be done. In particular, it is, I think, largely impossible to come up with a general scheme for the operationalization of invasion terminology. Instead, research is needed to develop specific criteria to apply the CM model to particular systems of study. It is likely that a number of unforeseen difficulties will become apparent during this process. Such problems can only be addressed as they arise, but I am confident that the CM model, as elaborated here, is an important first step in the elimination of ecological phlogiston and the operationalization of invasion concepts. I strongly believe that the use of a stage-based model as an integrative framework will help to unite the rift between invasion ecology and resource management to the benefit of both disciplines.

The Eurasian spiny waterflea (Bythotrephes longimanus) is a predacious zooplankter that has increased its range in Europe and is rapidly invading inland water-bodies throughout North America's Great Lakes region. To examine the genetics of these invasions, we isolated five microsatellite DNA loci with between 5 and 19 alleles per locus. We sampled three populations where B. longimanus has been historically present (Switzerland, Italy, and Finland) as well as an introduced European population (the Netherlands) and three North American populations (Lakes Erie, Superior, Shebandowan). Consistent with a bottleneck during colonization (i.e. founder effect), average heterozygosities of the four European populations ranged from 0.310 to 0.599, and were higher than that of three North American populations (0.151-0.220). Pairwise F(ST) estimates among North American populations (0.002-0.063) were not significantly different from zero and were much lower than among European populations (0.208-0.474). This is consistent with a scenario of high gene flow among North American populations relative to that of European ones. Contrary to an invasion bottleneck, however, Erie and Superior populations contained similar numbers of rare alleles as European populations. Assignment tests identified several migrant genotypes in all introduced populations (the Netherlands, Erie, Superior, Shebandowan), but rarely in native ones (Switzerland, Italy and Finland). A large number of genotypes from North America were assigned to our Italian population suggesting a second, previously unidentified, invasion source somewhere in the region of northern Italy. Together, our results support an invasion bottleneck for North American populations that has been largely offset by gene flow from multiple native sources, as well as gene flow among introduced populations.

In this essay, we have provided a brief review of the rapidly expanding literature on biological invasions. Many of the hypotheses explored above are not mutually exclusive, and the processes that underlie them may act simultaneously, in concert or in opposition, to determine the ultimate success or failure of an invader at each invasion stage. Though we have tried to be thorough, experienced researchers in the field will no doubt formulate additional hypotheses that warrant investigation. To date, most experiments have focused on only a single hypothesis for a small subset of invasive species under particular environmental conditions. To arrive at a realistic understanding of the ecological processes underlying biological invasions, a more integrative approach is warranted — one that examines the relative importance of each processes for a variety of species under myriad environmental conditions. This will prove to be a daunting task indeed, but one that will perpetuate the expansion of the field of invasion ecology for years to come, thereby ensuring plenty of fruitful lines of research for the keen investigators of the future.

Release from parasites, pathogens or predators (i.e. enemies) is a widely cited 'rule of thumb' to explain the proliferation of nonindigenous species in their introduced regions (i.e. the 'enemy release hypothesis', or ERH). Indeed, profound effects of some parasites and predators on host populations are well documented. However, some support for the ERH comes from studies that find a reduction in the species richness of enemies in the introduced range, relative to the native range, of particular hosts. For example, data on helminth parasites of the European starling in both its native Eurasia and in North America support a reduction of parasites in the latter. However, North American 'founder' starlings were likely not chosen randomly from across Eurasia. This could result in an overestimation of enemy release since enemies affect their hosts on a population level. We control for the effects of subsampling colonists and find, contrary to previous reports, no evidence that introduced populations of starlings experienced a reduction in the species richness of helminth parasites after colonization of North America. These results highlight the importance of choosing appropriate contrast groups in biogeographical analyses of biological invasions to minimize the confounding effects of 'propagule biases'. © Springer 2005.

Release of contaminated ballast water by transoceanic ships has been implicated in more than 70% of faunal nonindigenous species (NIS) introductions to the Great Lakes since the opening of the St. Lawrence Seaway in 1959. Contrary to expectation, the apparent invasion rate increased after the initiation of voluntary guidelines in 1989 and mandatory regulations in 1993 for open-ocean ballast water exchange by ships declaring ballast on board (BOB). However, more than 90% of vessels that entered during the 1990s declared no ballast on board (NOBOB) and were not required to exchange ballast, although their tanks contained residual sediments and water that would be discharged in the Great Lakes. Lake Superior receives a disproportionate number of discharges by both BOB and NOBOB ships, yet it has sustained surprisingly few initial invasions. Conversely, the waters connecting Lakes Huron and Erie are an invasion hotspot despite receiving disproportionately few ballast discharges. Other vectors, including canals and accidental release, have contributed NIS to the Great Lakes and may increase in relative importance in the future. Based on our knowledge of NIS previously established in the basin, we have developed a vector assignment protocol to systematically ascertain vectors by which invaders enter the Great Lakes.

Invasion ecology has been criticised for its lack of general principles. To explore this criticism, we conducted a meta-analysis that examined characteristics of invasiveness (i.e. the ability of species to establish in, spread to, or become abundant in novel communities) and invasibility (i.e. the susceptibility of habitats to the establishment or proliferation of invaders). There were few consistencies among invasiveness characteristics (3 of 13): established and abundant invaders generally occupy similar habitats as native species, while abundant species tend to be less affected by enemies; germination success and reproductive output were significantly positively associated with invasiveness when results from both stages (establishment/spread and abundance/impact) were combined. Two of six invasibility characteristics were also significant: communities experiencing more disturbance and with higher resource availability sustained greater establishment and proliferation of invaders. We also found that even though 'propagule pressure' was considered in only ∼29% of studies, it was a significant predictor of both invasiveness and invasibility (55 of 64 total cases). Given that nonindigenous species are likely introduced non-randomly, we contend that 'propagule biases' may confound current paradigms in invasion ecology. Examples of patterns that could be confounded by propagule biases include characteristics of good invaders and susceptible habitats, release from enemies, evolution of 'invasiveness', and invasional meltdown. We conclude that propagule pressure should serve as the basis of a null model for studies of biological invasions when inferring process from patterns of invasion. © Springer 2006.