Invasibility as an emergent property of native metapopulation structure

Biological invasions constitute major threats to global biodiversity. Eco‐evolutionary considerations highlight the importance of contemporary evolution in community responses to bioinvasions. However, effects of metapopulation structure on invasion success have been mostly overlooked even though metapopulation structure determines gene flow and is likely to affect evolutionary processes. Here, we investigate a stepping‐stone model with evolving alien native interaction strengths. We demonstrate analytically that the site of invasion can determine the success of an invading consumer because gene flow and demography of a local resource species interact to obstruct local resource adaptation. Our main results are 1) that invasion success is more likely in genetic sink populations of the native species and 2) that invasion is more likely to occur against the migrational flow of native species. These findings suggest that invasibility is best regarded as an emergent property not only of communities but of entire metapopulations. Since migration networks of aliens and natives are often mismatched due to anthropogenic interference, our results indicate how population structure eases the spread of invasives against the migrational flow of natives.     

The effect of mating system on invasiveness: some genetic load may be advantageous when invading new environments

The role of adaptation in determining invasion success has been acknowledged recently, notably through the accumulation of case studies of rapid evolution during bioinvasions. Despite this growing body of empirical evidence, there is still a need to develop the theoretical background of invasions with adaptation. Specifically, the impact of mating system on the dynamics of adaptation during invasion of a new environment remains only partially understood. Here, we analyze a simulation demo-genetic model of bioinvasion accounting for partial asexuality rates. We simulate two levels of recurrent immigration from a source population at mutation–drift–selection equilibrium to a new empty environment with a different adaptive landscape (black-hole sink). Adaptation relies on a quantitative trait coded explicitly by 10 loci under mutation, selection and genetic drift. Using this model, we confirm previous results on the positive effects on invasiveness of migration, mutation and similarity of local phenotypic optima. We further show how the invasion dynamics of the introduced population is affected by the rate of asexuality. Purely asexual species have lower invasion success in terms of probability and time to invasion than species with other mating systems. Among species with mixed mating systems, the greatest invasiveness is observed for species with high asexual rates. We suggest that this pattern is due to inflated genetic variance in the source population through the Hill-Robertson effect (i.e., clonal interference). An interesting consequence is that species with the highest genetic load in their source environment have greatest invasiveness in the new environment.     

High genetic variance in life-history strategies within invasive populations by way of multiple introductions

Biological invasions represent major threats to biodiversity as well as large-scale evolutionary experiments. Invasive populations have provided some of the best known examples of contemporary evolution [3-6], challenging the classical view that invasive species are genetically depauperate because of founder effects. Yet the origin of trait genetic variance in invasive populations largely remains a mystery, precluding a clear understanding of how evolution proceeds. In particular, despite the emerging molecular evidence that multiple introductions commonly occur in the same place, their contribution to the evolutionary potential of invasives remains unclear. Here, by using a long-term field survey, mtDNA sequences, and a large-scale quantitative genetic experiment on freshwater snails, we document how a spectacular adaptive potential for key ecological traits can be accumulated in invasive populations. We provide the first direct evidence that multiple introductions are primarily responsible for such an accumulation and that sexual reproduction amplifies this effect by generating novel trait combinations. Thus bioinvasions, destructive as they may be, are not synonyms of genetic uniformity and can be hotspots of evolutionary novelty.     

Disentangling the effects of land use, shrub cover and climate on the invasion speed of native and introduced pines in grasslands

Aims To determine how changes in land use, climate and shrub cover affect the invasion dynamics of native (Pinus sylvestris L.) and introduced (Pinus nigra Arn. subsp. nigra) pines in grasslands. To analyse how these factors interact and affect seedling recruitment, a bottleneck in the lifecycle of many trees. Such information is required to manage the dynamics of these species. Location Grands Causses, calcareous plateaus (Southern France). Methods We used both published and unpublished demographic and dispersal data to assess population growth and invasion speed of invading pines. A demographic and spatially explicit model, which included density dependence and stochasticity in dispersal, demography and environment, was run for different scenarios of sheep grazing pressure (nil, extensive or intensive), shrub cover (0, 10 or 20%) and drought frequency (past‐to‐present or future). For each scenario, population growth rate, invasion speed and elasticity of invasion speed to each demographic and dispersal parameter were computed. Results Grazing was the main factor for limiting invasion speed. Shrub cover reduced tree spread under nil or extensive grazing pressure, but increased it under intensive grazing pressure. Although dry years led to nil seedling establishment rates, an increase in their frequency had surprisingly few effects on pine invasion speed. This last result remained unchanged when very dry years, inducing seedling, but also sapling mortality were introduced. In most environmental conditions, population growth rate and invasion speed were higher for the introduced than for the native pine. Elasticity analysis highlighted the importance of demographic parameters on invasion speed, notably adult and sapling survival. Main conclusion Tree invasion speed may rely at least as much on human activities, like sheep grazing, tree cutting and non‐native trees introduction, as on changes in climate factors. Therefore, human activities need to be explicitly taken into account in the prediction and management of tree dynamics.     

Historical range expansion determines the phylogenetic diversity introduced during contemporary species invasion

For a species rapidly expanding its geographic range, such as during biological invasion, most alleles in the introduced range will have their evolutionary origins in the native range. Yet, the way in which historical processes occurring over evolutionary time in the native range contribute to the diversity sampled during contemporary invasion is largely unknown. We used chloroplast DNA (cpDNA) gene genealogies and coalescent methods to study two congeneric plants, Silene latifolia and S. vulgaris. We examined how phylogenetic diversity was shaped by demographic growth and historical range expansions in the native European range, and how this history affected the diversity sampled during their recent invasion of North America. Genealogies from both species depart from neutrality, likely as a result of demographic expansion in the ancestral range, the timing of which corresponds to shortly after each species originated. However, the species differ in the spatial distribution of cpDNA lineages across the native range. Silene latifolia shows a highly significant phylogeographic structure that most likely reflects different avenues of the post-glacial expansion into northern Europe from Mediterranean refugia. By contrast, cpDNA lineages in S. vulgaris have been widely scattered across Europe during, or since, the most recent post-glacial expansion. These different evolutionary histories resulted in dramatic differences in how phylogenetic diversity was sampled during invasion of North America. In S. latifolia, relatively few, discrete invasion events from a structured native range resulted in a rather severe genetic bottleneck, but also opportunities for admixture among previously isolated lineages. In S. vulgaris, lack of genetic structure was accompanied by more representative sampling of phylogenetic diversity during invasion, and reduced potential for admixture. Our results provide clear insights into how historical processes may feed forward to influence the phylogenetic diversity of species invading new geographic ranges.     

Demography and dispersal: invasion speeds and sensitivity analysis in periodic and stochastic environments

Invasion speeds can be calculated from matrix integrodifference equation models that incorporate stage-specific demography and dispersal. These models also permit the calculation of the sensitivity and elasticity of invasion speed to changes in demographic and dispersal parameters. Such calculations have been used to understand the factors determining invasion speed and to explore possible tactics to manage invasive species. In this paper, we extend these calculations to temporally varying environments. We present formulas for the invasion speed and its sensitivity and elasticity in both periodic and stochastic environments. Periodic models can describe seasonal variation within a year, or can be used to study the frequency of occurrence of events (e.g., floods, fires) on interannual time scales. Stochastic models can incorporate variances, covariances, and temporal autocorrelation of parameters. We show that the invasion speed is calculated from a growth rate which is in turn calculated from a periodic or stochastic product of moment-generating function matrices. We present a new formulation of sensitivity analysis, using matrix calculus, that applies equally to constant, periodic, and stochastic environments.     

Demographic Stochasticity, Environmental Variability, and Windows of Invasion Risk for Bythotrephes Longimanus in North America

Biological invasions are a leading threat to freshwater biodiversity worldwide. A central unanswered question of invasion ecology is why some introduced populations establish while most fail. Answering this question will allow resource managers to increase the specificity and effectiveness of control efforts and policy. We studied the establishment of spiny water flea (Bythotrephes longimanus) in the United States and Canada by modeling introduction failure caused by demographic stochasticity, environmental variation, and seasonal environmental forcing. We compared predicted establishment rates with observed invasions of inland lakes in Ontario, Canada. Our findings suggest that environmental forcing can cause “windows” of invasion opportunity so that timing of introductions might be a greater determinant of population establishment than demographic stochasticity and random environmental variation. We expect this phenomenon to be exhibited by species representing a wide range of life histories. For spiny water flea in North America, a large window of invasion opportunity opens around the fourth week of May, persists through the summer, and closes with decreasing water temperatures in autumn. These results show how timing of introductions with respect to seasonally forced environmental drivers can be a key determinant of establishment success. By focusing on introductions during windows of invasion opportunity, resource managers can more effectively control invasion rates.     

The effect of phenotypic variation on metapopulation persistence

Demographic stochasticity (due to the probabilistic nature of the birth–death process) and demographic heterogeneity (between-individual differences in demographic parameters) have long been seen as factors affecting extinction risk. While demographic stochasticity can be independent of underlying species traits, demographic heterogeneity may strongly depend on phenotypic variation. However, how phenotypic variation can affect extinction risk is largely unknown. Here, I develop a stochastic metapopulation model that takes into account the effects of demographic stochasticity and phenotypic variation in the traits controlling colonization rates to assess what the effect of phenotypic variation may be on the persistence of the metapopulation. Although phenotypic variation can lead to a decrease in metapopulation persistence under some conditions, it also may lead to an increase in persistence whenever phenotypic mismatch—or the distance between the optimal trait value and the population mean—is large. This mismatch can in turn arise from a variety of ecological and evolutionary reasons, including weak selection or a recent history of invasion. Last, the effect of phenotypic variation has a deterministic component on colonization rates, and a stochastic component on persistence through colonization rates, but both are important to understand the overall effect. These results have important implications for the conservation of threatened species and management practices that may historically have overlooked phenotypic variation as unimportant noise around mean values of interest.     

A stochastic model for integrating changes in species richness and community similarity across spatial scales

Human activities have elevated the extinction of natural populations as well as the invasion of new areas by non-native species. These dual processes of invasion and extinction may change the richness and similarity of communities, but the form these changes take is likely to depend on the manner in which invasions and extinctions occur and the spatial scale at which the changes are measured. Here, we explore the influence of differing patterns of extinction and invasion on the similarity and richness of a meta-community. In particular, we model simple stochastic processes analogous to realistic modes of human-mediated introduction of non-native species and range expansion by native species. We show that different modes of invasion and extinction can produce very different changes in diversity, and that the relative magnitude of these changes depends both on where in the meta-community diversity is measured and the degree of initial species aggregation. At any spatial scale of measurement, changes in the richness and similarity of communities following invasion and extinction are not necessarily strongly coupled: relatively large increases in richness may or may not also be associated with relatively large increases in similarity among communities. Thus, in real systems, the influence of human-induced invasions and extinctions on diversity will depend on both the precise mode of these processes (especially invasion), and how species populations are distributed across space.     

Rapid increase in dispersal during range expansion in the invasive ladybird Harmonia axyridis

The evolutionary trajectories associated with demographic, genetic and spatial disequilibrium have become an issue of growing interest in population biology. Invasive species provide unique opportunities to explore the impact of recent range expansion on life‐history traits, making it possible to test for a spatial arrangement of dispersal abilities along the expanding range, in particular. We carried out controlled experiments in laboratory conditions to test the hypothesis of an increase in dispersal capacity with range expansion in Harmonia axyridis, a ladybird that has been invading Europe since 2001. We found a marked increase in the flight speed of the insects from the core to the front of the invasion range in two independent sampling transects. By contrast, we found that two other traits associated with dispersal (endurance and motivation to fly off) did not follow the same spatial gradient. Our results provide a striking illustration of the way in which predictable directional genetic changes may occur rapidly for some traits associated with dispersal during biological invasions. We discuss the consequences of our results for invasion dynamics and the evolutionary outcomes of spatially expanding populations.     

Quantifying stochastic introgression processes with hazard rates

Introgression is the permanent incorporation of genes from one population into another through hybridization and backcrossing. It can have large environmental consequences, such as the spread of insecticide or herbicide resistant genes, the escape of transgenes from genetically modified crops, and the invasion of exotic genes into new habitats. Introgression usually involves strong random components, such as rare hybridization and backcrossing events, and demographic variation in reproduction and survival. Most introgression studies ignore these random effects, and consequently fail to accurately assess the risk of introgression. This paper presents a methodology for quantifying stochastic introgression processes, based on multitype branching process models. We derive a quantity called the hazard rate, which can be used to investigate how the risk of introgression depends on crop management and life history.     

The effects of climate change and land‐use change on demographic rates and population viability

Understanding the processes that lead to species extinctions is vital for lessening pressures on biodiversity. While species diversity, presence and abundance are most commonly used to measure the effects of human pressures, demographic responses give a more proximal indication of how pressures affect population viability and contribute to extinction risk. We reviewed how demographic rates are affected by the major anthropogenic pressures, changed landscape condition caused by human land use, and climate change. We synthesized the results of 147 empirical studies to compare the relative effect size of climate and landscape condition on birth, death, immigration and emigration rates in plant and animal populations. While changed landscape condition is recognized as the major driver of species declines and losses worldwide, we found that, on average, climate variables had equally strong effects on demographic rates in plant and animal populations. This is significant given that the pressures of climate change will continue to intensify in coming decades. The effects of climate change on some populations may be underestimated because changes in climate conditions during critical windows of species life cycles may have disproportionate effects on demographic rates. The combined pressures of land‐use change and climate change may result in species declines and extinctions occurring faster than otherwise predicted, particularly if their effects are multiplicative.     

The grass may not always be greener: projected reductions in climatic suitability for exotic grasses under future climates in Australia

Climate change presents a new challenge for the management of invasive exotic species that threaten both biodiversity and agricultural productivity. The invasion of exotic perennial grasses throughout the globe is particularly problematic given their impacts on a broad range of native plant communities and livelihoods. As the climate continues to change, pre-emptive long-term management strategies for exotic grasses will become increasingly important. Using species distribution modelling we investigated potential changes to the location of climatically suitable habitat for some exotic perennial grass species currently in Australia, under a range of future climate scenarios for the decade centred around 2050. We focus on eleven species shortlisted or declared as the Weeds of National Significance or Alert List species in Australia, which have also become successful invaders in other parts of the world. Our results indicate that the extent of climatically suitable habitat available for all of the exotic grasses modelled is projected to decrease under climate scenarios for 2050. This reduction is most severe for the three species of Needle Grass (genus Nassella) that currently have infestations in the south-east of the continent. Combined with information on other aspects of establishment risk (e.g. demographic rates, human-use, propagule pressure), predictions of reduced climatic suitability provide justification for re-assessing which weeds are prioritised for intensive management as the climate changes.     

High parasite infection level in non‐native invasive species: it is just a matter of time

The enemy release hypothesis is often used to explain the success of non‐native species invasions. Growing evidence indicates that parasite or pathogen species richness increases over time in invasive non‐native species; however, this increase should not directly translate into release from enemy pressure as infection intensity of parasites (number of parasites per host) has a more profound impact on host fitness. The changes in intensity of parasitic infections in invasive non‐native species have not yet been thoroughly analysed in newly colonized areas. The goal of this study was to determine whether gastrointestinal parasite (nematode and trematode) infection intensity has increased with time since the populations of American mink Neovison vison were established and how host demographic parameters affect infection intensity. We tested the enemy release hypothesis by substituting space for time, evaluating parasite abundance in American mink at six sites along a chronosequence of mink invasion history. Nematode and trematode abundance increased with time since mink introduction, from a few parasites on average per mink after 16 yr, to 200–250 parasites per mink after 34 yr. The rate of increase in parasite abundance varied among demographic groups of mink (sex and age). Both nematodes and trematodes were more abundant in males than in females, and in subadults than in adults. Higher nematode abundance negatively affected body condition of mink. Our results provide evidence that non‐native species are released from enemy pressure only in the first phase of invasion, and that infection is modulated by host demographics and season. These results contribute to the evaluation of the long‐term patterns of parasite accumulation in invasive non‐native species after their colonization of new territories.     

入侵植物的生理生态特性对碳积累的影响

随着国际贸易的发展和人们交往的增加以及全球环境的变化,生物种类在全球扩散的机会也大大增加,从而为生物入侵创造了机会。生物入侵不仅给农林牧生产造成损失,而且具有长期的生态学效应。外来种的成功入侵不是其自身某一个特性决定的,而是其特性与新的生境综合作用的结果。外来入侵种生理生态特性的研究对其预测和防治具有重要的意义。目前对入侵种生理生态特性的研究较少。已有的研究表明,与本地种相比入侵种可能通过提高光合能力、资源利用率、表型可塑性、化感作用,以及降低繁殖成本等增加植株碳积累,促进其入侵。但并不是所有的入侵种都同时具有这些特性。生境不同限制性资源不同,入侵机制就不同。成功的入侵种应该能够高效地利用生境中的限制性资源,并且能够较快地调节自身的生理特性以适应波动的资源环境。     

Bayesian inference of a complex invasion history revealed by nuclear and chloroplast genetic diversity in the colonizing plant,Silene latifolia

Species invading new ranges are subject to a series of demographic events that can strongly shape genetic diversity. Describing this demographic history is important for understanding where invasive species come from and how they spread, and is critical to testing hypotheses of postinvasion adaptation. Here, we analyse nuclear and chloroplast genetic diversity to study the invasion history of the widespread colonizing weed, Silene latifolia (Caryophyllaceae). Bayesian clustering and PCA revealed strong population structure in the native range of Europe, and although genotypes from multiple native sources were present in the introduced range of North America, the spatial distribution of genetic variance was dramatically reorganized. Using approximate Bayesian computation (ABC), we compared support for different invasion scenarios, including the number and size of independent introduction events and the amount of admixture occurring between sources of introduced genotypes. Our results supported independent introductions into eastern and western North America, with the latter forming a bridgehead for a secondary invasion into the Great Lakes region of central North America. Despite small estimated founder population sizes, the duration of the demographic bottleneck after the initial introduction appeared extremely short‐lived. This pattern of repeated colonization and rapid expansion has effectively eroded the strong population structure and cytonuclear associations present in Europe, but has retained overall high genetic diversity since invasion. Our results highlight the flexibility of the ABC approach for constructing a narrative of the demographic history of species invasions and provide baseline for future studies of evolutionary changes in introduced S. latifolia populations.     

History, chance and adaptation during biological invasion: separating stochastic phenotypic evolution from response to selection

Introduced species often exhibit changes in genetic variation, population structure, selection regime and phenotypic traits as they colonize and expand into new ranges. For these reasons, species invasions are increasingly recognized as promising systems for studying adaptive evolution over contemporary time scales. However, changes in phenotypic traits during invasion occur under non-equilibrium demographic conditions and may reflect the influences of prior evolutionary history and chance events, as well as selection. We briefly review the evidence for phenotypic evolution and the role of selection during invasion. While there is ample evidence for evolutionary change, it is less clear if selection is the primary mechanism. We then discuss the likelihood that stochastic events shift phenotypic distributions during invasion, and argue that hypotheses of adaptation should be tested against appropriate null models. We suggest two experimental frameworks for separating stochastic evolution from adaptation: statistically accounting for phenotypic variation among putative invasion sources identified by using phylogenetic or assignment methods and by comparing estimates of differentiation within and among ranges for both traits and neutral markers ( Q _ST vs. F _ST). Designs that incorporate a null expectation can reveal the role of history and chance in the evolutionary process, and provide greater insights into evolution during species invasions.     

Neither variation loss,nor change in selfing rate is associated with the worldwide invasion of Physa acuta from its native North America

Whether bioinvasions are associated with a loss of genetic diversity and a change in mating system is instrumental for understanding the evolutionary fate of invasive species. Little loss is expected under strong propagule pressure which might be a general situation in widespread, efficient invader. In hermaphroditic species, we have few examples of a transition between outcrossing and selfing as a consequence of invasion, though this is classically predicted (as a corollary to Baker’s law). We estimated microsatellite variation in 44 populations of the widespread freshwater snail Physa acuta sampled at worldwide scale (including several populations from its native North America). Neither loss of variation (or bottleneck), nor increase in selfing rate was detected in invaded areas. Moreover there was no isolation by distance at large geographic scale, and limited geographic coherence in genetic patterns was detected using STRUCTURE software—the West Mediterranean area being an exception. Such patterns might be explained by (1) multiple introductions in the invaded areas, presumably fostered by aquarium trade, followed by regional spread in some cases—in which case mating partners might be numerous enough to prevent transition towards higher selfing rates, and (2) invasions from the whole native area. This suggests that P. acuta is an exceptionally efficient invader (which is not true of related species), but the reasons of its success remain elusive.     

Patterns of grassland invasions by trees: insights from demographic and genetic spatial analyses

Aims Woody invasions into grasslands have increased globally due to changing land use, climate and introduced woody species, but spatial processes generating and sustaining these invasions are not well understood. To gain insight into the patterns of spread of tree populations within grasslands, and to propose a full spatial analytical toolbox for studying native and non-native woody species spread when long-term data are not available, we tested if 50 years of grassland invasion in Western Carpathians by Norway spruce (Picea abies Karst.) proceeded by one of the two traditionally competing hypotheses of species spread: (i) by frontier expansion, or (ii) by advanced groups established ahead of the population frontier. We also tested whether the pattern of invasion changed over time.Methods We analyzed the spatial demographic and genetic patterns of a Norway spruce population invading a Western Carpathian grassland using Ripley's L (t) and genetic kinship coefficients (F ij). We mapped and genotyped spruce trees across the invasion front (from the invasion leading edge to fully colonized grassland near the source population) using three demographic classes (adults, juveniles and seedlings) to approximate the temporal aspects of the invasion. We studied how the spatial patterns of invasion by individual demographic classes and their genetic kinship varied among adjacent plots established at different distances from the source population (ranging from 0 to 160 m, in 40-m distance increments).Important findings Juveniles were positively genetically related to adults on fine scales (<4 m), suggesting that adults within the grassland acted as a seed source and accelerated early invasion. However, adults did not act as nucleation centers for the formation of advanced juvenile groups. Instead, genetically unrelated juveniles formed groups independently of adults. These groups were small and separate at the leading edge but they increased in size and graded into a continuous zone near the source population. Thus, juvenile recruitment occurred as a frontier expansion near the source population and as advanced groups controlled by environmental variation at the leading edge. Unlike juveniles, seedlings were clustered on all scales across the invasion front and formed groups around adult crowns at the invasion leading edge. The bulk of seedling establishment occurred at intermediate distances from the source population, independently from the adults, suggesting that the invasion front continued to expand as a frontier, gradually coalescing with the advanced groups at the leading edge. Thus, the grassland invasion was driven by a gradual frontier expansion of the original population during the first 50 years, with advanced groups enhancing but not driving the invasion process. Frontier expansion appeared more important as a mechanism of woody species spread early in the invasion process in this study, while advanced groups may play a larger role over longer temporal scales.