Exploiting Allee effects for managing biological invasions

Biological invasions are a global and increasing threat to the function and diversity of ecosystems. Allee effects (positive density dependence) have been shown to play an important role in the establishment and spread of non-native species. Although Allee effects can be considered a bane in conservation efforts, they can be a benefit in attempts to manage non-native species. Many biological invaders are subject to some form of an Allee effect, whether due to a need to locate mates, cooperatively feed or reproduce or avoid becoming a meal, yet attempts to highlight the specific exploitation of Allee effects in biological invasions are surprisingly unprecedented. In this review, we highlight current strategies that effectively exploit an Allee effect, and propose novel means by which Allee effects can be manipulated to the detriment of biological invaders. We also illustrate how the concept of Allee effects can be integral in risk assessments and in the prioritization of resources allocated to manage non-native species, as some species beset by strong Allee effects could be less successful as invaders. We describe how tactics that strengthen an existing Allee effect or create new ones could be used to manage biological invasions more effectively.     

Exploring the potential of using priority effects during ecological restoration to resist biological invasions in the neotropics

Manipulating plant order of arrival, a process that creates priority effects, may be an unexplored powerful tool to hinder the establishment of invasive non‐native plants in sites under restoration. Knowledge and experimental studies on priority effects in the neotropics are scarce. Here, we propose a research agenda that investigates whether manipulating plant order of arrival can create priority effects in the neotropics, and if this strategy can be used to avoid the spread of undesired species in restoration projects. We also describe our view on expanding existing knowledge on priority effects to the neotropics and identifying key questions for future research.     

Climate change and biological invasions: evidence,expectations, and response options

A changing climate may directly or indirectly influence biological invasions by altering the likelihood of introduction or establishment, as well as modifying the geographic range, environmental impacts, economic costs or management of alien species. A comprehensive assessment of empirical and theoretical evidence identified how each of these processes is likely to be shaped by climate change for alien plants, animals and pathogens in terrestrial, freshwater and marine environments of Great Britain. The strongest contemporary evidence for the potential role of climate change in the establishment of new alien species is for terrestrial arthropods, as a result of their ectothermic physiology, often high dispersal rate and their strong association with trade as well as commensal relationships with human environments. By contrast, there is little empirical support for higher temperatures increasing the rate of alien plant establishment due to the stronger effects of residence time and propagule pressure. The magnitude of any direct climate effect on the number of new alien species will be small relative to human‐assisted introductions driven by socioeconomic factors. Casual alien species (sleepers) whose population persistence is limited by climate are expected to exhibit greater rates of establishment under climate change assuming that propagule pressure remains at least at current levels. Surveillance and management targeting sleeper pests and diseases may be the most cost‐effective option to reduce future impacts under climate change. Most established alien species will increase their distribution range in Great Britain over the next century. However, such range increases are very likely be the result of natural expansion of populations that have yet to reach equilibrium with their environment, rather than a direct consequence of climate change. To assess the potential realised range of alien species will require a spatially explicit approach that not only integrates bioclimatic suitability and population‐level demographic rates but also simulation of landscape‐level processes (e.g. dispersal, land‐use change, host/habitat distribution, non‐climatic edaphic constraints). In terms of invasive alien species that have known economic or biodiversity impacts, the taxa that are likely to be the most responsive are plant pathogens and insect pests of agricultural crops. However, the extent to which climate adaptation strategies lead to new crops, altered rotations, and different farming practices (e.g. irrigation, fertilization) will all shape the potential agricultural impacts of alien species. The greatest uncertainty in the effects of climate change on biological invasions exists with identifying the future character of new species introductions and predicting ecosystem impacts. Two complementary strategies may work under these conditions of high uncertainty: (i) prioritise ecosystems in terms of their perceived vulnerability to climate change and prevent ingress or expansion of alien species therein that may exacerbate problems; (ii) target those ecosystem already threatened by alien species and implement management to prevent the situation deteriorating under climate change.     

Understanding trends in biological invasions by introduced mammals in southern South America: a review of research and management

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The shape of the spatial kernel and its implications for biological invasions in patchy environments

Ecological and epidemiological invasions occur in a spatial context. We investigated how these processes correlate to the distance dependence of spread or dispersal between spatial entities such as habitat patches or epidemiological units. Distance dependence is described by a spatial kernel, characterized by its shape (kurtosis) and width (variance). We also developed a novel method to analyse and generate point-pattern landscapes based on spectral representation. This involves two measures: continuity, which is related to autocorrelation and contrast, which refers to variation in patch density. We also analysed some empirical data where our results are expected to have implications, namely distributions of trees (Quercus and Ulmus) and farms in Sweden. Through a simulation study, we found that kernel shape was not important for predicting the invasion speed in randomly distributed patches. However, the shape may be essential when the distribution of patches deviates from randomness, particularly when the contrast is high. We conclude that the speed of invasions depends on the spatial context and the effect of the spatial kernel is intertwined with the spatial structure. This implies substantial demands on the empirical data, because it requires knowledge of shape and width of the spatial kernel, and spatial structure.     

Boom‐bust dynamics in biological invasions: towards an improved application of the concept

Boom‐bust dynamics – the rise of a population to outbreak levels, followed by a dramatic decline – have been associated with biological invasions and offered as a reason not to manage troublesome invaders. However, boom‐bust dynamics rarely have been critically defined, analyzed, or interpreted. Here, we define boom‐bust dynamics and provide specific suggestions for improving the application of the boom‐bust concept. Boom‐bust dynamics can arise from many causes, some closely associated with invasions, but others occurring across a wide range of ecological settings, especially when environmental conditions are changing rapidly. As a result, it is difficult to infer cause or predict future trajectories merely by observing the dynamic. We use tests with simulated data to show that a common metric for detecting and describing boom‐bust dynamics, decline from an observed peak to a subsequent trough, tends to severely overestimate the frequency and severity of busts, and should be used cautiously if at all. We review and test other metrics that are better suited to describe boom‐bust dynamics. Understanding the frequency and importance of boom‐bust dynamics requires empirical studies of large, representative, long‐term data sets that use clear definitions of boom‐bust, appropriate analytical methods, and careful interpretations.     

Eight questions about invasions and ecosystem functioning

I pose eight questions central to understanding how biological invasions affect ecosystems, assess progress towards answering those questions and suggest ways in which progress might be made. The questions concern the frequency with which invasions affect ecosystems; the circumstances under which ecosystem change is most likely; the functions that are most often affected by invaders; the relationships between changes to ecosystems, communities, and populations; the long-term responses of ecosystems to invasions; interactions between biological invasions and other anthropogenic activities and the difficulty of managing undesirable impacts of non-native species. Some questions have been answered satisfactorily, others require more data and thought, and others might benefit from being reformulated or abandoned. Actions that might speed progress include careful development of trait-based approaches; strategic collection and publication of new data, including more frequent publication of negative results; replacement of expert opinion with hard data where needed; careful consideration of whether questions really need to be answered, especially in cases where answers are being provided for managers and policy-makers; explicit attention to and testing of the domains of theories; integrating invasions better into an ecosystem context; and remembering that our predictive ability is limited and will remain so for the foreseeable future.     

Evolutionary history predicts high‐impact invasions by herbivorous insects

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Callosciurus squirrels: worldwide introductions,ecological impacts and recommendations to prevent the establishment of new invasive populations

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The evolutionary consequences of biological invasions

A major challenge of invasion biology is the development of a predictive framework that prevents new invasions. This is inherently difficult because different biological characteristics are important at the different stages of invasion: opportunity/transport, establishment and spread. Here, we draw from recent research on a variety of taxa to examine the evolutionary causes and consequences of biological invasions. The process of introduction may favour species with characteristics that promote success in highly disturbed, human-dominated landscapes, thus exerting novel forms of selection on introduced populations. Moreover, evidence is accumulating that multiple introductions can often be critical to the successful establishment and spread of introduced species, as they may be important sources of genetic variation necessary for adaptation in new environments or may permit the introduction of novel traits. Thus, not only should the introduction of new species be prevented, but substantial effort should also be directed to preventing the secondary introduction of previously established species (and even movement of individuals among introduced populations). Modern molecular techniques can take advantage of genetic changes postintroduction to determine the source of introduced populations and their vectors of spread, and to elucidate the mechanisms of success of some invasive species. Moreover, the growing availability of genomic tools will permit the identification of underlying genetic causes of invasive success.     

Stronger regional biosecurity is essential to prevent hundreds of harmful biological invasions

Biological invasions often transcend political boundaries, but the capacity of countries to prevent invasions varies. How this variation in biosecurity affects the invasion risks posed to the countries involved is unclear. We aimed to improve the understanding of how the biosecurity of a country influences that of its neighbours. We developed six scenarios that describe biological invasions in regions with contiguous countries. Using data from alien species databases, socio‐economic and biodiversity data and species distribution models, we determined where 86 of 100 of the world's worst invasive species are likely to invade and have a negative impact in the future. Information on the capacity of countries to prevent invasions was used to determine whether such invasions could be avoided. For the selected species, we predicted 2,523 discrete invasions, most of which would have significant negative impacts and are unlikely to be prevented. Of these invasions, approximately a third were predicted to spread from the country in which the species first establishes to neighbouring countries where they would cause significant negative impacts. Most of these invasions are unlikely to be prevented as the country of first establishment has a low capacity to prevent invasions or has little incentive to do so as there will be no impact in that country. Regional biosecurity is therefore essential to prevent future harmful biological invasions. In consequence, we propose that the need for increased regional co‐operation to combat biological invasions be incorporated in global biodiversity targets.     

生物入侵对鸟类的生态影响

生物入侵是全球生物多样性面临的最主要威胁之一, 入侵种在改变入侵地环境的同时也使当地的生物受到极大影响。鸟类在生态系统中处于较高的营养级, 生态系统中任何一个环节的变化都可能对鸟类造成一定的影响。本文回顾了哺乳动物、鸟类、无脊椎动物和植物等不同生物类群的入侵对本地鸟类生态影响方面的研究进展。外来生物对鸟类的影响主要表现在以下几方面: (1)外来哺乳动物对成鸟、幼鸟或鸟卵的捕食作用; (2)外来鸟类与本地鸟类竞争栖息地和食物资源, 与当地的近缘种杂交而造成基因流失; (3)外来无脊椎动物改变本地鸟类的栖息环境和食物状况, 甚至直接捕食本地鸟类; (4)外来植物入侵改变入侵地的植物群落组成和结构, 造成本地鸟类的栖息地丧失或破碎化, 并通过改变入侵地生态系统的食物链结构而对高营养级的鸟类产生影响。最后, 作者还提出了该领域有待解决的问题和今后可能的研究方向。     

Contrasting patterns in the invasions of European terrestrial and freshwater habitats by alien plants,insects and vertebrates

Aim To provide the first comparative overview on the current numbers of alien species that invade representative European terrestrial and freshwater habitats for a range of taxonomic groups. Location Europe. Methods Numbers of naturalized alien species of plants, insects, herptiles, birds and mammals occurring in 10 habitats defined according to the European Nature Information System (EUNIS) were obtained from 115 regional data sets. Only species introduced after ad 1500 were considered. Data were analysed by ANCOVA and regression trees to assess whether differences exist among taxonomic groups in terms of their habitat affinity, and whether the pattern of occurrence of alien species in European habitats interacts with macroecological factors such as insularity, latitude or area. Results The highest numbers of alien plant and insect species were found in human‐made, urban or cultivated habitats; if controlled for habitat area in the region, wetland and riparian habitats appeared to support relatively high numbers of alien plant species too. Invasions by vertebrates were more evenly distributed among habitats, with aquatic and riparian, woodland and cultivated land most invaded. Mires, bogs and fens, grassland, heathland and scrub were generally less invaded. Habitat and taxonomic group explained most variation in the proportions of alien species occurring in individual habitats related to the total number of alien species in a region, and the basic pattern determined by these factors was fine‐tuned by geographical variables, namely by the mainland–island contrast and latitude, and differed among taxonomic groups. Main conclusions There are two ecologically distinct groups of alien species (plants and insects versus vertebrates) with strikingly different habitat affinities. Invasions by these two contrasting groups are complementary in terms of habitat use, which makes an overall assessment of habitat invasions in Europe possible. Since numbers of naturalized species in habitats are correlated among taxa within these two groups, the data collected for one group of vertebrates, for example, could be used to estimate the habitat‐specific numbers of alien species for other vertebrate groups with reasonable precision, and the same holds true for insects and plants.     

Porcine beta-lactoglobulin chemical unfolding: identification of a non-native alpha-helical intermediate

The chemical unfolding behavior of porcine beta-lactoglobulin (PLG) has been followed at pH 2 and 6 in the presence of guanidinium hydrochloride. The PLG unfolding transition, monitored by tryptophan fluorescence, far and near UV circular dichroism and 1D-NMR, can be described by a three-state transition suggesting the presence of at least one intermediate state that appears to display an excess of non-native alpha-helical structures. The thermodynamic parameters, as determined through a global analysis fitting procedure, give estimates of the free energy differences of the transitions connecting the native, the intermediate and the unfolded state: DeltaG(NI) (0) = 2.8 +/- 0.7 kcal mol(-1) (pH 2) and 4.2 +/- 0.5 kcal mol(-1) (pH 6) and DeltaG(NU) (0) = 7.2 +/- 0.6 kcal mol(-1) (pH 2) and 6.9 +/- 0.6 kcal mol(-1) (pH 6). CD unfolding data of the bovine species (BLG) have been collected here under the same experimental conditions of PLG to allow a careful comparison of the two beta-lactoglobulins. Intermediates with different characteristics have been identified for BLG and PLG, and their nature has been discussed on a structural analysis basis. The thermodynamic data reported here for PLG and BLG and the comparative analysis with data reported for equine beta lactoglobulin, show that homologous beta-barrel proteins, belonging to the same family and displaying high sequence identity (52-64%) populate unfolding intermediates to different extents, even though a common tendency to the formation of non-native alpha-helical intermediates, can be envisaged. The present results provide a prerequisite foundation of knowledge for the design and interpretation of future folding kinetic studies.