外来种隐蔽入侵： 类型及影响

隐蔽入侵（cryptic invasion）是指在形态上与土著种（或早期建群种）不能或难以区分的外来种在人们未觉察的状态下成功入侵的过程。人们对这类入侵方式往往视而不觉。本文综述了外来种隐蔽入侵的类型以及生态影响。隐蔽入侵的类型主要包括外来姊妹种形式的隐蔽入侵、 不同遗传支系的隐蔽入侵、 不同地理种群的隐蔽入侵以及“返传入”。其中, “返传入”目前还是一种假说。由于这类入侵外来种更容易与土著种（或早期建群种）杂交或基因渗透, 因而可对入侵种自身或土著种产生深远的生态影响。鉴于隐蔽入侵现象的广泛性, 建议进一步加强该方面的研究。     

福建外来物种入侵现状及对经济社会和生态的影响

福建已成为中国遭受外来物种入侵最严重的省份之一。本文概述了福建主要入侵物种的数量、种类、地理起源、分布、引入途径和入侵特点,系统分析了福建外来物种入侵严重的原因和外来物种入侵对福建生态安全、经济及社会的影响,在此基础上提出了针对外来物种入侵的防控对策。     

Secondary invasion: When invasion success is contingent on other invaders altering the properties of recipient ecosystems

Positive interactions between exotic species may increase ecosystem‐level impacts and potentially facilitate the entry and spread of other exotic species. Invader‐facilitated invasion success—”secondary invasion”—is a key conceptual aspect of the well‐known invasional meltdown hypothesis, but remains poorly defined and empirically underexplored. Drawing from heuristic models and published empirical studies, we explore this form of “secondary invasion” and discuss the phenomenon within the recognized conceptual framework of the determinants of invasion success. The term “secondary invasion” has been used haphazardly in the literature to refer to multiple invasion phenomena, most of which have other more accepted titles. Our usage of the term secondary invasion is akin to “invader‐facilitated invasion,” which we define as the phenomenon in which the invasion success of one exotic species is contingent on the presence, influence, and impacts of one or more other exotic species. We present case studies of secondary invasion whereby primary invaders facilitate the entry or establishment of exotic species into communities where they were previously excluded from becoming invasive. Our synthesis, discussion, and conceptual framework of this type of secondary invasion provides a useful reference to better explain how invasive species can alter key properties of recipient ecosystems that can ultimately determine the invasion success of other species. This study increases our appreciation for complex interactions following invasion and highlights the impacts of invasive species themselves as possible determinants of invasion success. We anticipate that highlighting “secondary invasion” in this way will enable studies reporting similar phenomena to be identified and linked through consistent terminology.     

Species composition,functional and phylogenetic distances correlate with success of invasive Chromolaena odorata in an experimental test

Biotic resistance may influence invasion success; however, the relative roles of species richness, functional or phylogenetic distance in predicting invasion success are not fully understood. We used biomass fraction of Chromolaena odorata, an invasive species in tropical and subtropical areas, as a measure of ‘invasion success’ in a series of artificial communities varying in species richness. Communities were constructed using species from Mexico (native range) or China (non‐native range). We found strong evidence of biotic resistance: species richness and community biomass were negatively related with invasion success; invader biomass was greater in plant communities from China than from Mexico. Harvesting time had a greater effect on invasion success in plant communities from China than on those from Mexico. Functional and phylogenetic distances both correlated with invasion success and more functionally distant communities were more easily invaded. The effects of plant‐soil fungi and plant allelochemical interactions on invasion success were species‐specific.     

Invasion Success and Community Resistance in Single and Multiple Species Invasion Models: Do the Models Support the Conclusions?

Elton's concept of community-level resistance to invasion has derived significant theoretical support from community assembly models in which species invade (colonize) singly at low densities. Several theoretical models have provided support to this concept and are frequently cited as providing evidence that invasion resistance occurs in nature. The underlying assumptions of these models however, are derived from island or island-like systems in which species invade infrequently at low abundances. We suggest that these island-like models cannot be generalized to systems in which species arrive in greater frequencies and densities. To investigate the effects of altering the basic assumptions of these original models, we utilized assembly algorithms similar to those used in previous studies, but allowed either two species to invade per time step or single species invasions at relatively high inoculation densities. In these models, invasion resistance only occurred when the invasion process was restricted to single species invading at low densities (as in previous models). When two species were allowed to invade per time step, invasion resistant states did not occur in any of 20 simulated communities, even after 10,000 invasion events. Relaxation of the assumption of invasion at low density also resulted in a lack of invasion resistance. These results may explain why the strict concept of complete invasion resistance appears only to operate in island and island-like systems.     

Running off the road: roadside non-native plants invading mountain vegetation

Prevention is regarded as a cost-effective management action to avoid unwanted impacts of non-native species. However, targeted prevention can be difficult if little is known about the traits of successfully invading non-native species or habitat characteristics that make native vegetation more resistant to invasion. Here, we surveyed mountain roads in seven regions worldwide, to investigate whether different species traits are beneficial during primary invasion (i.e. spread of non-native species along roadside dispersal corridors) and secondary invasion (i.e. percolation from roadsides into natural adjacent vegetation), and to determine if particular habitat characteristics increase biotic resistance to invasion. We found primary invasion up mountain roads tends to be by longer lived, non-ruderal species without seed dispersal traits. For secondary invasion, we demonstrate that both traits of the non-native species and attributes of the receiving natural vegetation contribute to the extent of invasion. Non-native species that invade natural adjacent vegetation tend to be shade and moisture tolerant. Furthermore, non-native species invasion was greater when the receiving vegetation was similarly rich in native species. Our results show how mountain roads define which non-native species are successful; first by favouring certain traits in mountain roadsides (the key dispersal pathway to the top), and secondly by requiring a different set of traits when species invade the natural adjacent vegetation. While patterns in species traits were observed at a global level, regional abiotic and biotic variables largely generated region-specific levels of response, suggesting that management should be regionally driven.     

Coexistence, saturation and invasion resistance in simulated plant assemblages

A popular hypothesis is that species-rich systems are less susceptible to invasion. This hypothesis is based on the idea that species richness correlates with community saturation so that establishment by a new species is more difficult in saturated communities. Little attention has been focussed on how changing assumptions about the processes regulating species richness will alter community properties such as invasion resistance. Here, we simulate plant community assembly using four models that have different underlying coexistence mechanisms (and so differ in the amount of available niche space) and subject them to invasion. We created species richness gradients by comparing between models or by considering the output of a single model with different parameter values. We found that the relation between species richness and invasion resistance depends critically on the model considered and the cause of the species richness gradient. Overall, our results suggest that species richness does not necessarily correlate with saturation and is likely to be a poor predictor of invasion resistance. These results provide a possible explanation for the variety of outcomes reported in recent experimental and observational studies that examine the relationship between species richness and invasion resistance. We conclude that consideration of the processes regulating species richness is crucial for a successful understanding of invasion resistance along species richness gradients.     

Invasion and coexistence of two phenotypically variable species

The effect of ecological polymorphism on the coexistence of species has not previously been studied. Using a model of competitive interactions within and among species I investigate invasion of both monomorphic and polymorphic species. Three comparisons are considered which place previous work in a broader context and extend it to cover the situations characterized by polymorphism. First, I examine invasion of a monomorphic species into the range of another monomorphic species. This is done in a manner which allows for generalization to the following more complex cases. Second, I examine invasion of a monomorphic species into the range of a polymorphic species. Finally, I examine invasion and coexistence of a polymorphic species in the range of another polymorphic species. This sequence of comparisons produces some simple generalizations concerning invasion and coexistence of species in a variety of ecological situations. Furthermore, it emphasizes polymorphism as a potentially important factor in shaping ecological communities.     

群落可侵入性及其影响因素

 可侵入性用于评价群落易遭受生物入侵的程度,受外来种死亡率、区域气候、干扰水平、生态系统抵抗入侵的能力、本地种竞争和抗干扰能力等因素的影响。当前对群落或区域间可侵入性的比较常以外来种数量或丰度为据,然而这两者仅代表了群落内单一的动态过程,不足以作为衡量群落可侵入性的广泛标准。借助一个描述外来种数量的简单模型阐明,由于影响可侵入性因素的复杂性,各地之间的可侵入性几乎不可比较。并从入侵过程、入侵种特性及本地种、本地群落对入侵的抵抗性几方面对群落可侵入性进行了阐述分析,其中着重介绍入侵生态中几个重要的概念,如可侵     

Beyond biodiversity: individualistic controls of invasion in a self-assembled community

Recent experimental and simulation results, and competition‐based ecological theory, predict a simple relationship between species richness and the invasibility of communities at small spatial scales – likelihood of invasion decreases with increasing richness. Here we show data from 42 continuous years of sampling old field succession that reveal quite different dynamics of plant invasion. Contrary to experimental studies, when richness was important in explaining invasion probability, it was typically positively associated with species invasion. Invasion of several species had a unimodal response to resident species richness, which appeared to be a mixture of compositional influences and a richness effect. Interestingly, invasions by native and exotic species did not fundamentally differ. Control of species invasion in this system is individualistic, caused by a variety of community‐level mechanisms rather than a single prevailing richness effect.     

外来种入侵的不确定性动态模拟

外来种的入侵性和生境的可入侵性是研究生物入侵机制的两个关键因素。总结多数学者的研究成果,认为外来种的拓殖率(迁移率或繁殖率)、死亡率及在新生境中的竞争力是衡量外来种能否入侵成功的内在实质。从全新的角度出发,将引入的外来种看作是对原有生态系统的一种干扰,并在此基础上结合Tilman的多物种共存模型提出了外来种干扰模型。模拟发现外来种在入侵时具有明显的不确定性。拓殖率小的外来种在新的生态系统中由于不能适应环境无法成功定殖,被排斥在系统之外;相反,拓殖率足够大的话,外来种由于自身优势具有很强的入侵性,在几年或几十年的时间内就会成功地在新的环境中建立种群并拓殖入侵,影响了当地物种的生存及原有生态系统的稳定。研究还发现,物种入侵也存在一定的不确定性,入侵不一定会一直持续下去,或许在百年或几百年的时间内入侵种会突然灭绝,原生态系统又恢复到原来的水平。     

In tallgrass prairie restorations,relatedness influences neighborhood-scale plant invasion while resource availability influences site-scale invasion

Attempting to control invasive plant species in tallgrass prairie restorations is time-consuming and costly, making improved approaches for predicting and reducing invasion imperative. Both biotic and abiotic factors mediate plant invasions, and can potentially be used by restoration managers to reduce invasion rates. Biotic factors such as plant species richness and phylogenetic diversity of the native community may impact invasion. Relatedness of invading species to those in recipient communities has also been shown to influence invasion success. However, the direction of this influence is variable, reflecting Darwin’s Naturalization Conundrum. Abiotic factors such as fire regime and soil factors may impact invasion by selecting against invasive species or indicating suitable habitats for them. We surveyed 17 tallgrass prairie restorations in Illinois, USA, to investigate the effects of biotic and abiotic factors on invasion by non-native plant species at two different scales. We predicted we would find support for Darwin’s Naturalization Hypothesis at the plot (neighborhood) scale with invasion by distantly related species, and find support for the Pre-adaptation Hypothesis at the site scale. We hypothesized that biotic factors would exert more influence at the neighborhood scale, while abiotic factors would be more influential at a coarser site scale. Contrary to our expectations, at the neighborhood scale we found that closely related invasive species are more likely to invade, supporting the Pre-adaptation Hypothesis. We found that native species richness and age of restoration were negatively correlated with invasion. At the site scale, soil organic matter [SOM] concentrations and heterogeneity in SOM were positively associated with the number of invasive species while pH heterogeneity was negatively associated. Restoration practitioners may be able to reduce plant invasions by increasing native species richness, and non-native species most closely related to the resident community should potentially be prioritized as those most likely to be highly invasive.     

Applying competition theory to invasion: resource impacts indicate invasion mechanisms in California shrublands

Despite widespread work documenting invasion, it remains a challenge to determine invasion mechanisms and incorporate them into invasive species management. Competition theory presents a strong model for evaluating the role of resource reduction and requirements in invasion. Additionally, alternative models suggest fluctuations in resources, niche differences, or non-resource priority effects are key factors determining invasion success. We propose a comparative framework that incorporates resource impacts of native and invasive species, performance in controlled invasion trials, and long-term natural invasion patterns to elucidate relative importance of these invasion mechanisms. To demonstrate this framework, we established monocultures of two representative native and two invasive plant species in Southern California’s coastal sage scrub (CSS), measured resource impacts (i.e., R*), and conducted invasion trials to test whether resource impacts predicted invasion success. We then related experimental results to field invasion patterns. Compared to exotic herbaceous species, native shrubs were associated with greater resource depletion of key resources: light, soil water (at multiple depths), and soil inorganic nitrogen (particularly at depth). In invasion trials, natives resisted invasion by the exotics, as resource depletion measures would predict. However, these results did not follow long-term natural invasion patterns indicating that these exotic species invade areas once dominated by native shrubland. Applying our results to the invasion framework, we conclude that disturbance, or a similar mechanism causing resources to fluctuate, is needed for exotics to invade CSS habitats. This resource-based comparative analysis of invasion mechanisms can point out important processes and help suggest effective management actions.     

Invasion of temperate deciduous broadleaf forests by N-fixing tree species – consequences for stream ecosystems

Biological invasions are a major threat to biodiversity and ecosystem functioning. Forest invasion by alien woody species can have cross-ecosystem effects. This is especially relevant in the case of stream–riparian forest meta-ecosystems as forest streams depend strongly on riparian vegetation for carbon, nutrients and energy. Forest invasion by woody species with dissimilar characteristics from native species may be particularly troublesome. The invasion of temperate deciduous broadleaf forests with low representation of nitrogen (N)-fixing species by N-fixers has the potential to induce ecosystem changes at the stream level. Although effects of tree invasion on stream ecosystems have been under assessed, knowledge of native and invasive tree characteristics allows prediction of invasion effects on streams. Here we present a conceptual model to predict the effects of forest invasion by alien N-fixing species on streams, using as a background the invasion of temperate deciduous broadleaf forests by leguminous Acacia species, which are among the most aggressive invaders worldwide. Effects are discussed using a trait-based approach to allow the model to be applied to other pairs of invaded ecosystem–invasive species, taking into account differences in species traits and environmental conditions. Anticipated effects of N-fixing species invasions include changes in water quality (increase in N concentration) and quantity (decrease in flow) and changes in litter input characteristics (altered diversity, seasonality, typology, quantity and quality). The magnitude of these changes will depend on the magnitude of differences in species traits, the extent and duration of the invasion and stream characteristics (e.g. basal nutrient concentration). The extensive literature on effects of nutrient enrichment of stream water, water scarcity and changes in litter input characteristics on aquatic communities and processes allows prediction of invasion effects on stream structure and function. The magnitude of invasion effects on aquatic communities and processes may, however, depend on interactions among different pathways (e.g. effects mediated by increases in stream nutrient concentration may contrast with those mediated by decreases in water availability or by decreases in litter nutritional quality). A review of the literature addressing effects of increasing cover of N-fixing species on streams suggests a wide application of the model, while it highlights the need to consider differences in the type of system and species when making generalizations. Changes induced by N-fixing species invasion on streams can jeopardize multiple ecosystem services (e.g. good quality water, hydroelectricity, leisure activities), with relevant social and economic consequences.     

The role of plant species size in invasibility: a field experiment

Large plant species self-thin to disproportionately lower densities than smaller plant species, and therefore may leave more patches of unused space suitable for invasion. Using experimental monocultures of 11 old-field perennial plant species differing in maximum size, as well as mixtures composed of all monoculture species, we tested our primary hypothesis that monocultures of larger species will be more susceptible to natural invasion. After 3 years, monocultures of larger species were invaded by a significantly greater number of species, and more ramets, from the surrounding vegetation. Invading plant species were significantly smaller than the monoculture species being invaded, suggesting that smaller plant species may be better invaders. Thus, we quantified a trade-off between species size, which is frequently associated with increased competitive ability for light, and invasibility, suggesting one reason why large and small species coexist in virtually all plant communities. Although we expected that invasion would enhance biomass production by more fully capturing available resources, we found that the most highly invaded plots of each species produced significantly less biomass. This suggests that increased diversity resulting from invasion did not result in complementary resource use. Mixture plots containing all experimental species did not admit a significantly different number of invading ramets or species than most monocultures, indicating no obvious role for diversity in resistance to invasion, or complementary resource use. Our results suggest that relatively large species may be limited in their capacity to competitively exclude other, smaller species from communities because pure stands of the former are more susceptible to invasion by the latter.     

Disentangling the effects of plant species invasion and urban development on arthropod community composition

Urban development and species invasion are two major global threats to biodiversity. These threats often co‐occur, as developed areas are more prone to species invasion. However, few empirical studies have tested if both factors affect biodiversity in similar ways. Here we study the individual and combined effects of urban development and plant invasion on the composition of arthropod communities. We assessed 36 paired invaded and non‐invaded sample plots, invaded by the plant Antigonon leptopus, with half of these pairs located in natural and the other half in developed land‐use types on the Caribbean island of St. Eustatius. We used several taxonomic and functional variables to describe community composition and diversity. Our results show that both urban development and A. leptopus invasion affected community composition, albeit in different ways. Development significantly increased species richness and exponential Shannon diversity, while invasion had no effect on these variables. However, invasion significantly increased arthropod abundance and caused biotic homogenization. Specifically, uninvaded arthropod communities were distinctly different in species composition between developed and natural sites, while they became undistinguishable after A. leptopus invasion. Moreover, functional variables were significantly affected by species invasion, but not by urban development. Invaded communities had higher community‐weighted mean body size and the feeding guild composition of invaded arthropod communities was characterized by the exceptional numbers of nectarivores, herbivores, and detritivores. With the exception of species richness and exponential Shannon diversity, invasion influenced four out of six response variables to a greater degree than urban development did. Hence, we can conclude that species invasion is not just a passenger of urban development but also a driver of change.     

Effects of plant invasions on the species richness of abandoned agricultural land

While exotic plant invasions are thought to lead to declines in native species, the long-term impacts of such invasions on community structure are poorly known. Furthermore, it is unknown how exotic plant invasions compare to invasions by native species. We present data from 40 yr of continuous vegetation sampling of 10 fields released from agriculture to examine the effects of invasions on species richness. The effects of both exotic and native species invasions on species richness were largely driven by variations among fields with most species not significantly affecting species richness. However, invasion and dominance by the exotics Agropyron repens, Lonicera japonica. Rosa multiflora. Trifolium pratense and the native Solidago canadensis were associated with declines in richness. Invasions by exotic and native species during old field succession have similar effects on species richness with dominance by species of either group being associated with loss of species richness. Exotic species invasions tended to have stronger effects on richness than native invasions. No evidence was found of residual effects of invasions because the impact of the invasion disappeared with the decline of the invading population. When pooled across species, heavy invasion by exotic species resulted in greater loss o species richness than invasion by native species. Studies of invasion that utilize multiple sites must account for variability among sites. In our study, had we no included field as a factor we would have incorrectly concluded that invasion consistently resulted in changes in species richness.     

Interaction of Species Traits and Environmental Disturbance Predicts Invasion Success of Aquatic Microorganisms

Factors such as increased mobility of humans, global trade and climate change are affecting the range of many species, and cause large-scale translocations of species beyond their native range. Many introduced species have a strong negative influence on the new local environment and lead to high economic costs. There is a strong interest to understand why some species are successful in invading new environments and others not. Most of our understanding and generalizations thereof, however, are based on studies of plants and animals, and little is known on invasion processes of microorganisms. We conducted a microcosm experiment to understand factors promoting the success of biological invasions of aquatic microorganisms. In a controlled lab experiment, protist and rotifer species originally isolated in North America invaded into a natural, field-collected community of microorganisms of European origin. To identify the importance of environmental disturbances on invasion success, we either repeatedly disturbed the local patches, or kept them as undisturbed controls. We measured both short-term establishment and long-term invasion success, and correlated it with species-specific life-history traits. We found that environmental disturbances significantly affected invasion success. Depending on the invading species’ identity, disturbances were either promoting or decreasing invasion success. The interaction between habitat disturbance and species identity was especially pronounced for long-term invasion success. Growth rate was the most important trait promoting invasion success, especially when the species invaded into a disturbed local community. We conclude that neither species traits nor environmental factors alone conclusively predict invasion success, but an integration of both of them is necessary.     

Biodiversity theory applied to the real world of ecological restoration

One of the perceived benefits of biodiversity is resistance to invasion by exotic species. This has relevance for vegetation restoration: according to theory, sowing more species of the desired type would help to exclude the invasion of undesired ones. Oakley & Knox (Applied Vegetation Science, this issue) tested this in a real restoration situation: the revegetation of bare compacted clay after construction or commercial activity. Higher sown diversity did indeed reduce the invasion of non‐sown species and, of particular practical relevance, reduced the invasion of exotic species.     

Quantifying levels of biological invasion: towards the objective classification of invaded and invasible ecosystems

Biological invasions are a global phenomenon that threatens biodiversity, and few, if any, ecosystems are free from alien species. The outcome of human‐mediated introductions is affected by the invasiveness of species and invasibility of ecosystems, but research has primarily focused on defining, characterizing and identifying invasive species; ecosystem invasibility has received much less attention. A prerequisite for characterizing invasibility is the ability to compare levels of invasion across ecosystems. In this paper, we aim to identify the best way to quantify the level of invasion by nonnative animals and plants by reviewing the advantages and disadvantages of different metrics. We explore how interpretation and choice of these measures can depend on the objective of a study or management intervention. Based on our review, we recommend two invasion indices and illustrate their use by applying them to two case studies. Relative alien species richness and relative alien species abundance indicate the contribution that alien species make to a community. They are easy to measure, can be applied to various taxa, are independent of scale and are comparable across regions and ecosystems, and historical data are often available. The relationship between relative alien richness and abundance can indicate the presence of dominant alien species and the trajectory of invasion over time, and can highlight ecosystems and sites that are heavily invaded or especially susceptible to invasion. Splitting species into functional groups and examining invasion patterns of transformer species may be particularly instructive for gauging effects of alien invasion on ecosystem structure and function. Establishing standard, transparent ways to define and quantify invasion level will facilitate meaningful comparisons among studies, ecosystem types and regions. It is essential for progress in ecology and will help guide ecosystem restoration and management.