Biological Invasions: Mechanisms, Impacts and Management
Biological invasions have caused tremendous ecological and socio-economic damages worldwide. Therefore, it is important to develop methods for their effective management. Biological invasion is a process of adaptive evolution in which hybridization and introgression play an important role in promoting invasive species by changing their invasiveness. Therefore, understanding how the genetic mechanisms of hybridization and introgression influence biological invasion will facilitate effective control of invasive species. The escape of transgenes with special functions into populations of wild relatives through hybridization and introgression may change the invasiveness and weediness of the wild relatives, causing undesired environmental problems. This paper introduces the role of hybridization and introgression in adaptive evolution and speciation, and discusses how an alien species can change its adaptability, competitive ability, and invasiveness in new habitats through introgressive hybridization. Hybridization and introgression can cause polyploid and homoploid evolution of plant species, thereby influencing the fitness of new species and promoting the formation of an invasive species in new habitats. At the same time, with the rapid development of transgenic technologies, transgenic crops are being extensively released into the environment for commercial production. Biological invasion is a complicated evolutionary and ecological process, and future research should investigate the roles of hybridization and introgression in biological invasions in the context of the myriad factors that influence the process.
An exotic invasive pest, the sycamore lace bug, Corythucha ciliata (Say) (Hemiptera: Tingidae), was first found in Changsha of Hunan Province in 2002 and has since been recorded in 25 cities in 11 Chinese provinces or municipalities. This species feeds mainly on leaves of Platanus spp. trees. Heavy infestations have been concentrated in the Yangtze River basin. Here, we briefly describe this species, focusing on its invasion biology. In so doing, we first review the lace bug’s biological characteristics, including its life history, feeding habits and reproduction, and then focus on its invasion ecology, covering invasion pathways, adaptation during invasion (cold and heat tolerance), and major environmental factors affecting its invasion success and its impact on native urban ecosystems. Finally, we make some suggestions for managing this bug in Chinese cities.
Interactions between vector insects, plant viruses and host plants are complex and diverse. Although much work has been done to study the tripartite relationships, their roles in biological invasions have been rarely explored. The limited case studies available indicate that the interactions may be mediated by the host plant susceptibility to viruses, the suitability of host plants to vector insects, and the insects’ capacity to utilize host plants. When a host plant is highly susceptible to the virus but shows a low level of suitability to the insect, and the insect has a strong capacity to use different host plants, an indirect mutualistic relationship is likely to occur between vector insect and plant virus via their shared host plants. This kind of mutualism can contribute to the widespread invasion of vector insects as well as the epidemics of plant viruses. In view of the significance of the tripartite interactions in biological invasions, future effort should be made to investigate comparatively many more combinations of different species, and various technologies can be used to reveal the physiological and molecular mechanisms of the interactions.
Biological invasion is one of the three most intractable environmental problems. As an important component of global change, biological invasion can be influenced by the other components of global change. As the transitional zone between ocean and land, the coastal ecosystem is the most important buffer which has undergone the impact of sea level rise induced by global change. Furthermore, coastal zone, which has been suffered from frequently and intensively anthropogenic disturbance, is an ecological and environmental sensitive area, and one of the most frequently invaded ecosystems. In this paper, the effects of elevated CO2, sea level rise and eutrophication on the process of biological invasion have been analyzed based on the invasive status of coastal ecosystem. The results indicate that elevated CO2 disturbs the competitive balance between invasive and native species. Sea level rise adjusts the spatial pattern of invasive species and eutrophication of coastal ecosystem promotes further spread of alien species. At last, the research hotspots about interactions between global change and biological invasion in coastal ecosystem are discussed.
The impacts of invasive alien species on the genetic diversity and evolutionary responses of native species are poorly understood. Accumulating evidence shows that invasive plant species can lead to genetic erosion of natives directly through hybridization and gene infiltration, or even affect genetic diversity of natives through creation of new “genotypes”. Exotic species can also alter genetic diversity of natives indirectly through habitat fragmentation and modification, processes which influence gene flow within and among populations and result in inbreeding and genetic drift. On the other hand, some studies show that native species can respond evolutionarily to invasive plants, thereby reducing or eliminating invasive impacts. While interacting with invasive species, native species in both above- and below-ground ecosystems exhibit a series of evolutionary events such as adaptation, speciation or extinction. To more comprehensively evaluate the ecological impacts of biological invasions and the adaptive potential of natives, here we review the impacts of invasive plants on biological (genetic) diversity of native species, and the evolutionary responses of natives. We also discuss relationships between the genetic and evolutionary responses of natives and the success of invasive plants, and propose topics for further research.
Alligator weed (Alternanthera philoxeroides), a worldwide noxious weed, has caused enormous ecological and economic losses in China. To better understand the species’ successful invasion, and to predict relationships between expansion of its population and environmental conditions, we compared morphological traits of alligator weed with two closely related species, i.e. the geographically restricted alien A. pungens and the native A. sessilis, under different water and nutrient conditions in a common garden experiment. Compared to A. pungens and A. sessilis, alligator weed produced more biomass with high availability of water and nutrients, but did not in conditions of low water availability. These results suggest that the invasiveness of alligator weed may be influenced by environmental conditions. In addition, alligator weed had marginally greater specific leaf area (SLA) and phenotypic plasticity than its congeners. Phenotypic plasticity and SLA may thus have predictive value for invasiveness of alien species under multiple environmental conditions.
Genetic diversity is hypothesized to affect the invasiveness of alien species. Although many alien species have experienced genetic bottlenecks during invasion, multiple introductions and intra- or inter- specific hybridization may result in high levels of genetic variation that are comparable to native conspecifics. This genetic variation could enable plants to adapt to novel habitats through rapid evolution. However, research has shown that genetic uniformity sometimes has little effect on invasiveness, and instead, may even aid successful invasion by some alien species. These results suggest that high levels of genetic diversity are not always necessary in biological invasions. In this review, we explore relationships between genetic diversity and successful invasions. We examine the possible mechanisms through which genetic diversity can influence the short-term success and long-term evolution of alien species. Additionally, we raise and discuss issues that deserve attention in future studies. Future research should unite invasion ecology and other ecological disciplines in order to deepen our understanding of the ecological and evolutionary processes involved with biological invasions.
Plant invasion is a significant element of global change. Recently, exotic Spartina alterniflora have rapidly displaced native Phragmites australis on China’s East Coast. In the Yangtze Estuary’s Jiuduansha wetlands, nitrogen content in aerial standing litter for both Spartina and Phragmites was measured from November 2003 to April 2004. Spartina had larger nitrogen content (N g/m2) in aerial litter than Phragmites. Nitrogen content in aerial litter of Spartina sheathes and stems increased during our study period; this trend was not observed for Phragmites. In January 2006, new (produced in winter of 2005) and old (produced in winter of 2004) Spartina aerial sheath and stem litter was collected from six sites on China’s East Coast. Aerial litter of old sheathes and stems had higher nitrogen concentrations than new sheathes and stems, suggesting that the increase of nitrogen content in Spartina sheath and stem aerial litter was consistent across sites. Further greenhouse experiments showed that the increase of nitrogen content in Spartina aerial litter was associated with microbial epiphytic nitrogen fixation. Our results suggest that Spartina invasion alters aerial litter nitrogen dynamics, which might increase nitrogen input into invaded ecosystems and facilitate rapid expansion of Spartina into additional habitats.
A database of alien flora is important for better understanding the patterns of plant invasions, as well as explicating the mechanisms promoting plant invasion at local, regional and global scales. However, little attention has been paid to the construction of such a database of terrestrial alien herbs in China. Here, we attempt to summarize literature and reveal chronological and biogeographical patterns of alien herbaceous plant invasions. In total, 800 herbaceous alien species were identified, belonging to 37 orders and 72 families, ten families which account for 60% of the total flora. The families containing large numbers of species include Asteraceae, Fabaceae, Cactaceae, Poaceae and Brassicaceae. Species originating from the Americas made up 47% of total species. In relation to life forms, most species are perennial herbs (293 species, 40%) and annuals (272 species, 37%). About 46% of species are often found in highly disturbed habitats with rich resources. Our results suggest that, in China, terrestrial alien herbs are distributed throughout the entire country, with particularly high species richness in the Southeast (23% of total richness, 4 species per 10,000 km2). Eighty percent of alien plants were introduced into China intentionally, and there has been a steady increase in the number of neophyte species over the last two centuries, of which about 90% were introduced after 1800s and 60% post-1950s. The information on diversity and ecological characteristics of terrestrial alien herbs provided in this paper can be used for designing national management strategies for alien plants in China.
Invasive alien species threaten both native ecosystems and local economies and thus, considerable efforts have been expended to understand why certain alien species invade their non-native ecosystems so successfully and what consequences their successful invasions have for the invaded ecosystems. However, the mechanisms underlying the successful invasions by alien plants remain highly controversial as little is known about the determinants of plant invasiveness; this information is critical in understanding the mechanisms of successful invasions. With the development of new techniques, a new area, ‘invasive plant genomics’, emerged recently, in which genomic approaches are used for understanding plant invasiveness, and hence plant invasions at the genetic level. Here we review three genomic approaches that could be used in plant invasion biology, and discuss the selection of model invasive species and future research directions in invasive plant genomics. First, comparative genomics offers a way to analyze genome constitution and structure based on genome mapping and full sequencing. Second, functional genomics allows us to identify candidate genes that contribute to the invasive characteristics through natural selection experiments, and determine their functions using “forward ecology” methods. Third, epigenetic genomics complements to the above two approaches, and provides information on gene expression patterns, thereby helping us to understand interactions between genes and the environment. With these genomic approaches, it is possible to dissect the invasiveness-related genes and their expression and regulation patterns, identify invasive genotypes and hence understand plant invasiveness. With this information it is theoretically possible to disassemble plant invasion mechanisms and define the evolutionary patterns during plant invasions. Through use of genomic tools, much progress has already been made in two aspects of plant invasions, namely the molecular basis of weed herbicide-resistance and rhizome development in invasive plants. However, invasive plant genomics is still at an early stage. We urgently need to identify ideal model invasive plants or model systems for use in invasive plant genomics. Other important issues that need to be addressed in future research include genomic information accumulation, molecular responses to varying environmental conditions and systems biology of invasive plants.
Agricultural ecosystems are vulnerable to biological invasions. There are a total of 239 invasive alien species in a variety of agricultural ecosystems in China. Among them, 155 species are plants, 55 are animals and 29 are microbes. The number of invasive alien species decreases from south to north, and from east to west in China. Invasive alien plant species are generally introduced intentionally, while animals and microbes are mainly unintentionally introduced. Among these invaders, 45.04% have a geographical origin in the Americas, 22.90% in Europe, and 16.41% in Asia. Of these species, 64.85% and 66.53% occur in vegetable gardens (including greenhouses) and orchards, respectively, while 34.31%, 23.85% and 6.28% occur in summer-harvested crop dry land, autumn-harvested crop dry land and paddy field, respectively. Among these 239 invaders, 17 plant species, 10 animals and 7 microbes are noxious pests which, we feel, deserve closer management attention. Currently, chemical control is the principal approach to managing these species in agricultural ecosystems. However, long-term application of pesticides has led to pesticide resistance in some invasive species, with 51 of the 239 invasive alien species reported as exhibiting pesticide-resistant populations worldwide. Therefore, more attention should be paid to management which integrates biological control, ecological measures, agronomic means and quarantine. We suggest that research into the following issues would be fruitful: patterns and mechanisms of and trends in biological invasions in agricultural ecosystems, origin sources and invasion pathways, influences of biological invasions and pesticide resistance on successions of pest communities, and biological invasions caused by transgenic crops.
The regions invaded by alien species are normally disjunct from their native ranges, so it is difficult to understand the reasons for successful invasion through studies conducted only in native or invasive ranges. Many researchers have engaged in whole-range studies of invasive species, i.e. studying the exotics both in their introduced and native ranges, in order to explain the present geographical patterns and invasion mechanisms of alien plants. Here, we review progress in whole-range studies on invasive plants by summarizing their main contents, achievements and significance. We also point out the problems and shortcomings of existing studies and provide prospects for further studies. There are two main approaches in whole-range studies: (1) comparison of phenotypic traits (e.g. growth, reproduction, and ecophysiology) between invasive and native populations through direct observation and common garden experiments; (2) genetic diversity analysis and phylogeographic research using molecular markers. Such studies have tested major hypotheses of plant invasion mechanisms, and provided advice for management and control of invasive plants. However, the methods and contents of existing whole-range studies are imperfect, and further improvements based on increased international cooperation are needed.
Understanding the functional traits and ecological mechanisms associated with successful invasions of alien plants is a key role of the field of invasion ecology. Through literature review and analysis of plant functional traits contributing to successful plant invasions and the demands for functional traits at different invasion stages, we discuss the relationships between the functional traits and invasiveness of alien plants as well as related ecological mechanisms. Functional traits that have been studied in relation to their invasions mainly include seed characters, and morphological, developmental, physiological, clonal and propagation characteristics, as well as genetic variation and plasticity of phenotype. The impacts of these functional traits on invasion success vary from one stage to another. At the introduction stage, plant invasions are mainly affected by seed characters. At the establishment stage, stress-tolerance related physiology and propagation traits exert important influences. At the explosion stage, clonal characters and physiological traits related to competitive ability largely contribute to invasion success. Because plant invasions result from interactions between plant functional traits and environmental features, further studies on plant invasions should consider both the effects of invasion stage and specific environmental variables on invasion success.
Biological invasions represent a growing threat to biodiversity. The movement of organisms among continents by humans has caused profound changes in structure and function of the ecosystem to which they have been introduced. By testing the differences in the relative contributions of the various origins of the invasive plants to each region, we found intercontinental invasions were more prevalent than intracontinental invasions, primarily including the exchange of species among Eastern Asia–North America, Eastern Asia–South America, Europe–South Africa, Europe–North America, Europe–Eastern Asia, and North America–Oceania. They have posed a higher threat than intracontinental invasions. Thus, preventing future invasions is the most cost-effective form of management. Species distribution models (SDMs) are increasingly used to estimate risks of biological invasions. Niche stasis, the tendency of a species niche to remain unchanged across space and time, is often assumed when applying these models to predict and explain biogeographical patterns. Yet, both niche change and conservatism have occurred for intercontinental invasive exotic plants, which were severely disconnected from their source populations and often adapt rapidly to conditions in the new range. To further understand the niche characteristics of invasive species, it is therefore necessary to consider which factors limit range expansion in the native region. After comparing the similarities and differences of invasive species expansion across continents to within continents, we propose that the probabilities of niche shifts occurring depended primarily on the ecological and evolutionary processes limiting the species in its native range such as dispersal limit, species interaction, adapted evolution, ecological plasticity and population characteristics. Most limited factors of species niche properties are more consistent with there being a niche shift than niche stasis in the new range. Therefore, we suggest the following areas for future research: (1) multi-scales studies on niche attributes across spatial, temporal, environmental and phylogenetic investigations; (2) comparative studies that identify both the groups of species that are characterized by environmental niche stasis or shifts, and the traits that the species are more prone to niche change; and (3) niche dynamics over time to estimate the propensity, historical rate, and magnitude of niche shift. Such understanding will improve our confidence in SDM-based predictions of the impacts of climate change and species invasions on species distributions and biodiversity.
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