Biological invasions cause ecological and economic impacts across the globe. However, it is unclear whether there are strong patterns in terms of their major effects, how the vulnerability of different ecosystems varies and which ecosystem services are at greatest risk. We present a global meta-analysis of 199 articles reporting 1041 field studies that in total describe the impacts of 135 alien plant taxa on resident species, communities and ecosystems. Across studies, alien plants had a significant effect in 11 of 24 different types of impact assessed. The magnitude and direction of the impact varied both within and between different types of impact. On average, abundance and diversity of the resident species decreased in invaded sites, whereas primary production and several ecosystem processes were enhanced. While alien N-fixing species had greater impacts on N-cycling variables, they did not consistently affect other impact types. The magnitude of the impacts was not significantly different between island and mainland ecosystems. Overall, alien species impacts are heterogeneous and not unidirectional even within particular impact types. Our analysis also reveals that by the time changes in nutrient cycling are detected, major impacts on plant species and communities are likely to have already occurred.

There has been a dramatic growth in research on biological invasions over the past 20 years, but a mature understanding of the field has been hampered because invasion biologists concerned with different taxa and different environments have largely adopted different model frameworks for the invasion process, resulting in a confusing range of concepts, terms and definitions. In this review, we propose a unified framework for biological invasions that reconciles and integrates the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions. The unified framework combines previous stage-based and barrier models, and provides a terminology and categorisation for populations at different points in the invasion process.

With the growing body of literature assessing the impact of invasive alien plants on resident species and ecosystems, a comprehensive assessment of the relationship between invasive species traits and environmental settings of invasion on the characteristics of impacts is needed. Based on 287 publications with 1551 individual cases that addressed the impact of 167 invasive plant species belonging to 49 families, we present the first global overview of frequencies of significant and non-significant ecological impacts and their directions on 15 outcomes related to the responses of resident populations, species, communities and ecosystems. Species and community outcomes tend to decline following invasions, especially those for plants, but the abundance and richness of the soil biota, as well as concentrations of soil nutrients and water, more often increase than decrease following invasion. Data mining tools revealed that invasive plants exert consistent significant impacts on some outcomes (survival of resident biota, activity of resident animals, resident community productivity, mineral and nutrient content in plant tissues, and fire frequency and intensity), whereas for outcomes at the community level, such as species richness, diversity and soil resources, the significance of impacts is determined by interactions between species traits and the biome invaded. The latter outcomes are most likely to be impacted by annual grasses, and by wind pollinated trees invading mediterranean or tropical biomes. One of the clearest signals in this analysis is that invasive plants are far more likely to cause significant impacts on resident plant and animal richness on islands rather than mainland. This study shows that there is no universal measure of impact and the pattern observed depends on the ecological measure examined. Although impact is strongly context dependent, some species traits, especially life form, stature and pollination syndrome, may provide a means to predict impact, regardless of the particular habitat and geographical region invaded.

Summary 1.Much attention has been paid to negative effects of alien species on resident communities but studies that quantify community-level effects of a number of invasive plants are scarce. We address this issue by assessing the impact of 13 species invasive in the Czech Republic on a wide range of plant communities. 2. Vegetation in invaded and uninvaded plots with similar site conditions was sampled. All species of vascular plants were recorded, their covers were estimated and used as importance values for calculating the Shannon diversity index H ′ , evenness J and Sørensen index of similarity between invaded and uninvaded vegetation. 3. With the exception of two invasive species, species richness, diversity and evenness were reduced in invaded plots. Species exhibiting the greatest impact reduced species numbers per plot and the total number of species recorded in the communities sampled by almost 90%. A strong reduction of species number at the plot scale resulted in a marked reduction in the total species number at the landscape scale, and in less similarity between invaded and uninvaded vegetation. The decrease in species richness in invaded compared to uninvaded plots is largely driven by the identity of the invading species, whereas the major determinants of the decrease in Shannon diversity and evenness are the cover and height of invading species, and differences between height and cover of invading and dominant native species, independent of species identity. 4. Synthesis. Management decisions based on impact need to distinguish between invasive species, as their effects on diversity and composition of resident vegetation differ largely. Tall invading species capable of forming populations with the cover markedly greater than that of native dominant species exert the most severe effects on species diversity and evenness. Since a strong impact on the community scale is associated with reduction in species diversity at higher scales, invaders with a high impact represent a serious hazard to the landscape.

Invasive alien species come from most taxonomic groups, and invasion biology is searching for robust cross-taxon generalizations and principles. An analysis of 2,670 papers dealing with 892 invasive species showed that all major groups of invaders are well studied, but that most information on the mechanisms of invasion has emerged from work on a limited number of the most harmful invaders. A strong geographical bias, with Africa and Asia understudied, inhibits a balanced understanding of invasion, because we might be lacking knowledge of specific invasion mechanisms from poorly studied, regionally specific habitats. International cooperation is required to achieve a more geographically balanced picture of biological invasions. Invasive species with the greatest impact are best studied, but more studies of species that are naturalized but not (yet) invasive are needed to improve understanding of the mechanisms acting during the naturalization phase of invasions and leading to successful invasion.

The accelerating rates of international trade, travel, and transport in the latter half of the twentieth century have led to the progressive mixing of biota from across the world and the number of species introduced to new regions continues to increase. The importance of biogeographic, climatic, economic, and demographic factors as drivers of this trend is increasingly being realized but as yet there is no consensus regarding their relative importance. Whereas little may be done to mitigate the effects of geography and climate on invasions, a wider range of options may exist to moderate the impacts of economic and demographic drivers. Here we use the most recent data available from Europe to partition between macroecological, economic, and demographic variables the variation in alien species richness of bryophytes, fungi, vascular plants, terrestrial insects, aquatic invertebrates, fish, amphibians, reptiles, birds, and mammals. Only national wealth and human population density were statistically significant predictors in the majority of models when analyzed jointly with climate, geography, and land cover. The economic and demographic variables reflect the intensity of human activities and integrate the effect of factors that directly determine the outcome of invasion such as propagule pressure, pathways of introduction, eutrophication, and the intensity of anthropogenic disturbance. The strong influence of economic and demographic variables on the levels of invasion by alien species demonstrates that future solutions to the problem of biological invasions at a national scale lie in mitigating the negative environmental consequences of human activities that generate wealth and by promoting more sustainable population growth.climate | economy | exotic plants and animals | geography | prediction

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Globalization and economic growth are widely recognized as important drivers of biological invasions. Consequently, there is an increasing need for governments to address the role of international trade in their strategies to prevent species introductions. However, many of the most problematic alien species are not recent arrivals but were introduced several decades ago. Hence, current patterns of alien-species richness may better reflect historical rather than contemporary human activities, a phenomenon which might be called "invasion debt." Here, we show that across 10 taxonomic groups (vascular plants, bryophytes, fungi, birds, mammals, reptiles, amphibians, fish, terrestrial insects, and aquatic invertebrates) in 28 European countries, current numbers of alien species established in the wild are indeed more closely related to indicators of socioeconomic activity from the year 1900 than to those from 2000, although the majority of species introductions occurred during the second half of the 20th century. The strength of the historical signal varies among taxonomic groups, with those possessing good capabilities for dispersal (birds, insects) more strongly associated with recent socioeconomic drivers. Nevertheless, our results suggest a considerable historical legacy for the majority of the taxa analyzed. The consequences of the current high levels of socioeconomic activity on the extent of biological invasions will thus probably not be completely realized until several decades into the future.exotic plants and animals | species establishment | time lag