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
Human activities have altered the composition of biotas through two fundamental processes: native extinctions and alien introductions. Both processes affect the taxonomic (i.e., species identity) and phylogenetic (i.e., species evolutionary history) structure of species assemblages. However, it is not known what the relative magnitude of these effects is at large spatial scales. Here we analyze the large-scale effects of plant extinctions and introductions on taxonomic and phylogenetic diversity of floras across Europe, using data from 23 regions. Considering both native losses and alien additions in concert reveals that plant invasions since AD 1500 exceeded extinctions, resulting in (i) increased taxonomic diversity (i.e., species richness) but decreased phylogenetic diversity within European regions, and (ii) increased taxonomic and phylogenetic similarity among European regions. Those extinct species were phylogenetically and taxonomically unique and typical of individual regions, and extinctions usually were not continent-wide and therefore led to differentiation. By contrast, because introduced alien species tended to be closely related to native species, the floristic differentiation due to species extinction was lessened by taxonomic and phylogenetic homogenization effects. This was especially due to species that are alien to a region but native to other parts of Europe. As a result, floras of many European regions have partly lost and will continue to lose their uniqueness. The results suggest that biodiversity needs to be assessed in terms of both species taxonomic and phylogenetic identity, but the latter is rarely used as a metric of the biodiversity dynamics.alien species ͉ alpha diversity ͉ biodiversity ͉ phylogenetic beta diversity ͉ phylogeny G lobalization is progressively altering the composition of biotas worldwide (1-3). The interplay of two fundamental processes -extinctions of native species and introductions and successful establishment of alien species (sensu 4; hereafter referred to as invasion) -has been known to reduce the distinctiveness of species communities. Global species extinctions lead to a continuous decrease of overall species richness (i.e., ␥-diversity) (5). However, at the scale of continents, regions, and countries, invasions exceed local extinctions and result in an increase in local or regional species richness (i.e., ␣-diversity) (5-9).Changes in species composition driven by the combined effects of invasions and extinctions can result in decreasing (i.e., homogenization), increasing (i.e., differentiation), or unchanged compositional turnover of species (i.e., -diversity) (10, 11) and traits between and within continents (12, 13). The higher magnitude of invasions compared with extinctions is known to lead to decreased -diversity between regions at continental (14-17) and regional scales (18-21).Previous evidence of biotic homogenization at continental and regional scales has largely examined impacts on the taxonomic structure of species assemblages, yet extinctions...
Summary1 Species attributes have been used to explain invasion patterns assuming the prevalence of biological mechanisms, although this approach often suffers several methodological and conceptual limitations, such as local idiosyncrasies, differences among habitats, phylogenetic constraints and insufficient sample size. 2 The relative importance of 15 species traits for explaining the abundance over 350 naturalized alien plant species was assessed across five Mediterranean islands (Corsica, Crete, Majorca, Malta and Sardinia). A comparative analysis accounting for phylogeny was used to examine variation in semi-quantitative estimates of species abundance in comparable habitats across the five island floras. 3 Species were divided into those with affinity for semi-natural, agricultural and ruderal habitats. Both vegetative and reproductive attributes were evaluated for individual islands and averaged across all islands. 4 Vegetative propagation, large leaf size, summer flowering, long flowering period and dispersal by wind or vertebrates were positively associated with average alien abundance across all five islands. Fewer significant trends were found in island-specific patterns. 5 The relative importance of a few reproductive traits is reflected in over-representation of Caryophyllales, Asterales and Poales (late flowering, large seed size and anemochory). Although significant covariation in traits was found there was no evidence for welldefined invasive syndromes. 6 Succulence was important in ruderal habitats, long flowering period in agricultural habitats and vertebrate seed dispersal in semi-natural habitats, suggesting that empty niches, avoidance of competitors and exploitation of mutualists, respectively, are important. 7 The study highlights the importance of estimating invasion success across a wide region, but analyses of specific invasion stages are also needed. Reproductive traits, which may be more relevant for long-distance colonization, and vegetative traits, which determine local dominance and persistence, were, nevertheless, both related to abundance within islands.
Aims Although biological invasions occur throughout the world, and some invaders are widespread in many habitats, few studies on the ecological impact of invaders have examined multiple sites. We tested how the impact of three widespread plant invaders changed depending on the identity of the species and the invaded island. We also tested whether relative species loss was lower in species-rich communities than in species-poor ones.Location We conducted floristic surveys and soil analyses in eight Mediterranean Basin islands: Crete and Lesbos (Greece), Sardinia (Italy), Corsica, Bagaud and Porquerolles (France), and Mallorca and Menorca (Spain).Methods We compared native species richness and diversity, proportion of life forms, soil percentage nitrogen, percentage organic carbon, C/N, and soil pH in nearby paired plots of 2 · 2 m: one control and one invaded by either the deciduous tree Ailanthus altissima, the succulent subshrubs Carpobrotus spp. or the annual geophyte Oxalis pes-caprae, across eight Mediterranean Basin islands.Results On average, the presence of invaders reduced species diversity, Carpobrotus spp. exhibiting the largest impact and Oxalis the least. However, the relative impact was island-dependent, and was positively but weakly associated with the species richness of the recipient community. Therophytes were the life form that experienced the largest decrease across islands. The effects of invasion on soil properties were very variable. Total N changed (increased) only in plots invaded by Ailanthus, significantly decreasing the C/N ratio. The presence of this tree increased soil pH, whereas the opposite was found in plots invaded by the other two species. Organic C increased in plots invaded by Ailanthus and Carpobrotus species.Main conclusions By conducting an analysis at multiple sites, we found that the three plant invaders had an impact on plant community structure not entirely concordant with changes in soil properties. The impacts depended on the identity of the species and of the invaded island, suggesting that impact of invaders is context-specific. The impact in terms of species loss was not lower in species-rich than in species-poor communities.
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