The paper provides the first estimate of the composition and structure of alien plants occurring in the wild in the European continent, based on the results of the DAISIE project (2004–2008), funded by the 6th Framework Programme of the European Union and aimed at “creating an inventory of invasive species that threaten European terrestrial, freshwater and marine environments”. The plant section of the DAISIE database is based on national checklists from 48 European countries/regions and Israel; for many of them the data were compiled during the project and for some countries DAISIE collected the first comprehensive checklists of alien species, based on primary data (e.g., Cyprus, Greece, F. Y. R. O. Macedonia, Slovenia, Ukraine). In total, the database contains records of 5789 alien plant species in Europe (including those native to a part of Europe but alien to another part), of which 2843 are alien to Europe (of extra-European origin). The research focus was on naturalized species; there are in total 3749 naturalized aliens in Europe, of which 1780 are alien to Europe. This represents a marked increase compared to 1568 alien species reported by a previous analysis of data in Flora Europaea (1964–1980). Casual aliens were marginally considered and are represented by 1507 species with European origins and 872 species whose native range falls outside Europe. The highest diversity of alien species is concentrated in industrialized countries with a tradition of good botanical recording or intensive recent research. The highest number of all alien species, regardless of status, is reported from Belgium (1969), the United Kingdom (1779) and Czech Republic (1378). The United Kingdom (857), Germany (450), Belgium (447) and Italy (440) are countries with the most naturalized neophytes. The number of naturalized neophytes in European countries is determined mainly by the interaction of temperature and precipitation; it increases with increasing precipitation but only in climatically warm and moderately warm regions. Of the nowadays naturalized neophytes alien to Europe, 50% arrived after 1899, 25% after 1962 and 10% after 1989. At present, approximately 6.2 new species, that are capable of naturalization, are arriving each year. Most alien species have relatively restricted European distributions; half of all naturalized species occur in four or fewer countries/regions, whereas 70% of non-naturalized species occur in only one region. Alien species are drawn from 213 families, dominated by large global plant families which have a weedy tendency and have undergone major radiations in temperate regions (Asteraceae, Poaceae, Rosaceae, Fabaceae, Brassicaceae). There are 1567 genera, which have alien members in European countries, the commonest being globally-diverse genera comprising mainly urban and agricultural weeds (e.g., Amaranthus, Chenopodium and Solanum) or cultivated for ornamental purposes (Cotoneaster, the genus richest in alien species). Only a few large genera which have successfully invaded (e.g., Oenothera, Oxalis, Panicum, Helianthus) are predominantly of non-European origin. Conyza canadensis, Helianthus tuberosus and Robinia pseudoacacia are most widely distributed alien species. Of all naturalized aliens present in Europe, 64.1% occur in industrial habitats and 58.5% on arable land and in parks and gardens. Grasslands and woodlands are also highly invaded, with 37.4 and 31.5%, respectively, of all naturalized aliens in Europe present in these habitats. Mires, bogs and fens are least invaded; only approximately 10% of aliens in Europe occur there. Intentional introductions to Europe (62.8% of the total number of naturalized aliens) prevail over unintentional (37.2%). Ornamental and horticultural introductions escaped from cultivation account for the highest number of species, 52.2% of the total. Among unintentional introductions, contaminants of seed, mineral materials and other commodities are responsible for 1091 alien species introductions to Europe (76.6% of all species introduced unintentionally) and 363 species are assumed to have arrived as stowaways (directly associated with human transport but arriving independently of commodity). Most aliens in Europe have a native range in the same continent (28.6% of all donor region records are from another part of Europe where the plant is native); in terms of species numbers the contribution of Europe as a region of origin is 53.2%. Considering aliens to Europe separately, 45.8% of species have their native distribution in North and South America, 45.9% in Asia, 20.7% in Africa and 5.3% in Australasia. Based on species composition, European alien flora can be classified into five major groups: (1) north-western, comprising Scandinavia and the UK; (2) west-central, extending from Belgium and the Netherlands to Germany and Switzerland; (3) Baltic, including only the former Soviet Baltic states; (4) east-central, comprizing the remainder of central and eastern Europe; (5) southern, covering the entire Mediterranean region. The clustering patterns cut across some European bioclimatic zones; cultural factors such as regional trade links and traditional local preferences for crop, forestry and ornamental species are also important by influencing the introduced species pool. Finally, the paper evaluates a state of the art in the field of plant invasions in Europe, points to research gaps and outlines avenues of further research towards documenting alien plant invasions in Europe. The data are of varying quality and need to be further assessed with respect to the invasion status and residence time of the species included. This concerns especially the naturalized/casual status; so far, this information is available comprehensively for only 19 countries/regions of the 49 considered. Collating an integrated database on the alien flora of Europe can form a principal contribution to developing a European-wide management strategy of alien species.
A list of alien plant species recorded from Svalbard in the summer of 1988 is presented. Two localities, the Russian settlements of Barentsburg and Pyramiden on the Isfjorden, Spitsbergen, were studied. Prior to this study, almost 60 alien species were recorded from Svalbard by other investigators. During the research reported here, 44 taxa were found, 14 of which are new records for the Svalbard archipelago. Six species are considered to be possibly naturalized; however, it is difficult to assess their naturalization status because of the severity of the climate in the study area. A complete list of species is presented, with information on height and phenological stage of particular specimens. Most of the alien plants recorded at the two settlements belong to the family Brassicaceae.
Conyza bonariensis (L.) Cronquist and C. triloba Decne. are reported as new alien species to the Czech Republic and Europe, respectively. Conyza bonariensis was collected in 1964 and 1965 in two localities in N Bohemia; the specimens are deposited in PRA. In both cases, the plants were introduced with cotton and occurred in areas of textile factories. Conyza triloba was collected once at the pond in the village of Černivsko in S Bohemia in 1971; the specimen is deposited in PR. The mode of introduction is unknown.
Genome size has been suggested as one of the traits associated with invasiveness of plant species. To provide a quantitative insight into the role of this trait, we estimated nuclearDNAcontent in 93 alien species naturalized in the Czech Republic, belonging to 32 families, by using flow cytometry, and compared it with the values reported for non-invading congeneric and confamilial species from the Plant DNA C-values database. Species naturalized in the Czech Republic have significantly smaller genomes than their congeners not known to be naturalized or invasive in any part of the world. This trend is supported at the family level: alien species naturalized in the Czech flora have on average a smaller genome than is the mean value for non-invading confamilials. Moreover, naturalized and non-invading species clearly differed in the frequency of five genome size categories; this difference was mainly due to very small genomes prevailing and intermediate to very large genomes underrepresented in the former group. Our results provide the first quantitative support for association of genome size with invasiveness, based on a large set of alien species across a number of plant families. However, there was no difference in the genome size of invasive species compared to naturalized but non-invasive. This suggests that small genome size provides alien plants with an advantage already at the stage of naturalization and need not be necessarily associated with the final stage of the process, i.e. invasion.
Invasions by alien plant species significantly affect biodiversity and ecosystem functioning. Investigations of the soil seed banks of invasive plant species and changes in the composition and structure of resident seed banks following plant invasions can provide valuable insight into the long-term implications of plant invasions. Soil seed banks play a major role as reservoirs of species and genetic diversity and allow for the persistence of a species at a locality, buffering environmental changes that may occur over time. Despite the emerging body of literature on ecological impacts of invasive plants on the diversity of resident communities, the long-term implications of impoverished soil seed banks for vegetation dynamics and ecosystem functioning have only recently begun receiving attention. Evidence has so far indicated that there is a correlation between the invasiveness of a species and the characteristics of its seed bank, and that changes in the seed banks of resident communities associated with plant invasions affect their biotic resistance to primary and secondary invasions. To promote the study of soil seed banks in the context of invasive species, we (i) summarize the functional roles of soil seed banks; (ii) describe how the capacity to form a seed bank may contribute to a species’ invasiveness using data from the flora of the Czech Republic, showing an increasing representation of species capable of forming long-term persistent seed bank from casual to naturalized to invasion stage; (iii) assess the impact of invasive plants on seed banks of resident communities, including the potential creation of conditions that favour secondary invasions by other alien species or native weeds, and long-term implications of such impact; and (iv) describe the potential effects of climate change on the soil seed bank in the context of plant invasions. We conclude with highlighting promising avenues for future research on invaded soil seed banks, and emphasize the importance of this knowledge in the development of control programs and restoration strategies.
The citation frequency of papers on invasion ecology published between 1981 and 2003 and that had accumulated at least 30 citations on the Web of Science on 9 August 2006 was analysed. The dataset comprised 329 papers and 27,240 citations. For each paper, the total number of citations was recorded and the annual citation rate (number of citations per year) was calculated. Papers were classified into broad research fields: plant invasions, animal invasions, biological control, and general papers (reviews and syntheses). Eight papers were cited more than 300 times, five of them dealt with general topics, and the mean value of the total number of citations across the whole data set is 82.8±73.1. The mean annual citation rate is 11.5±11.3 citations per year; six studies received on average at least 50 citations each year. About a half (50.8%) of papers in the data set deal with plant invasions. General papers are significantly more cited than papers from the other categories. The annual citation rate increased with time over the analysed period (1981–2003), by 1.0 citations per year. To compare the trends in invasion ecology with those in other fields of ecology, comparable data were compiled for population ecology and dynamics, and global change. The annual citation rate for invasion ecology as a whole increased faster than that for population ecology and dynamics, but not exponentially as is the case with studies on global change. The best-cited papers on invasion ecology were distributed among most of the top ecology journals. Those published in Oikos, Journal of Ecology, Ecological Applications and BioScience are cited 3.8–5.8 times more than the average for these journals (based on the impact factor). Papers on biodiversity, community ecology, impact, invasibility, dispersal, population ecology, competition, resources, genetical issues, biological control and species invasiveness received the highest total number of citations. However, measured by the annual citation rate, the hottest current topics in invasion ecology are the effect of global change on invasions, the role of natural enemies, character of the invasion process, evolutionary aspects, invasibility of communities and ecosystem processes. Some topics are disproportionally more cited than studied and vice versa. Studies on plant and animal invasions differ in focus: the topics of invasibility, biodiversity, resources, species invasiveness and population genetics are more emphasized in botanical studies, dispersal, competition, impact and pathways in papers dealing with animal invasions. Studies of grasslands and marine environment are most frequently cited in botanical and zoological studies, respectively. Most of the highly cited papers deal with multiple species; only 14 plant species and four animal species are the primary focus of one or more of the highly-cited papers. Twenty-two authors (4.5% of the total involved in the papers analysed), each with seven or more contributions cited at least 30 times, together contributed 49.4% of the most-cited papers, and attracted 55.6% of the total number of citations.