Comparing naturalized alien plants and recipient habitats across an east–west gradient in the Mediterranean Basin

Aim To investigate alien plant species invasion levels in different habitats and alien species traits by comparing the naturalized flora in different areas of the same biogeographical region. Location Spain, Italy, Greece and Cyprus. Methods Comparison of floristic composition, species traits and recipient habitats of naturalized alien neophytes across an east–west gradient comprising four countries in the European Mediterranean basin. Results A total of 782 naturalized neophytes were recorded; only 30 species were present in all four countries. Although floristic similarity is low, the four alien floras share the same patterns of growth form (mostly herbs), life cycle (mostly perennials) and life form (mostly therophytes, hemicryptophytes and phanerophytes). The majority of the recipient habitats were artificial. Wetlands were the natural habitats, with the highest numbers of naturalized species. Floristic similarity analyses revealed: (1) the highest floristic similarity between Italy and Spain, both of which were more similar to Greece than to Cyprus; (2) two groups of floristic similarity between habitat categories in each country (Greece–Cyprus and Italy–Spain); (3) a higher degree of homogenization in the plant assemblages in different habitats in Greece and Cyprus and a lower degree of homogenization in those in Italy and Spain; and (4) a higher degree of homogenization in artificial and natural fresh‐water habitats than in the other natural habitats. Main conclusions The floristic similarity of naturalized neophytes between the four countries is low, although the overall analysis indicates that the western group (Italy–Spain) is separated from the eastern group (Greece–Cyprus). Similar patterns emerged regarding the life‐history traits and recipient habitats. The artificial habitats and the natural wet habitats are those that are invaded most and display the greatest homogenization in all four countries. Coastal habitats display a lower degree of homogenization but a high frequency of aliens. Dry shrubs and rocky habitats display a lower degree of homogenization and a low frequency of aliens.     

Determinants of native and alien species richness in the urban flora of Rome

Until now, analytical studies on European urban floras have mostly concentrated on the central and north‐western parts of the continent. In this paper, factors determining species richness of urban flora were studied for the city of Rome, Italy, based on a comprehensive floristic survey carried out between 1985 and 1994, and updated in 2005. All species were recorded in grid cells of 1.6 km² and classified into native and alien (the latter divided into archaeophytes and neophytes). The grids were classified with respect to the prevailing habitat type, area available to vegetation, level of disturbance and geographical position within the city. Data were analysed using minimal adequate models. Total species number was determined by habitat and its interaction with position on the north‐west gradient; other variables explained much less variance. Holding other variables constant, the average species number per grid cell was highest in archaeological sites and parks, followed by woodlands and rivers, and grasslands and recent developments. Residential areas and the historical centre were poorest in species number. Towards the north of the city, species richness in corresponding habitats increases because of higher landscape heterogeneity and closer association with diaspore pools in the surroundings. Native species make up on average 84% of the total species numbers, and trends opposite to those for the total number of species were found for the proportional representation of aliens. The occurrence of alien and native species in the flora of Rome is driven by similar factors, but factors that increase representation of aliens decrease that of natives and vice versa. The representation of neophytes and native species in grid cells was easier to explain (74% of variation accounted for) than that of archaeophytes (27%); this result reflects that in terms of ecology and response to factors, archaeophytes take an intermediate position between native plants and neophytes. Proportional representation of neophytes decreased with increasing area available to vegetation, reflecting that semi‐natural vegetation is better developed where less fragmented.     

Seed dispersal of alien and native plants by vertebrate herbivores

Seed dispersal is crucial for the success and spread of alien plants. Herbivores often establish a dual relationship with plants: antagonist, through herbivory, and mutualist, through seed dispersal. By consuming plants, herbivores may disperse large amounts of seeds, and can facilitate the spread of alien plants. However, seed dispersal of alien plants by herbivores has been largely uninvestigated. I studied factors associated with dispersal of alien and native seeds by the three most important vertebrate herbivores in SW Australia: emus (Dromaius novaehollandia), western grey kangaroos (Macropus fuliginosus) and European rabbits (Oryctolagus cuniculus). Overall frequencies of alien and native seeds dispersed by these herbivores were determined by differences among them in (1) the plant groups they predominantly disperse, that differed in frequencies of aliens versus natives, and (2) the predominant dispersal of aliens or natives within those plant groups. Emus and kangaroos (natives) tended to disperse predominantly alien seeds within plant groups (defined by life forms, dispersal syndromes, and diaspore size), whereas rabbits (alien) tended to disperse predominantly natives. This agrees with the hypothesis that herbivores will use predominantly plants that have evolved in different areas, because of less effective defences against new enemies. Overall frequencies were consistent with this pattern in kangaroos and rabbits, but not in emus. Kangaroos dispersed mostly plant groups that were mainly aliens (herbaceous species and small and medium sized dispersal units and seeds), which together with their predominant use of aliens over natives within groups resulted in the highest overall frequency of alien seeds (73%). Rabbits were similar to kangaroos in the type of plants dispersed, but their predominant use of natives over aliens within groups contributed to an overall predominance of native seeds in their pellets (88%). Emus dispersed mostly plant groups that were mainly natives (e.g. woody species with big diaspores), resulting in low overall frequency of alien seeds (11%), despite their predominant use of aliens over natives within plant groups. Thus, the within-groups trend pointed to a facilitative role of native herbivores of plant invasions through seed dispersal, but was obscured by the different use by herbivores of plant groups with different frequency of aliens.     

Alien and native species in Central European urban floras: a quantitative comparison

The paper provides quantitative information on the occurrence of alien species in Central European cities and analyses factors determining the richness of alien and native floras in this habitat type. Data for 54 cities (25 Polish, 24 German, 4 Czech and 1 Austrian) were gathered, and the representation of archaeophytes (i.e. aliens introduced before 1500 ad ), neophytes (introduced after that date) and native species was expressed. In an average city there were 87.4 archaeophytes (15.2% of the city flora) and 172.4 neophytes (25.2%) giving a total of 259.7 for alien species (40.3%). The number of native species averaged 386.5. The numbers of species in each category of immigration status increased significantly with city size. For neophytes, the species-area relationship showed a higher slope (0.49) on log–log axes than for both archaeophytes (0.16) and native species (0.30). Not only the number, but also the relative contribution of neophytes to the total flora, increased with city size, indicating that neophytes are the group which are most closely associated with human activity. On the other hand, archaeophytes were better represented in smaller cities, as they were confined to rural environment. A step-wise multiple regression was used to test for environmental variables acting as significant predictors, and explained between 40 and 65% of variation in the species numbers for particular categories of immigration status, providing the best fit for neophytes. City size was the best predictor for each characteristic, except of the proportion of total aliens, where the percentage of explained variability was low (8.2%), with latitude being the only significant predictor. Temperature was another highly significant predictor for the number of archaeophytes and total aliens, reflecting the origin of aliens in warmer areas. There was an effect of region on some flora characteristics. Polish cities had significantly higher proportion of archaeophytes and of total aliens than German cities. It is concluded that the occurrence of native and alien species in urban floras follows rather different pattern.     

An updated checklist of the vascular flora alien to Italy

An updated inventory of the vascular flora alien to Italy, providing details on the occurrence at regional level, is presented. The checklist includes 1597 species, subspecies, and hybrids, distributed in 725 genera and 152 families; 2 taxa are lycophytes, 11 ferns and fern allies, 33 gymnosperms, and 1551 angiosperms. 157 taxa are archaeophytes and 1440 neophytes. The alien taxa currently established in Italy are 791 (570 naturalized and 221 invasive), while 705 taxa are casual aliens, 4 are not assessed, 7 are of unknown regional distribution, 47 have not been confirmed in recent times, 3 are considered extinct or possibly extinct in the country, and 40 are doubtfully occurring in Italy. This checklist allows to establish an up-to-date number (9792) of taxa constituting the whole (native and alien) Italian flora.     

Alien invasive neophytes of the Southeastern part of the Pannonian Plain

Floristic diversity in the Pannonian Plain is the result of mosaic habitat composition, characterized by steppes, saline meadows, and lowland forests along major rivers. In the last few centuries, the flora has become enriched by species introduced by human activities, such as colonization and industrialization. Presently it consists of nearly 1800 species. As a result of 30 years of field research as well as herbarium and literature searches, a list of invasive alien neophytes has been compiled and is presented here. The list includes 152 species, classified as either naturalized, casual, or unknown. One of the species is listed in Red Book of Serbian Flora. Eighty species are naturalized, and 61 are casual species. The majority of the introduced flora originates from North America (66) and the Old World (43). Most alien invasive species of the Southeastern part of the Pannonian Plain, were introduced accidentally by seed material or deliberately by nursery saplings. The annual and perennial herbaceous plants, which flower in the summer and summer-autumn periods, are represented in the greatest numbers. The alien flora was further analyzed with respect to taxonomy, life forms, pollination, and seed dispersal mode. The greatest number of alien species appears in anthropogenic and semi-natural habitats (59). In natural habitats, 45 invasive taxa were recorded, representing a conservation threat to biodiversity.     

Plant Invasions on an Oceanic Archipelago

Plant invasions are particularly noticeable on oceanic islands. For many ecological or evolutionary phenomena, oceanic islands offer advantages in comparison to continental regions, because they are often simpler systems. The Juan Fernández (Robinson Crusoe) Islands, located 667 km west of continental Chile, provide an especially favorable case study of plant invasions on an oceanic archipelago. They have little flora, no influence from aboriginal peoples, and good historical and recent documentation of flora, vegetation and human impacts. The total flora of the archipelago consists of 441 vascular plants, of which 209 are native (125 of them endemic) and 232 are aliens. Many alien species exist on the Juan Fernández Archipelago, but three shrubs are particularly invasive: Aristotelia chilensis, Rubus ulmifolius, and Ugni molinae. About 15% of the total area of the island has been impacted by at least one of these shrubs. In addition, the herbaceous Acaena argentea, now occurs at high abundance in 12% of the total area of the island. Comparisons of earlier and recent surveys of vegetation reveal that the area impacted by Aristotelia, Rubus, and Ugni molinae has increased tremendously. Among the most frequent aliens are Euro-Mediterranean taxa, also present on other archipelagos. A few aliens that are serious invasives on other archipelagos have been recently observed near San Juan Bautisata, the only village on the island. Effective measures involving stronger monitoring and sanitation of incoming materials (foodstuffs, building materials, etc.), cutting or poisoning of invasives, and reseeding with native species, are necessary to help preserve the native and endemic flora (and biota) of these islands.     

Czech alien flora and the historical pattern of its formation: what came first to Central Europe?

Temporal patterns of immigration to the country were analysed using 668 alien species in the flora of the Czech Republic for which the dates of the first record were available (64.8% of the total number of 1031 so-called neophytes, i.e. aliens introduced after the year 1500). After a period of initial slow increase lasting to the 1840s, the accumulation of neophytes over time could be best fitted by a linear model that explained 97% of the variance. The intensity of floristic research, which varied between periods, did not significantly affect the overall increase in the number of aliens. The effect of species traits on the year of introduction was evaluated, with continent of origin, introduction type (deliberate or accidental), life history, Grime's life strategy, onset of flowering, mode of dispersal and propagule size as explanatory variables. Species of European origin and CSR strategists arrived earlier than those with other origins and strategies. Deliberately introduced species appeared earlier than accidental arrivals, and those cultivated for utilitary reasons on average arrived earlier than ornamentals. Species capable of early flowering were remarkably more prevalent among early newcomers. A separate analysis of accidentally introduced American species also identified life history as a significant predictor of immigration time, with annuals being introduced earlier than biennials and perennials. The data contribute to an understanding of a crucial stage of the invasion process that has received little attention in the literature. The model "early alien" to Central Europe is a European species with a CSR strategy deliberately brought for cultivation as a utilitary plant. Once it escaped from cultivation, its establishment in the wild was favoured by its ability to flower early and, therefore, complete the life cycle.     

Urban domestic gardens (I): Putting small‐scale plant diversity in context

Abstract. As part of a larger survey of biodiversity in gardens in Sheffield, UK, we examined the composition and diversity of the flora in two 1‐m² quadrats in each of 60 gardens, and compared this with floristic data from semi‐natural habitats in central England and derelict urban land in Birmingham, UK. Garden quadrats contained more than twice as many taxa as those from any other habitat type. Ca. 33 % of garden plants were natives and 67 % aliens, mainly from Europe and Asia. A higher proportion of garden aliens originated from Asia and New Zealand than in the UK alien flora as a whole; 18 of the 20 most frequent plants in garden quadrats were natives, mostly common weeds. Garden quadrats showed no evidence of ‘nestedness’, i.e. a tendency for scarce species to be confined to the highest diversity quadrats. Conversely, species in all semi‐natural and derelict land data sets were significantly nested. Compared to a range of semi‐natural habitats, species richness of garden quadrats was intermediate, and strikingly similar to the richness of derelict land quadrats. Although species accumulation curves for all other habitats showed signs of saturation at 120 quadrats, gardens did not. Correlations between Sørensen similarity index and physical distance were insignificant for all habitat types, i.e. there was little evidence that physical distance played any part in structuring the composition of the quadrats in any of the data sets. However, garden quadrats were much less similar to each other than quadrats from semi‐natural habitats or derelict land.     

Native and alien plant species richness in relation to spatial heterogeneity on a regional scale in Germany

Aim The aim of our study was to reveal relationships between richness patterns of native vs. alien plant species and spatial heterogeneity across varying landscape patterns at a regional scale. Location The study was carried out in the administrative district of Dessau (Germany), covering around 4000 km². Methods Data on plant distribution of the German vascular flora available in grid cells covering 5′ longitude and 3′ latitude (c. 32 km²) were divided into three status groups: native plants, archaeophytes (pre 1500 AD aliens) and neophytes (post 1500 AD aliens). Land use and abiotic data layers were intersected with 125 grid cells comprising the selected area. Using novel landscape ecological methods, we calculated 38 indices of landscape composition and configuration for each grid cell. Principal components analysis (PCA) with a set of 29 selected, low correlated landscape indices was followed by multiple linear regression analysis. Results PCA reduced 29 indices to eight principal components (PCs) that explained 80% cumulative variance. Multiple linear regression analysis was highly significant and explained 41% to 60% variance in plant species distribution (adjusted R²) with three significant PCs (tested for spatial autocorrelation) expressing moderate to high disturbance levels and high spatial heterogeneity. Comparing the significance of the PCs for the species groups, native plant species richness is most strongly associated with riverine ecosystems, followed by urban ecosystems, and then small‐scale rural ecosystems. Archaeophyte and neophyte richness are most strongly associated with urban ecosystems, followed by small‐scale rural ecosystems and riverine ecosystems for archaeophytes, and riverine ecosystems and small‐scale rural ecosystems for neophytes. Main conclusions Our overall results suggest that species richness of native and alien plants increases with moderate levels of natural and/or anthropogenic disturbances, coupled with high levels of habitat and structural heterogeneity in urban, riverine, and small‐scale rural ecosystems. Despite differences in the order of relevance of PCs for the three plant groups, we conclude that at the regional scale species richness patterns of native plants as well as alien plants are promoted by similar factors.     

Widely distributed native and alien plant species differ in arbuscular mycorrhizal associations and related functional trait interactions

It is debated whether alien plants in new environments benefit from being mycorrhizal and whether widely distributed natives and aliens differ in their associations with mycorrhizal fungi. Here, we compared whether species differing in their origin status, i.e. natives, archaeophytes (alien species introduced before the year 1500) and neophytes (introduced after the year 1500), and arbuscular mycorrhizal (AM) status (obligate, facultative, non‐mycorrhizal) differ in their area of occupancy in Germany (i.e. number of occupied grid cells, each ~130 km²). We used generalized linear models, incorporating main effects and up to three‐way interactions combining AM status, origin status and plant functional traits. The latter were chosen to describe the possible trade‐off in carbon allocation either towards the symbiosis or to other plant structures, such as storage organs (significant interactions involving traits were assumed to indicate the existence of such trade‐offs). AM status significantly explained the area of occupancy of natives and neophytes – with facultative mycorrhizal species occupying the largest area in both groups – but was less pronounced among archaeophytes. Archaeophytes may have reduced dependency on AM fungi, as they are generally agricultural weeds and the symbiosis potentially becomes obsolete for plants growing in habitats providing a steady provision of nutrients. Trait interactions between AM status and other functional traits were almost exclusively detected for neophytes. While facultative mycorrhizal neophytes benefit from trade‐offs with other traits related to high C cost in terms of area of occupancy, such trade‐offs were almost absent among natives. This indicates that natives and neophytes benefit differently from the symbiosis and suggests that native AM fungal partners might be less important for neophytic than for native plant species or that more time is required to establish similar relationships between neophytes and native fungal symbionts.     

Alien plants in central Australia

The central Australian flora currently contains 78 alien species, of which 17 are predominantly summer-growing, 19 are southern winter-growing species reaching roadsides, stockyards and watercourses in the southern Northern Territory and 42 are at present confined to gardens in Alice Springs. A further 11 species reach the far north of South Australia or die far southwest of Queensland but not the Northern Territory. The central Australian alien flora may be classified by growing season and drought-tolerance, or broadly categorized on the basis of habitat and dispersal agent into 'garden weeds', 'tourist weeds' and 'stock weeds'. Present ecological knowledge of central Australian aliens is limited, but indicates that whilst some will probably remain confined to better-watered habitats, many are likely to spread into the arid areas, becoming obvious only after a succession of wet years. The central Australian alien flora is increasing at present and this increase is expected to continue.     

Phyloecology of urban alien floras

1.  Understanding the mechanisms that affect invasion success of alien species is a major issue in current ecological research. Although many studies have searched for either functional or habitat attributes that drive invasion mechanisms, few researchers have addressed the role of phylogenetic diversity of alien species.
2.  Here, using data from 21 urban floras located in Europe and eight in the USA, we show that the phylogenetic diversity of alien species is significantly lower than that of native species, both at the continental scale and at the scale of single cities.
3.  Second, we show that if archaeophytes and neophytes (non-native species introduced into Europe before and after AD 1500, respectively) are analysed separately, archaeophytes show lower phylogenetic diversity than neophytes, while the phylogenetic structure of neophytes is indistinguishable from a random sample of species from the entire species pool.
4.  Our results suggest that urban aliens are subject to environmental filters that constrain their phylogenetic diversity, although these filters act more strongly upon archaeophytes than neophytes.
5.   Synthesis. Despite the huge taxonomic diversity of plants imported into European and American cities, the strong environmental filters imposed by cities constrain the functional diversity of urban floras, which is reflected in their generally low phylogenetic diversity. Urban alien floras are mainly composed of phylogenetically related species that are well adapted to anthropogenic habitats, although these filters are stronger for species groups with longer residence times.     

Native and alien floras in urban habitats: a comparison across 32 cities of central Europe

Aim To determine relative effects of habitat type, climate and spatial pattern on species richness and composition of native and alien plant assemblages in central European cities. Location Central Europe, Belgium and the Netherlands. Methods The diversity of native and alien flora was analysed in 32 cities. In each city, plant species were recorded in seven 1‐ha plots that represented seven urban habitat types with specific disturbance regimes. Plants were classified into native species, archaeophytes (introduced before ad 1500) and neophytes (introduced later). Two sets of explanatory variables were obtained for each city: climatic data and all‐scale spatial variables generated by analysis of principal coordinates of neighbour matrices. For each group of species, the effect of habitat type, climate and spatial variables on variation in species composition was determined by variation partitioning. Responses of individual plant species to climatic variables were tested using a set of binomial regression models. Effects of climatic variables on the proportion of alien species were determined by linear regression. Results In all cities, 562 native plant species, 188 archaeophytes and 386 neophytes were recorded. Proportions of alien species varied among urban habitats. The proportion of native species decreased with increasing range and mean annual temperature, and increased with increasing precipitation. In contrast, proportions of archaeophytes and neophytes increased with mean annual temperature. However, spatial pattern explained a larger proportion of variation in species composition of the urban flora than climate. Archaeophytes were more uniformly distributed across the studied cities than the native species and neophytes. Urban habitats rich in native species also tended to be rich in archaeophytes and neophytes. Main conclusions Species richness and composition of central European urban floras are significantly affected by urban habitat types, climate and spatial pattern. Native species, archaeophytes and neophytes differ in their response to these factors.     

The non-native vascular flora of Campania (southern Italy)

We discuss several aspects of the alien flora of Campania (southern Italy), on the basis of an updated and annotated checklist. The non-native flora includes at present 346 taxa in 231 genera, representing 82 families. However, other 55 taxa were not found later than 1950. Trends in alien plants invasion throughout the investigated area are discussed. Results are therefore compared with similar data from other Italian regions and other European countries. In the Appendix, taxa are listed with essential synonymy, local and general distribution, status, biological form, habitat, diffusion mechanisms, potential threats, literature data, voucher specimens, and miscellaneous ethnobotanical, chronological and taxonomical notes. In addition, 49 escaped taxa are known only for Botanical and private gardens, while 66 taxa are regarded as doubtful aliens. By including also these two categories, the non-native flora of Campania reaches 516 taxa. In addition, numerous doubtful and erroneous indications were discussed. Finally, a reference list for the exotic regional flora from 1500 C.E. to present is proposed.     

Alien flora of the Korean Peninsula

The Korean Peninsula has been divided into the countries of North and South Korea since 1948. This adoption of different political and socio-economic regimes has significantly changed economic activity and international exchange by those nations, thus affecting the introduction of alien plants. Our study objective was to provide a comprehensive, up-to-date inventory of the alien flora on the Korean Peninsula and to analyse how the partitioning between North and South Korea has influenced the numbers and status of those alien plants. We identified pre-1876 aliens that were brought to the Korean Peninsula before 1876, when Korea opened up to foreign countries, and also determined the post-1876 aliens that were introduced after 1876. Plants in the latter group were further classified into those that arrived before the outbreak of the Korean War in 1950 (1876–1950 aliens) and those introduced since then (post-1950 aliens). We investigated the regions of origin, family compositions, and invasion status of those alien taxa. In total, 504 alien taxa were identified, with 102 taxa being pre-1876 and 117 being 1876–1950 aliens. After the Korean War, the number of alien plants in South Korea more than doubled (276 taxa), whereas the number reported for North Korea was considerably lower (33 taxa). This might be due to enhanced economic activity and a larger human population in South Korea. Climatic differences as well as variations in the recording intensity for alien flora between North and South Korea may also have contributed to the large contrast in numbers of new alien species recorded during the last decades. Coordinated measures between the two nations are necessary if the spread of invasive alien plants onto the Korean Peninsula is to be reduced.     

Do alien plants on Mediterranean islands tend to invade different niches from native species?

In order to understand invasions, it is important to know how alien species exploit opportunities in unfamiliar ecosystems. For example, are aliens concentrated in niches under-exploited by native communities, or widely distributed across the ecological spectrum? To explore this question, we compared the niches occupied by 394 naturalized alien plants with a representative sample from the native flora of Mediterranean islands. When niche structure was described by a functional group categorization, the distribution of native and alien species was remarkably similar, although “succulent shrubs” and “trees with specialized animal pollination mechanisms” were under-represented in the native species pool. When niche structure was described by Grime’s CSR strategy, the positioning of aliens and natives differed more strongly. Stress-tolerance was much rarer amongst the aliens, and a competitive strategy was more prevalent at the habitat level. This pattern is similar to previous findings in temperate Europe, although in those regions it closely reflects patterns of native diversity. Stressed environments are much more dominant in the Mediterranean. We discuss a number of factors which may contribute to this difference, e.g., competitive and ruderal niches are often associated with anthropogenic habitats, and their high invasibility may be due partly to introduction patterns rather than to a greater efficiency of aliens at exploiting them. Thus far, the reasons for invasion success amongst introduced species have proved difficult to unravel. Despite some differences, our evidence suggests that alien species naturalize across a wide range of niches. Given that their ecologies therefore vary greatly, one may ask why such species should be expected to share predictable traits at all?     

Do non‐native species invasions lead to biotic homogenization at small scales? The similarity and functional diversity of habitats compared for alien and native components of Mediterranean floras

Although a number of recent studies have demonstrated biotic homogenization, these have mainly focused on larger spatial scales. Homogenizing effects are equally important at finer resolutions, e.g. through increasing similarity between habitats, which may result in a simplification of ecosystem structure and function. One major cause of homogenization is the expanding ranges of alien species, although it is not clear whether they are inherently homogenizing at smaller scales. We therefore assessed whether the alien flora is less complex across habitats than the resident native flora of Mediterranean Islands. From a regional data base, we examined floristic lists for between‐habitat taxonomic and functional similarity, and within‐habitat functional diversity, using resampled data sets to control for sample size biases. Aliens and natives showed equivalent complexity in most respects. At the taxonomic level, between‐island and between‐habitat similarities were almost identical, and when ecosystem function was measured by a functional group classification system, this was also true of between‐habitat similarities and within‐habitat diversities. When ecosystem function was measured using Grime's CSR classification, aliens were found to be more functionally homogenous between‐habitats and less functionally diverse within habitats. However, since the CSR profiles of aliens and natives differed, simplification is not inevitable due to ecological segregation of the two floras (aliens tend to be recruited to disturbed habitats rather than displacing natives). One deficiency is a lack of large scale species abundance data. A simple simulation exercise indicated that this is likely to lead to substantial overestimation of true levels of similarity, although would only influence the comparison between aliens and natives if they have different abundance distribution curves. The results indicate that alien floras are not intrinsically more simple than natives, but a higher proportion of competitive strategists among aliens may still cause small‐scale homogenization as these include many strong competitors that are likely to dominate communities.     

外来植物成功入侵的生物学特征

外来植物的入侵能力与其性状之间的关系是入侵生态学中的基本问题之一.成功的入侵种常常能占据多样化的生境,具有较强的适应性、繁殖力和散布力.表型可塑性和遗传分化是外来入侵植物对生境异质性的两种适应策略;散布体多态型和散布途径多种化,使外来入侵植物迅速占领入侵生境,并进行远距离扩散;无性生殖和有性生殖并存,并根据生境和入侵阶段权衡的繁育对策不仅使入侵种群大面积暴发成为可能,而且直接影响散布机制并对种群遗传结构具有调节作用.高效的资源利用性竞争,加之以化感作用为基础的干扰性竞争使植物更具入侵性.     

Interactions between alien plant species traits and habitat characteristics in agricultural landscapes in Finland

The survival and success of alien plant species is determined by species traits (i.e., invasiveness) and the characteristics of the habitats in the region of introduction (i.e., invasibility). However, little is known about species traits as related to habitat characteristics. We assessed the characteristics of successful invaders and the interaction of environmental factors and life-history traits for alien plant species. The vascular plants were recorded from 52 agricultural landscapes in Finland. We compared the traits of native and alien plant species with Fisher’s exact test and used a three table ordination analysis, RLQ analysis, to relate species traits to environmental conditions. Species were clustered according to their position on the RLQ axes, and the clusters were tested for phylogenetic independence. The successful alien plant species were associated with life form and preferences for moisture and nitrogen, but the trait composition varied according to the habitat type. Two RLQ axes explained 80.5% of the variation, and the species traits were significantly associated with environmental variables. The clustering showed that the occurrence of alien plant species in agricultural habitats was driven by invasion history, traits related to dispersal (dispersal type, seed mass) and habitat preferences, as well as environmental features, such as geographical location, temperature and the quality and disturbance regime of the habitats. All clusters were phylogenetically non-independent. Thus, the clusters of alien species comprised species of diverse taxonomic affinities, although, they shared the traits explaining their occurrence in particular habitats. This information is useful for understanding the link between species traits and the environmental conditions of the habitats, and complexity of the invasion process.