Patterns of native and exotic species richness in the urban flora of Brussels: rejecting the ‘rich get richer’ model

In this study we analyzed patterns of native and exotic species richness in the urban flora of Brussels (Belgium) using a coarse-scale systematic sampling grid of 1 km². The observed correlation between native and exotic richness within the grid cells sampled was then compared to the results of an adequate null model assuming no species interactions. In addition, ordinary least-squares and quantile regressions were used to analyze the relationship between the ratio of exotics to natives and the proportion of densely built up areas in each cell. Though the results obtained conform to the Eltonian expectation that exotic species preferably invade areas of low native species diversity, traditional niche-filling mechanisms seems inadequate to explain the observed pattern. Rather, aliens simply tend to have different environmental requirements than natives.     

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.     

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.     

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.     

Diversity of native and alien vascular plant species of dry grasslands in central Europe

Question: Which factors determine diversity of native and alien vascular plant species in semi‐natural dry grasslands? Location: Northern limestone Alps to the southern rim of the Bohemian massif in northern Austria. Methods: In 70 randomly chosen dry grassland patches (0.008 ha ‐ 7 ha) we sampled a complete inventory of vascular plant species at each site. We analysed the correlation between species diversity of natives, archaeophytes (pre‐1500 aliens) and neophytes (post‐1500 aliens). We used GLM to study the relationship of species number (natives, neophytes, archaeophytes) to five explanatory variables (altitude, within habitat diversity, habitat diversity of adjacent areas, within land‐use diversity and land‐use in adjacent areas). Orthogonal components of these variables were derived with a PCA and used in the models. We also tested the influence of minimum residence time (MRT) and the covariables origin, mode of introduction and life form on the number of grassland sites with neophytes with analogous GLMs. Results: Native species diversity species was positively correlated with the species diversity of new, but not old invaders. GLM explains 70% of the variance in the number of native species. Patch size explained the largest part of the variation in the number of native species. PCA axes 1 and 3 were significantly related to the number of native species. Axis 1was related to on‐site habitat and land‐use diversity. The GLM of the archaeophyte diversity explains 18% of the variance. Altitude and presence of fields and grassland in the neighbourhood mainly explained archaeophyte species diversity. The GLM of neophyte diversity explains 12% of the variance. The number of neophytes was positively related to that of archaeophytes. Only PCA axis 3, which is mainly influenced by adjacent land‐use types, showed a relationship with neophytes. MRT, mode of introduction and region of origin (but not life form) were significantly related to the number of grassland sites invaded by neophytes, explaining 35% of the variance. Conclusion: Most factors governing native species diversity are not significantly related to alien species diversity. Additional determinants of the local scale diversity of alien species exist such as region of origin and historical factors (MRT, mode of introduction).     

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.     

Do urban areas act as foci for the spread of alien plant species? An assessment of temporal trends in the UK

Aim Given that urban landscapes often act as a point of entry for many non‐native species and urban development continues to increase as the human population rapidly expands, an understanding of the interaction between urbanization and non‐native plant species is important both in the control of potentially invasive species and in the conservation of native biodiversity. We investigated the spatial and temporal relationship between urban land cover and the distribution of non‐native species in Britain using two floristic data sets collected at two different time periods: 1987–88 and 2003–04. Location UK. Methods Using floristic data collected by the Botanical Society of the British Isles in 1987–88 (Monitoring Scheme) and 2003–04 (Local Change) in conjunction with habitat data obtained from the Land Cover Map of the UK, we conducted multiple regression analyses both within and between years on both groups of species (natives, neophytes and archaeophytes) and individual species. Results Neophytes (alien species introduced after 1500) were very strongly associated with urban land cover in both time periods and do not appear to be spreading out of urban habitats into the wider countryside. Archaeophytes (alien species introduced before 1500), however, showed a strong association with urban habitats in the earlier 1988 data set but no longer showed this association in the 2004 data set. Analysis at the individual species level showed that a large percentage of alien plant species, particularly archaeophytes, were not strongly associated with urban land cover or were negatively associated with such habitats. Main conclusions Our results suggest that there has been a reduction in the urban association of archaeophytes that is likely to have resulted from the recovery of archaeophytes associated with non‐urban (especially arable) habitats, following their decline in mid‐20th century, rather than from the movement of aliens into the wider countryside from urban habitats.     

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.     

Scale dependence in the phylogenetic relatedness of alien and native taxa

达尔文的归化假说提出,由于生态位的不同,成功建群的外来物种与本地物种的关系不太密切。先前的研究对这一假设有支持也有反对,其中一个原因是外来物种和本地物种在大的空间尺度上有系统发育聚类的倾向,而在细微尺度上存在过度分散的倾向。然而,对于外来物种的系统发育关系如何改变其入侵群落的系统发育结构,以及在何种空间尺度上可能表现出这种影响,人们知之甚少。在本研究中,我们调查被入侵的森林下层植物群落在系统发育上是聚集的还是或过度分散的,亲缘关系如何随空间尺度变化,以及外来物种如何影响下层群落的系统发育模式。在澳大利亚东南部干旱森林的下层群落进行了5个空间尺度(1, 20, 500, 1500和4500 m²)的实地调查。使用两个指标的标准化效应量[(i)平均成对距离和(ii)平均最近分类单元距离]来量化群落与其外来和本地亚群落之间的系统发育关系,并研究系统发育模 式如何随空间尺度变化。研究结果表明,外来物种之间的亲缘关系非常密切,而且这种亲缘关系会随着尺度的增加而增加。在中等空间尺 度下(20–500 m²), 整个群落呈随机分布趋势,而本地物种高度分散,外来亚群落高度聚集。这说明亲缘关系密切的外来物种入侵使群落系统发育结构由过度分散向随机分布转变。外来物种和本地物种在空间尺度上是远亲,这支持了达尔文的归化假说,但只是在系统发育距离被量化为平均最近分类单元距离时成立。外来物种和本地物种的系统发育差异随着空间尺度的增加而增加,这与预期的模式相反。我们的研究结果表明,外来物种强大的系统发育聚类是由人类干预的引入驱动的,牵涉能够成功建群和传播的密切相关的类群。系统发育相关性的尺度依赖模式可能是由火灾和散布等随机过程引起的,这表明竞争和生境过滤并不是分别在小和大尺度上控制系统发育关系的唯一因素。区分不同进化深度的指标很重要,因为不同的指标可以显示不同的尺度依赖模式。     

Phylogenetic beta diversity of native and alien species in European urban floras

Aim Human activities have weakened biogeographical barriers to dispersal, increasing the rate of introduction of alien plants. However, their impact on beta diversity and floristic homogenization is poorly understood. Our goal is to compare the phylogenetic beta diversity of native species with that of two groups of alien species, archaeophytes and neophytes (introduced before and after ad 1500, respectively), across European urban floras to explore how biological invasions affect phylogenetic turnover at a continental scale. Location Twenty European cities located in six countries between 49 and 53° N latitude in continental Europe and the British Isles. Methods To compare the phylogenetic beta diversity of native and alien species we use the average phylogenetic dissimilarity of individual floras from their group centroid in multivariate space. Differences in phylogenetic beta diversity among different species groups are then assessed using a randomization test for homogeneity of multivariate dispersions. Results Across European urban floras, and when contrasted with natives, archaeophytes are usually associated with lower levels of phylogenetic beta diversity while neophytes tend to increase phylogenetic differentiation. Main conclusions While archaeophytes tend to promote limited homogenization in phylogenetic beta diversity, because of their diverse geographical origin together with short residence times in the invaded regions, neophytes are not promoting biotic homogenization of urban floras across Europe. Therefore, in spite of the increasing rate of alien invasion, an intense phylogenetic homogenization of urban cities is not to be expected soon.     

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.     

Effects of urban land cover on the local species pool in Britain

Regression models were used to quantify the relationship between the amount of urban land and composition of local species pools. There was no evidence that urban land cover increases the richness of plant species, based on a survey of 785 2-km squares of which 157 had > 10% urban land cover. However, the number of alien plants is significantly higher in urban areas. Complete urbanization approximately doubles the proportion of alien species, and the proportion of aliens is twice as high in southern Britain as in the north. The flora of urban tetrads consists of ubiquitous native species and introduced species characteristic of waste ground, but woodland species are poorly represented. At the tetrad scale. enhanced dispersal by man is not the main factor for maintaining the urban flora; availability of urban habitats and high levels of disturbance are more important. The planned housing expansion to greenfield sites in Britain will increase the proportion of alien species, yet the majority of native species should persist in urban areas if existing woodland is preserved.     

Predicting and explaining plant invasions through analysis of source area floras: some critical considerations

Comparing species that become invasive with others from the same regional species pool that do not invade raises several issues about the accuracy of analyses attempting to define the determinants of invasiveness. The delimitation of the source area and deciding which species group(s) to include are especially relevant in analyses focusing on species originating in Europe. Historical patterns of immigration of alien species into Europe must be considered since European floras comprise a complex mix of native species, historical introductions (archaeophytes) and relative newcomers (neophytes). We make three main points: (1) Archaeophytes (species introduced to Europe before the discovery of America) differ from natives in a number of traits and in historical association with people; it is misleading to lump archaeophytes with native taxa. (2) Taxa from climatically and geographically different regions, representing distinct floristic geoelements, need to be treated separately, and not as a homogenous pool of potential invaders. Restricting the source species pool to native taxa with comparable phytogeographical characteristics reduces the variation associated with chance of dispersal by humans from the source area. (3) For prediction, a clear distinction should be made between accuracy (the proportion of those found to be alien that were also predicted to be there) and reliability (or predictive value, the proportion of those predicted to become aliens that do so). Information accumulated over centuries by botanists in Central Europe provides an excellent opportunity to deal with these issues and avoid spurious results. To illustrate these issues, we revisit a recently published study of Central‐European plant species as invaders in two Argentinean provinces ( Prinzing et al., 2002 ) to explore and demonstrate the implications of the above points. We hope that future studies will build on these points to achieve more reliable predictions.     

Alien plant species and factors of invasiveness of anthropogenic vegetation in the Northwestern Balkans — a phytosociological approach

We studied the anthropogenic vegetation of the Northwest Balkans in order to determine its susceptibility to invasion by alien plant species. We compiled a dataset of 3089 vegetation plots sampled between 1939 and 2009, recording a set of variables for each sample plot in order to determine which factors have the most effect on a habitat’s vulnerability to invaders. We calculated the proportion of native species, archaeophytes and neophytes for each plot. We used regression tree models to determine the site conditions of the most invaded anthropogenic habitats. The sample plots contained an average of 12.7% alien plant species, with a low proportion of archaeophytes (4.3%) and 8.4% neophytes. Local habitat conditions proved to have the largest effect, rather than climatic variables or propagule pressure. The proportion of archaeophytes follows a different pattern than that seen in central and northern Europe, indicating that macroecological factors are more important. Neophytes show a similar distribution to other European locations.     

Floristic homogenization by native ruderal and alien plants in north-east Spain: the effect of environmental differences on a regional scale

Aim To evaluate the relative potential contribution of native ruderals and aliens to plant homogenization at a regional scale, after taking into account the effect of diverse environmental distances. Location Catalonia (north‐east Spain) Methods We have used the flora module of the BDBC project (Catalonian Database of Biodiversity), which provides information on plant species distribution per 10 × 10 km Universal Transverse Mercator (UTM) cell. Pairwise floristic similarities of: (1) total, (2) native non‐ruderal, (3) native ruderal, and (4) alien vascular plant species have been calculated for a particularly well‐sampled subset of UTM cells, using a modified version of the Simpson index. These similarities have been compared per UTM pair using Mantel tests, before and after considering their relative association with geographical, climatic and landscape distances from linear regression models. The floristic similarity of both total and native non‐ruderal species was also correlated with the proportion of alien and native ruderal species after discounting the effects of environmental distances. Results The proportion of variance explained by environmental correlates was highest for the floristic similarity of native non‐ruderal plants and lowest for that of aliens. In all plant groups, climatic distance was the main significant variable of species similarity. Geographical distance was only significant for total and native non‐ruderal species and was of secondary importance in both cases. Landscape distance was not significant in any case. Similarities among both aliens and native ruderals were significantly higher than among native non‐ruderals, but these differences disappeared after removing the effect of environmental distances. Main conclusions Species similarity between sites may depend on differences in environmental factors other than geographical distance. This has to be taken into account when exploring the implications for biotic homogenization. In the case of Catalonian flora, the potentially homogenizing effect of native ruderal and alien species seems to be associated with their lower dependence on geographical distance and climatic factors compared with those of native, non‐ruderal species.     

Taxonomic and biogeographic patterns in the native and alien woody flora of Kashmir Himalaya,India

Biodiversity inventories unravel insightful taxonomic and biogeographic patterns that provide valuable inputs in guiding conservation practices and policymaking. Here we present an updated inventory of the native and alien woody flora of the Kashmir Himalaya, India, together with analyses of the patterns of taxonomic composition, geographic distribution, and invasion status of the alien species. The data was assembled from the floristic literature of the last two centuries, supplemented with herbarium records and validated by field surveys over the past decade. In total, the woody flora comprised of 520 species; out of these 322 species were native and 198 alien, yielding a proportion of 38% aliens. Amongst the natives, 16 species are on the IUCN red list 2009. While 37 genera included both native and alien species indicating a potential for hybrid formation, only 15 families were represented by alien species exclusively. Regarding the invasion status, presently 37 alien woody species are naturalized and 7 are invasive in the region. The results show that the Kashmir Himalaya harbours a diverse woody flora. It is concluded that a distinction between the native and the alien flora, as adopted in the present study, is crucial for a meaningful analysis of any biodiversity patterns. We predict that in the near future many alien woody species, presently naturalized in the region, can become serious plant invaders, and therefore need further research efforts and timely management.     

A meta-analysis of impacts of alien vs. native plants on pollinator visitation and reproductive success of co-flowering native plants

Alien plant species can alter pollinator visitation and, in turn, the sexual reproduction of natives. Using a conventional and a phylogenetically controlled meta-analytical approach on a data set of 40 studies, we evaluated the effect of alien neighbour plant species (aliens) on visitation to and reproduction of native co-flowering focal species (focals), and compared such effect to that of native neighbours (natives). An overall significantly negative effect of aliens on visitation to and reproduction of focals was confirmed. Interestingly, aliens differed from natives in their effect on visitation, but not on reproductive success. The negative effect of aliens on visitation and reproductive success increased at high relative alien plant abundance, but this increase was proportionally lower than the increase in relative plant abundance. Likewise, effect of aliens on visitation and reproductive success was most detrimental when alien and focal species had similar flower symmetry or colour. The phylogenetic relatedness between alien neighbours and focals influenced the reproductive success effect size. Results of the phylogenetic meta-analysis were only partly consistent with those of the conventional meta-analysis, depending on the response variable and on whether we controlled for the phylogeny of neighbour or focal species, which calls for special attention to control for species relatedness in this type of review. This study demonstrates the predominant detrimental impact of alien plants on pollination and reproduction of natives, and highlights the importance of phenotypic similarity to the outcome of the interaction.     

It takes one to know one: Similarity to resident alien species increases establishment success of new invaders

Aim

Darwin's naturalization hypothesis states that dissimilarity to native species may benefit alien species establishment due to empty niches and reduced competition. We here add a new dimension to large‐scale tests of community invasibility, investigating the role that previously established alien species play in facilitating or hindering new invasions in plant communities.

Location

Permanent grasslands across France (including mainland and Corsica), as a receding ecosystem of great conservation importance.

Methods

Focusing on 121 alien plant species occurring in 7,215 vegetation plots, we quantified biotic similarity between new invaders and resident alien species (i.e., alien species with longer residence times) based on phylogenetic and trait distances. Additionally, we calculated distances to native species for each alien species and plot. Using multispecies distribution models, we analysed the influence of these biotic similarity measures and additional covariates on establishment success (presence/absence) of new invaders.

Results

We found that biotic similarity to resident alien species consistently increased establishment success of more recently introduced species. Phylogenetic relatedness to previous invaders had an equally strong positive effect as relatedness to native species. Conversely, trait similarity to natives hindered alien establishment as predicted by Darwin's naturalization hypothesis. These results highlight that various mechanisms may act simultaneously to determine alien establishment success.

Main conclusions

Our results suggest that, with greater similarity among alien species, invasion success increases. Such a pattern may arise either due to actual facilitation among invaders or as a result of weaker competitive interactions among invaders than between native and alien species, leading to an indirect facilitative effect. Alternatively, recent environmental changes (e.g., eutrophication, climate change) may have added new environmental filters. Determining how initial invasions might pave the road for subsequent invasions is crucial for effective multispecies management decisions and contributes a new aspect to our understanding of community assembly.

     

Exotic plant species invade diversity hot spots: the alien flora of northwestern Kenya

We analysed the distribution of native and alien plant species across 20 ecogeographic zones of northwestern Kenya. The source pool for the majority of aliens was Europe and America. Thus, the source pool has a biogeographic bias which explains the low proportion of aliens in the tropics: most species in the European or American source pool are not well adapted to tropical conditions. As expected, native and alien plant species showed an area effect. Correcting for this area effect. species rich zones showed a higher proportion of alien plant species in their flora. At the analysed scale, species richness of native plant communities does not increase the resistance to invasions and alien plant species invade diversity hotspots. Compared to the other ecogeographic zone, the urban area around Nairobi showed an increased richness in alien and native plant species. This is very similar to findings in Europe, although the history of urbanisation is much shorter in Kenya. The species turnover between zones (β-diversity) shows a similar pattern in native and alien plant species. Within a very short time scale the alien plant species mapped the biogeographic patterns of natives, although the geography of human activities influences the propagule pressure.