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.     

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.     

Linking biotic homogenization to habitat type, invasiveness and growth form of naturalized alien plants in North America

Aim Biotic homogenization is a growing phenomenon and has recently attracted much attention. Here, we analyse a large dataset of native and alien plants in North America to examine whether biotic homogenization is related to several ecological and biological attributes. Location North America (north of Mexico). Methods We assembled species lists of native and alien vascular plants for each of the 64 state‐ and province‐level geographical units in North America. Each alien species was characterized with respect to habitat (wetland versus upland), invasiveness (invasive versus non‐invasive), life cycle (annual/biennial versus perennial) and habit (herbaceous versus woody). We calculated a Jaccard similarity index separately for native, for alien, and for native and alien species. We used the average of Jaccard dissimilarity index (1 ? Jaccard index) of all paired localities as a measure of the mean beta diversity of alien species for each set of localities examined in an analysis. We used a homogenization index to quantify the effect of homogenization or differentiation. Results We found that (1) wetland, invasive, annual/biennial and herbaceous alien plants markedly homogenized the state‐level floras whereas non‐invasive and woody alien plants tended to differentiate the floras; (2) beta diversity was significantly lower for wetland, invasive, annual/biennial and herbaceous alien plants than their counterparts (i.e. upland, non‐invasive, perennial and woody alien plants, respectively); and (3) upland and perennial alien plants each played an equal role in homogenizing and differentiating the state‐level floras. Main conclusions Our study shows that biotic homogenization is clearly related to habitat type (e.g. wetland versus uplands), species invasiveness and life‐history traits such as life cycle (e.g. annual/biennial and herbaceous versus woody species) at the spatial scale examined. These observations help to understand the process of biotic homogenization resulting from alien vascular plants in North America.     

Invasiveness of alien plants in Brussels is related to their phylogenetic similarity to native species

Aim Understanding the processes that drive invasion success of alien species has received considerable attention in current ecological research. From an evolutionary point of view, many studies have shown that the phylogenetic similarity between the invader species and the members of the native community may be an important aspect of invasiveness. In this study, using a coarse‐scale systematic sampling grid of 1 km², we explore whether the occupancy frequency of two groups of alien species, archaeophytes and neophytes, in the urban angiosperm flora of Brussels is influenced by their phylogenetic relatedness to native species. Location The city of Brussels (Belgium). Methods We used ordinary least‐squares regressions and quantile regressions for analysing the relationship between the occupancy frequency of alien species in the sampled grid and their phylogenetic distance to the native species pool. Results Alien species with high occupancy frequency in the sampled grid are, on average, more phylogenetically related to native species than are less frequent aliens, although this relationship is significant only for archaeophytes. In addition, as shown by the quantile regressions, the relationship between phylogenetic relatedness to the native flora and occupancy frequency is much stronger for the most frequent aliens than for rare aliens. Main conclusions Our data suggest that it is unlikely that species with very low phylogenetic relatedness to natives will become successful invaders with very high distribution in the area studied. To the contrary, under future climate warming scenarios, present‐day urban aliens of high occupancy frequency are likely to become successful invaders even outside urban areas.     

Measuring floristic homogenization by non-native plants in North America

Aim To determine if non‐native plant species are homogenizing species composition among widely dispersed plant communities. Location Twenty localities in North America. Methods Species lists among localities were compared to measure the influence of non‐native species richness at each locality on the Jaccard Index (JI) of similarity between localities. Results After removing the effects of distance, because shared native species decreased with distance, three (nonexclusive) lines of evidence indicate that non‐native species promote homogenization. First, pairs of sites with a high combined total of non‐native species tend to have higher similarity than those with a low total of non‐natives. Second, for a given distance, more non‐native than native species tended to be shared among localities. Third, whereas most of the site comparisons with high total non‐native richness have a non‐native/native JI ratio greater than 1 (often much greater), only half of the comparisons with low total non‐native richness have a ratio greater than one. Main conclusions These findings provide quantitative support for the widely held, but rarely tested, notion that non‐native species tend to homogenize biological communities because they are more commonly shared among communities. Such testing is important as non‐native species could theoretically have no impact or even reduce homogenization among communities, if non‐native colonizers consist of different species pools.     

Plant invasions along mountain roads: the altitudinal amplitude of alien Asteraceae forbs in their native and introduced ranges

Studying plant invasions along environmental gradients is a promising approach to dissect the relative importance of multiple interacting factors that affect the spread of a species in a new range. Along altitudinal gradients, factors such as propagule pressure, climatic conditions and biotic interactions change simultaneously across rather small geographic scales. Here we investigate the distribution of eight Asteraceae forbs along mountain roads in both their native and introduced ranges in the Valais (southern Swiss Alps) and the Wallowa Mountains (northeastern Oregon, USA). We hypothesised that a lack of adaptation and more limiting propagule pressure at higher altitudes in the new range restricts the altitudinal distribution of aliens relative to the native range. However, all but one of the species reached the same or even a higher altitude in the new range. Thus neither the need to adapt to changing climatic conditions nor lower propagule pressure at higher altitudes appears to have prevented the altitudinal spread of introduced populations. We found clear differences between regions in the relative occurrence of alien species in ruderal sites compared to roadsides, and in the degree of invasion away from the roadside, presumably reflecting differences in disturbance patterns between regions. Whilst the upper altitudinal limits of these plant invasions are apparently climatically constrained, factors such as anthropogenic disturbance and competition with native vegetation appear to have greater influence than changing climatic conditions on the distribution of these alien species along altitudinal gradients.     

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.     

Diversity and abundance patterns of phytophagous insect communities on alien and native host plants in the Brassicaceae

The herbivore load (abundance and species richness of herbivores) on alien plants is supposed to be one of the keys to understand the invasiveness of species. We investigate the phytophagous insect communities on cabbage plants (Brassicaceae) in Europe. We compare the communities of endophagous and ectophagous insects as well as of Coleoptera and Lepidoptera on native and alien cabbage plant species. Contrary to many other reports, we found no differences in the herbivore load between native and alien hosts. The majority of insect species attacked alien as well as native hosts. Across insect species, there was no difference in the patterns of host range on native and on alien hosts. Likewise the similarity of insect communities across pairs of host species was not different between natives and aliens. We conclude that the general similarity in the community patterns between native and alien cabbage plant species are due to the chemical characteristics of this plant family. All cabbage plants share glucosinolates. This may facilitate host switches from natives to aliens. Hence the presence of native congeners may influence invasiveness of alien plants.     

Transit towards floristic homogenization on oceanic islands in the south-eastern Pacific: comparing pre-European and current floras

Aim  To quantify the occurrence of processes of homogenization or differentiation in the vascular flora of six oceanic islands.
Location  Six islands in the south-eastern Pacific drawn from the Desventuradas Archipelago, Easter Island and the Juan Fernández Archipelago.
Methods  Using published floristic studies, we determined the floristic composition of each island at two different stages: (1) pre-European colonization and (2) current flora. We compared changes in the number of shared plants and the floristic similarity among islands for each stage.
Results  The number of plant species doubled from 263 in pre-European flora to 531 species currently. Only three native species became extinct, four natives were translocated among the islands and 271 plant species were introduced from outside. The frequency of plant species shared by two or more islands is higher in the post-European floras than prior to European contact, and the level of floristic similarity between islands increased slightly.
Main conclusions  Considering the low naturalization rate of alien plants, the small number of extinctions and the meagre increase in floristic similarity, these islands are undergoing a slow process of floristic homogenization.     

Widespread plant species: natives versus aliens in our changing world

Estimates of the level of invasion for a region are traditionally based on relative numbers of native and alien species. However, alien species differ dramatically in the size of their invasive ranges. Here we present the first study to quantify the level of invasion for several regions of the world in terms of the most widely distributed plant species (natives vs. aliens). Aliens accounted for 51.3% of the 120 most widely distributed plant species in North America, 43.3% in New South Wales (Australia), 34.2% in Chile, 29.7% in Argentina, and 22.5% in the Republic of South Africa. However, Europe had only 1% of alien species among the most widespread species of the flora. Across regions, alien species relative to native species were either as well-distributed (10 comparisons) or more widely distributed (5 comparisons). These striking patterns highlight the profound contribution that widespread invasive alien plants make to floristic dominance patterns across different regions. Many of the most widespread species are alien plants, and, in particular, Europe and Asia appear as major contributors to the homogenization of the floras in the Americas. We recommend that spatial extent of invasion should be explicitly incorporated in assessments of invasibility, globalization, and risk assessments.     

Alien and native plant life‐forms respond differently to human and climate pressures

Aim To investigate whether differences in the elevational trend in native and alien species richness were dependent on climate or human pressures. Specifically we tested whether life‐form and/or alien/native status modifies the response of plant species richness to human population and temperature along: (1) a complete elevational gradient, and (2) within separate elevational bands that, by keeping temperature within a narrow range, elucidate the effects of human pressures more clearly. Location Two provinces (c. 7507 km²) on the southern border of the European Alps (Italy), subdivided into 240 contiguous sampling cells (c. 35.7 km²). Methods We used an extensive dataset on alien and native species richness across an elevation gradient (20–2900 m a.s.l.). Richness of natives and naturalized aliens were separately related to temperature, human population and Raunkiaer life‐form using general linear mixed models. Life‐form describes different plant strategies for survival during seasons with adverse cold/arid conditions. Results The relationship between species richness and temperature for natives was strongly dependent on life‐form, while aliens showed a consistent positive trend. Similar trends across alien and native life‐forms were found for the relationship between species richness and human population along the whole gradient and within separate elevational bands. Main conclusions The absence of life‐form‐dependent responses amongst aliens supports the hypothesis that the distribution of alien plant species richness was more related to propagule pressure and availability of novel niches created by human activities than to climatic filtering. While climate change will potentially contribute to relaxing species thermal constraints, the response of alien species to future warming will also be contingent on changes in anthropogenic pressures.     

How general are global trends in biotic homogenization? Floristic tracking in Chile,South America

Aim   To quantify the occurrence of floristic change in the vascular flora of Chile. We test whether continental areas have experienced floristic modification leading to either homogenization, differentiation or tracking.
Location   Continental Chile.
Methods   On the basis of the geographical distribution of native (1806 species) and naturalized plants (552 species) in continental Chile, we quantified change between two floristic stages: (1) pre-European flora, including native extant and extinct species; and (2) current flora, including native and naturalized species, but excluding extinct plants. We compared changes in compositional similarity (calculated by Jaccard's index, Δ J ) between pairs of regions, and similarity decay with respect to geographical distance. Additionally, by means of Whittaker's index, we examined species turnover, distinguishing between native and naturalized plants.
Results   Between floristic stages (pre-European vs. current flora) no significant changes in floristic similarity were noted at national or regional scales. Similarity decay showed no statistical differences between pre-European and current flora. Analysing patterns of geographical turnover, we found that species turnover of naturalized plants over their geographical range is similar to that of native plants.
Conclusions   The composition of the continental flora of Chile does not show significant modifications in similarity patterns after considering naturalized species, thus indicating floristic tracking. The causes of this phenomenon may be related to the current geographical distribution of naturalized plants, which closely parallels that of native plants. Our results differ from those obtained in Northern Hemisphere continents, thus indicating that trends of biotic change may differ between hemispheres.     

Role of non-established plants in determining biotic homogenization patterns in Pacific Oceanic Islands

Numerous organisms have been relocated by human agency among remote regions of the world. One of the consequences of alien invasions (and associated native extinctions) is that the taxonomic similarity between areas trends to increase, a process called biotic homogenization. This process is studied in plants using naturalized species, while the role of non-established (but nevertheless present) plants is not investigated. Here we evaluate if adding the non-established component to the pool of alien plants modifies similarity patterns recorded in a preceding study of biotic homogenization in six Oceanic Islands of the South-eastern Pacific. Although our analyses confirm previous findings of floristic homogenization for these Islands, the effect of adding the non-established component results in several changes. By consideration of only naturalized plants we detected two cases of increased similarity (i.e. floristic homogenization), while the other 13 comparisons yielded non-significant changes. By adding non-established plants, four pairs of Islands show increased similarity, while in one case there is decreased similarity (i.e. floristic differentiation). These results support the hypothesis that non-established species can modify qualitative and quantitative trends of floristic change, revealing the complexity of biotic change. Although discrimination between only native and naturalized species appears reasonable for some groups, among plants it becomes a critical decision because of the importance of non-established species as ecological players and as purveyors of early information on future biotic change.     

Disentangling the relative effects of environmental versus human factors on the abundance of native and alien plant species in Mediterranean sandy shores

Aim Mediterranean coastal sand dunes are characterized by both very stressful environmental conditions and intense human pressure. This work aims to separate the relative contributions of environmental and human factors in determining the presence/abundance of native and alien plant species in such an extreme environment at a regional scale. Location 250 km of the Italian Tyrrhenian coast (Region Lazio). Methods We analysed alien and native plant richness and fitted generalized additive models in a multimodel‐inference framework with comprehensive randomizations to evaluate the relative contribution of environmental and human correlates in explaining the observed patterns. Results Native and alien richness are positively correlated, but different variables influence their spatial patterns. For natives, human population density is the most important factor and is negatively related to richness. Numbers of natives are unexpectedly lower in areas with a high proportion of natural land cover (probably attributable to local farming practices) and, to a lesser degree, affected by the movement of the coastline. On the other hand, alien species richness is strongly related to climatic factors, and more aliens are found in sectors with high rainfall. Secondarily, alien introductions appear to be related to recent urban sprawl and associated gardening. Main conclusions Well‐adapted native species in a fragile equilibrium with their natural environment are extremely sensitive to human‐driven modifications. On the contrary, for more generalist alien species, the availability of limited resources plays a predominant role.     

Forest fragmentation drives Atlantic forest of northeastern Brazil to biotic homogenization

Aim To examine whether the tree flora of the Atlantic forest of northeastern Brazil has experienced detectable taxonomic homogenization via the proliferation of native pioneer species in response to habitat loss and fragmentation. Location Biotic homogenization (BH) was examined across the Atlantic forest of northeast Brazil, i.e. a 56,000 km² piece of tropical forest and a distinct centre of species endemism in South America. Methods We assessed a dataset consisting of 5122 tree records and compared the similarity of tree floras from 12 semi‐natural sub‐regions of the Atlantic forest between two time periods: pre‐1980 (plant records between 1902 and 1980), and post‐1980 (between 1981 and 2006). To understand the mechanisms leading to BH (1) tree floras were ordered (via non‐metric multidimensional scaling – NMDS) by date (pre/post 1980) based on species occurrence and frequency, (2) NMDS axes were regressed against the proportion of those species that increased their occurrence post‐1980 (i.e. the winner species), and (3) patterns of geographic distribution and frequency of particular life‐history traits were examined across winner species and a control group. Results Tree floras across the Atlantic forest became c. 20–40% more similar to each other post‐1980, but patterns of species similarity were also influenced by between‐plot geographical distance. NMDS ordination clearly segregated pre‐ and post‐1980 floras with a clear signal of floristic convergence. Furthermore, winner tree species were largely composed of short‐lived and small‐seeded pioneer species that exhibit wide geographic distributions. Main conclusions Our results suggest that tropical forest biotas are susceptible to taxonomic homogenization (i.e. increasing levels of similarity) in the context of severe human‐disturbance via the proliferation of particular groups of native species comprised mainly by ecologically‐plastic, generalist species. We are thus extending the concept of homogenization to address and highlight a pervasive biological shift in the structure of tropical forest communities currently taking place across hyper‐fragmented landscapes.     

Contrasting patterns and mechanisms of spatial turnover for native and exotic freshwater fish in Europe

Aim We compare the distribution patterns of native and exotic freshwater fish in Europe, and test whether the same mechanisms (environmental filtering and/or dispersal limitation) govern patterns of decrease in similarity of native and exotic species composition over geographical distance (spatial species turnover). Locations Major river basins of Europe. Methods Data related to geography, habitat diversity, regional climate and species composition of native and exotic freshwater fish were collated for 26 major European river basins. We explored the degree of nestedness in native and exotic species composition, and quantified compositional similarity between river basins according to the beta‐sim (independent of richness gradient) and Jaccard (dependent of richness gradient) indices of similarity. Multiple regression on distance matrices and variation‐partitioning approaches were used to quantify the relative roles of environmental filtering and dispersal limitation in shaping patterns of decreasing compositional similarity over geographical distance. Results Native and exotic species exhibited significant nested patterns of species composition, indicating that differences in fish species composition between river basins are primarily the result of species loss, rather than species replacement. Both native and exotic compositional similarity decreased significantly with increasing geographical distance between river basins. However, gradual changes in species composition with geographical distance were found only for exotic species. In addition, exotic species displayed a higher rate of similarity decay (higher species turnover rate) with geographical distance, compared with native species. Lastly, the majority of explained variation in exotic compositional similarity was uniquely related to geography, whereas native compositional similarity was either uniquely explained by geography or jointly explained by environment and geography. Main conclusions Our study suggests that large‐scale patterns of spatial turnover for exotic freshwater fish in Europe are generated by human‐mediated dispersal limitation, whereas patterns of spatial turnover for native fish result from both dispersal limitation relative to historical events (isolation by mountain ranges, glacial history) and environmental filtering.     

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.

     

Effects of introduced species on floristic similarity: Comparing two US states

This study aims to examine the effects of introduced species on increasing (homogenizing) or decreasing (differentiating) floristic similarity of plant composition. We calculated the Jaccard index for each pair of counties within two states of USA, California and Florida. We computed the Jaccard index separately for all (native plus exotic) species, for native species, and for exotic species. We further calculated a homogenization index between all species and native species for each pair of counties by subtracting similarity index for native species from that for all species. We correlated the Jaccard and homogenization indices to geographic distance, latitude separation, and longitude separation between pairs of counties and to average human population density and average land area of the two counties. We find a very strong pattern of differentiation for introduced species among nearly all Florida counties. In California, introduced species have a differentiating effect in about half the comparisons. We also find that introduced species tend to have a more homogenizing (or less differentiating) effect with increasing distances between counties. Our results do not show a clear relationship between human population density and the homogenization index.     

Different flowering phenology of alien invasive species in Spain: evidence for the use of an empty temporal niche?

Flowering phenology is an important and poorly understood plant trait that may possibly be related to the invasiveness potential of alien species. The present work evaluates whether flowering time of invasive alien species is a key trait to overcome the climatic filters operating in continental Mediterranean ecosystems of Spain (characterised by summer drought and low temperatures in winter). We conducted comparisons between the flowering phenology of the invasive species in their native range and in Spain, and between flowering phenology of 91 coexisting invasive–native species pairs. For the alien species, geographical change from the native to the invaded region did not result in shifts in the start and the length of the flowering period. Overall, climatic conditions in the native range of species selected for a flowering pattern is maintained after translocation of the species to another region. Flowering of tropical and temperate invasive alien species peaked in summer, which contrasts with the spring flowering of native and invasive alien species of Mediterranean climate origin. By exploiting this new temporal niche, these invasive alien species native to tropical and temperate regions benefit from reduced competition with natives for abiotic and biotic resources. We suggest that human-mediated actions have reduced the strength of the summer drought filter in particular microhabitats, permitting the invasion of many summer-flowering aliens.     

Patterns of alien plant diversity in the urban landscapes of global biodiversity hotspots: a case study from the Himalayas

In an age of Anthropocene, the urban landscapes are recognised as the ‘hotspots’ of human-mediated alien species introductions. As the cities provide an ideal natural experimental system to investigate the patterns of alien plant diversity in urban landscapes, the present study aimed to unravel the taxonomic, biogeographic and ecological patterns of alien flora of Srinagar—one of the largest urban centres in the Himalayan biodiversity hotspot. The alien flora of Srinagar comprises 325 species, constituting ca.35% of total flora of the city. Out of the 325 alien species documented, 157 species (43%) were recorded to be under cultivation, while 168 species (57%) were growing in the wild (i.e., outside cultivation); those growing in the wild, in turn, comprised 110 cultivation escapes and 58 accidentally introduced plant species. Biogeographically, two-third of the alien plant diversity reported from Srinagar is native to Asia-Temperate. This indicates that climatic similarity between Asia-Temperate and Kashmir Himalayas facilitate in flourishing similar floristic diversity. The study highlights a relatively higher proportion of herbaceous growth form in the aliens growing in the wild (80%) than those under cultivation (43%). Similarly, 82% of the alien species under cultivation had a perennial life span, but those growing in the wild were dominated by annuals (44%). Currently, 45 species are growing as casuals and 124 species are naturalised (including 105 naturalised non-invasive and 19 naturalised invasive). Along the continuum of casual-naturalised-invasive categories, the contribution of cultivation escapes and accidently introduced aliens contrastingly shows decreasing and increasing trends respectively. Interestingly, the results revealed that the human practice of stopping cultivation of alien escapes increased rapidly as we move along the continuum. Thus, the present study has investigated the patterns of alien plant diversity in the urban landscape of Srinagar, and the results obtained offer scientific insights toward better scientific understanding and management of plant invasions in this Himalayan city, with wider policy implications for neighbouring urbanised landscapes in the Himalayas and other mountainous regions across the world.