Floristic homogenization as a teleconnected trend in oceanic islands

Aim We searched for evidence of floristic homogenization in widespread oceanic archipelagos. Location Twelve oceanic archipelagos in the Atlantic and Pacific oceans: Ascension, Azores, Canary, Cape Verde, Desventuradas, Easter, Galápagos, Hawaii, Juan Fernández, Madeira, Puerto Rico and Savage. Methods By using Jaccard’s index, we established the floristic similarity between pairs of archipelagos at two stages: original (pre‐European; J_o) and current flora (J_c). Then, we calculated ΔJ = J_c–J_o, where positive differences imply that similarity has increased floristic homogenization. Results We found that floristic similarity increased fourfold on average, from 1.6% to 6.3% for original and current floras, respectively. In fact, we recorded 64 cases in which floristic similarity increased and only two in which it decreased. The importance of invasions exceeds that of extirpations as a driver of biotic change by more than an order of magnitude (2679 versus 142 species, respectively). Main conclusions The vascular floras of these 12 insular oceanic systems have increased in compositional similarity, a phenomenon consistent with the trend towards biotic homogenization. It can be characterized as a teleconnected process that operates across vast geographical distances, driven by the unprecedented capacity of humans for translocation. Trends in biotic homogenization differ depending on the geographical location (i.e. Pacific < Atlantic archipelagos) and phylogenetic groups (island vascular plants > island birds reported in a previous study), emphasizing the complexity of biotic change.     

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.     

Distribution of alien plant species in relation to human disturbance on the Georgia Sea Islands

Abstract. This study investigates the effects of human disturbance and environmental factors on the distribution of alien plant species on the Georgia Sea Islands (GSI), USA. We sampled the absolute cover of native and alien plant species on two tourist islands (St. Simons Island and Jekyll Island) and on two protected National Wildlife Refuge Islands (Blackbeard Island and Wassaw Island). On each island, vegetation composition and environmental variables (soil properties and salt spray) were measured in two habitats that differed substantially in their degree of environmental stress, the more exposed primary dune and the more sheltered and inland maritime forest. Sites were further stratified within each habitat into areas that had different levels of human disturbance. Many alien species were present on all islands and the absolute cover of alien species was not significantly different among islands even though they varied substantially in their degree of accessibility and overall land use. Alien plant cover was appreciably greater in severely disturbed sites than in less disturbed sites on all islands and within both habitats. However, the difference between disturbance categories was much less pronounced in the primary dunes where human disturbance agents do not mitigate the harsh environmental conditions of this habitat (salt spray and saline soils). Alien plant abundance on the GSI is evidently more dependent upon the availability of disturbed ground than the degree of accessibility or overall island development. It appears that human disturbance increases alien cover in general, but in environments where the stress levels are not mitigated, human disturbance does little to foster alien invasions.     

Spatial aspects of trait homogenization within the German flora

Aim Biotic homogenization, the replacement of local biota by non‐indigenous and locally expanding species, is among the major effects of species invasions. Almost all studies related to this topic are focused on the species level, on taxonomic homogenization. Homogenization effects on trait diversity (functional homogenization) may be very important for ecosystem functioning, but they are rarely analysed, especially not at different spatial scales within a single study. This paper aims to examine how the presence of alien species (species introduced since 1500 ad ) affects the distribution of ploidy levels in the German flora at different spatial scales. Location Germany. Methods We used a resampling method and the Morisita–Horn dissimilarity index to calculate dissimilarities of ploidy level within and between alien and indigenous plants at three different spatial scales in Germany: (1) the entire country as one location, (2) between grid cells, and (3) within grid cells of a lattice covering Germany. Results We found a significant differentiation effect within grid cells of c. 130 km², with alien plants increasing the variability of ploidy levels. The differentiation effect varies according to the scale used. At the coarsest scale (entire Germany), ploidy levels of alien plants tend to be more homogeneous in composition compared with those of native plants. At the intermediate scale (between grid cells), and even more pronounced at a small scale (within grid cells), ploidy levels are more heterogeneously distributed across Germany than those of native plant species. Main conclusions Homogenization of ploidy levels at a large scale (entire Germany) and differentiation at a small scale (within grid cells) is comparable with the patterns found in taxonomic homogenization analyses. This analysis is a first step towards understanding the impacts of plant invasions on a trait level. Differentiation and homogenization of ploidy levels might mirror the distribution of related ecological traits and therefore might have an impact on ecosystem functioning.     

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.     

The biogeography of naturalization in alien plants

Aim  This paper reviews the main geographical determinants of naturalization in plants.
Location  Global.
Methods  Comparative studies of large data sets of alien floras are the main source of information on global patterns of naturalization.
Results  Temperate mainland regions are more invaded than tropical mainland regions but there seems to be no difference in invasibility of temperate and tropical islands. Islands are more invaded than the mainland. The number of naturalized species in temperate regions decreases with latitude and their geographical ranges increase with latitude. The number of naturalized species on islands increases with temperature. Naturalized species contribute to floristic homogenization, but the phenomenon is scale-dependent.
Main conclusions  Some robust patterns are evident from currently available data, but further research is needed on several aspects to advance our understanding of the biogeography of naturalization of alien plants. For example, measures of propagule pressure are needed to determine the invasibility of communities/ecosystems/regions. The patterns discussed in this paper are derived largely from numbers and proportions of naturalized species, and little is known about the proportion of introduced species that become naturalized. Further insights on naturalization rates, i.e. the proportion of aliens that successfully naturalize within regions, and on geographical and other determinants of its variation would provide us with better understanding of the invasion process. Comparative studies, and resulting generalizations, are almost exclusively based on numbers of species, but alien species differ in their impact on native biodiversity and ecosystem processes.     

Detailed study of a river corridor plant distribution pattern provides implications for river valley conservation

River valleys have been subjected to human-induced changes for centuries, but they are still considered regional hotspots of biodiversity. In central Europe, some vascular plant species demonstrate confinement to the corridors of large rivers. They are termed river corridor plants (RCPs). RCPs are an important component of regional biodiversity and include a high proportion of threatened species, thus they deserve attention. Here we examine: (1) the detailed distribution pattern of RCPs within a river valley, (2) the habitat preferences of RCP species, and (3) the correlation between the richness of RCP species and selected variables. The studied variables include: river bed proximity, distance from the river mouth, floodplain coverage, richness of native, red listed and invasive species, and number of habitats considered to be of Europaean Community importance. Surveys were conducted in 10 transects running perpendicularly to the San River bed (Poland, central Europe). Each transect was divided into 14 plots (1 km × 1 km). In each plot, the site locations of RCPs as well as their habitats were recorded. The occurrence of all vascular plant species in a particular plot was also noted. The richness and abundance of RCP species depended on the distance from the river and the floodplain coverage in a plot. The plots located in the vicinity of the river were the richest in RCP species and usually harbored the largest number of native, red-listed and invasive species. They were also characterized by the largest number of habitats considered to be of importance to the European Community. RCP species differed in the degree of confinement to habitats regarded as typical for them. Some of the RCP species were recorded only within typical habitats while others were found in several different types of habitats, including anthropogenic ones. Knowledge concerning the RCP distribution pattern and its correlates can make restoration initiatives in river valleys more effective. While implementing conservation measures in river valleys, one should keep in mind that: (1) hotspots of RCP and invasive species spatially overlap and (2) anthropogenic linear elements occurring within river valleys constitute important habitats for some RCP species.     

Assessing the vulnerability of riparian vegetation to invasion by Mimulus guttatus: relative importance of biotic and abiotic variables in determining species occurrence and abundance

Riparian habitats are particularly susceptible to invasion by non‐native plants. At present, attempts to build consensus as to what the primary drivers of plant invasion in riparian ecosystems might be is hindered by the absence of common standards for data collected on plant species (e.g. occurrence, or relative abundance). Mimulus guttatus L., a non‐native riparian plant species, was used as a model to determine how environmental drivers influence two aspects of invasibility: species occurrence and abundance (assessed in relation to three variables number of patches, patch area and number of stems per patch). Mimulus occurrence and abundance, together with 20 environmental variables, were surveyed in almost 700 contiguous 50‐m‐long riverbank segments within a catchment in north‐east Scotland. More than half of the segments had been colonized by Mimulus. Occurrence and number of patches responded to similar environmental gradients, particularly bare sediment, boulders, high soil moisture, short‐statured ruderal communities, and open canopies, and tended to be highest downstream where the river was widest. In contrast to occurrence and patch number, patch area and stem number per patch were higher in the upper reaches of the catchment and were positively associated with low tree canopy and vegetation dominated by light‐demanding species and smaller‐statured species. Patch area and stem number per patch were also positively related to grazing. This study has highlighted the importance of assessing more than one measure of invasion success (occurrence or patch number and either patch area or stem number per patch), as they are each determined by a different suite of environmental variables. Abiotic factors, such as sediment availability and presence of boulders, appeared to be the major determinants of occurrence and patch number, whereas biotic factors, such as interspecific competition and grazing, were more important ecological determinants underlying area and stem number per patch.     

The varying role of population abundance in structuring indices of biotic homogenization

Aim An important component of human‐induced global change is the decrease or increase in community distinctiveness (taxonomic homogenization or differentiation, respectively) that follows the loss of native species and gain of non‐native species. We use simulation approaches to assess the extent to which conclusions about the outcome of the homogenization process depend on whether or not abundance data are incorporated. Location Data were produced through computer simulation. Methods The frequency with which occurrence‐based similarity indices and abundance‐based similarity indices give different views of changes in community similarity, and the conditions under which such differences occurred were assessed using both deterministic and stochastic modelling approaches to simulate species assemblage states. Results Occurrence‐based and abundance‐based indices were positively correlated across the set of simulations for both the deterministic and stochastic models. However, in both situations approximately one quarter (25%) of models resulted in contrasting outcomes for the two approaches of calculating changes in compositional similarity; that is, one data type showed a positive value (homogenization), whereas the other showed a negative value (differentiation). Main conclusions In the majority of cases, species abundances will not change drastically enough after perturbation to produce large differences between homogenization scores measured using occurrence versus abundance information. However, in cases where these changes are large, it is important to recognize that the choice of metric to analyse homogenization trends will influence the qualitative and quantitative conclusions drawn. Studies of real assemblages are therefore necessary to evaluate the role of species abundance in defining the magnitude and direction of changes in community composition across space, and the implications of these changes for native biodiversity.     

Impact of invasions by alien plants on soil seed bank communities: Emerging patterns

Much of our current understanding of the impact of invasive species on plant communities is based on patterns occurring in the above-ground vegetation, while only few studies have examined changes in soil seed banks associated with plant invasions, despite their important role as determinants of vegetation dynamics. Here, we reviewed the literature on the impact of plant invasions on the seed bank and we provide a quantitative synthesis using a meta-analysis approach. Specifically, (1) we quantified the impact of 18 invasive alien plants on (i) species richness and (ii) density of the seed banks of invaded communities, based on 58 pair-wise invaded-uninvaded comparisons (cases); we identified (2) the invasive taxa that are responsible for the largest changes in the seed bank; and (3) the habitats where substantial changes occur. Our study showed three major findings: (1) species richness (68% of cases) and density (58% of cases) were significantly lower in native seed banks invaded by alien plants; (2) species richness and density of native and alien species were remarkably lower in seed banks invaded by large, perennial herbs compared to uninvaded sites; and (3) invaded seed banks were often associated with a larger richness and/or abundance of alien species. This study indicates a need for additional seed bank data in invasion ecology to characterize species-specific and habitat-specific impacts of plant invasions, and to determine whether changes in the seed banks of native and alien species are a symptom of environmental degradation prior to a plant invasion or whether they are its direct result. The findings of this study help improve our capacity to predict the long-term implications of plant invasions, including limitations in the recruitment of native species from the seed bank and the potential for secondary invasions by seeds of other alien species.     

The Pacific Islands: a biotechnology resource bank of medicinal plants and traditional intellectual property

Traditional medicines, the mainstay of medical treatment for virtually all minor ailments in many developing countries, have been tapped for the production of new therapeutics in the sustenance of human health and well-being. Vulnerable to the vicissitudes of globalization issues such as intellectual property rights, trade and gender are of relevance in the Pacific region that is a source of some unique traditional healing systems.     

Maintaining Biodiversity in Freshwater Ecosystems on Oceanic Islands of the Tropical Pacific

Duringthepastfiveyears,aresearchgroupfromtheHawai'iDivisionofAquaticRe-sourcesandtheLouisianaStaeUniversityMuseumofNaturalSciencehascollaboratedonaseriesofstudiesconcerningthebiologyandconservaionofstreamanimalsintheHawaiianIs-lands.Fromtheviewpointsofbothgeologyandbiology,theseislandsrepresentthenorth-ernmostextensonofPolynesia.StreamanimalsinHawai'ihaverelativesattheleveloffami-ly,genus,andoccasionallyevenspeciesnotonlyinPolynesiabutthroughoutMelanesiaandMicronesiaaswell.Forthisreason…     

The role of species traits and taxonomic patterns in alien bird impacts

Aim To test whether the distribution of alien bird impacts varies across bird families and regions of origin, and to investigate whether species traits associated with successful introductions can predict which species will have negative impacts in the new area of introduction. Location Europe and the Mediterranean Basin. Methods Combining historical information and published literature about negative economic, biological and human health impacts, we compared the distribution of impacts among bird families and native origins of bird species for three major types of impact (economic, biodiversity and human health). We examined the relationships between ecological, biological and reproductive characteristics of species and the severity of the impacts. Results The majority of alien species with reported impacts originated from the Afrotropical, Indo‐Malayan and Palaearctic biogeographical regions. The distribution of alien bird species in Europe with reported impacts shows a taxonomic bias and largely mirrors patterns of establishment. While most species had primarily either economic or biodiversity impacts, several species in the Anatidae, Corvidae, Passeridae, Phasianidae and Sturnidae families were associated with moderate to serious negative impacts on both economic resources and native biodiversity. After controlling for taxonomic effects, species with the greatest overall impacts were habitat generalists and multi‐brooded, while species with smaller bodies and the tendency to form large feeding or roosting flocks were linked with greater impacts on native biodiversity. Main conclusions This study presents the first synthesis of published impact data for alien birds and provides a broad‐scale perspective on factors that contribute to their impacts. The results show that accounting for both species traits and taxonomy improves our ability to predict the impacts of alien bird species. Because several species are currently in the early stages of establishment in Europe, there may be an opportunity to limit negative impacts with efforts that promote proactive strategies against species and families possessing the above characteristics.