Invasiveness and homogenization: synergism of wide dispersal and high local abundance

Aim To determine whether invasive and locally abundant non‐native species have a more homogenizing effect on plant communities than non‐invasive and less abundant non‐native species. Location California and Florida counties, conservation areas in the USA, and eight US cities. Methods Species lists among counties, conservation areas and cities were compared to see whether invasive and abundant non‐native species increased the Jaccard index of similarity between localities beyond any increases caused by non‐invasive and less abundant non‐native species. Results For all comparisons, we found that invasive non‐native species have a significantly greater homogenizing effect than non‐invasive non‐native species. For the US conservation areas, we found that locally abundant invasive species tend to be more widespread and more widely shared than less abundant invasive species. There is also a positive relationship between homogenization by invasive species and the magnitude of human disturbance. Main conclusions Invasive non‐native species tend to be disproportionately shared among communities relative to non‐invasive non‐native species. This effect is enhanced by human disturbance, as measured by the ratio of non‐native to native species. There is a synergism between abundance and geographical range which enhances the homogenizing effects of abundant species. Invasive species, with wide ecological niches, are more widely shared among communities and more locally abundant. Abundant invasive species are thus more spatially homogenizing, and more ecologically dominant (functionally homogenizing). Also, ‘perceived homogenization’ is probably greater than homogenization measured by the increase in shared species. The abundant species typically seen by the casual observer in a biological community are probably more commonly shared between communities than less common species. Studies that lack abundance data and measure homogenization only on the basis of shared species, which includes most homogenization studies to date, probably underestimate the homogenizing impacts of non‐native species as perceived by people.     

Compositional similarity among urban floras within and across continents: biogeographical consequences of human-mediated biotic interchange

Anthropogenic activities have weakened biogeographical barriers to dispersal resulting in the global spread and establishment of an increasing number of non‐native species. We examine the broad‐scale consequences of this phenomenon based on an analysis of compositional similarity across urban floras in the northeastern United States and Europe. We test the prediction that homogenization of species composition is uniquely defined within vs. between continents based on the time and place of origin of non‐native species. In this case, for archaeophytes and neophytes in Europe (introduced before and after ad 1500, respectively) and non‐native species originating from within and outside the United States. More species in urban floras were shared within than between continents. Within Europe, archaeophytes shared more species across urban floras compared with neophytes; strong associations were not observed for non‐native species across US urban floras. Between the two continents, non‐native species in the United States that originated from outside the United States shared species primarily with archaeophytes but also with European natives and neophytes. These results suggest that the direction of biotic interchange was unidirectional with species moving primarily from Europe to the United States with archaeophytes playing a primary and non‐native species originating from outside the two continents a secondary role as a homogenizing source. Archaeophytes, based on combination of biogeographical, evolutionary, and ecological factors in association with a long history of anthropogenic influence, appear to have played a prominent role in the continental and intercontinental homogenization of species composition. This suggests that the uniform homogenization of the Earth's biota is not imminent and is presently directed by a combination of biogeographically defined anthropogenic and historical factors.     

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.     

The local introduction of strongly interacting species and the loss of geographic variation in species and species interactions

Species introductions into nearby communities may seem innocuous, however, these introductions, like long-distance introductions (e.g. trans- and intercontinental), can cause extinctions and alter the evolutionary trajectories of remaining community members. These 'local introductions' can also more cryptically homogenize formerly distinct populations within a species. We focus on several characteristics and the potential consequences of local introductions. First, local introductions are commonly successful because the species being introduced is compatible with existing abiotic and biotic conditions; many nearby communities differ because of historical factors and the absence of certain species is simply the result of barriers to dispersal. Moreover, the species with which they interact most strongly (e.g. prey) may have, for example, lost defences making the establishment even more likely. The loss or absence of defences is especially likely when the absent species is a strongly interacting species, which we argue often includes mammals in terrestrial communities. Second, the effects of the introduction may be difficult to detect because the community is likely to converge onto nearby communities that naturally have the introduced species (hence the perceived innocuousness). This homogenization of formerly distinct populations eliminates the geographic diversity of species interactions and the geographic potential for speciation, and reduces regional species diversity. We illustrate these ideas by focusing on the introduction of tree squirrels into formerly squirrel-less forest patches. Such introductions have eliminated incipient species of crossbills (Loxia spp.) co-evolving in arms races with conifers and will likely have considerable impacts on community structure and ecosystem processes.     

Compositional similarity and the distribution of geographical range size for assemblages of native and non-native species in urban floras

There is evidence that, within a region, non-native species introduced from nearby sources (extralimital native) promote homogenization and non-native species introduced from distant sources (exotic) promote differentiation of species composition. A possible explanation for these associations is that they are related to differences in the distribution of geographical range size. We test this by examining geographical ranges, delineated within a defined region, for assemblages of vascular plants in eight urban floras in the USA. Across floras, native species had the largest, least variable ranges and the greatest proportion of shared species. Exotic species had the most variable ranges with concentrations of species with small and large ranges and the lowest proportion of shared species. Extralimital natives had concentrations of species with intermediate-sized ranges and intermediate proportions of shared species. These results suggest that patterns of compositional similarity were associated with the relative strength and equality of two opposing patterns within species range size distributions: species with small vs. large ranges. In general, concentrations of species with small ranges promoted low levels and concentrations of species with large ranges promoted high levels of compositional similarity. However, patterns documented for exotic species will likely continue to develop, possibly taking on new forms, depending on how geographical distributions and the rate of introductions of exotic species change over time. Our findings also suggest that processes underlying these patterns have operated at two spatiotemporal scales. The first scale reflects historical consequences of anthropogenic activities occurring within regional extents that have promoted the introduction of extralimital natives; the second scale reflects modern consequences of anthropogenic activities operating at an increasingly global extent that have promoted the introduction of exotic species.     

Locating the sources of an invasive pest,grape phylloxera,using a mitochondrial DNA gene genealogy

Range expansions through human introductions have increased with global commerce and have led to the extinction of native species, alterations in community structure and pest status of the invasive species. Inferring the evolutionary history of invasive species can help to build a firmer footing for management tactics. This study used mitochondrial DNA (mtDNA) sequence comparisons of samples collected from the native and introduced ranges of a pest herbivore of cultivated grapes, grape phylloxera (Daktulosphaira vitifoliae Fitch, Phylloxeridae) to infer the sources and pattern of introductions into worldwide viticulture. Introductions into viticulture from its native North American range first occurred in the mid-19th century. The pattern of spread has suggested a focus of introduction into France, but independent introductions may have occurred elsewhere. The results show that the introduced population represents a limited subsample of the native genetic diversity. The data suggest that most grape phylloxera in viticulture, including all European, have originated in the northeastern USA where the grape species Vitis riparia dominates. There was evidence for independent introductions into South Africa and California. Most California haplotypes were most closely related to native grape phylloxera from the Atlantic Coast on V. vulpina. It is likely that subsequent spread from California into Australia, New Zealand and Peru has occurred.     

Biotic homogenization of three insect groups due to urbanization

Cities are growing rapidly, thereby expected to cause a large‐scale global biotic homogenization. Evidence for the homogenization hypothesis is mostly derived from plants and birds, whereas arthropods have so far been neglected. Here, I tested the homogenization hypothesis with three insect indicator groups, namely true bugs, leafhoppers, and beetles. In particular, I was interested whether insect species community composition differs between urban and rural areas, whether they are more similar between cities than between rural areas, and whether the found pattern is explained by true species turnover, species diversity gradients and geographic distance, by non‐native or specialist species, respectively. I analyzed insect species communities sampled on birch trees in a total of six Swiss cities and six rural areas nearby. In all indicator groups, urban and rural community composition was significantly dissimilar due to native species turnover. Further, for bug and leafhopper communities, I found evidence for large‐scale homogenization due to urbanization, which was driven by reduced species turnover of specialist species in cities. Species turnover of beetle communities was similar between cities and rural areas. Interestingly, when specialist species of beetles were excluded from the analyses, cities were more dissimilar than rural areas, suggesting biotic differentiation of beetle communities in cities. Non‐native species did not affect species turnover of the insect groups. However, given non‐native arthropod species are increasing rapidly, their homogenizing effect might be detected more often in future. Overall, the results show that urbanization has a negative large‐scale impact on the diversity specialist species of the investigated insect groups. Specific measures in cities targeted at increasing the persistence of specialist species typical for the respective biogeographic region could help to stop the loss of biodiversity.     

The desire for variety: Italian wall lizard (Podarcis siculus) populations introduced to the United States via the pet trade are derived from multiple native-range sources

Tests of invasion success often require comparisons between introduced and native populations, but determining the native-range sources for introduced populations can be difficult. Molecular markers can help clarify the geographic extent of native-range sources, helping to identify which populations are appropriate for comparative studies. The Italian Wall Lizard (Podarcis siculus) was introduced multiple times to the United States with extant populations in California, Kansas, New Jersey, and New York. We used phylogeographic analysis of mtDNA sequences (cytb gene) for individuals sampled from these introduced populations and across the native range to identify the number of independent introductions and the location of the source populations. Haplotypes sampled from introduced populations were nested within three geographically distinct, well-supported clades that together encompassed a large portion of the native range. Combining these phylogeographic results with documentation of the introductions revealed putative sources: California individuals are derived from Sicily; Kansas and New York populations are from Tuscany near Florence; and the New Jersey population is likely from the Adriatic coastal region, but a more specific locality is not possible. The pet trade dominates the invasion pathway for P. siculus introductions to the US. The genetically and geographically diverse sampling of its native range may be driven by the desire for phenotypic variety in the pet trade, a hypothesis that needs future testing.     

Alien species richness is currently unbounded in all but the most urbanized bird communities

Urban areas suffer high pressure of introductions of alien species compared to other habitats due to intensive human activities. As trading globally continues to rise, more species will likely be introduced into urban areas. To determine whether this increase in introduction pressure will lead to increased alien species richness in urban areas, or whether other processes would act to impose an upper limit on species richness, we examined how the shape of the relationship between alien species richness and the number of introduced species over time (i.e. introduction pressure) varies along gradients of urbanization. We collected species composition data from urban bird surveys worldwide and used a global database of alien bird introductions to quantify how many species have been introduced over time at different sites. We found that urbanization gradually modified the shape of the studied relationship from linear to asymptotic. Only communities in extremely urbanized environments were associated with an asymptotic relationship, suggesting that alien bird richness has likely not reached its ecological limit in most urban areas. Our results show that urbanization can reduce the importance of introduction pressure in determining alien species richness. Additionally, the results predict that alien species richness will increase at finer spatial scales, especially if the introduced species can survive in urban areas outside of their native range.     

Freshwater fish introductions in mediterranean‐climate regions: are there commonalities in the conservation problem?

Aim To compare patterns and drivers of freshwater fish introductions across five climatically similar regions and evaluate similarities and differences in the non‐native species introduced. Location Five mediterranean‐climate regions: California (USA), central Chile, south‐western Australia, the Iberian peninsula (Spain and Portugal) and the south‐western Cape (South Africa). Methods Species presence–absence for native and non‐native fishes were collated across the regions, and patterns of faunal change were examined using univariate and multivariate statistical approaches. Taxonomic patterns in freshwater fish introductions were evaluated by comparing the number of species introduced by order to the numbers expected from binomial probabilities. Factors influencing multiple introductions of freshwater fish species in mediterranean regions were determined using generalized linear modelling. Results High levels of endemism (70–90%) were revealed for south‐western Cape, south‐western Australia and Chile. Despite their high rates of endemism, all regions currently have more non‐native species than endemic species. Taxonomic selection was found for five orders, although this was only significant for Salmoniformes across regions. The average increase in regional compositional similarity of fish faunas resulting from non‐native fish introductions was 8.0%. Important factors predicting multiple introductions of a species include previous introduction success and mean latitude of its distribution Main conclusions The mediterranean‐climate regions of the world, separated by vast distances, originally had a few fish species in common but are now more similar, owing to species introductions, illustrating the extent and importance of taxonomic homogenization. Introductions are largely driven by taxonomically biased human interests in recreational fisheries, aquaculture and ornamental pet species.     

Homogenization of freshwater lakes: Recent compositional shifts in fish communities are explained by gamefish movement and not climate change

Globally, lake fish communities are being subjected to a range of scale‐dependent anthropogenic pressures, from climate change to eutrophication, and from overexploitation to species introductions. As a consequence, the composition of these communities is being reshuffled, in most cases leading to a surge in taxonomic similarity at the regional scale termed homogenization. The drivers of homogenization remain unclear, which may be a reflection of interactions between various environmental changes. In this study, we investigate two potential drivers of the recent changes in the composition of freshwater fish communities: recreational fishing and climate change. Our results, derived from 524 lakes of Ontario, Canada sampled in two periods (1965–1982 and 2008–2012), demonstrate that the main contributors to homogenization are the dispersal of gamefish species, most of which are large predators. Alternative explanations relating to lake habitat (e.g., area, phosphorus) or variations in climate have limited explanatory power. Our analysis suggests that human‐assisted migration is the primary driver of the observed compositional shifts, homogenizing freshwater fish community among Ontario lakes and generating food webs dominated by gamefish species.     

Standing variation boosted by multiple sources of introduction contributes to the success of the introduced species, Lotus corniculatus

Although ecological differences between native and introduced ranges have been considered to drive rapid expansion of invasive species, recent studies suggest that rapid evolutionary responses of invasive species to local environments may also be common. Such expansion across heterogeneous environments by adaptation to local habitats requires genetic variation. In this study, we investigated the source and role of standing variation in successful invasion of heterogeneous abiotic environments in a self-incompatible species, Lotus corniculatus. We compared phenotypic and genetic variation among cultivars, natives, and introduced genotypes, and found substantial genetic variation within both native and introduced populations. Introduced populations possessed genotypes derived from both cultivars and native populations, and had lower population differentiation, indicating multiple sources of introduction and population admixture among the sources in the introduced range. Both cultivars and introduced populations had similarly outperforming phenotypes on average, with increased biomass and earlier flowering compared with native populations, but those phenotypes were within the range of the variation in phenotypes of the native populations. In addition, clinal variation within introduced populations was detected along a climatic gradient. Multiple introductions from different sources, including cultivars, may have contributed to pre-adaptive standing variation in the current introduced populations. We conclude that both introduction of cultivar genotypes and natural selection in local environments contributed to current patterns of genetic and phenotypic variation observed in the introduced populations.     

Taxonomic homogenization in ungulates: patterns and mechanisms at local and global scales

Aim The aim of this paper is to examine taxonomic homogenization in ungulates globally and at the local scale in South Africa. Specifically, we aim to examine the roles of distance, scale, time, extinctions vs. introductions, and extralimital vs. extraregional introductions in the homogenization of ungulate biotas, and to determine pathways of introduction of ungulate species globally and the proximate explanatory variables of ungulate introductions in South Africa. Location Forty‐one countries globally and three spatial resolutions in South Africa. Methods Indigenous, extirpated and established introduced ungulate species data were obtained for countries globally, and at a quarter‐degree grid‐cell resolution in South Africa. Homogenization was calculated using Jaccard’s index of similarity (JI) for countries globally and for three spatial resolutions in South Africa. Zoo holdings and transfer data from the International Species Information System database were used to investigate the relationship between non‐indigenous ungulate species introductions and the number of non‐indigenous ungulate species in zoos. Relationships between JI and species richness, and between numbers of introductions and several environmental and social factors were examined using generalized linear models. Results Homogenization in ungulates was 2% for countries globally and 8% at the coarsest resolution in South Africa. Homogenization increased with increasing resolution and with time, but it decreased with increasing percentage change in species richness. Globally, introductions contributed more to homogenization than did extinctions. Within South Africa, extralimital introductions contributed more to the homogenization of ungulate assemblages than did extraregional ones, and ungulates were typically introduced to high‐income areas with high human population and livestock densities. The same was not true in the past, when ungulates were introduced to ungulate species‐poor areas. The number of non‐indigenous ungulate species established in a country is significantly related to the number of non‐indigenous ungulate species in zoos in the country, possibly owing to sales of surplus animals from zoos. Main conclusions Ungulate faunas are homogenized at both the global scale and in South Africa, with extralimital introductions being of considerable significance regionally. In consequence, increasing attention will have to be given to the conservation consequences of ungulate translocations, both within particular geopolitical regions and across the globe.     

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.     

Widespread primary,but geographically restricted secondary,human introductions of wall lizards,Podarcis muralis

Establishing the introduction pathways of alien species is a fundamental task in invasion biology. The common wall lizard, Podarcis muralis, has been widely introduced outside of its native range in both Europe and North America, primarily through escaped pets or deliberate release of animals from captive or wild populations. Here, we use Bayesian clustering, approximate Bayesian computation (ABC) methods and network analyses to reconstruct the origin and colonization history of 23 non‐native populations of wall lizards in England. Our analyses show that established populations in southern England originate from at least nine separate sources of animals from native populations in France and Italy. Secondary introductions from previously established non‐native populations were supported for eleven (47%) populations. In contrast to the primary introductions, secondary introductions were highly restricted geographically and appear to have occurred within a limited time frame rather than being increasingly common. Together, these data suggest that extant wall lizard populations in England are the result of isolated accidental and deliberate releases of imported animals since the 1970s, with only local translocation of animals from established non‐native populations. Given that populations introduced as recently as 25 years ago show evidence of having adapted to cool climate, discouraging further translocations may be important to prevent more extensive establishment on the south coast of England.     

Species invasions and the changing biogeography of Australian freshwater fishes

Aim By dissolving natural physical barriers to movement, human‐mediated species introductions have dramatically reshuffled the present‐day biogeography of freshwater fishes. The present study investigates whether the antiquity of Australia's freshwater ichthyofauna has been altered by the widespread invasion of non‐indigenous fish species. Location Australia. Methods Using fish presence–absence data for historical and present‐day species pools, we quantified changes in faunal similarity among major Australian drainage divisions and among river basins of north‐eastern Australia according to the Sørensen index, and related these changes to major factors of catchment disturbance that significantly alter river processes. Results Human‐mediated fish introductions have increased faunal similarity among primary drainages by an average of 3.0% (from 17.1% to 20.1% similarity). Over three‐quarters of the pairwise changes in drainage similarity were positive, indicating a strong tendency for taxonomic homogenization caused primarily by the widespread introduction of Carassius auratus, Gambusia holbrooki, Oncorhynchus mykiss and Poecilia reticulata. Faunal homogenization was highest in drainages subjected to the greatest degree of disturbance associated with human settlement, infrastructure and change in land use. Scenarios of future species invasions and extinctions indicate the continued homogenization of Australian drainages. In contrast, highly idiosyncratic introductions of species in river basins of north‐eastern Australia have decreased fish faunal similarity by an average of 1.4%. Main conclusions We found that invasive species have significantly changed the present‐day biogeography of fish by homogenizing Australian drainages and differentiating north‐eastern river basins. Decreased faunal similarity at smaller spatial scales is a result of high historical similarity in this region and reflects the dynamic nature of the homogenization process whereby sporadic introductions of new species initially decrease faunal similarity across basins. Our study points to the importance of understanding the role of invasive species in defining patterns of present‐day biogeography and preserving the antiquity of Australia's freshwater biodiversity.     

Relationships among native and introduced populations of the Argentine ant (Linepithema humile) and the source of introduced populations

The Argentine ant (Linepithema humile) is a damaging invasive species that has become established in many Mediterranean-type ecosystems worldwide. To identify likely sources of introduced populations we examined the relationships among native Linepithema populations from Argentina and Brazil and introduced populations of L. humile using mitochondrial cytochrome b sequence data and nuclear microsatellite allele frequencies. The mitochondrial phylogeny revealed that the populations in Brazil were only distantly related to both the introduced populations and the native populations in Argentina, and confirmed that populations in Brazil, previously identified as L. humile, are likely a different species. The microsatellite-based analysis provided resolution among native and introduced populations of L. humile that could not be resolved using the mitochondrial sequences. In the native range, colonies that were geographically close to one another tended to be genetically similar, whereas more distant colonies were genetically different. Most samples from the introduced range were genetically similar, although some exceptions were noted. Most introduced populations were similar to native populations from the southern Rio Parana and were particularly similar to a population from Rosario, Argentina. These findings implicate populations from the southern Rio Parana as the most likely source of introduced populations. Moreover, these data suggest that current efforts to identify natural enemies of the Argentine ant for biological control should focus on native populations in the southern Rio Parana watershed.     

The worldwide expansion of the Argentine ant

Aim The aim of this study was to determine the number of successful establishments of the invasive Argentine ant outside native range and to see whether introduced supercolonies have resulted from single or multiple introductions. We also compared the genetic diversity of native versus introduced supercolonies to assess the size of the propagules (i.e. the number of founding individuals) at the origin of the introduced supercolonies. Location Global. Methods We used mitochondrial DNA (mtDNA) markers and microsatellite loci to study 39 supercolonies of the Argentine ant Linepithema humile covering both the native (n = 25) and introduced range (n = 14). Results Data from three mitochondrial genes and 13 nuclear microsatellites suggest that the introduced supercolonies studied originated from at least seven founding events out of the native area in Argentina (primary introductions). The distribution of mtDNA haplotypes also suggests that supercolonies in the introduced range each derive from a single source supercolony and that one of these source supercolonies has been particularly successful, being the basis of many introduced populations spread across the world. Comparison of the genetic diversity of supercolonies based on the five most diverse loci also revealed that native and introduced supercolonies have greatly overlapping ranges of diversity, although the genetic diversity is on average less in introduced than in native supercolonies. Main conclusions Both primary introductions (from the native range) and secondary introductions (from sites with established invasive supercolonies) were important in the global expansion of the Argentine ant. In combination with the similar social organization of colonies in the native and introduced range, this indicates that invasiveness did not evolve recently as a unique and historically contingent event (e.g. reduction of genetic diversity) in this species. Rather, native L. humile supercolonies have characteristics that make them pre‐adapted to invade new – and in particular disturbed – habitats when given the opportunity. These results have important implications with regard to possible strategies to be used to control invasive ants.     

Dispersal mechanisms for zebra mussels: population genetics supports clustered invasions over spread from hub lakes in Minnesota

Invasion genetic studies show great promise for inferring sources, pathways and vectors of spread, but have to date focused on introductions from native ranges at continental scales and larger. Here we present genetic analyses of post-introduction (i.e. secondary) spread of zebra mussels at the scale of a US state, Minnesota (MN). We genotyped 9 microsatellite DNA loci in 2047 zebra mussels collected from 40 lakes and 4 rivers that geographically and chronologically span the MN invasion. We analyzed genetic variation, geographic differentiation, and genetic clustering of populations across water bodies, and performed invasion-scenario contrasts using Approximate Bayesian Computation. Our population genetic analyses revealed that the pattern of spread of zebra mussels to inland lakes did not conform to the hub-lake model that is often proposed for this species. Consistent with a stratified dispersal model, lake rich regions were colonized from afar, followed by spread within regions by short-distance dispersal, but high-traffic hub lakes made no contribution. For the first time, we obtained evidence about the pattern of spread of zebra mussels to inland lakes that is not based on inference from analysis of boater movements; rather it derives from direct evaluation of genetic attributes of invasive populations. Local introductions overwhelmed more distant ones in three independently invaded regions, suggesting a need to understand the lake colonization process and identify the vectors responsible.     

Toward a mechanistic understanding and prediction of biotic homogenization

The widespread replacement of native species with cosmopolitan, nonnative species is homogenizing the global fauna and flora. While the empirical study of biotic homogenization is substantial and growing, theoretical aspects have yet to be explored. Consequently, the breadth of possible ecological mechanisms that can shape current and future patterns and rates of homogenization remain largely unknown. Here, we develop a conceptual model that describes 14 potential scenarios by which species invasions and/or extinctions can lead to various trajectories of biotic homogenization (increased community similarity) or differentiation (decreased community similarity); we then use a simulation approach to explore the model's predictions. We found changes in community similarity to vary with the type and number of nonnative and native species, the historical degree of similarity among the communities, and, to a lesser degree, the richness of the recipient communities. Homogenization is greatest when similar species invade communities, causing either no extinction or differential extinction of native species. The model predictions are consistent with current empirical data for fish, bird, and plant communities and therefore may represent the dominant mechanisms of contemporary homogenization. We present a unifying model illustrating how the balance between invading and extinct species dictates the outcome of biotic homogenization. We conclude by discussing a number of critical but largely unrecognized issues that bear on the empirical study of biotic homogenization, including the importance of spatial scale, temporal scale, and data resolution. We argue that the study of biotic homogenization needs to be placed in a more mechanistic and predictive framework in order for studies to provide adequate guidance in conservation efforts to maintain regional distinctness of the global biota.