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Reservoir construction and the introduction of nonnative species are major anthropogenic drivers of biotic change in freshwater ecosystems. To understand the influence of these drivers, we quantified the degree to which fish faunas have either homogenized or differentiated at multiple spatial scales across the Great Plains—Rocky Mountain continuum (Wyoming, USA), given that homogenization processes are scale-dependent. Homogenization was most prevalent at the largest scale, with an average increase in similarity of 6.8% among river basins. At an intermediate scale, sub-basins with reservoirs had homogenized faunas in comparison to sub-basins without reservoirs, which were more differentiated. Differentiation was dominant at the smallest scale with a 7.8% average decrease in similarity among individual sampling sites. Reservoirs had only localized homogenization impacts along stream systems, and homogenization was greater for streams connected to large reservoirs. Large-sized streams appeared to trend towards homogenization, whereas small and medium streams trended towards differentiation. Reservoirs altered fish faunas from historical conditions, but did not result in cross-stream homogenization because of the idiosyncratic nature of reservoir introductions that reflect environmental gradients and socio-economic factors. Our results provide insight into how spatial scale, reservoirs, and nonnative species interact to influence the degree of homogenization and differentiation.

Aim The introduction of exotic plants can both increase (homogenize) and decrease (differentiate) floristic similarity between areas. We have a poor understanding of the degree to which plant species introductions tend to homogenize or differentiate floras, and relevant studies covering large spatial extent are scarce. China has been heavily invaded by exotic plants. Here, we analyse a comprehensive dataset of vascular plants to determine whether the introduction of exotic plant species has homogenized or differentiated species composition in regions across China. Location China. Methods We calculated the Jaccard index and Simpson index of similarity for each pair of province‐level regions for native and exotic species separately and jointly, and calculated a homogenization index for each pair of regional floras. We correlated species richness of native and exotic plants to climatic factors, and correlated the Jaccard index and Simpson index to geographic and climatic distances. We used variation partitioning analysis to determine the relative importance of geographic and climatic distances on species turnover. We also examined the effect of human population density on florisitic similarity of exotic species. Results We found that the geographic range of each species was, on average, larger for exotics than for natives; floristic similarity between regions was greater for exotics than for natives; the vast majority of pairwise regional floras have been homogenized; the introduction of exotic species has caused stronger biotic homogenization for pairwise floras with greater dissimilarity in their species composition; geographic distributions of exotic and native species were determined by different sets of climatic factors; and distributions of exotic species were determined by climatic factors more strongly, compared to those of native species. Human population density had a moderate effect on florisitic similarity of exotic species. Main conclusions The introduction of exotic plant species has homogenized regional floras across China. Because strong international trades between China and other countries and dramatic development of transportation systems are continuing in China, which help spread of exotic species, we predict that exotic species will continue to spread and will strengthen biotic homogenization in China.

Question: How does urbanization and associated declines in fire frequency alter the floristic composition of native temperate grasslands? Does it lead to: (1) biotic homogenization, i.e. compositional similarity between remnants increases; (2) biotic differentiation, whereby similarity between remnants declines, or; (3) clustered differentiation, where similarity between remnants remains unchanged, but composition shifts from the historical state? Location: Victoria, Australia. Methods: Using site-level surveys, we examined changes in the floristic similarity of 29 urban grasslands from 1992 to 2013 and compared these changes to those of 63 rural grasslands from 1989 to 2014. Community-level changes in the representation of key functional traits were also examined in urban grasslands, with traits advantaged following disturbance regime change and urban fragmentation predicted to increase in frequency. Results: Our results supported the biotic homogenization hypothesis in urban grasslands. Compositional similarity between grasslands increased principally because of an increase in commonly shared non-native species, with change in native composition comparatively minor. However, no evidence of biotic homogenization was found in rural grasslands, with no significant change in overall composition identified. The most urbanized sites had the highest number of non-native species in both the current and historical data sets, yet non-native composition over the past two decades changed the most in sites on the urban fringe, becoming more similar to sites closer to the urban core. As expected, following declines in fire frequency and increased urbanization, the overall composition of urban grasslands shifted to taller plant species, while native species capable of vegetative reproduction and exotic species with an annual life span increased in frequency. Conclusion: Urbanization was an important driver of biodiversity change in the investigated system, with increasing competition intensity in response to disturbance regime change a likely cause of biotic homogenization. Our results demonstrate that non-native species are a key driver of biotic homogenization, emphasizing the importance of managing non-native immigration and maintaining historical disturbance processes once native ecosystems become urbanized.

The addition of nonnative species and loss of native species has modified the composition of communities globally. Although changes in β-diversity have been well documented, there is a need for studies incorporating multiple time periods, more than one dimension of biodiversity, and inclusion of nestedness and turnover components to understand the underlying mechanisms structuring community composition and assembly. Here, we examined temporal changes in functional dissimilarity of fish communities of the Laurentian Great Lakes and compared these changes to those of taxonomic dissimilarity by decade from 1870 to 2010. Jaccard-derived functional dissimilarity index was used to quantify changes in functional β-diversity within communities, between all possible pairs of communities, and using a multiple-site index among all communities. β-diversity was partitioned into components of nestedness and turnover, and changes were examined over time. Similar to patterns in taxonomic dissimilarity, each community functionally differentiated from the historical community of 1870, with Lake Superior changing the most (~24%) and Lake Ontario the least (~14%). Although communities have become taxonomically homogenized, functional β-diversity among all communities has increased over time, indicating functional differentiation. This is likely due to functional similarity between the communities being historically high (i.e., ~88% similar in 1870). The higher taxonomic relative to functional turnover indicates that the species being replaced between communities are functionally redundant, which could occur given the harsh environmental conditions of the region and/or as a result of the recent glacial history of the region. High functional nestedness across communities reflects dispersal limitations, with smaller communities being functional subsets of large communities closer to source populations. The functional differentiation observed is likely due to nonnative species with functional traits unique to the region establishing or the loss of functionally redundant native species; however, it is important to note that patterns of homogenization were periodically observed through time. Our study demonstrates the possible factors regulating diversity in the Laurentian Great Lakes fish communities, that patterns of taxonomic and functional β-diversity are dynamic over time and vary in the magnitude and direction of change, and that taxonomic diversity should not be used to predict changes in functional diversity.

Species respond to environmental changes at different rates, resulting in no change, increased, or decreased resemblance among species assemblages. We explored the patterns of rate of change in bird diversity in five ecoregions of the United States across 30 years. We characterized the rate of change in breeding avian biodiversity using measures of species richness and assemblage dissimilarity, detecting changes in the same for 50% and 70% of the assemblages, respectively. Fast richness declines and species replacement were associated with rapid biotic differentiation within ecoregions, while rapid increases in richness and slowed species replacement were tied to high within-ecoregion biotic homogenization rates. Further, it was exceedingly rare for any biodiversity measured to change slowly over time; most changes were rapid. For the species assemblages studied here, changes in assemblage dissimilarity patterns were more common than changes in species richness, even though species richness has received more research attention. These results underscore the need to combine measures capturing different aspects of biodiversity (e.g., species richness and assemblage differentiation) to provide greater insight into the underlying mechanisms and pathways driving changes in biodiversity patterns.

ContextA core theme in ecohydrology is understanding how hydrology affects spatial variation in the composition of species assemblages (i.e., beta diversity). However, most empirical evidence is from research in upland rivers spanning small spatial extents. Relatively little is known of the consequences of hydrological variation for beta diversity across multiple spatial scales in lowland rivers.ObjectivesWe sought to examine how spatial variation in hydrology and fish beta diversity within and among rivers changed over time in response to intensification and cessation of hydrological drought.MethodsWe used monitoring data of fish assemblages, coupled with hydrological and biophysical data, to test how spatial variation in hydrology and multiple components of fish beta diversity in lowland rivers of the Murray—Darling Basin (Australia) varied across spatial scales during contrasting hydrological phases.ResultsSpatial variation in hydrology among rivers declined with increasing duration of drought before increasing during a return to above-average flows. Spatial variation in hydrology within rivers did not show consistent changes between hydrological phases. Beta diversity among and within rivers showed variable, river-specific changes among hydrological phases for both incidence- and abundance-based components of assemblage composition.ConclusionsInconsistent hydrology—beta diversity patterns found here suggest that mechanisms and outcomes of drought and flooding impacts to beta diversity are context-dependent and not broadly generalisable. Our findings indicate that hydrological fluctuations occurring in the Murray—Darling Basin in the period analysed here did not cause significant or consistent homogenisation or differentiation of freshwater fish assemblages.

Aim: Biotic homogenization describes the process by which species invasions and extinctions increase the genetic, taxonomic or functional similarity of two or more biotas over a specified time interval. The study of biotic homogenization is a young and rapidly emerging research area in the budding field of conservation biogeography, and this paper aims to synthesize our current knowledge of this process and advocate a more systematic approach to its investigation. Methods: Based on a comprehensive examination of the primary literature this paper reviews the process of biotic homogenization, including its definition, quantification, underlying ecological mechanisms, environmental drivers, the empirical evidence for different taxonomic groups, and the potential ecological and evolutionary implications. Important gaps in our knowledge are then identified, and areas of new research that show the greatest promise for advancing our current thinking on biotic homogenization are highlighted. Results: Current knowledge of the patterns, mechanisms and implications of biotic homogenization is highly variable across taxonomic groups, but in general is incomplete. Quantitative estimates are almost exclusively limited to freshwater fishes and plants in the United States, and the principal mechanisms and drivers of homogenization remain elusive. To date research has focused on taxonomic homogenization, and genetic and functional homogenization has received inadequate attention. Trends over the past decade, however, suggest that biotic homogenization is emerging as a topic of greater research interest. Main conclusions: My investigation revealed a number of important knowledge gaps and priority research needs in the science of biotic homogenization. Future studies should examine the homogenization process for different community properties (species occurrence and abundance) at multiple spatial and temporal scales, with careful attention paid to the various biological mechanisms (invasions vs. extinctions) and environmental drivers (environmental alteration vs. biotic interactions) involved. Perhaps most importantly, this research should recognize that there are multiple possible outcomes resulting from the accumulation of species invasions and extinctions, including biotic differentiation whereby genetic, taxonomic or functional similarity of biotas decreases over time.

The invasion of non-native species, and declines or extinctions of native species, can act together to drive either biotic homogenization or differentiation between regions over time. To date, no studies have investigated whether high incidence of biological invasions, as well as high risk of extinction, among freshwater crayfishes has resulted in biotic homogenization consistent with other taxonomic groups globally. We used Jaccard’s dissimilarity index to calculate taxonomic beta diversity of native crayfishes between and within major regions of North America, the most species-rich region on the planet for these organisms, and subsequently decomposed beta diversity into its turnover and nestedness components. To estimate biotic homogenization or differentiation, we next added introduced and established non-native crayfishes to these beta diversity calculations, and then incrementally subtracted native crayfishes on a gradient of high to lower vulnerability to extinction. We found that North American native crayfishes have extremely high beta diversity between major geographic regions of this continent, driven by turnover rather than nestedness, whereas beta diversity and its components are more heterogenous within major geographic regions. Further, we found that biological invasions by introduced non-native crayfishes drive biotic homogenization, rather than differentiation, between all regions of North America, and this effect was substantially greater than that of potential future extinctions. Within major regions of North America, biological invasions resulted in either biotic homogenization or differentiation, although the magnitude of homogenization when present was greater than that of differentiation. Cumulatively, biological invasions by non-native crayfishes are driving biotic homogenization of North American crayfishes. Management interventions are urgently needed to protect native species and ecosystems from the negative effects of these invasive crayfishes, as well as to conserve the antiquity and novelty of crayfish communities across North America.

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 Sorensen 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.

Biotic homogenization, the process of gradual replacement of native biotas by nonindigeous and locally expanding nonnative species, is rapidly diminishing the regional distinctiveness of global terrestrial and aquatic ecosystems. Although the empirical study of biotic homogenization is substantial and growing, the mechanisms underlying its dynamics remain poorly understood. We recently developed a theoretical model that predicts levels of biotic homogenization or differentiation (i.e., decreased community similarity) according to a series of distinct mechanisms that describe the outcomes of various interactions between native species, nonnative species, and the environment. Here, we test this model using empirical data for freshwater fish faunas in the United States at three spatial scales: the entire continent, Zoogeographic provinces in California, and watersheds within these provinces. Our analysis reveals that, in general, mechanisms depicting widespread introductions of cosmopolitan species and either no or differential spatial patterns of native-species extirpations explain fish-community homogenization across multiple spatial scales. Our results also highlight the potential effect of spatial grain on the perceived importance of different invasion-extinction scenarios shaping patterns of homogenization and differentiation. Next, we discuss the utility of the model for providing insight into the dominant ecological processes likely driving the homogenization of other major taxonomic groups that currently lack quantitative estimates of community change. Our study is the first to quantitatively examine the relative importance of different ecological mechanisms that can generate observed patterns of biotic homogenization. Using this model may allow advance prediction of future patterns of homogenization by explicitly considering underlying ecological processes and mechanisms.