The global loss of floristic uniqueness

The introduction of non-native species, either intentionally or accidentally, has significantly altered biogeography by allowing species to overcome natural dispersal barriers. These naturalized species establish self-sustaining populations and can profoundly impact the ecological and evolutionary dynamics of their new environments. Such changes include alterations in biotic interactions, hybridization, and most importantly for this study, a shift in the biotic uniqueness of regions. The distinctiveness of a region’s flora, compared to others, becomes less pronounced. This homogenization, a growing similarity in species composition between regions, has been documented across various taxa and ecosystems but lacks comprehensive global-scale analysis for vascular plants. Previous studies have focused on continental scales, leaving a critical gap in understanding the global-scale impacts of plant naturalization. This study aims to fill this gap by analyzing the loss of regional floristic uniqueness globally, considering both taxonomic and phylogenetic aspects. The research investigates how geographic distance, climatic similarity, and historical/current administrative relationships between regions influence the degree of floristic homogenization. The economic impact of such homogenization is also considered, as it may reduce the attractiveness of a region for tourism.

Existing literature extensively documents the ecological and evolutionary consequences of biological invasions. Studies have shown that introduced species can alter biotic interactions, leading to hybridization and changes in community structure. The concept of biotic homogenization, the increasing similarity in species composition across geographical regions, has been established in various taxa and ecosystems. Previous research highlights the role of human activity in accelerating this process through increased trade and travel. However, large-scale studies specifically focusing on the global-scale floristic homogenization in vascular plants are limited, with most previous research concentrating on continental-level analyses. While these continental-scale studies have provided valuable insights, a global perspective is necessary to fully understand the extent and implications of this phenomenon. The current study builds upon these previous findings to provide a comprehensive global assessment of floristic homogenization.

This study utilized data from native and naturalized alien floras of 658 regions globally, encompassing countries and provinces. The data encompassed approximately 65.7% of the tree-lined Earth's surface and 62.3% of flowering plant richness. For each pair of regions, taxonomic and phylogenetic similarities were calculated for both native floras and combined native and naturalized floras. The change in similarity, representing the degree of homogenization due to naturalization, was quantified using log-response ratios. The researchers analyzed the associations between changes in floristic similarity and various factors including geographic distance, climatic distance (calculated using 19 bioclimatic variables and PCA), and administrative relationships (both current and historical, considering colonial ties). Multiple regression on distance matrices (MRM) and generalized additive models (GAMs) were employed to account for the non-independence of region pairs and to assess linear and non-linear relationships. Finally, the mean degree of homogenization for each region was calculated, and its associations with region characteristics (size, native species richness, endemism, naturalized species richness, and island/mainland status) were assessed using linear multiple regression, accounting for spatial autocorrelation. Data sources included the GloNAF database and the Global Inventory of Flora and Fauna (GIFT).

The analysis revealed strong evidence of taxonomic and phylogenetic floristic homogenization globally. The natural decline in floristic similarity with increasing geographic distance was significantly weakened by the presence of naturalized species. The degree of floristic homogenization increased with climatic similarity between regions, emphasizing the role of climate matching in plant naturalization. Importantly, homogenization was notably greater between regions sharing current or past administrative relationships. This association suggests that intensified trade and transport between such regions facilitated the exchange of species. Geographic distance increased the halving distance of taxonomic similarity from 1791 to 2638 km and phylogenetic similarity from 7008 to 7489 km. Regions with current or past administrative links showed a stronger increase in taxonomic homogenization with geographic distance compared to regions without such links. This effect was observed both within countries and between regions with historical colonial ties. Analysis of homogenization hotspots revealed Australasia as a primary region experiencing high taxonomic homogenization, likely due to its previously unique flora and the introduction of numerous alien species. Many oceanic islands, also characterized by unique endemic floras, were hotspots for taxonomic homogenization, but showed less phylogenetic homogenization suggesting that unique evolutionary lineages were retained, despite introduction of many new species. The study further demonstrated that taxonomic homogenization decreased with greater native species richness and the donor role of a region (average number of non-native species naturalized in), while increasing with a greater proportion of endemic native species and naturalized species richness. Phylogenetic homogenization exhibited similar patterns, decreasing with region size, native species richness and the donor role.

The findings strongly support the hypothesis that the naturalization of alien plant species is driving floristic homogenization at a global scale. The weakening of the distance-decay relationship in floristic similarity underscores the ability of humans to circumvent natural dispersal barriers. The strong association between climatic similarity and homogenization highlights the importance of climate matching in determining the success of naturalized plants. The influence of administrative relationships reflects the impact of human activities on biotic exchange. The identification of homogenization hotspots, particularly in Australasia and on oceanic islands, emphasizes the vulnerability of regions with unique and often endemic floras. The contrasting patterns of taxonomic and phylogenetic homogenization on islands warrant further investigation. While the study primarily focuses on vascular plants, the findings may have broader implications for other taxonomic groups and ecosystems. Future research could explore the specific mechanisms driving homogenization, including the traits of successful invaders and the ecological consequences of floristic homogenization, such as potential impacts on ecosystem function and services.

This study provides compelling evidence of widespread floristic homogenization globally due to the naturalization of alien plants. The results highlight the interconnectedness of biogeography, climate, and human activities in shaping global biodiversity patterns. The findings underscore the urgent need for effective biodiversity conservation strategies to mitigate the ongoing loss of floristic uniqueness. Future research could focus on exploring the mechanisms of homogenization, developing more precise predictive models, and implementing targeted conservation measures to preserve regional biodiversity.

The study acknowledges limitations related to data availability and completeness. The analysis relied on available species lists, which may not represent all species present in a region. The incomplete representation of some geographic areas and taxonomic groups might have introduced biases into the estimations of homogenization. The study also notes that species lists are inherently variable across administrative regions, depending on factors like size and environmental heterogeneity. While the authors attempted to mitigate these issues using strict selection criteria, future research should consider supplementing available data to enhance the representativeness of the analysis. The focus on presence-absence data instead of abundance data is another limitation; abundance data could provide more nuanced insights into the homogenization process.