Abstract
Urban areas are being affected by rapidly increasing human-made pressures that can strongly homogenize biodiversity, reduce habitat heterogeneity, and facilitate the invasion of alien species. One of the key concerns in invaded urban areas is comparing the trait–environment relationships between alien and native species, to determine the underlying causes of invasiveness. In the current study, we used a trait–environment dataset of 130 native plants and 33 alien plants, recorded in 100 plots covering 50 urban areas and 50 non-urban ones in an urbanization gradient in the arid mountainous Saint-Katherine protected area in Egypt. We measured eleven morphological plant traits for each plant species and ten environmental variables in each plot, including soil resources and human-made pressures, to construct trait–environment associations using a fourth-corner analysis. In addition, we measured the mean functional and phylogenetic distances between the two species groups along an urbanization gradient. Our results revealed strongly significant relationships of alien species traits with human-made pressures and soil resources in urban areas. However, in non-urban areas, alien species traits showed weak and non-significant associations with the environment. Simultaneously, native plants showed consistency in their trait–environment relationships in urban and non-urban areas. In line with these results, the functional and phylogenetic distances declined between the aliens and natives in urban areas, indicating biotic homogenization with increasing urbanization, and increased in non-urban areas, indicating greater divergence between the two species groups. Thereby, this study provided evidence that urbanization can reveal the plasticity of alien species and can also be the leading cause of homogenization in an arid urban area. Future urban studies should investigate the potential causes of taxonomic, genetic, and functional homogenization in species composition in formerly more diverse urbanized areas.
Highlights
Urbanization is an important factor that facilitates invasion and can be considered a causative driver for the introduction of alien species [1,2]
Alien plants were well adapted to urban environmental conditions, which is evidenced by the strong positive relationships between most of their traits and all environmental variables expressing human-made pressures (Figure 1a)
Our results provide evidence that urbanization can reveal the plasticity of alien species in an arid protected area and that can drive alien invasiveness
Summary
Urbanization is an important factor that facilitates invasion and can be considered a causative driver for the introduction of alien species [1,2]. Anthropogenic modifications of the landcover, such as plant-collection, road construction, and humanmade activities, have been implicated as a cause of increasing alien species richness and abundance [3]. This increase could be attributed to the fact that alien species tend to exhibit wide environmental tolerance and high phenotypic plasticity, which may enhance their capacity to survive under changes in environmental conditions [4–8]. Alien species abundance is supposed to be promoted by increasing land-use intensity [10–12], and thereby, the presence of alien species can be favored when urban cover or human-made pressures increase [13] This is not the case for native species, as intensive urban land use, associated with habitat fragmentation [14,15] and changes in ecosystem functioning [16], decreases the range of the specialized native species pool [17–19]. A comparative study between native and alien species groups cannot be performed in isolation from the environmental determinants of their distribution
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