Abstract
Research Highlights: Urban ecosystems are claimed to be more invaded than natural vegetation. Despite numerous studies, the patterns of alien species occurrence in urban forests are rarely linked to invasion ecology hypotheses. Background and Objectives: We assumed that patterns of invasion level (i.e., neophyte richness) and neophyte ecological success (cover) are context-dependent, i.e., depend on the type of vegetation, and that hypotheses connected with empty niche and biotic acceptance will have the strongest support in urban forests. We also tested biotic resistance, habitat filtering, disturbance, resource availability, and environmental heterogeneity hypotheses. Materials and Methods: Using a random forest algorithm, we tested the importance of factors related to invasion ecology hypotheses in a dataset of urban forest vegetation plots (n = 120). We studied seven types of forest plant communities occurring in Poznań (W Poland) and we assessed the vegetation’s taxonomic and functional composition. Results: We found that models of alien species richness and cover explained 28.5% and 35.0% of variance, respectively. Vegetation type was of the highest importance in both cases, suggesting that the occurrence of alien plant species is context-dependent. Resource availability and disturbance ecological indicator values were also of high importance. Conclusions: Our study supported resource availability and habitat filtering hypotheses as explanations of the level of invasion and ecological success of alien species in an urban forest, with partial support for the disturbance hypothesis. Our study revealed that predictors of invasion level are context-dependent, as patterns of alien species richness and cover differed among vegetation types. We highlight context-dependence of alien species invasion patterns in different vegetation types due to the habitat-forming role of dominant tree species and different availability of resources and disturbance levels, as well as different pools of native species. Thus, prevention and management of biological invasions in urban forests should account for forest vegetation type.
Highlights
Biological invasions are one of the most important threats to biodiversity worldwide [1,2,3].Their impact is especially severe in urban ecosystems, where human influence and habitat destruction are responsible for the retreat of numerous indigenous species and spread of alien species [4,5,6].In cities, alien species often replace more specialized native species, which are locally extinct and would otherwise provide substantial ecosystem services [7]
Our study revealed that predictors of invasion level are context-dependent, as patterns of alien species richness and cover differed among vegetation types
We propose the following three hypotheses: (H1) Patterns of both level of invasion and alien species ecological success are context-dependent, i.e., depend mostly on type of vegetation, which considers the forest plant community type to be of utmost importance. (H2) Due to biotic homogenization and extinctions of specialized species in urban environments [4,5], factors related to empty niches and biotic acceptance hypotheses will have the strongest support in urban forests. (H3) Forest vegetation more transformed by humans will host a higher richness and cover of alien species, according to the invasional meltdown concept [46]
Summary
Biological invasions are one of the most important threats to biodiversity worldwide [1,2,3].Their impact is especially severe in urban ecosystems, where human influence and habitat destruction are responsible for the retreat of numerous indigenous species and spread of alien species [4,5,6].In cities, alien species often replace more specialized native species, which are locally extinct and would otherwise provide substantial ecosystem services [7]. Biological invasions are one of the most important threats to biodiversity worldwide [1,2,3] Their impact is especially severe in urban ecosystems, where human influence and habitat destruction are responsible for the retreat of numerous indigenous species and spread of alien species [4,5,6]. Invasive species may achieve ecological success due to several mechanisms. These are usually divided into three main groups of factors: propagule pressure, habitat invasibility, and species invasiveness [15,16,17]. Successful spread of alien species in their exotic ranges is claimed to be an effect of interactions among all three groups of factors [17,18,19]. Ecological success of alien species (single or as a group) is expressed by their biomass [23,28,29], density [21,30,31], or cover [32,33,34]
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