Documenting how alien species alter native communities structure from a functional and
phylogenetic point of view is a critical approximation to understanding the mechanisms
beyond the magnitude of alien impacts on native diversity. Here, we propose a framework that investigated whether alien plants expand or decline the functional and phylogenetic space of native communities and the effects of alien relatedness, i.e. being functionally and phylogenetically similar or dissimilar to natives, and the availability of resources on the magnitude of alien relative impact (ARI) on native diversity. We tested this framework using a trait-environment dataset of 33 alien and 130 native plants in 83 pair plots (invaded/non-invaded) covering a gradient of soil resources (organic matter, nitrogen, soil moisture) in Saint-Katherine Protectorate, an arid protected area in Egypt. We compared the changes in native diversity indices (e.g. abundance, richness, functional dispersion, functional richness and the phylogenetic diversity) among invaded and non-invaded plots using Tukey HSD test. We also estimated the magnitude of ARI on native diversity in each pair to be positive if the native diversity increases and negative if native diversity declines. Additionally, we calculated the mean functional and phylogenetic distance among aliens and natives in each pair. Altogether, we constructed series of generalized additive models to test the effects of soil resources availability and alien relatedness to natives on the magnitude of ARI.
Results/Conclusions
Our results revealed that in invaded plots, native diversity was phylogenetically less, but functionally more diverse in addition to a relative decline in native richness and abundance compared to non-invaded ones. Moreover, the magnitude of ARI was significantly positive when aliens were more distant to natives in resources richer plots but negative in limited plots when aliens were closely related to natives. While demonstrated in only one arid system, our results suggest that linking soil resource availability with alien relatedness to natives could be used as an informative strategy to assess alien impacts on native diversity measures. Thus, future invasion studies should investigate the magnitude of ARI in different environments to test the generality of our findings. It would further be interesting to test the effects of different nutrient‐niches as well as the plausible biotic interactions among aliens and natives on the magnitude of alien impacts that might be an effective strategy for risk assessment of plant invasions.