Soil biota can facilitate exotic plant invasions and these effects can be influenced by specific phylogenetic relationships among plant taxa. We measured the effects of sterilizing soils from different native plant monocultures on the growth of Potentilla recta, an exotic invasive forb in North America, and conducted plant–soil feedback experiments with P. recta, two native congeners, a close confamilial, and Festuca idahoensis, a native grass species. We also reanalyzed data comparing the ability of P. recta to invade experimentally constructed congeneric monocultures vs. monocultures of a broad suite of non‐congeners. We found that monocultures as a group, other than those of the native P. arguta, were highly invasible by P. recta. In contrast, this was not the case for monocultures of P. arguta. In our first experiment, the biomass of P. recta was 50% greater when grown in soil from F. idahoensis monocultures compared to when it was grown in soils from P. arguta or P. recta monocultures. Sterilizing soil from F. idahoensis rhizospheres had no effect on the biomass of P. recta, but sterilizing soil from P. arguta and P. recta rhizospheres increased the biomass of P. recta by 108% and 90%, respectively. In a second experiment, soil trained by F. idahoensis resulted in a positive feedback for P. recta. In contrast, soils trained independently by each of the two native Potentilla species, or the closely related Dasiphora (formerly Potentilla) resulted in decreases in the total biomass of the invasive P. recta indicating strong negative feedbacks. Soil trained by P. recta also resulted in intraspecific negative feedbacks. Our results demonstrate substantial negative feedbacks for an invader in its nonnative range under certain conditions, and that native congeners can mount strong biotic resistance to an invader through the accumulation of deleterious soil biota.

Anthropogenic biodiversity decline threatens the functioning of ecosystems and the many benefits they provide to humanity¹. As well as causing species losses in directly affected locations, human influence might also reduce biodiversity in relatively unmodified vegetation if far-reaching anthropogenic effects trigger local extinctions and hinder recolonization. Here we show that local plant diversity is globally negatively related to the level of anthropogenic activity in the surrounding region. Impoverishment of natural vegetation was evident only when we considered community completeness: the proportion of all suitable species in the region that are present at a site. To estimate community completeness, we compared the number of recorded species with the dark diversity-ecologically suitable species that are absent from a site but present in the surrounding region². In the sampled regions with a minimal human footprint index, an average of 35% of suitable plant species were present locally, compared with less than 20% in highly affected regions. Besides having the potential to uncover overlooked threats to biodiversity, dark diversity also provides guidance for nature conservation. Species in the dark diversity remain regionally present, and their local populations might be restored through measures that improve connectivity between natural vegetation fragments and reduce threats to population persistence.