No negative impact of Erigeron annuus on native plant diversity: a case study from Northern Switzerland

In Switzerland, the neophyte Erigeron annuus is considered to be harmful to the environment, particularly to native plant diversity and thus is listed as “invasive alien species” (IAS) in the normative sense. Since the evidence for this listing is limited and contradictory, we studied it empirically. We searched for sites in which the species occurs with high local cover values, irrespective of the vegetation type. We sampled clusters of four 1-m2 plots in 10 different sites around Zurich, Northern Switzerland. Each cluster represented a local “invasion gradient” from no (or very low) to the maximum Erigeron annuus cover of the site, within the same plant community and all other factors being as similar as possible. We then applied (generalised) linear mixed-effect models to test whether plant diversity, mean ecological indicator values (temperature, light, moisture, reaction, nutrients) or CSR strategy types are influenced by the cover of Erigeron annuus. All response variables were calculated without Erigeron annuus (“resident community”) and diversity indices additionally without all neophytes (“native community”). Most of the sites belonged to grasslands of the Arrhenatherion elatioris, the Erigeron annuus cover ranged from 0% to 55% and the mean vascular plant species richness was 24.6. We found that the cover of Erigeron annuus did not influence native and resident species richness or Shannon evenness, nor native Shannon diversity significantly. Only on resident Shannon diversity it had a minimal positive effect. None of the tested mean ecological indicator values or CSR strategies was influenced by the cover of Erigeron annuus. Our results demonstrate that Erigeron annuus regularly invades relatively species rich grassland communities, but its effect on the resident vascular plant community in our study region remains negligible. This is consistent with our literature review according to which other studies with sound methodology either found no or even a positive effect of Erigeron annuus on native plant diversity. As our sample is limited in size and geographic scope, we recommend repeating such impact studies in other regions with similar methods to test for the generality of our results. Beyond Erigeron annuus, our results suggest that the determination of “invasive alien neophytes” should be based on solid scientific evidence to spend conservation money effectively on measures that improve the state of native biodiversity.