Impact of invasive and native dominants on species richness and diversity of plant communities

Invasive alien plants are known to reduce the diversity of recipient communities. However, there is an ongoing debate on whether or not native dominant species have similar effects. To answer this question, we compared herbaceous dominant species of plant communities in central Europe, 10 of which were native and nine alien to this region. We sampled 5–16 populations per species, selected to reflect a gradient from a low to a high cover of the dominant species studied and include a range of typical habitats. To reveal the possible effect of scale, we sampled the vegetation in 4 × 4 m (large scale) and 1 × 1 m (small scale) plots. All vascular plant species and their percentage covers were recorded in each plot. LMM regressions models were used to relate the dominant species’ cover to the richness and diversity of the plant community and ANCOVAs to test for differences between the impacts of native vs. invasive dominants. On the large scale, 17 dominants (nine native and eight invasive) significantly reduced community species richness, and seven (four native and three alien) decreased species diversity measured using the Shannon H’ index. Reynoutria ×bohemica, Calamagrostis epigejos and Phalaris arundinacea had the strongest negative impact on species richness, while Reynoutria ×bohemica, Phalaris arundinacea and Urtica dioica had the strongest impact on species diversity H’; the results at the small scale were very similar. No significant differences in impacts were detected with regard to the origin of the dominant species when all 19 dominants were included in one model. Further, we used indirect gradient ordination analysis (DCA) to identify pairs of native and invasive dominants that grow in similar habitats and, thus, their impacts can be compared and tested for the effect of origin (native vs. alien). This procedure yielded 27 pairs in total, as some dominants occur in more than one type of habitat and could, therefore, be coupled with more than one species from the other group. At the large scale, native dominants had stronger impacts on species richness in three cases (Calamagrostis epigejos, Cirsium oleraceum and Phalaris arundinacea) and invasive dominants in two (Aster novi-belgii agg. and Rumex alpinus), making up 11.1% and 7.4% of the total number of pairs examined, respectively. Only the invasive dominants (Reynoutria ×bohemica, Rumex alpinus) had stronger impacts on species diversity H’, in four pairwise comparisons (14.8%). The differences were not significant at the small scale in all but one comparison. The results show that both native and invasive dominants can reduce the diversity of vegetation. To conserve biodiversity, measures should be adopted to mitigate not only the impacts of invasive species but also those of native dominants, spreading in the current landscape; this would be best achieved by promoting traditional management and land-use.