Impacts of an invasive tree across trophic levels: Species richness, community composition and resident species' traits

Aim: To investigate the community-level impacts of woody plant invasions using Robinia pseudoacacia as a model species, affecting organisms on different trophic levels: vascular plants, nocturnal Lepidoptera and birds. Location: Czech Republic, central Europe. Methods: Nineteen plots with strong dominance of the invader were compared to 20 plots with native deciduous trees on sites with similar conditions. Species richness was compared using marginal models, species composition and the distribution of species traits by ordination analyses. Functional relationships between the three groups of organisms were investigated using a path analysis. Results: Only minor differences in species richness between invaded and uninvaded plots were detected for plants and birds, but the invaded stands hosted significantly fewer species of nocturnal Lepidoptera. On the contrary, all three groups differed in species composition and in the distribution of traits between the invaded and uninvaded stands. Nitrophilous plants, supported by human disturbances, were more represented in the invaded stands, while habitat specialist birds preferred uninvaded forest. Within nocturnal Lepidoptera, species of open habitats and fast life cycle preferred the invaded stands, and forest and canopy species and habitat generalists of larger sizes preferred the uninvaded stands. Path analysis showed a minor effect of R. pseudoacacia on the numbers of plants which were unrelated to species richness at higher trophic levels. However, R. pseudoacacia had a negative direct effect on nocturnal Lepidoptera, contributing to a weak negative indirect impact on birds. Main conclusions: The impacts of R. pseudoacacia on species richness differed across the trophic levels, questioning the existence of simple cascading effects as a consequence of its invasion. Invasive plants do not always reduce the diversity of species per se, but cause shifts in species composition by replacing some of the pre-invasion biota by species with traits enabling tolerance to the invaded habitat.