Patterns of plant invasions at small spatial scale correspond with that at the whole country scale

Invasions were studied at different spatial scales, however these scales were rarely compared. The aim of this study is to compare the rate of expansion of eight alien plant species within the borders of the city with expansion in the whole country and to analyse changes in distribution of these plant species in the last 30 years. We hypothesised that at the scale of the city, despite of stronger human impact, invasion processes will perform in a similar way as at the scale of whole country. Distribution changes of Bidens frondosa, Conyza canadensis, Echinocystis lobata, Helianthus tuberosus, Impatiens parviflora, Reynoutria japonica, Solidago canadensis and S. gigantea in 1 km squares grid was examined in the Warta River Valley in Poznan (W Poland) in 2013 and compared to data collected during 1980–1984. All species increased their distribution within the study area. E. lobata had the highest increment of occupied grid squares (from 0 in 1980’s to 28 in 2013), and S. gigantea had the lowest (from 8 to 9). Echinocystis lobata reached the highest invasion success, which results from species traits (hydrochory) and large propagule pressure from garden plots. Between species with different ecological traits residence time has relatively little influence on invasion success, however in cases of species with similar traits (S. canadensis and S. gigantea) residence time is a crucial factor determining invasion success. Although urban ecosystems are more vulnerable to biological invasions, this process occurs in a similar way as in the whole country. This similarity is connected with similar factors responsible for spread of invasive species: propagule pressure, time since introduction and spatial distributions of invasible habitats.