Differences in invasiveness and competitiveness of hybrids of different ploidy levels
The evolution of invasive species in their new ranges and the formation of new hybrids following invasion are of high interest in current research of invasion biology. The hybrid Rorippa x armoracioides derives from the native Rorippa sylvestris and the invasive Rorippa austriaca. It is very invasive in the Elbe valley (Germany) and may have displaced R. austriaca, the latter having declined in the last decade in this region. This tetraploid hybrid now forms autonomous stands. A pentaploid form has been found on the river Ruhr near Mülheim (Germany), and a triploid form along the river Main near Würzburg (Germany). These two forms are not considered invasive in the region.
Experiments have been conducted to compare soil parameters, vegetation cover and plant diversity in these different situations. The results showed that the soil near the river Elbe, especially in flood plain meadows where the hybrid has its highest cover, is more acid than the soil in the regions Ruhr and Main. There was a trend towards reduced plant species diversity when the hybrid from the river Elbe (tetraploid) occurred. In experiments, the Elbe hybrid (tetraploid) was more competitive than R. sylvestris while its competition intensity was similar to that of R. austriaca. In contrast, the hybrid from the river Ruhr (pentaploid) was a weak competitor. The Elbe hybrid (tetraploid) allocated more resources into its belowground biomass than the other Rorippa taxa and had the longest roots. While R. austriaca grew only vegetatively in the first year, this hybrid was able to produce fruits shortly after establishment.
The results may indicate that the Elbe hybrid (tetraploid) profits from a higher availability of soil nutrients due to a lower soil pH by investing more resources in the root system than its parent species. Moreover, this hybrid may have an even higher plasticity in resource allocation than R. austriaca, which may promote its invasiveness. Although these results suggest that the hybrid has replaced R. austriaca in flood plain meadows of the river Elbe, other unknown factors could also be responsible of the decline of the latter. Due to its invasiveness and deep reaching root system, an ongoing invasion of drier habitats of the middle Elbe valley by the Elbe hybrid (tetraploid) is expected.
Sources
Buschmann H, Melz K (2006) Differences in invasiveness and competitiveness of hybrids of different ploidy levels. In: Neobiota. From Ecology to Conservation. 4th European Conference on Biological Invasions. Vienna (Austria), 2006-09-27/29, BfN-Skripten 184: page 90 (abst.).