Responses of naturalized alien plants to soil heterogeneity and competition vary with the global naturalization success of the native competitors

Abstract

1. Soil heterogeneity (i.e. the heterogeneous distribution of nutrients, soil types or other soil characteristics) has been found to promote the invasion success of naturalized alien plants (i.e. plants that have been introduced by humans to new ranges, where they have established persistent populations) when competing with native plants. However, native species that compete with the naturalized aliens may themselves be naturalized alien species elsewhere. It has also been found that common species, irrespective of whether they are alien or native, benefit more from high resource availability than rare species. Therefore, it remains unknown how regional commonness of alien plants and the extent of the worldwide naturalized geographical distribution of native species impact the invasion process on heterogeneous soils.

2. We considered the geographical distributions of native and alien species in Germany at two scales: commonness (i.e. grid-cell frequency) in Germany and the global naturalized distribution. Then, to test whether the performance of common (n = 7) and rare (n = 7) alien species in Germany depends on soil heterogeneity and on whether the natives they compete with are themselves widely naturalized elsewhere (i.e. outside of Germany; n = 10) or not (n = 5), we conducted a large outdoor pot experiment.

3. We found that the growth of the alien plants, irrespective of whether they are relatively rare or common in Germany, was reduced by competition, and particularly if the native competitor is widely naturalized elsewhere. Consequently, the relative biomass of the alien plants (i.e. the ratio of alien to total biomass) was reduced when growing with a native plant that is widely naturalized elsewhere. Soil heterogeneity overall resulted in a reduced growth, but this was not the case for the common aliens when they were competing with a native that is widely naturalized elsewhere.

4. Synthesis. Our findings thus show that soil heterogeneity does not always promote alien plants over native plants. Moreover, our findings show that when testing the effects of native species on naturalized alien species, it is important to consider whether the native species themselves are widely naturalized around the globe or not.