Guan-Wen Wei and Mark van Kleunen, from the Ecology Lab at the University of Konstanz in Germany, discuss their article: Responses of naturalized alien plants to soil heterogeneity and competition vary with the global naturalization success of the native competitors
Biological invasions are happening
With the development of transportation and globalization, species have been introduced across geographical barriers into new regions. Some of these introduced alien species have formed self-sustaining populations in nature and are considered to be naturalized. Globally, at least 13,000 plant species have become naturalized due to human activities, and the number is still increasing. Those naturalized species that spread rapidly and have a negative impact on the environment are considered invasive. Therefore, studying the mechanisms underlying naturalization of alien species into local communities—as a central stage of the invasion process—is crucial.
Who are more competitive, aliens or native plants?
Competition between alien and native plants is commonly thought to play an important role in the invasion process. Key questions are: what determines the competitiveness of alien species, and are alien species really different from native species? Concerning the latter, it is important to consider that a species native to one region may simultaneously be a naturalized alien in another. While it is commonly taken into account how successful the alien species are around the world, the global naturalization success of the native species is often overlooked and should be taken into consideration.
Soil heterogeneity promotes invasion?
Soil heterogeneity, i.e., the uneven distribution of resources (water, nutrients, etc.) and other characteristics (soil types, etc.), is ubiquitous in nature. Heterogeneous distributions of resources in the soil could benefit widely naturalized alien species by reducing the competitive pressure asserted by native plants. However, how this depends on whether the native competitors themselves are widespread as naturalized alien species elsewhere remains unknown.
Let us answer these questions with an experiment!
We selected 29 species that differ in their commonness in Germany and in their naturalization success globally. Seedlings of species that were either common or rare aliens in Germany and native species that were globally non-widely or widely naturalized were planted in heterogeneous and homogeneous soils alone and in native-alien species pairs, to test the effects of competition and soil heterogeneity on plant growth, and if these effects depend on how common or widely naturalized the species are.
The global naturalization success of native species matters
We found that soil heterogeneity does not always increase the growth of alien species, and it might depend on the commonness of the alien plant and the naturalization success of the native competitor elsewhere.
Moreover, the growth of alien plants in the presence of native competitors was more strongly related to the global naturalization success of their native competitors than to their own commonness. Therefore, considering the global naturalization success of native species can provide us with important information on their competitiveness.
Our findings suggest that native species that are widely naturalized elsewhere can inhibit the growth of alien plants, reduce their establishment chances and thus slow down the invasion process.
