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

Abstract

1 INTRODUCTION

Together with the increasing movement of humans, culture and goods, more and more species have been introduced across geographical barriers into novel regions, leading to floristic homogenization (Kinlock et al., 2022; Yang et al., 2021). A subset of these so-called alien species has formed self-sustaining populations in nature (i.e. are naturalized alien species), and the naturalized species that spread rapidly and frequently have negative impacts on the environment are considered invasive (Blackburn et al., 2011; Richardson et al., 2000). Naturalization, however, is a central stage of the invasion process, and it is of high importance to study and understand the mechanisms underlying the invasion (i.e. establishment) of naturalized alien species into local communities (Richardson & Pyšek, 2012).

Worldwide, at least 13,000 plant species have become naturalized due to human activities during the past centuries, and the pace of biological invasions does not seem to slow down in the foreseeable future (Seebens et al., 2017; Seebens et al., 2021; van Kleunen et al., 2015). Thus, how introduced alien plants interact with the local plant species remains one of the key questions in ecology. Answering this question will be necessary in order to reduce biological invasions and maintain native biodiversity and the functioning of ecosystems under global change (IPBES, 2023; Valladares et al., 2015).

Competition between alien and native plants is commonly thought to play an important role in the invasion process (Gioria & Osborne, 2014; Kuebbing & Nuñez, 2015). A large number of studies have tested whether aliens are more competitive than natives in different aspects, yet no agreement has been reached (Kuebbing & Nuñez, 2016; Vilà & Weiner, 2004; Zhang & van Kleunen, 2019). So, key questions are what determines the competitiveness of alien species, and are they really different from native species. With regard to the latter, it is important to consider that a species that is native to a region may at the same time be a naturalized alien in other regions (van Kleunen et al., 2010). So, when comparing alien to native species, it might be important to consider how successful the native species are as naturalized aliens in other parts of the world.

Similarly, not all naturalized alien species are equally competitive and successful. Some occur in relatively few regions or are relatively rare within their non-native range. So, both among the natives and aliens in a region, we have successful and less successful species both at the regional scale and at the global scale. Frequently, species that are widespread globally grow faster than less widely distributed species (Dawson et al., 2011). A recent study also found that common aliens are more competitive than rare natives but are not necessarily more competitive than common natives due to the high intrinsic growth rates of both groups of common species (Zhang & van Kleunen, 2019). Furthermore, it has been shown that common species, and especially common aliens, may take more advantage of nutrient addition (i.e. eutrophication), resulting in a higher competitiveness (Dawson et al., 2012). However, whether this is a general pattern and how it depends on the spatial distribution of resources remains unclear.

Soil heterogeneity, that is, the heterogeneous distribution of nutrients, soil types and various physical and chemical characteristics of the soil (Xue et al., 2019), is ubiquitous in nature. Heterogeneous soils can impact seed germination (Liu & Hou, 2021), ramet placement of clonal plants (Dong et al., 2015), root foraging (Keser et al., 2014), as well as growth and biomass allocation of plants (Liu, Li, et al., 2021). In addition, heterogeneous soils are thought to create more niche opportunities, and thus could promote species coexistence (Beck & Givnish, 2021; Liu, Bortier, et al., 2021; Stover & Henry, 2019). Indeed, some recent studies found that soil heterogeneity benefited common alien species by alleviating the competitive pressure from native plant communities (Gao et al., 2021; Wei & van Kleunen, 2022). However, whether this is also true for less common alien species, and how it depends on whether the native competitors themselves are widespread as alien species elsewhere, remains unknown.

Here, we conducted an experiment with seven rare and seven common alien species in Germany. We grew these 14 alien species on homogeneous and heterogeneous substrates alone or in pairwise competition with 15 native species, of which 10 are widely naturalized elsewhere (outside of Germany) and 5 are not widely naturalized. With the 210 alien–native competitor pairs on heterogeneous and homogeneous soils, we aimed to answer the following questions: (1) Do the alien species take more advantage of the heterogeneous soil conditions than the native species and is this effect stronger for the common alien species than for the rare ones? (2) Are the alien species, and particularly the rare ones, less competitive when they compete with natives that are widely naturalized elsewhere in the world?