Abstract
The novel-weapons and homeland-security hypotheses are based on the idea that aliens and natives are not adapted to each other’s allelochemicals as they did not co-evolve. However, as only a few studies have tested this, it remains unclear how important co-evolutionary history is in determining the strength of allelopathic interactions between aliens and natives. Here, we tested for potential pairwise allelopathic effects on each other of five alien and five native herbaceous species in China. We did a germination experiment and a competition experiment. In the germination experiment, we tested whether aqueous extracts of the ten study species had allelopathic effects on each other’s seed germination. In the competition experiment, we tested whether the alien and native species differed in their competitive effects and responses, and whether these were changed by the presence of activated carbon—a presumed allelopathy neutralizer– in the soil. Plant extracts had negative allelopathic effects on seed germination. This was particularly the case for extracts from the native species. Moreover, aqueous extracts had slightly stronger negative effects on germination of the aliens than on germination of the natives. In the competition experiment, on the other hand, the natives suffered more from competition than the alien species did, but we could not relate this to allelopathy. Alien plants had negative competitive and allelopathic effects on native plants, but the reverse was also true. These alien-native interactions, however, were not consistently stronger or weaker than native-native or alien-alien interactions.
Highlights
With increasing globalization, thousands of plant species have been introduced from their native areas into foreign lands
We addressed the following questions: (1) Do the effects of aqueous plant extracts on germination and the germination responses to the aqueous extracts differ between the native and alien species? (2) Do the native and alien species differ in their competitive effects and responses to competition, and are the competitive interactions mediated by allelopathy?
Germination success depended on the species used as source for the extract, with the lowest germination on extracts made of the alien Lepidium virginicum (6%) and the native Achyranthes bidentata (6%), and the highest germination on extracts made of the alien Capsella bursa-pastoris (49%; Fig. 1a)
Summary
Thousands of plant species have been introduced from their native areas into foreign lands Many of those alien plant species are found in cultivation only (van Kleunen et al 2018a), over 13,000 alien species worldwide have established persistent wild populations (i.e., have become naturalized; van Kleunen et al 2015). As already pointed out by Baker (1974), the high competitiveness could be due to the production of allelochemicals that suppress germination and growth of neighboring plants. The novel-weapons hypothesis predicts that some alien plants are successful invaders because they produce allelopathic compounds to which the native plants are not adapted (Callaway and Ridenour 2004). Several studies provided support for the novel-weapons hypothesis (e.g. Ridenour and Callaway 2001; Prati and Bossdorf 2004; Abhilasha et al 2008; GomezAparicio and Canham 2008; Thorpe et al 2009; Li and Jin 2010; Inderjit et al 2011; Becerra et al 2018; Irimia et al 2019), suggesting that allelopathy can play an important role in the ability of invasive plants to suppress native plants
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