Phylogenetic and functional mechanisms of direct and indirect interactions among alien and native plants

Abstract

Summary1. Disentangling direct and indirect interactions among alien and native plants is essential to understanding the success of alien plants in multispecies communities, but studies have only focused on direct pairwise interactions. Moreover, it is also essential to explore phylogenetic and functional mechanisms driving these interactions. 2. In a greenhouse experiment, we selected nine groups of alien and native plant species from the herbaceous flora of Germany to disentangle their direct and indirect interactions. Each group had an alien (A) that is common or rare in Germany (i.e. non-native range), two natives that are phylogenetically closely related (N close ) and distantly related (N dist ) to A respectively, and a distantly related "target" native (T). We grew the four species of each group alone, and in two-species and three-species combinations. Specifically, we tested whether competition is greater between A and N close than between A and N dist , whether presence of N close rather than N dist indirectly alleviates competition of A on T, and whether these interaction patterns depend on commonness of A. Moreover, we tested how intensity of these interactions is explained by phylogenetic distance, functional traits (height, seed mass, SLA, leaf size, specific root length, leaf area ratio, root length ratio (root length/plant mass), shoot weight ratio) and traits-based functional distance. 3. We found A had stronger competition on N close than on N dist . In turn, A was more suppressed by N close than by N dist , but this was only true for rare rather than common A. The presence of N dist rather than N close indirectly reduced competition of A on T. The intensity of these interactions was not explained by phylogenetic or functional distance, but by some of the functional traits. Specifically, a plant experienced stronger competition when it was shorter and had smaller leaves and lower shoot weight ratio, and when its neighbours were taller, had greater SLA, leaf area ratio and shoot weight ratio, and had a lower root length ratio. 4. Synthesis. Functional traits can help explain competitive interactions. While direct competition tended to be stronger between more closely related alien and native plants, this did not indirectly facilitate other co-occurring native plants.

Linking Darwin’s naturalisation hypothesis and Elton’s diversity–invasibility hypothesis in experimental grassland communities

“…Possibly, our findings indicate that the large seedlings of large‐seeded species are more likely to survive, but that this is at a cost of reproduction (Díaz et al, ), at least during the period that our experiment lasted. In line with several other studies (e.g., Conti et al, ; Feng & van Kleunen, ), our results showed that the effects of univariate hierarchical functional distance based on plant height or leaf size on the performance of the alien species were positive, whereas those based on SLA were negative. This shows that alien plants are more successful if they are taller and have larger leaves with a lower SLA than the native residents.…”

“…In support of the predictions of Darwin's naturalisation hypothesis, increasing values of the different measures of alien-native distance resulted in enhanced performance (from germination to reproduction) of our five target invasive species. This is in line with some previous studies which also found evidence in support of the hypothesis (Feng & van Kleunen, 2016;Jiang et al, 2010;Strauss et al, 2006), but others showed that the effects of alien-native distances were dependent on whether phylogenetic distance indicates niche preadaptation or competitive interactions, and on the stages of invasion (Diez et al, 2008;Duncan & Williams, 2002;Li, Guo, et al, 2015). Moreover, in our study, the effects of multivariate functional distance were much stronger than those of phylogenetic distance, which suggests that multiple traits-based functional distance is better than phylogenetic distance in capturing niche differences that favour coexistence between the aliens and the native residents.…”

“…This shows that alien plants are more successful if they are taller and have larger leaves with a lower SLA than the native residents. Although a high SLA is usually associated with a fast relative growth rate (e.g., Poorter & Remkes, ), several recent studies found that a greater SLA tends to be associated with greater competitive disadvantage (Feng & van Kleunen, ; Kraft et al, ; Kunstler et al, ). In short, our findings thus confirm that these univariate hierarchical functional distances are indeed able to capture alien–native competitive inequalities.…”

“…Numerous studies have tested Darwin's naturalisation hypothesis, but the results are mixed (Diez, Sullivan, Hulme, Edwards, & Duncan, ; Duncan & Williams, ; Feng & van Kleunen, ; Finerty et al, ; Jiang, Tan, & Pu, ; Li, Cadotte, et al, ; Strauss, Webb, & Salamin, ; Yannelli, Koch, Jeschke, & Kollmann, ). Potential explanations are that the studies differ in the spatial and phylogenetic scales considered (Thuiller et al, ) and stages of invasions (Cadotte, Campbell, Li, Sodhi, & Mandrak, ; Li, Guo, et al, ), and are based on observations rather than experiments.…”

Soil-microbes-mediated invasional meltdown in plants

“…Most studies, however, focused on pairwise interactions 6 (i.e. interaction between two species; but see refs 7,8 for two exceptions), although in nature most species interact with multiple species. With regard to invasive alien organisms, only few hypotheses guide us in testing interactions among more than two species.…”