Understanding the strength and modes of interspecific interactions between introduced and resident species (native or previously introduced) is necessary to predict invasion success. We evaluated different mechanisms of interspecific competition among four species of polyphagous fruit flies (Diptera: Tephritidae) from the island of La Reunion: one endemic species, Ceratitis catoirii, and three exotic species, C. capitata, C. rosa, and Bactrocera zonata, that have successively invaded the island. Larval competition experiments, i.e., co-infestations of the same fruit, and behavioral interference experiments measuring the ability of one female to displace another from a fruit, were performed among all pairs of the four species. We observed asymmetric and hierarchical interactions among species in both larval and adult interference competition. In agreement with the hypothesis that invasion is competition-limited, the competitive hierarchy coincided with the temporal sequence of establishment on the island, i.e., each newly established species tended to be competitively dominant over previously established ones.

While most alien species fail to establish, some invade native communities and become widespread. Our understanding of invasion success is derived mainly from pairwise interactions between aliens and natives, while interactions among more than two species remain largely unexplored. Here, we experimentally tested whether and how a third plant species, either native or alien, affected the competitive outcomes between alien and native plants through its soil legacy. We first conditioned soil with one of ten species (six natives and four aliens) or without plants. We then grew on these 11 soils five aliens and five natives without competition, or with intra-or interspecific competition. We found that aliens were not more competitive than natives when grown on soil conditioned by other natives or on non-conditioned soil. However, aliens were more competitive than natives on soil conditioned by other aliens (that is, invasional meltdown). Soil conditioning did not change competitive outcomes by affecting the strength of competition between later plants. lnstead, soil conditioned by aliens pushed competitive outcomes towards later aliens by affecting the growth of aliens less negatively than that of natives. Microbiome analysis verified this finding, as we showed that the soil-legacy effects of a species on later species were less negative when their fungal endophyte communities were less similar, and that fungal endophyte communities were less similar between two aliens than between aliens and natives. Our study reveals invasional meltdown in multispecies communities and identifies soil microorganisms as a driver of the invasion success of alien plants.

It is generally accepted that plants locally influence the composition and activity of their rhizosphere microbiome, and that rhizosphere community assembly further involves a hierarchy of constraints with varying strengths across spatial and temporal scales. However, our knowledge of rhizosphere microbiomes is largely based on single-location and time-point studies. Consequently, it remains difficult to predict patterns at large landscape scales, and we lack a clear understanding of how the rhizosphere microbiome forms and is maintained by drivers beyond the influence of the plant. By synthesizing recent literature and collating data on rhizosphere microbiomes, we point out the opportunities and challenges offered by advances in molecular biology, bioinformatics, and data availability. Specifically, we highlight the use of exact sequence variants, coupled with existing and newly generated data to decipher the rules of rhizosphere community assembly across large spatial and taxonomic scales. Unearthing the Macroecology of Rhizosphere Soil Microbes Recent advances in sequencing technologies have expanded our ability to study plantassociated microbial communities. This has transformed our perception of the interactions between the plant and its microbiome (see Glossary) [1], which are now increasingly regarded jointly as holobionts [2-5]. The rhizosphere (i.e., the interface of plant roots and soils [6]) hosts diverse communities of microorganisms that are crucial to the plants they associate with. The rhizosphere microbiome can supply plants with nutrients [7], and protect plants against pathogens [8]. Furthermore, microbiomes can stimulate plant growth by producing phytohormones [9,10], and improve plant resistance and tolerance to abiotic stressors. Recent research suggests that rhizosphere microbiomes can alter plant phenology (e.g., flowering time) [11], modify morphological and size-related traits (e.g., shoot and root length and biomass, and number of secondary roots and leaves) [12], have a major role in plant community dynamics [13-15], and mediate plant responses to global change [16,17]. While much progress has been made, we are still far from understanding the mechanisms that control rhizosphere microbiome assembly and maintain community structure and composition. It has long been hypothesized that the macroecological patterns (i.e., ecological patterns across large spatial scales) of the rhizosphere microbiome will relate to the macroecological patterns of plants [18]. However, few studies have found clear relationships between plant and rhizosphere diversity [19,20]. This is likely because there is a disconnect between the plant and the microbial scales (Box 1). For other (non-rhizosphere) microbiomes, large-scale sampling campaigns are leading the way by generating standardized raw data and metadata (e.g., the Earth Microbiome Project [21] or the Human Microbiome Project [22]). For macro-organisms, macroecological patterns are frequently identified by collating existing and newly generated data from ...