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Variations in the patterns of diversity of symbionts have been described worldwide on Mimosa pudica, a pan-tropical invasive species that interacts with both alpha and beta-rhizobia. In this study, we investigated if symbiont competitiveness can explain these variations and the apparent prevalence of beta- over alpha-rhizobia. We developed an indirect method to measure the proportion of nodulation against a GFP reference strain and tested its reproducibility and efficiency. We estimated the competitiveness of 54 strains belonging to four species of beta-rhizobia and four of alpha-rhizobia, and the influence of the host genotype on their competitiveness. Our results were compared with biogeographical patterns of symbionts and host varieties. We found: (i) a strong strain effect on competitiveness largely explained by the rhizobial species, with Burkholderia phymatum being the most competitive species, followed by B. tuberum, whereas all other species shared similar and reduced levels of competitiveness; (ii) plant genotype can increase the competitiveness of Cupriavidus taiwanensis. The latter data support the likelihood of the strong adaptation of C. taiwanensis with the M. pudica var. unijuga and help explain its prevalence as a symbiont of this variety over Burkholderia species in some environments, most notably in Taiwan.