The phenotypic plasticity and the competitive ability of the invasive Acacia longifolia v. the indigenous Mediterranean dune species Halimium halimifolium and Pinus pinea were evaluated. In particular, we explored the hypothesis that phenotypic plasticity in response to biotic and abiotic factors explains the observed differences in competitiveness between invasive and native species. The seedlings’ ability to exploit different resource availabilities was examined in a two factorial experimental design of light and nutrient treatments by analysing 20 physiological and morphological traits. Competitiveness was tested using an additive experimental design in combination with ¹⁵N-labelling experiments. Light and nutrient availability had only minor effects on most physiological traits and differences between species were not significant. Plasticity in response to changes in resource availability occurred in morphological and allocation traits, revealing A. longifolia to be a species of intermediate responsiveness. The major competitive advantage of A. longifolia was its constitutively high shoot elongation rate at most resource treatments and its effective nutrient acquisition. Further, A. longifolia was found to be highly tolerant against competition from native species. In contrast to common expectations, the competition experiment indicated that A. longifolia expressed a constant allocation pattern and a phenotypic plasticity similar to that of the native species.