A major aim in invasion ecology is to understand the role of exotic species in plant communities. Whereas most studies have explored the traits of exotic species in the context of the introduced community, functional comparisons of entire assemblages of species in their native and introduced communities have rarely been analysed. Taking advantage of the unidirectional invasion of plant species of European origin (i.e. colonizers) into California, this study aims to investigate the relative importance of plant traits, environmental factors, and invasion status in biological invasions. We compared the functional structure (i.e. trait composition and diversity) along with resource availability gradients in recipient and native Mediterranean grassland communities in California and Spain, respectively. Traits were related to resource use in above- and belowground organs and reproductive strategy. We also investigated how niche differences vary along environmental gradients between coexisting colonizer and native species assemblages within communities. There were clear differences in the functional structure of Mediterranean grassland communities between regions, which were associated with the resource availability gradient. Paradoxically, the most acquisitive communities occurred in resource-poor sites, highlighting that rapid acquisition and use of resources permit species to cope with environmental stress through stress avoidance. In Spain, colonizer species had greater SLA than non-colonizers. Yet, differences between colonizer and non-colonizer species in Spain for other traits were mostly absent and did not change along the gradient. This might be a result of the greater native species richness as a consequence of the agricultural practices that have taken place in Europe for millennia and reflect that the entire species pool of grasslands is adapted to agricultural landscapes. In comparison, in California, colonizer species were more acquisitive in their use of resources than natives under favourable conditions, but functionally converged in resource-limited sites. This result in the recipient region underscores that the importance of niche differences between native and colonizer species as a community assembly mechanism is strongly subject to the influence of habitat filtering. Trait comparisons are context-dependent, and a correct interpretation of filtering processes in community assembly requires a regional perspective.

From March 2018 to May 2018, we sampled 60 vegetation plots across 4 sites in Spain; and from March 2019 to May 2019, we sampled 60 vegetation plots across 4 sites in California. Sites were weekly monitored and sampled during the two weeks following the flowering blooming peak; therefore on mature plants. We selected sites that, over the last 30 years, had been moderately grazed by wild and domestic ungulates and had not been burned, ploughed, or planted. In each site, we collected plant species composition and relative cover abundance in fifteen plots (0.50 × 0.50 m) located along a 1 km transect. For each combination of species × site, we collected 10 flowering individuals of the species that contributed to 90% of the cumulative cover and measured 8 traits representative of plant resource-use and acquisition strategies: Specific Leaf Area (SLA), Leaf Dry Matter Content (LDMC), Leaf Nitrogen Content (LNC), Isotopic Carbon Fraction (δ13C), Specific Root Length (SRL), Root Dry Matter Content (RDMC) and Reproductive Height (distance from the latest inflorescence to the ground). Measurements were performed following the protocols included in the handbook for standardised measurement of plant functional traits (Pérez-Harguindeguy et al., 2013). Additionally, seed mass was retrieved from the Seed Information Database. We classified species as “colonizers” if they were native to Spain and known to be successfully established as exotic species elsewhere. We also recorded species growth form and life form traits. We estimated average soil nutrient concentration per site by taking 5 cores: percentage total nitrogen (N) by Kjeldahl digestion, organic matter (OM) by the Walkley and Black method, and available phosphorus (P) by the Olsen method.

Trait and soil data are available in csv format, and abundance data in txt format.