Functional assembly of grassland plant species in donor and recipient communities

Montserrat Vilà Planella, Estación Biológica de Doñana (EBD-CSIC), Sevilla, Spain, Javier Galán, Estación Biológica de Doñana (EBD-CSIC), Spain, Ingrid M. Parker, Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, CA and Enrique G De la Riva, Brandenburg University of Technology, Germany

Background/Question/Methods
Biogeographical comparisons of species in their native and introduced ranges are important to elucidate whether invasion changes fundamental features of population regulation, and how the structure of “novel communities” incorporating exotics may differ from that of their communities of origin. Yet functional comparisons of entire assemblages of exotic species in their native and introduced communities have rarely been analysed. We took advantage of the large unidirectional invasion of annual plant species of Spanish origin into California grasslands as a large natural experiment to understand community functional convergence after invasion. A previous analysis indicated that these invaded California grasslands are more similar in species composition to Spanish grasslands than some Spanish grasslands are among themselves.
By conducting coordinated floristic surveys and analysis of plant traits in grasslands in California and Spain, we investigated the role of exotic plant species in the functional assembly of donor and recipient communities. Specifically, we explored (1) the relationship between environmental constraints and community function, and (2) how habitat filtering and niche segregation operate along environmental gradients.
Results/Conclusions

Overall, we found 139 plant species in Spain and 95 in California. In Spain, all species were native. In California, half were exotics. All exotic species found in California grasslands were native to Spain.
There were clear differences in the functional structure of Mediterranean grassland communities both between and within ranges in relation to the environmental gradient. Paradoxically, the most acquisitive communities occurred in less productive sites, highlighting that a rapid resource acquisition strategy might allow species to avoid long periods of drought. In Spain, species that invaded California were more acquisitive than species not known to be invasive somewhere. Yet, we did not find significant differences for most traits. This might reflect preadaptation of the entire species pool to the management practices that have taken place in Europe for millennia. In contrast, in California, exotic species differed with natives in most traits. Exotic species were more acquisitive in resource use than native species especially under favourable conditions, but functionally converged with natives in less productive sites. This indicates that niche differentiation between native and exotic species in California is subject to the influence of abiotic filters. Our results show that plant trait comparisons between native and exotic species are context dependent and that a correct interpretation of filtering processes in community assembly requires a biogeographical perspective.