Linking the impacts of plant invasion on community functional structure and ecosystem properties

Questions: Many studies report a decrease of native species richness in communities after plant invasion by exotic taxa, but the implications of species losses on community functional structure and ecosystem processes have been less explored. The questions addressed are: (1) what are the impacts of invasive plant taxa on the functional structure of the recipient community; and (2) are there links between such functional structure and ecosystem properties representing key ecosystem processes? Location: Costal habitats of Mallorca and Menorca, Balearic Islands, Spain. Methods: In paired invaded and non-invaded plots, species frequencies and ecosystem properties related to biogeochemical cycles (soil C and N content, soil moisture and decomposition rate) were assessed. For every native species we obtained values of several functional traits (plant height, woodiness, N fixation and evergreenness). Then we calculated indices of community functional structure (community-weighted means for each trait; functional diversity: richness, evenness, divergence and dispersion; and functional redundancy) and compared them between paired invaded and non-invaded plots. The relationship with ecosystem properties was assessed using GLM. Results: The spectra of trait distribution changed in the invaded assemblages towards a higher frequency of woodiness and evergreenness. Invaded communities showed decreased species richness, and lower functional richness, divergence, dispersion and redundancy. In contrast, ecosystem properties were scarcely sensitive to invasion. Functional redundancy was negatively related to soil N of invaded plots and soil moisture in the dry season, which suggests higher depletion of soil resources when several species share the same combination of trait values. Conclusions: Carpobrotus invasion led to a functional homogenization within communities, which probably reduces their resilience. The functional structure of the community was more responsive to invasion than the ecosystem properties. The weak relationship between functional indices and ecosystem properties suggests that they may respond to invasion at different rates.