Community assembly of exotic plant species across Mediterranean regions

Abstract

Invasions by plant species are an increasing threat which is reducing species diversity across regions, changing community composition and altering ecosystems functioning. While most investigations on impacts of exotic plants are conducted in their areas of introduction, the study of the assembly of exotic species in their native areas is emerging as a framework to better understand their roles in the invaded communities. In this regard, functional traits reflect the ecological strategy of plants and their interactions with coexistent species and the environment, therefore plant traits are a key tool to understand the rol of exotic plant species in the structure of their communities. One of the main objectives of this thesis is to identify the functional strategies of exotic species in invaded Mediterranean ecosystems and disentangle the rules that govern the assembly of invaded communities. For this, we analysed traits from several plant organs (i.e. leafs, seeds, roots) of 285 species from two vegetation types (woodlands and grasslands) at different spatial scales of resolution (i.e. Biome, environmental gradients, communities, plants). We used the Mediterranean biome as a study system because of the mostly unidirectional invasion of plants from the Mediterranean Basin to California, Chile, Australia and South Africa. Many exotic species were introduced with the arrival of the European settlers and their naturalisation was facilitated by the simultaneous intensification of agriculture and farming. Currently, these exotic species are locally very abundant and cause great impacts on the diversity and functioning of the invaded communities. We first investigated the assembly of exotic herbaceous species in their donor and recipient grassland communities in Spain and California, respectively. We found that exotic species were more abundant than other coexisting species in communities of both the donor and recipient regions (Chapter 1). The abundance of exotic species in California was similar to that in Spain except for invasive species which were more abundant in the introduced grasslands. Overall, this resulted in striking similarities in the taxonomic structure of Spanish and California grassland communities. We also found that productivity influences the functional composition of grassland communities by filtering traits related to resource conservation (Chapter 2). Exotic species had different traits than coexisting species in both their donor and recipient communities, but trait differences were greater in the most productive sites of the recipient communities in California. Trait differences suggest a competitive advantage of exotic species in grasslands, and supports that water and resource availability might determine niche segregation between native and exotic species in invaded California grasslands. Altogether, the results of the first two chapters indicate that the invasiveness of an exotic species could be predicted by understanding its role in its native communities. Finally, we explored the functional strategies and trait variability of coexisting native and invasive plant species across eight highly invaded Mediterranean communities of the World (Chapter 3). Invasive species were more frequently herbaceous than natives, and had a more acquisitive resource-use strategy across the studied Mediterranean communities. Also, invasive species showed higher trait diversity in half of the communities. We also found that intraspecific variance constitutes a non-negligible source of community trait diversity accounting for 11%–27% of total trait variation. Intraspecific trait variability was on average greater in invasive than in native species, which probably reflects the ecological versatility of invasive species but also the greater susceptibility to environmental heterogeneity of herbaceous plants. Overall, by comparing the abundance, the traits and the intraspecific variability of exotic species across communities of the Mediterranean Biome, this thesis provides insights to disentangle the assembly processes of invaded Mediterranean communities and advances our understanding on the biogeography of plant invasions.