Plant Community Assembly in Invaded Recipient Californian Grasslands and Putative Donor Grasslands in Spain

Abstract

The introduction of exotic species to new regions offers opportunities to test fundamental questions in ecology, such as the context-dependency of community structure and assembly. Annual grasslands provide a model system of a major unidirectional introduction of plant species from Europe to North America. We compared the community structure of grasslands in two Mediterranean regions by surveying plots in Spain and in California with similar environmental and management conditions. All species found in Spanish grasslands were native to Spain, and over half of them (74 of 139 species) are known to have colonized California. In contrast, in California, over half of the species (52 of 95 species) were exotic species, all of them native to Spain. Nineteen species were found in multiple plots in both regions (i.e., shared species). The abundance of shared species in California was either similar to (13 species) or greater than (6 species) in Spain. In California, plants considered pests were more likely than non-pest species to have higher abundance. Co-occurring shared species tended to maintain their relative abundance in native and introduced communities, which indicates that pools of exotic species might assemble similarly at home and away. These findings provide interesting insights into community assembly in novel ecosystems. They also highlight an example of startling global and local floristic homogenization.

Functional assembly of grassland plant species in native communities in Spain and recipient communities in California

“…As hypothesized, we found that, in both regions, colonizer species assemblages were functionally more diverse than the other coexisting species. This adds to previous evidence that Spanish grassland communities hold greater species richness than California grasslands (Galán Díaz, de la Riva, Parker, et al, 2021;Leiva et al, 1997), and reflects that colonizer species might also dominate the trait space. Some authors suggest that ancient agricultural landscapes in Europe may have created human-mediated niches promoting high levels of biodiversity (Eriksson, 2013).…”

“…In Spain, we found 139 species, all of them native to Spain (Valdés et al, 1987). In all, 79 were colonizers; 73 are established in California (Calflora., 2014; Galán Díaz, de la Riva, Parker, et al, 2021) and 6 more are established in other Mediterranean regions but were not introduced to California. These six species are expected to share similar traits and so were also considered colonizers (Arianoutsou et al, 2013).…”

“…We classified species as 'colonizers' if they were native to Spain and known to be successfully established as exotic species elsewhere (Galán Díaz, de la Riva, Parker, et al, 2021). In Spain, we found 139 species, all of them native to Spain (Valdés et al, 1987).…”

“…Mediterranean grasslands are an excellent study system to compare differences in community assembly between introduced and native ranges. Over the last four centuries, there has been an overwhelmingly unidirectional movement of herbaceous species from Mediterranean Europe to other Mediterranean regions (Casado et al, 2018; Galán Díaz, de la Riva, Parker, et al, 2021). For example, in California, 672 out of the 975 herbaceous exotic plant species are from the Mediterranean Basin (Rejmanek & Randall, 1994).…”