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

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.