Make it simpler: Alien species decrease functional diversity of coastal plant communities

Abstract

Questions: (a) Are there differences in abundance-weighted functional trait values between native and alien species in coastal plant communities? (b) Which functional traits are associated with a higher level of invasion in these communities? (c) Do functional diversity patterns differ between native and alien species? (d) Is alien species occurrence linked to small-scale functional homogenization effects on the resident native species? Location: N-Adriatic coastal ecosystems (Marano and Grado lagoon, Friuli Venezia Giulia region, Italy). Methods:We sampled coastal vegetation within two habitats (foredunes and saltmarshes) along nine belt transects in two sampling sites. Plant species richness and abundance were assessed in 128 plots along with a suite of plant functional traits.We tested for differences in community-weighted means between native and alien species within the two habitats, and Linear Mixed Models provided insights into traits fostering the invasion success among alien species. To check for potential functional homogenization driven by alien species invasion, we explored functional diversity patterns of native and alien species (β and γ functional diversity) and the relationship between α functional diversity and alien cover.Results: Alien species had lower functional diversity than natives and were characterized by lower leaf construction costs coupled with lower drought resistance and higher water transport efficiency. The most abundant aliens were the ones minimizing carbon investment for leaf construction. In addition, we also found evidence for small-scale functional homogenization driven by alien invasion. Conclusions:Our results suggested that native species adopt a resource conservative strategy whereas alien species are characterized by a higher resource acquisition capacity (i.e., acquisitive strategy). Our data also confirmed that alien species are less functionally diverse than natives, potentially driving the community towards small-scale functional homogenization, resulting in a loss of species and a reduction in the functional space. K E Y W O R D S biological invasion, community ecology, community weighted mean, functional homogenization, functional rarefaction, functional traits, functional uniqueness, quantitative methods | 499 Journal of Vegetation Science TORDONI eT al.

Functional Divergence Drives Invasibility of Plant Communities at the Edges of a Resource Availability Gradient

“…IAS were often reported to be more performant than native species displaying higher values for traits related to resource acquisition such as photosynthetic capacity and specific leaf area [18,42,76], and this signal was detected for most of the traits measured in this study (i.e., higher peaks of TPDs in invasive assemblage in LMA, LDMC, δ 13 C). Greater phenotypic plasticity, the ability to spread over long distances, and functional features that were not yet present in the native resident community are usually considered as key-factors which may promote a greater community invasibility [20,77], even though there is still an open debate since these factors may vary across time, and invasion stages [78].…”

“…In Site 2 (n = 131), which is characterized by a complex mosaic of small habitat patches and a steep ecological gradient, we used random belt transects since their effectiveness in sampling coastal systems has already been proved [47]. Transects were displaced with variable length (from 16 to 168 m) according to dune extension and coast morphology (see Tordoni et al [42] for more details). Vascular plants occurring within each plot were identified following Pignatti [48], and nomenclature was standardized with the Taxonomic Name Resolution Service ( [49]; http://tnrs.iplantcollaborative.org/).…”

“…where C alien is the alien cover in the plot and C total was obtained summing up native and alien species cover. This procedure was necessary to exclude eventual empty spaces and make CWM values independent from I norm [42]. Functional composition niches (i.e., region of the functional space containing all the trait combinations displayed by a selected sampling unit) were computed using the trait probability density approach (TPD, [65]), which reflects the probability of observing different trait values in a given unit (e.g., species, community, region), thus incorporating the probabilistic nature of functional niches.…”

“…Biotic interactions (i.e., competition and facilitation) may become more and more important along the subsequent stages of the invasion continuum [31]. One of the main consequences of the invasion processes could be the disruption and homogenization of the native resident community [92][93][94][95][96], resulting in a loss of species and a reduction in the functional space [42]. Probably, accounting for phylogenetic signal and intraspecific trait variation along with the use of long-term experiments [78] may help to comprehensively understand invasion dynamics, even though not all traits are phylogenetically conserved.…”

“…To the best of our knowledge, only a few studies aimed at disentangling functional diversity patterns using mechanistic traits (see, for instance, [41] for a subtropical forest; [42] for a coastal ecosystem), but these have never been used to test for functional niche overlap between native and IAS and, in particular, to infer how the functional space determined by native species could influence community invasibility in two sites characterized by a different degree of resource availability. In this study, we aimed at (i) determining the functional strategies of native and invasive assemblage (i.e., environmental filtering hypothesis vs. niche divergence) and (ii) assessing the combination of community-weighted functional traits of native assemblage related with vulnerability to the invasion of natural plant communities.…”