Patterns of plant community assembly in invaded and non‐invaded communities along a natural environmental gradient

Abstract

Questions: Is the stress gradient hypothesis (SGH) effective in predicting\ud patterns of community assembly in coastal dune plant communities along the\ud sea–inland environmental gradient? Does the introduction of invasive plant species\ud disrupt these patterns, leading to a collapse in community structure?\ud Location: Sandy coastal dunes of the Lazio region (Central Italy).\ud Methods:We randomly sampled coastal dune plant species in 2 m 9 2 mplots\ud (4 m2). Multivariate techniques were used to classify these plots and allowed\ud identification of four plant communities along the sea–inland environmental\ud gradient, three of which were invaded by Carpobrotus aff. acinaciformis (iceplant).\ud For each community, we computed two different indices of co-occurrence that\ud take into account species abundance, and then used Monte Carlo permutations\ud alongside appropriate null models to determine whether overlap in the use of\ud space was greater (aggregated community structure) or smaller (segregated\ud community structure) than what would be expected by chance alone.\ud Results: For the four non-invaded communities, the analysis highlighted how\ud community assemblage patterns were strongly tied to the sea–inland environmental\ud gradient. In the two foredune communities, overlap in the use of space\ud was greater than expected by chance, indicating an aggregated assemblage. In\ud contrast, progressing along the zonation, assemblage patterns first shifted to random\ud in the transition dune community and then became segregated in the\ud fixed-dune community. As for the three communities that were also found in\ud an invaded state, the presence of iceplant was associated with a random structure\ud in community assemblage.\ud Conclusions: Taken together, our results are consistent with the SGH, which\ud predicts that along an environmental gradient, facilitation drives the assembly of\ud plant communities where conditions are harsh, whereas competition is predominant\ud where conditions are less severe. The comparison of the three invaded\ud communities with their non-invaded counterparts revealed how iceplant invasion\ud has led to a shift to randomness in community structure, as has been documented\ud forwildfires and other disturbances

Direct and indirect effects of invasion by the alien tree Ailanthus altissima on riparian plant communities and ecosystem multifunctionality

“…Most of the existing studies regarding the effect of invasive plants on biodiversity focus solely on the number of species as a measure of biodiversity ignoring other indicators of biodiversity which are more closely related to ecosystem functioning, such as the diversity of evolutionary histories or phylogenetic diversity (Forest et al 2007;Maherali and Klironomos 2007; see also Santoro et al 2012;Hejda and de Bello 2013;Bennett et al 2014). Numerous important ecological traits, such as co-evolved relationships with N-fixing bacteria or other soil organisms, are preserved through evolutionary times (Flynn et al 2011;Milcu et al 2013).…”

The response of plant community diversity to alien invasion: evidence from a sand dune time series

“…Coastal ecosystems are regarded as threatened by alien invasion worldwide (McLachlan and Brown 2006;Santoro et al 2012). Their high sensitivity to alien establishment seems to be related to natural and anthropogenic factors such as habitat heterogeneity, intensive propagule pressure from ancient times, and frequent and diverse disturbances (Acosta et al 2007(Acosta et al , 2008.…”

Population structure and growth of a non-native invasive clonal plant on coastal dunes in Southern Brazil

“…Conversely, regarding our second hypothesis, although F. foetida plants showed significant increase in cover and number of leaves over the one-year period of the study no difference was found for native species richness, diversity, and cover between invaded and uninvaded plots. Other authors have showed that species richness remained stable in invaded and uninvaded plots by different invasive plant species although some of them only displaced the dominant native species and had no effects on non-dominant species (Sax, 2002;Mason and French, 2008;Powell et al, 2013;Dong et al, 2015). Hejda et al (2009) found that decreases in species richness in invaded plots by different invasive plant species significantly interacted with species-specific differences in cover between the invading and native dominant species.…”

“…Propagules may also arrive on coastal dunes from the ocean especially with high tides (Aptekar and Rejmánek, 2000). Non-native invasive species impacts on costal dune ecosystems include native species displacement, decreasing soil stabilization functions (Kim, 2005), competition for resources (Santoro et al, 2012), and native species richness or diversity decline (Badano and Pugnaire, 2004;Carboni et al, 2010).…”

“…In coastal dune systems, different microhabitats may be distinctly susceptible to invasion by non-native species as a consequence of resource availability and stress level (Carboni et al, 2010;Santoro et al, 2012). In coastal ecosystems, a strong environmental gradient along the sea-inland is mainly due to salinity, wind action, temperature, drought and sand instability (Kolb et al, 2002;Lortie and Cushman, 2007;Santoro et al, 2012). Kolb et al (2002) showed that coastal grasslands which occur in milder conditions (with higher levels of nitrogen and soil moisture) are more susceptible to invasion by nonnative plants than those subjected to harsher conditions (lower levels of nitrogen and soil moisture).…”