Biological invasions drive biodiversity loss and ecosystem change on tropical islands. However, we know little about the implications of species losses on the functional structure of both resident and novel communities. Herein, we examined the potential effect of a non-native palm species, Pinanga coronata, on the taxonomic and functional assemblages of understory plant species in a Fijian rainforest. We predicted that competition from this invasive species would lead to trait convergence according to the competitive hierarchy hypothesis. Using a trait-based approach, we sampled plant communities in 280 plots across a gradient of P. coronata densities. We measured five functional traits, including height and leaf traits related to nutrient acquisition. We found that an increase in P. coronata density is strongly correlated with a decrease in taxonomic diversity (i.e., about − 50% for species richness and − 33% for Shannon diversity index) and a decrease in functional richness. Community-weighted mean values of traits of resident species (i.e., excluding P. coronata) converged toward competitive strategies such as higher leaf nitrogen content (LNC), lower carbon-to-nitrogen (C:N) ratios and leaf dry matter content (LDMC), a pattern that is significantly non-random for LDMC and C:N. This study demonstrates that P. coronata might act as a strong biotic filter responsible for species loss and functional changes. Our findings suggest that in response to increasing competition with this invasive plant, resident and novel plant communities shift toward less diverse and more competitive assemblages. Nevertheless, the intensity of this filtering is habitat dependent (e.g. less filtering effect under mahogany trees). Lastly, changes in resource acquisition strategies (mainly nutrient-based) in particular in low nutrient status of rainforest soils, could lead to long-term impacts on tree regeneration, in turn causing large-scale changes in ecosystem properties.

In island ecosystems, biological invasions are one of the major threats to native biodiversity and to ecosystem functioning. Invasive ornamental plants such as the alien palm tree Pinanga coronata in the Fiji islands can form mono-dominant stands in rainforests and displace native species. Using a functional trait-based approach, we investigated the impact of P. coronata on both above and belowground communities (i.e. plants and Collembola). Within a rainforest reserve on Fiji´s principal island, we sampled a total of 10 invaded and non-invaded plots and recorded five functional traits for plants and six for Collembola. We found that invasion by P. coronata led to a strong and significant decrease of native plants and Collembola taxonomic diversity. Ingress of P. coronata also induced a decrease in the functional diversity of plant communities and to a lesser extent of Collembola communities. P. coronata invasion led to a decrease of leaf carbon-nitrogen ratio (LCN), Leaf nitrogen content (LN), specific leaf area (SLA) and leaf dry matter content (LDMC) of plant communities, suggesting a change in litter properties compared to non-invaded communities. Plots with P. coronata were associated with large Collembola living at the soil surface with more trichobothria and pseudocelli, which are used as defence mechanisms. Using trait-matching, we also found that the strength of plant-soil relationships was higher (i.e. more stable) in non-invaded plots than in P. coronata-invaded plots. Lastly this study suggests that the main mechanism through which P. coronata alters soil communities is a change in plant properties rather than a change in the abiotic environment. Our trait-based approach underlines the negative impact of this alien palm invasion on native rainforest plant and soil fauna in Fiji, and the urgent need for on-the-ground action to conserve terrestrial island biodiversity in Fiji´s rainforests.

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