Aim To examine how native plant native communities, environment and geography are associated with alien plant species invasion in tropical island forests. Location Four US national parks in Hawai'i and American Samoa. Methods We analysed the richness, coverage and composition of alien plant species communities located across 204 plots, five islands and two archipelagos. We used generalized linear mixed‐effects models and generalized dissimilarity models to determine the relative importance of native plant species communities (richness, structure and composition), environment (temperatures, precipitation, and soil age) and geography (elevation, geographical distance between plots, identity of islands and archipelagos) on alien plant species communities. Results Alien plant species were found in >90% of plots with on average four species covering ~15% of the understorey. Variation in alien species richness and coverage was primarily explained by archipelago and island identity and elevation. Plots located on younger islands tended to exhibit higher richness and coverage of alien plants. Native tree density and canopy height were negatively correlated with alien species coverage but not with alien species richness. Elevation was negatively correlated with both the richness and coverage of alien species. Differences in elevation, mean annual precipitation and geographical distances explained about 40% of the observed spatial turnover in alien species composition with differences in elevation being the main driver. Turnover in alien species composition was weakly correlated with turnover in native species. Main conclusions In the studied islands, higher elevations seem to be relatively spared from alien species invasion so far. However, our results suggest that the negative effect of elevation on alien species coverage may be larger than on alien species richness. Maintenance of tall and dense canopy may limit the coverage of alien species but not the number of alien species. Our results also support that alien species invasion may be particularly high on isolated archipelagos and younger islands.

No abstract

Premise Rarity is a complex and central concept in ecology and conservation biology. Yet, it is still poorly understood why some species are rare and others common. Here, we aimed to understand the drivers of species rarity patterns in woody plant communities. Methods We analyzed the local abundance and landscape frequency of 121 woody plant species across 238 plots on American Samoa and Hawaiian islands. We first assessed whether taxonomy, life form (shrub, small tree, large tree), and dispersal syndrome (dispersed by animals or by other means) are associated with the rarity of species. We then analyzed phylogenetic patterns in plant rarity and tested whether rarity patterns are associated with species evolutionary distinctiveness and the number of species within genera and families. Results Large trees were less abundant but more frequent than shrub species. Animal‐dispersed species tended to be less abundant than species dispersed by other means, while species frequency was not associated with dispersal syndromes. Relative frequency in Hawai′i exhibited a more robust phylogenetic signal than did abundance. Both evolutionary distinctiveness and taxa species richness were significantly associated with the frequency of shrub species in Hawai′i. Conclusions Life form appears consistently associated with the rarity of species. High diversification rate is probably a key factor explaining landscape‐scale rarity of native species on isolated archipelagos like Hawai′i. At the landscape scale, rarity appears to be inversely associated with evolutionary distinctiveness, but at the local scale, species abundance may be not associated with evolutionary distinctiveness.

No abstract