Ferns are the only major lineage of vascular plants that have nutritionally independent sporophyte (diploid) and gametophyte (haploid) life stages. However, the implications of this unique life cycle for fern community ecology have rarely been considered. To compare patterns of community structure between fern sporophytes and gametophytes, we conducted a survey of the ferns of the islands of Moorea and Tahiti (French Polynesia). We first constructed a DNA barcode library (plastid rbcL and trnH‐psbA) for the two island floras including 145 fern species. We then used these DNA barcodes to identify more than 1300 field‐collected gametophytes from 25 plots spanning an elevational gradient from 200 to 2000 m. We found that species richness of fern sporophytes conforms to the well‐known unimodal (i.e., mid‐elevation peak) pattern, reaching a maximum at ~1000–1200 m. Moreover, we found that fern sporophyte communities become increasingly phylogenetically clustered at high elevations. In contrast, species richness of fern gametophytes was consistent across sites, and gametophytes showed no correlation of phylogenetic community structure with elevation. Turnover of sporophyte and gametophyte communities was closely linked with elevation at shallow phylogenetic levels, but not at deeper nodes in the tree. Finally, we found several species for which gametophytes had broader ranges than sporophytes, including a vittarioid fern with abundant gametophytes but extremely rare sporophytes. Our study highlights the importance of including diverse life history stages in surveys of community structure, and has implications for the possible impacts of climate change on the distribution of fern diversity.

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Inherent characteristics of island species make them particularly susceptible to anthropogenic changes and need to be assessed to implement appropriate conservation strategies. The impacts of climate change are increasingly being investigated along elevational gradients since they provide natural laboratories to study how species respond to climatic variation. Ferns are particularly sensitive to air humidity and temperature and are therefore potentially useful as bio-indicators. This study addresses the question of whether the distributions of fern species richness and abundance have climatic correlates along an elevational gradient on the tropical volcanic island of Tahiti (French Polynesia). Analyses were conducted on two datasets: island-wide richness was estimated using published data on species elevational ranges, and local richness and abundance were addressed through a transect survey. Correlations with water availability, temperature, area availability, and a randomly-generated species richness pattern were investigated. Results showed that both diversity and abundance varied in association with climate. Rainfall was collinear with diversity in the lower half of the elevational gradient (R 2 = 0.97), while temperature was the most important climatic correlate Climatic Change for diversity in the upper half (R 2 = 0.98). The number of terrestrial fern individuals and epiphytic fern cover were both correlated with temperature (R 2 = 0.86 and 0.81, respectively). Our results imply that impacts of climate change on ferns on Tahiti might include change in diversity and abundance, and increased extinction risk due to low overlap between current and projected species distributions. Ferns represent important indicator organisms that can be used to study species distributional responses to climate change in island ecosystems.

Long-term monitoring of biological control agents in their areas of introduction is essential to assess their effectiveness. There is a need to monitor and evaluate agent dispersal and impacts so that the degree of success can be quantified or reasons for failure can be clearly understood. A fungal pathogen, Colletotrichum gloeosporioides f. sp. miconiae Killgore & L. Sugiyama (Melanconiales, Coelomycetes, Deuteromycetinae), found in Brazil in 1997 was released in the tropical oceanic island of Tahiti (Society Islands, French Polynesia, South Pacific) to control miconia, Miconia calvescens DC (Melastomataceae), a small tree native to Tropical America, which has invaded native rainforests. The plant pathogen, proven to be highly specific to miconia, causes leaf spots, defoliation and eventually death of young seedlings in laboratory conditions. Two permanent plots in Tahiti (Taravao Plateau and Lake Vaihiria) were monitored for a period of 6 years to assess the pathogen's dispersal and impacts on miconia in the wild. Leaf spots were observed approximately 30 days after inoculation. Percentage of infected plants reached 100% after 3 months, and between 90% and 99% of leaves were infected. Subsequent re-infection occurred after 3 months at Vaihiria and 18 months at Taravao. Mortality rate for monitored plants was 15% and reached 30% for seedlings less than 50 cm tall. Within 3 years, the fungus had disseminated throughout the island of Tahiti and had infected nearly all the miconia plants up to 1400 m in montane rainforests. It was also found on the neighbouring island of Moorea without any intentional release there. Leaf damage on miconia canopy trees increased from 4% to 34% with elevation in permanent plots set up between 600 and 1020 m. Our study showed that rainfall and temperature were two limiting environmental factors that affected fungal spread and disease development. Although this plant pathogenic agent is successfully established, has spread efficiently and has caused significant impacts on seedlings, additional biocontrol agents are still needed to fully control the massive invasion by miconia in the Society Islands.

Success of biological control programs is commonly assessed by studying the direct negative impacts of released agents on the target invasive species. Very few quantitative studies have focused on the indirect positive effects on native biodiversity. In this study, we monitored the response of the plant community (both native and alien species) in permanent plots located in four different sites in montane rainforests of the tropical island of Tahiti (South Pacific) severely invaded over decades by the alien invasive tree Miconia calvescens DC (Melastomataceae), after the release of a defoliating fungal pathogen Colletotrichum gloeosporioides f. sp. miconiae Killgore & L. Sugiyama. Results of five years of monitoring showed that total native and endemic species richness and plant cover increased in all sites and plots. Partial defoliation of miconia canopy trees (between 6% and 36%) led to significant recruitment of light-demanding pioneer species, but also to the appearance of some semi-shade and shade tolerant rare endemic species. Native ferns and angiosperms remained dominant (ca. 80%) in the forest understorey during the monitoring period. Colonization by a small number of alien plant species occurred in one permanent plot located at the lower elevation. We conclude that biological control may be considered a tool for partial habitat restoration and recovery of native and endemic species, but long-term monitoring is needed to confirm the stability and resilience of the ''novel plant assemblage''.