# Vulnerability and resilience of rainforests to plant invasions in a tropical island of the south pacific: a sixteen-year survey

Jean-Yves Meyer, Solène Fabre Barroso, Louise Mazoyer, Méryl Jordan, Julie

Fraisse, Marie Fourdrigniez, Ravahere Taputuarai, Robin Pouteau

► To cite this version:

Jean-Yves Meyer, Solène Fabre Barroso, Louise Mazoyer, Méryl Jordan, Julie Fraisse, et al. Vulnerability and resilience of rainforests to plant invasions in a tropical island of the south pacific: a sixteen-year survey. Biodiversity and Conservation, 2025, 34, pp.3985-3999. 10.1007/s10531-025-03143-5. hal-05288027

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# Vulnerability and resilience of rainforests to plant invasions in a tropical island of the south pacific: a sixteen-year survey

Jean-Yves Meyer¹,²☐, Solène Fabre Barroso³, Louise Mazoyer⁴, Méryl Jordan⁵, Julie Fraisse⁶, Marie Fourdrigniez⁷, Ravahere Taputuarai⁸, Robin Pouteau⁹

Received: 27 August 2024 / Revised: 12 August 2025 / Accepted: 22 August 2025
© The Author(s), under exclusive licence to Springer Nature B.V. 2025

## Abstract

Island biodiversity is considered to be particularly vulnerable to biological invasions. However, the direct impacts of invasive alien plants on native and endemic flora are often difficult to assess. Indeed, invasion by woody species is a relatively slow process, and natural or anthropogenic disturbances that favor some plant invaders, also contribute to biodiversity loss, and might act as confounding factors. Hence, we conducted a long-term monitoring of rainforest composition and structure in the small tropical high volcanic island of Moorea (French Polynesia, South Pacific) over a 16-year (2006–2022) period without any major natural or human disturbances. All endemic, indigenous and alien woody species with a stem ≥ 1 cm in diameter were identified and measured every four years in ten 20 m × 20 m plots. The variation of species richness, Shannon diversity, Pielou evenness and abundance over time was analyzed using generalized linear mixed-effects models. Results revealed an increase in the number of stems of alien plants (+50% in 16 years), especially the two most common invasive alien trees, *Miconia calvescens* (Melastomataceae) and *Spathodea campanulata* (Bignoniaceae). In contrast, we found a decrease in the richness (from 28 species to 19) and abundance (−20% of stems) of endemic and indigenous species, especially in the higher-elevation plots. Three plots remained relatively stable in terms of invasive species diversity according to Shannon and Pielou indices, and some common indigenous and endemic woody species persisted over time. Although a continuous decline of plant diversity, including species extirpation, was observed, our results also suggest a relative resilience of certain forest types and associated communities to plant invasions. Efforts to control invasive alien plants should therefore target the most vulnerable native habitats.

**Keywords** Disturbance · Endemic flora · Homogenization · Long-term monitoring
**Introduction**

Island terrestrial biodiversity is considered particularly vulnerable to global and local envi-
ronmental changes, mainly to biological invasions (Brockie et al. 1988; Loope and Mueller-
Dombois 1989); Kueffer et al. 2010; Russell et al. 2017; Fernández-Palascios et al. 2021).
The invasion by cosmopolitan alien (or introduced) species combined with the extirpation
or extinction of indigenous (or native) and endemic ones (a process termed “biotic homog-
enization”, Holden et al. 2004; Castro and Jaksic 2008) threatens the distinctiveness of
island biotas, with potentially serious ecological and evolutionary consequences (Yang et
al. 2021). However, the direct impacts of invasive alien plants on endemic and native flora
are often difficult to assess, with little evidence that invasive alien plants have led to the
extirpation (i.e., population extinction) or global extinction of indigenous plants (Sax and
Gaines 2008; Jäger et al. 2009; Downey and Richardson 2016; but see Baider and Florens
2011). One reason is probably that invasions by plants, especially by woody plants, are a
relatively slow process because of their growth rate and first age of reproduction compared
to annual herbs or grasses. Some indirect evidences of species extirpations caused by inva-
sive woody plants have been provided through some manipulative experiments comparing
areas where alien plants have been removed with areas where they were left (as controls),
leading to increased species richness and community diversity in treated areas (see refer-
ences in Caujapé-Castells et al. 2010).

Another possible reason might be the difficulty to disentangle the direct impact of plant invaders from that of natural (e.g., cyclones, landslides) or anthropogenic (e.g., logging, intentional or accidental fires) disturbances that may also contribute to habitat changes and biodiversity losses (MacDougall and Turkington 2005). The important role of natural or human-caused disturbances have often been proposed to explain plant invasions in primary forests. Indeed, many invasive alien plants are fast-growing light-demanding, early successfulional pioneer species that require disturbances to invade closed-canopy forests (Hobbs and Huennecke 1992); Fine 2002). Thus, continental tropical rainforests usually show some sort of resilience to plant invasions or lower “invasibility” (Cronk and Fuller 1995; Rejmanek et al. 2005) because of their higher native biota diversity and complexity (but see Peters 2001; Delnatte and Meyer 2012). It is possible that closed-canopy forest ecosystems found in islands may also be more resilient to plant invasions. Therefore, long-term vegetation monitoring appears crucial to study island forest dynamics and vegetation changes in the presence of invasive alien species (Barbé et al. 2015; Li et al. 2015; Florens et al. 2017; Denslow et al. 2024) but also according to disturbance regime and forest types.

Here, we monitored rainforest dynamics in 10 plots established in the small tropical
high volcanic island of Moorea (French Polynesia, South Pacific) over a 16-year period
(2006–2022) to address the following questions: (1) How do woody invasive alien species
alter the species richness, diversity and abundance of endemic and indigenous plants over
a relatively long period of time (16 years), i.e. how vulnerable are island forest ecosystems
to woody plant invasions? (2) Does this effect vary among different forest types and
associated plant communities, depending on their relative resilience to natural and human
disturbances? Those issues are crucial to prioritize control efforts and to identify the most
vulnerable habitats to plant invasions.
# Methods

## Study area

Moorea is a high volcanic island (17°32′33″S, 149°49′50″W) located ca. 18 km to the north-west of Tahiti, the main island of the Society archipelago in French Polynesia, South Pacific Ocean (Fig. 1A). The island is small (ca. 135 km²), between 1.15 and 2.45 My old, and its highest summit reaches 1,207 m (Mt Tohiea). The climate is tropical oceanic with a relatively dry and cool season between April and October and a warm and rainy season between November and March, when cyclones may occasionally occur (Laurent and Maamaatuaiahutapu 2019). The native vascular flora of Moorea comprises 300 species, includ-

<figure><img src="image_3.png" /><figcaption>**Fig. 1** A Location of the island of Moorea (Society archipelago, French Polynesia) in the South Pacific Ocean. B Location of the 10 permanent plots on the three study sites (Google satellite image)</figcaption></figure>
ing 122 ferns and 178 flowering plants (Meyer et al. 2015). Among them, 76 species are endemic (17 to French Polynesia or to Southeastern Polynesia, 47 to the Society Islands, six to the two islands of Tahiti and Moorea, and six are single island endemics). At least 767 alien plant species have been introduced on the island and half of them (327) are now naturalized, including 182 introduced as ornamentals or cultivated, and 145 agricultural weeds, adventives, and ruderals. The most dominant woody species include the large *Falcataria moluccana* (syn. *Albizia moluccana*, Fabaceae) and the small tree *Leucaena leucocephala* (Fabaceae) in dry to mesic forests, and the large Africa tulip tree *Spathodea campanulata* (Bignoniaceae), and the small tree *Miconia calvescens* (Melastomataceae) in wet forests. The alien flora of Moorea also exceeds its native flora in terms of distributional range since 17% of the island are highly invaded, 45% are currently occupied by “hybrid” habitats where indigenous and endemic plant species co-occur with alien species, and primary forests occurs on only 6% (8 km²), mainly on the highest summits of Mt Tohiea, Mt Rotui (899 m) and Mt Mouaputa (830 m) (Meyer et al. 2015).

## Permanent plots

Ten 20 m × 20 m plots, demarcated by small iron poles at each corner, were set up in 2006 in three sites of the island of Moorea, and accessible by hiking trails (Chevillotte et al. 2014): Opunohu (OPU), Vaianae (VAI) and Mouaroa (MOU) (Fig. 1B). These plots are located at elevations ranging from 230 to 500 m. The vegetation is described as tropical low- to mid-elevation rainforests, also called « hygrophilous forests » (Florence 1993), or « valley forests » and « ridge forests » in Tahiti and Moorea (Mueller-Dombois and Fosberg 1998). Average annual temperature over the plots is ca. 25 °C and annual precipitation ca. 3,000 mm (Laurent and Maamaatuaiahutapu 2019). The OPU site is made of valley forests on the north side of the island, the VAI site harbours a similar vegetation on the south side and the MOU site is composed of ridge forests between both valleys (Fig. 1B). The composition and structure of woody species in each plot was first described in 2006 (Chevillotte et al. 2014). This first study showed that the three sites differ in their level of invasion, with OPU and VAI being more heavily invaded both in the canopy and understory strata and MOU having more abundant indigenous and endemic species (Table 1). Since their establishment, the plots were monitored every four years until 2022 (2006, 2010, 2014, 2018 and 2022), i.e., for a period of 16 years.

## Woody species richness, diversity and abundance

All endemic, indigenous and alien woody species with a stem diameter at breast height (taken at 1.3 m) ≥ 1 cm were identified. The number of species identified in each plot (alpha diversity) and the total number of species in the 10 plots (gamma diversity) were analyzed over time as well as two diversity indices:

* the Shannon index (Shannon and Weaver 1949), defined as: $H' = \sum_{i=1}^{S} p_i \log_2 p_i$ where S stands for the number of species and $p_i = n_i/N$ the relative importance of species;

* the Pielou evenness index (Pielou 1966): $J' = \frac{H'}{H'_{max}} = \frac{H'}{\log_2(S)}$
<table><caption>**Table 1** Location and characteristics of the 10 permanent plots</caption><thead><tr><th>Site</th><th>Plot name</th><th>Latitude S</th><th>Longitude W</th><th>Elev. (m)</th><th>Slope (°)</th><th>Forest type</th><th>Dominant woody species (by order or abundance)</th></tr></thead><tbody><tr><td rowspan="3">Opunohu</td><td>OPU 1</td><td>17°32′36.24″</td><td>149°49′46.56″</td><td>250</td><td>8–12</td><td>Valley forest (north side)</td><td>Spathodea campanulata**-Neonuclea forsteri-Talipariti tiliaceum forest with Ixora mooreensis-Cyclophyllum barbatum in the understory</td></tr><tr><td>OPU 2</td><td>17°32′37.32″</td><td>149°49′47.64″</td><td>230</td><td>8–10</td><td></td><td>Neonauclea-Barringtonia asiatica forest with Ixora-Cyclophyllum</td></tr><tr><td>OPU 3</td><td>17°32′48.48″</td><td>149°50′22.56″</td><td>300</td><td>0–5</td><td></td><td>Spathodea**-Aleurites moluccana* forest with Miconia calvescens**-Talipariti</td></tr><tr><td rowspan="3">Mouaroa</td><td>MOU 1</td><td>17°32′50.64″</td><td>149°50′33.36″</td><td>420</td><td>15–25</td><td>Ridge forest</td><td>Neonauclea-Talipariti forest with Ixora-Syzygium malaccense*</td></tr><tr><td>MOU 2</td><td>17°32′49.20″</td><td>149°50′44.16″</td><td>480</td><td>30–40</td><td></td><td>Spathodea**-Neonauclea forest with Miconia**-Crossostylis biflora</td></tr><tr><td>MOU 3</td><td>17°32′49.20″</td><td>149°50′54.96″</td><td>500</td><td>30–40</td><td></td><td>Pterophylla (syn. Weinmannia) parviflora-Astronidium cf. fraternum forest with Miconia**</td></tr><tr><td rowspan="4">Vaianae</td><td>VAI 1</td><td>17°33′04.32″</td><td>149°50′29.47″</td><td>230</td><td>12–18</td><td>Valley forest (south side)</td><td>Spathodea*-Neonauclea forest with Miconia**</td></tr><tr><td>VAI 2</td><td>17°32′53.81″</td><td>149°50′32.10″</td><td>340</td><td>0–5</td><td></td><td>Neonauclea-Talipariti forest with Miconia**-Ixora</td></tr><tr><td>VAI 3</td><td>17°33′06.41″</td><td>149°50′30.48″</td><td>260</td><td>8–12</td><td></td><td>Spathodea**-Neonauclea forest with Miconia**-Syzygium*</td></tr><tr><td>VAI 4</td><td>17°32′53.52″</td><td>149°50′25.08″</td><td>380</td><td>20–30</td><td></td><td>Spathodea**-Neonauclea forest with Talipariti-Miconia**</td></tr></tbody></table>

Forest types with dominant woody species found in canopy and in the understory are taken from Chevillotte et al. (2014). *naturalized alien species (ancient Polynesian introduction); **invasive alien species (modern introductions). Plant species names are according the Inventaire National du Patrimoine Naturel (INPN) taxonomic repository (https://inpn.mnhn.fr)

The Shannon index considers both species richness and evenness and represents the uncertainty with which we can predict which species will be one randomly selected individual in the community. If the community contains only one monodominant species, the uncertainty is zero then the more species-rich the community is, the more uncertainty increases. The maximum value of *H*′ for community of given richness occurs at situation where all species represent the same relative proportion. The Pielou index is the ratio of the observed value of Shannon index to its maximum value. It is minimum (*J*′ = 0) when one species dominates.

Diversity indices were calculated using the *diversity* function of the R package *vegan* (Oksanen et al. 2019). Woody plants were also measured and their cumulative basal area (in cm²/m²) in each permanent plot was calculated every four years between 2006 and 2022.
We analyzed how species richness, diversity and abundance (expressed in both num-
ber of stems and basal area) of endemic, indigenous, naturalized and invasive alien plants
changed from 2006 to 2022. We built generalized linear mixed-effects models with a Pois-
son distribution for count data (richness and abundance) and linear mixed-effects models for
diversity indices. Richness, diversity and abundance were included as response variables,
the interaction between time and sites as explanatory variable and identities of the plots as
random factors. Mixed-effects models were fitted using the R package ‘glmmTMB’ (Brooks
et al. 2025). All data analyses were computed with the R software (R Core Team 2019).

Results

Changes in woody species richness and diversity

A total of 38 woody plant species were observed in the ten 400 m² permanent plots at the beginning of our survey in 2006, including 15 endemic species, 13 indigenous species (excluding endemics), eight naturalized alien species and two widespread invasive alien trees: *Miconia calvescens* (Melastomataceae) and *Spathodea campanulata* (Bignoniaceae). After 16 years, we observed a decrease in the gamma diversity of endemic and indigenous species (falling from 28 species to 19) whereas the number of naturalized and invasive alien species remained stable (Fig. 2, Table S1). Four endemic and five indigenous woody species have been extirpated in 2022 (Table S1): the endemic trees *Macaranga attenuata* (Euphorbiaceae), *Meryta lanceolata* (Araliaceae) and *Myrsine ovalis* (Myrsinaceae), the endemic shrub *Cyrtandra cf. mooreaensis* (Gesneriaceae), as well as the indigenous trees *Serianthes myriadenia* (Fabaceae), *Tarenna sambucina* (Rubiaceae) and the shrubs *Boehmeria virgata* (Urticaceae), *Eugenia reinwardtiana* (Myrtaceae) and *Piper latifolium* (Piperaceae).

The alpha diversity reflected the same tendency with a significant decrease in the number of endemic and indigenous species, especially for the higher-elevation plots (MOU), and no variation in the number of naturalized and invasive alien species (Fig. 3). However, the Shannon and Pielou indices did not change significantly with time, indicating a stable diversity and equitability over time (Fig. 4).

Changes in woody plant abundance

The number of stems of the two dominant invasive alien trees, *M. calvescens* and *S. cam-
panulata*, increased by 50% over this 16-year vegetation survey, and this was associated
with a 20% loss in the number of stems of endemic species, but these changes were found
not to be significant across the 10 plots (Figs. 5 and 6). A similar tendency was observed
with abundance as estimated with basal area (Fig. S1). Some woody species suffered serious
population decrease such as the common endemic small tree *Glochidion manono* (Euphor-
biaceae) and the indigenous trees *Crossostylis biflora* (Rhizophoraceae), *Alstonia costata*
(Apocynaceae), as well as the tree fern *Cyathea affinis* (Cyatheaceae) especially in the
higher-elevation plots located between 400 and 500 m elevation in upper rainforest (Table
S1). The conjunction of the increasing abundance in invasive alien species and the decreas-
ing abundance in some endemic species suggests a slow ongoing biotic homogenization
(Fig. 6).
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Change in species richness of the ten 400 m² plots combined (gamma diversity) between 2006 and 2022 by separating species based on their biogeographical status (END=endemic species, IND=indigenous species, NAT=naturalized alien species, INV=invasive alien species)</figcaption></figure>

# Discussion

## Forest vulnerability versus resilience to plant invasions

We monitored over a period of 16 years the composition and structure of 10 low- to mid-elevation rainforest plots in the island of Moorea. We found a general trend toward biotic homogenization associated with an increase in the abundance of the two dominant invasive alien species (Table S1), *M. calvescens*, a shade-tolerant small tree with fleshy fruits actively dispersed by native and non-native birds (Meyer 1998; Spotswood et al. 2012), and *S. campanulata*, a large tree with wind-dispersed seeds whose seedlings are also able to established in closed-canopy rainforests (Larrue et al. 2016), both introduced as garden ornamentals in the early 1930's in Tahiti, French Polynesia (Meyer and Florence 1996). In response, we revealed the extirpation of several endemic and indigenous woody species, mainly small trees and shrubs found in the understory of these two major plant invaders.

Through which mechanisms invasive alien woody plants replace indigenous ones remains unclear but several well-established hypotheses can serve as a basis for explaining this process. *M. calvescens* and *S. campanulata* are both large-leaved species so that they
A $P < 0.050$, pseudo-R² = 0.21

<figure><img src="image_5.png" /></figure>

B $P < 0.050$, pseudo-R² = 0.34

<figure><img src="image_6.png" /></figure>

C $P = 0.150$, pseudo-R² = 0.72

<figure><img src="image_7.png" /></figure>

D $P = 0.075$, pseudo-R² = 0.20

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

**Fig. 3** Change in species richness (alpha diversity) in the ten 400 m² plots between 2006 and 2022 by separating species based on their biogeographical status (endemic, indigenous, naturalized and invasive alien species). Plots are grouped by site: OPU (three plots), VAI (four plots) and MOU (three plots). Data have been fitted with generalized linear mixed-effects models with species richness as response variable, the interaction between time and sites as explanatory variable and identities of the plots as random factors. Shaded areas represent the 95% confidence interval for significant relationships

are likely to be strong competitors for light (but perhaps also for ground resources), and to inhibit germination and growth of indigenous plant species (Vilà and Weiner 2004; but see Barton and Fortunel 2023). Because they are liberated from their co-evolved natural enemies, they might also be less impacted by herbivores and pathogens than native species (Liu and Stiling 2006). These invasive alien trees could also alter natural interaction networks by disrupting mutualistic associations between native species (involving plant-plant and plant-animal partners) and favour the establishment and maintenance of other invasives in the understory (Dunn et al. 2009; Zhang et al. 2020). The two species are hypothesized
A $P = 0.076$, pseudo-R² = 0.33

<figure><img src="image_10.png" /></figure>

B $P = 0.072$, pseudo-R² = 0.30

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

**Fig. 4** Change in Shannon and Pielou indices in the ten 400 m² plots between 2006 and 2022 for indigenous and alien species taken together. Plots are grouped by site: OPU (three plots), VAI (four plots) and MOU (three plots). Data have been fitted with linear mixed-effects models with diversities as response variables, the interaction between time and sites as explanatory variable and identities of the plots as random factors

to be able to alter the natural disturbance regime (increasing soil erosion for *M. calvescens* and windfalls for *S. campanulata*), which, as aggressive colonizers, might favour their persistence via positive feedbacks at the expense of native species (Nanko et al. 2015). Further studies directly testing these hypotheses should provide additional insights into the impact of invasive alien trees on indigenous plant communities.

Three plots remained relatively stable with regard to diversity indices despite plant invasions. Some common indigenous trees such as *Cyclophyllum barbatum* (Rubiaceae) and *Neonauclea forsteri* (Rubiaceae), a small number of woody species such as the endemic shrub *Ixora mooreensis* (Rubiaceae), the relatively rare endemic trees *Ceodes taitensis* (syn. *Pisonia taitensis*, *Nyctaginaceae*), *Claoxylon taitense* (Euphorbiaceae), *Lepinia taitensis* (Apocynaceae) and *Pittosporum taitense* (Pittosporaceae), also persisted over time (Table S1). This shows the relative resilience of a few number of native forest taxa (Table S1).

The introduction and dispersal of a fungal pathogen to control *M. calvescens* in Tahiti and Moorea since the 2000–2002 period, causing its partial defoliation in the canopy, might also explain the survival of some understory indigenous and endemic shrubs and small trees (Meyer and Fourdrigniez 2011). By examining invasion dynamics of 480 plots over 40 years, Li et al. (2015) showed that native plants more closely related to invasive alien species were more likely to go locally extinct. This remains to be tested on islands and further work needs to be carried out to understand interspecific variations in plant responses to biological invasions, and functional ecology seems to be a promising approach (Barton and Fortunel 2023).
A $P = 0.089$, pseudo-R² = 0.14

<figure><img src="image_13.png" /></figure>

B $P = 0.950$, pseudo-R² = 0.13

<figure><img src="image_14.png" /></figure>

C $P = 0.070$, pseudo-R² = 0.02

<figure><img src="image_15.png" /></figure>

D $P = 0.154$, pseudo-R² = 0.39

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

**Fig. 5** Change in stem number in the ten 400 m² plots between 2006 and 2022 by separating species based on their biogeographical status (endemic, indigenous, naturalized and invasive alien species). Plots are grouped by site: OPU (three plots), VAI (four plots) and MOU (three plots). Data have been fitted with generalized linear mixed-effects models with stem number as response variable, the interaction between time and sites as explanatory variable and identities of the plots as random factors

## The role of natural and anthropogenic disturbances

Cyclones or hurricanes are perhaps the most important and large-scale natural disturbances in tropical islands, with huge impact on forest composition, structure, and dynamics, including the recruitment and spread of alien plant species (Walker et al. 1992; Horvitz et al. 1998; Thomson et al. 2007). However, these extreme climatic events are relatively rare in French Polynesia. Only 23 of them (with wind speed >110 km/h) occurred between 1969 and 2018, i.e. a frequency of one cyclone every two years (Laurent and Maamaatuaiahutapu 2019). Among them, only one cyclone (named “Oli”) hit Moorea in 2010 during our 16-year
<figure><img src="image_18.png" /><figcaption>**Fig. 6** Change in the proportion of stem number and of basal area between 2006 and 2022 according to biogeographical status</figcaption></figure>

survey, causing some windfalls (to which *S. campanulata* was particularly prone) and partial defoliation of the canopy (especially that of *M. calvescens*) (J-Y M, pers. obs.). This cyclone of moderate intensity was detrimental to some endemic woody species, such as *Meryta lanceolata* with observed broken stems or trunks, but has also promoted the seedling recruitment of a few other endemic species, such as the fast-growing and light-demanding pioneer tree *Macaranga attenuata* (unpub. data). Thus endemic, indigenous and alien plants have complex responses to disturbances that can be hardly captured by a simplistic snapshot approach.

Furthermore, long-term vegetation monitoring allows to better document the impact of human disturbances at smaller scales. Although our study sites are located in forested areas without strong anthropogenic impacts (such as logging, agriculture, housing development), few forest changes were caused by human activities near the hiking trails. We noticed a slight decrease in the number of stems of invasive alien species between 2018 and 2022 (Fig. 5) that we attributed to an operation carried out by a local nature protection group (called “Moorea Biodiversité”) aimed at controlling *M. calvescens* in the valley of Opunohu (cut stems were observed during field sampling at the OPU site, J-Y M, pers. obs.). As the majority of our study plots are located off-trails and permanently demarcated by iron poles, we will be able to document any future vegetation changes caused by both natural and human disturbances over a longer period of time.

## Priorities for conservation and restoration

Our findings illustrate that long-term monitoring is crucial to study the impact of invasive alien woody species on forest structure, composition and dynamics according to forest types
and associated plant communities as they may show differences in their resilience to plant invasions. On Moorea, we demonstrated a continuous decline of indigenous and endemic plants, including species extirpation or local extinction, associated with increasingly abundant invasive alien trees in our higher elevation rainforest sites. This phenomenon of biotic homogenization where alien naturalized plants overwhelm the native flora, well documented in many other tropical island forests (see e.g., Meyer 2004; Mascaro et al. 2008; Florens et al. 2016; Wohlwend et al. 2021), could be slowed down, if not completely stopped, only by using adapted and efficient control methods against invasive alien plants (e.g. Jäger 2018 in the Galapagos; Bissessur et al. 2023 in Mauritius), including biological control (e.g. Cybèle et al. 2021 in La Réunion; Meyer and Fourdrigniez 2011 in Tahiti), in order to mitigate the impact of dominant invasive species in the most vulnerable habitats, such as the endemic species-rich upland rain- and cloud forests.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10531-025-03143-5.

**Acknowledgements** We thank Hervé Chevillotte and Dr. Jacques Florence (formerly at IRD, now both retired) for the initial plant surveys conducted together in 2006, and Teva Maruhi (formerly at IRD, now deceased), Kevin Heimanu (formerly at the Délégation à la Recherche) and Elie Poroi (from the “Te Rau Ati Ati a Tau a Hiti Noa Tu” nature protection group) for field assistance in 2010.

**Author contributions** J.-Y. M. selected the study sites and designed the field protocol. S. F. B. and R. P. performed the statistical analyses. All authors conducted the field work, read and approved the final draft of the manuscript.

**Funding** The Délégation à la Recherche de la Polynésie française (Research Department of the Government of French Polynesia) provided the funding support, and the Richard Gump biological research station (University of California, Berkeley), the CRIOBE (EPHE-CNRS-Université de Perpignan) and the Eco-Musée “Te Fare Natura” in Moorea logistical support and accommodation for the monitoring team during the 16 years of survey. Robin Pouteau acknowledges funding by the French National Research Agency through the EDENE project (ANR-22-CE32-0006).

**Data availability** No datasets were generated or analysed during the current study.

## Declarations

**Competing interests** The authors declare no competing interests.

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## Authors and Affiliations

Jean-Yves Meyer<fnref n="1" />,<fnref n="2" />✉, Solène Fabre Barroso<fnref n="3" />, Louise Mazoyer<fnref n="4" />, Méryl Jordan<fnref n="5" />, Julie Fraisse<fnref n="6" />, Marie Fourdrigniez<fnref n="7" />, Ravahere Taputuarai<fnref n="8" />, Robin Pouteau<fnref n="9" />

<figure><img src="image_19.png" /></figure>

Jean-Yves Meyer
jean-yves.meyer@recherche.gov.pf

<footnote n="1">¹ Délégation à la Recherche, B.P. 20981, 98713 Papeete, Tahiti, French Polynesia</footnote>

<footnote n="2">² UMR SECOPOL, UPF, Ifremer, ILM, IRD, 98702, Faaa, Tahiti,, French Polynesia</footnote>

<footnote n="3">³ Fenua Ecologie, B.P. 64564, Faaa, Tahiti 98702, French Polynesia</footnote>

<footnote n="4">⁴ Auzat, Villeneuve d'Allier 43380, France</footnote>

<footnote n="5">⁵ 1633 chemin du Château, Roquemaure 30150, France</footnote>

<footnote n="6">⁶ 2 rue des Vosges, Raedersheim 68190, France</footnote>

<footnote n="7">⁷ BioConsulting, B.P. 50902, Pirae, Tahiti 98716, French Polynesia</footnote>

<footnote n="8">⁸ B.P. 5819, Pirae, Tahiti 98716, French Polynesia</footnote>

<footnote n="9">⁹ Pôle de Protection des Plantes, AMAP, IRD, 7 chemin de l'IRAT, 97410 Saint-Pierre, La Réunion, France</footnote>