Study of alteration of terrestrial ecosystems functioning by three introduced species on a polynesian atoll

This project aims to understand how the presence of rats, yellow crazy ants (Anoplolepis gracilipes), and coconut palms influences the dynamics of Tetiaroa's terrestrial ecosystems, particularly through their effects on land crabs, seabirds, and the overall balance of the motu.

Collaborators: Michael Demortier, UPF doctoral student | Simon Ducatez, IRD research fellow | Jayna DeVore, UPF research engineer
Affiliations: University of French Polynesia, UMR 241 SECOP (UPF, IFREMER, ILM, IRD), French Polynesian Research Delegation, Tetiaroa Society, ANR invecof
Dates: 2025

Update January 2026 : Study of alteration of terrestrial ecosystems functionning by three introduced species on a polynesian atoll

Main objectives:

The disruption of ecosystem functioning following the introduction of invasive alien species is particularly damaging, both for the environment and for economic activities. This project aims to characterize the effects on biodiversity and ecosystem functioning of three species introduced on numerous islands in French Polynesia. Using the Tetiaroa atoll as a natural laboratory, we will focus on the coconut palm Cocos nucifera, which is the target of a management plan on the atoll, the mosquito Aedes polynesiensis, which is the target of a future eradication program in Tetiaroa, and the honeybee Apis mellifera, whose ecological functions on the atolls remain unknown.

This thesis will study certain effects of these species on land crabs, soil fauna, seabird and pollinating insect communities, and on a series of key ecosystem functions (nutrient flow, litter decomposition, pollination). This project will thus facilitate decision-making regarding the management of introduced species by anticipating the likely indirect effects of management measures. Developed for and with Michaël Demortier, a student who aspires to pursue an academic career in the field, and with Hervé Bossin (ILM), this thesis will bridge three ongoing projects led by various organizations with complementary expertise: the InvEcoF project, led by IRD and UPF and financially supported by ANR; the TAME project, led by ILM and Pacific Beachcomber; and the TARP project, led by Tetiaroa Society FP.

Summary of fieldwork:

measuring an egg — Michael measuring and counting brown booby eggs

An inventory of brown booby (Sula leucogaster) nests was carried out on the three motu hosting the largest breeding colonies, namely Horoatera, Tiarauunu, and Tahuna Iti. The aim of this action was to assess the impact of mosquitoes on the reproductive success of the species.

Mosquito capture campaigns were conducted simultaneously within the colonies. Cameras were installed near the nests to document any irritation or stress behaviors related to high mosquito density.

Several parameters were measured, including the distance from the nests to the vegetation line, the size of the eggs laid, and the development of the hatched eggs, from hatching to the flight stage.

Mosquitoes were captured within the colonies of the three motu in triplicate using BG-Pro traps, provided by Hervé Bossin's team (ILM de Paea). Targeted captures were also carried out directly at individual nests using adhesive devices.

Preliminary results:

Between January and July 2025, a total of 590 brown booby (Sula leucogaster) nests were recorded. The distribution of nests varies according to the sites studied, with 176 nests on Tahuna Iti, 265 nests on Horoatera, and 149 nests on Tiarauunu.

Outlook:

The next steps in the project will involve in-depth analysis of the data collected and correlating the nest inventory results with data from mosquito captures to assess their potential influence on the reproductive success of brown boobies (Sula leucogaster).

This analysis phase will be part of a scientific collaboration with Beth Gardner and Sarah Converse's team, aiming to cross-reference ecological parameters measured in the field with indicators of reproductive success.

In addition, a new field campaign is planned for October–November, in connection with the study of Tahinu phenology, in order to integrate seasonal vegetation dynamics into the interpretation of interactions between habitat, mosquitoes, and brown booby reproduction.

Study of alteration of terrestrial ecosystems functionning by three introduced species on a polynesian atoll

Project start: July-August, 2025

The researchers are interested in the complex ecological interactions between these components: the role of seabirds in nitrogen enrichment of soils according to seasons and locations, the ecological functions performed by land crabs in decomposing litter or dispersing seeds, and how these balances change after the elimination of invasive species. The main objective is to assess, in the medium and long term, whether rat eradication allows for natural recolonization by seabirds and a resumption of essential ecological roles by land crabs. This monitoring should also allow for a better understanding of how these two groups together contribute to the functioning and resilience of island ecosystems.

Summary of field activities

Two main missions were carried out in July and late August 2025, bringing together three researchers in the field. These campaigns aimed to ensure post-eradication monitoring of crabs, seabirds and vegetation on several motu of the atoll. In July, the team conducted capture-mark-recapture sessions of crabs on six beaches to estimate population sizes and analyze the morphology of the different species. These surveys, initiated before the 2022 eradication, now make it possible to measure the changes observed since the disappearance of the rats, particularly among ghost crabs. At the same time, the vegetation was characterized along 25 kilometers of coastal transects used since 2021 for monitoring seabirds. The researchers also carried out maintenance on the camera traps—replacing batteries and SD cards—used to monitor the reproduction of the main seabird species, including red-footed boobies, lesser and Pacific frigatebirds, gray-backed terns, brown noddies, sooty terns, masked boobies, and great crested terns.

Camera traps were also installed on the sampled beaches, and acoustic recordings were made for two species of ghost crabs. During the late August mission, vegetation characterization was completed along the last ten kilometers of coastal transects. The researchers recovered the camera traps installed in July and continued monitoring brown booby nests, in collaboration with the team of B. Gardner and S. Converse from the University of Washington. Sites were selected for the ATTRACT program, and several nighttime capture sessions documented the morphology of ghost crabs on two beaches, including one at Tiaraunu, to expand the comparative database. These observations complete a coherent set of ecological monitoring designed to measure the rapid response of terrestrial fauna to the disappearance of introduced predators.

Impacts and preliminary results

Initial results confirm a very positive evolution of terrestrial communities on motu cleared of rats. The diversity and abundance of crabs have increased significantly, particularly on islets where black rats were previously present. This rapid recolonization illustrates the central role of crabs in soil rehabilitation and the resumption of natural ecological cycles. Among seabirds, several encouraging signs have been observed. A pair of masked boobies successfully bred again, and the chick observed in August was healthy and about to fledge. This reproductive success demonstrates a gradual return to favorable nesting conditions and supports the idea that restoring terrestrial ecosystems directly benefits the marine species that depend on them. These preliminary results suggest that the eradication of rats on Tetiaroa has already led to a significant revitalization of terrestrial ecological networks, and that prolonged monitoring will allow us to better quantify the magnitude and stability of these changes.

Perspectives

The work continues with several complementary research areas. Monitoring the reproductive success of seabirds will continue thanks to the network of camera traps deployed in the colonies. At the same time, Michaël's thesis, currently underway as part of the project, focuses on the functioning of post-eradication ecological interactions: tahinu flowering, leaf dispersal by crabs, and characterization of plant-pollinator networks.

A new mission is planned for the end of October 2025, primarily to welcome two teams of journalists wishing to document the ecological effects of the restoration of Tetiaroa. This mission will also be an opportunity to check and replace the SD cards and batteries of the automatic cameras installed on the seabird colonies. All of this work is part of the long-term program aimed at monitoring the ecological trajectory of the motu and better understanding the resilience processes of island ecosystems after the eradication of invasive species.