Atoll ecological response to simulated crab loss

This work will demonstrate the role healthy land crab populations play in nutrient dynamics for atoll ecosystems, and provide evidence for the benefits of conservation efforts that include recovery and sustainable management of land crab populations in areas where they have been lost.

Research Team: Alexander Wegmann, Charlie Braman, Abigail Youngblood, Finnegan Menn, Hillary Young
Collaborators: Université de Californie, Santa Barbara
Dates: February 2024 - ongoing

Exclosure on Hiraaanea
tupa
Charlie Braman with a trencher
Alex Wegman & Andrew Petit finalizing an enclosure on Tiaraunu

Progress Updates 2025

Main Objectives

This project aims to understand how the loss of terrestrial crabs affects forest ecosystems on Tetiaroa Atoll. By simulating their absence using 3x3 meter exclosures installed in coconut and broadleaf native forests, researchers seek to measure changes in vegetation, soil nutrients -particularly carbon and nitrogen - and invertebrate communities. The goal is to demonstrate the critical role of crabs as bioturbation agents and nutrient vectors between marine and terrestrial systems, and to position them as a conservation lever for atoll ecosystems.

Field Actions and Preliminary Results

In July 2025, Charles Braman and his team continued a study initiated in 2024 across several motu - Tiaraunu, Rimatu’u, Reiono, and Oroatera -installing and monitoring exclosures designed to exclude crabs. Trail cameras were deployed at 12 sites to monitor crab activity, while soil and leaf samples were collected to analyze shifts in carbon and nitrogen isotope signatures. Soil carbon flux robots (fluxbots) were installed inside and outside exclosures to track real-time carbon exchange. Ten-meter transects were used to map crab burrow density, and GPS units traced colony boundaries.

Early results suggest that crab absence alters plant composition - particularly by favoring rare species they typically consume - and reduces nutrient availability in the soil. These observations confirm that crabs are key drivers of soil fertility and forest ecosystem dynamics.

Perspectives

The project will continue through 2027, with annual monitoring to assess long-term exclosure effects. Data will be analyzed in the United States and shared with the Tetiaroa Society to integrate crab management into atoll conservation strategies. Collaboration with The Brando staff is planned -particularly for monthly exclosure checks and data collection -to strengthen local involvement and project sustainability.

This work contributes to a broader understanding of terrestrial crabs’ role in Pacific island ecosystems -often overlooked compared to seabirds. It reinforces the Tetiaroa Society’s mission by identifying crabs as a pillar of atoll ecological resilience.

The Project: 2024

Initial installation of 12 research sites occurred during February and June of 2024 to start a three year study. Areas were selected across a gradient of high to low crab activity to investigate the impact of land crabs on bio-available carbon, nitrogen, phosphorous, and micronutrients throughout the forest ecosystem.

Summary of field activities:

Each site has a fenced exclosure which simulates the removal of land crabs from an area. Fencing was installed, buried into a trench, and wrapped in flashing to prevent crabs from accessing the interior with crabs removed from the interior. A paired unfenced site was established along with each exclosure so the change in nutrient pools in the soil and plants between the “crab-free” exclosure and the paired site where crabs maintain access can be observed over time. Soil samples at the surface and depth were taken in June, and leaves were collected for stable isotope analysis. Preliminary data show areas with high crab densities have higher levels of soil organic carbon, nitrogen, phosphorous indicating crab activity is an important part of making a more nutrient rich soil within atoll forests.

Perspectives:

Robotic sensors have also been deployed at the sites that measure soil carbon flux. These “fluxbots” are a first of their kind technology that create highresolution data of how active the soil is through measuring how much CO2 is produced. These robots will not only elucidate how land crabs affect the soil activity in atoll forests, but also provide the first ever such biogeochemical carbon flux dataset from an atoll.