This Task Agreement addresses the land crab knowledge gap by exploring the effects of land crabs on land-sea connectivity and nutrient cycling. Given that crab populations can quickly recover following restoration activities, this may provide a relatively low-cost and high-impact way to restore ecosystem functionality and ultimately resilience to climate impacts in historically crab-dominated systems. We are confident this research will provide valuable insights into the role of land crabs in atoll ecosystems and will help to inform future conservation and management efforts. Dr. Hillary Young, UCSB-MSI, Dr. Alex Wegmann, TNC, and Dr. Elizabeth Forbes (Yale University) will direct a Graduate Student (Charlie Braman) and Undergraduate Assistant (TBD) in designing and executing a multisite exclosure study simulating removal of land crabs from atoll ecosystems. The team will conduct this work in two relatively pristine and extremely well studied atoll ecosystems: Palmyra Atoll (USA) and Tetiaroa Atoll (French Polynesia). These two sites have minimal present or historical human presence, are ecologically similar with robust land crab populations, and both have been historically colonized by invasive rats and experienced habitat conversion to coconut palm. However, Palmyra Atoll has benefited from nearly two decades of active restoration including a 2011 rat eradication and an ongoing forest management program, whereas similar programs do not exist (coconut palm) or have only just been instigated (rats) at Tetiaroa Atoll. By studying both habitats, we hope to describe the ecological role of land crabs in atoll systems so as to justify and inform conservation action towards the restoration and protection of atolls for nature and people. The preliminary results from the study will be summarized in a report with intent to eventually publish the study in one or more peer-reviewed papers.

By installing a series of fenced exclosures, portions of the forested habitats will be without land crab burrowing, bioturbation, and nutrient deposition. These exclosures can be precisely monitored long-term, allowing for measurement of rapid and long-term impacts of crab population declines. Monitoring the community composition and nutrients within the exclosures, along with nearby paired open habitat where crabs will remain, will enable us to understand the impacts of the loss of these critical behaviors. Three rapid changes anticipated that will be monitored in this proposal will be 1) shifts in forest vegetation community composition (and thus above and below ground carbon pools), 2) changes in plant available nitrogen, and 3) changes in carbon soil flux, the movement of carbon from one pool to another.

Carbon flux rates help signify the biologically available soil organic carbon and carbon off-gassed as carbon dioxide. This respiration fluctuates throughout the day, making true assessments of flux notoriously challenging. However, work in the Young lab has developed low-cost carbon “fluxbots” that can be installed at remote field sites and allow for high-resolution, long-term, carbon flux data throughout each day al. 2023). We will use fluxbots to monitor remotely and in real time, the effects of crab removal on carbon flux dynamics.  

By installing exclosures on multiple islets on both atolls, we can pinpoint which crabs are the keystone species for nutrient movement, allowing for more effectively targeted interventions initially prioritizing these ecosystem engineers. Our proposed study will create the foundation for better informed atoll management interventions, and the knowledge necessary to create another lever to increase atoll resilience through maintaining healthy crab populations.

C. Braman will also analyse seabird distribution and abundance data collected by TNC at Palmyra Atoll. This task is generally related to the land crab nutrient study in that the objective of the seabird data analysis is to estimate seabird nutrient contribution to Palmyra’s island and near-shore reef ecosystems; however, it will be completed parallel to the land crab nutrient study. Dr. Wegmann will work with C. Braman on this task.