Tropical seabirds play a key role in the ecosystems of coral reefs, and in turn depend on islands associated with these reefs in order to reproduce. To improve conservation of these birds, researchers are calling on the scientific community to develop international communication networks devoted to studying and protecting these species and ecosystems.
Coral islands are formed by the accumulation of sediment debris from coral reefs. Varying widely in shape and size, they occur in most reef ecosystems in the tropical belt, and are home to unique communities of plants and animals, including some of the largest colonies of tropical seabirds in the world. Recently, studiesconducted by IRD in New Caledonia: a).- Lorrain, A., Houlbrèque, F., Benzoni, F., Barjon, L., Tremblay-Boyer, L., Menkes, C., ... & Vidal, E. (2017). Seabirds supply nitrogen to reef-building corals on remote Pacific islets. Scientific Reports, 7(1), 3721. and b).- Thibault, M., Houlbreque, F., Duprey, N. N., Choisnard, N., Gillikin, D. P., Meunier, V., ... & Lorrain, A. (2022). Seabird-derived nutrient supply modulates the trophic strategies of mixotrophic corals. Frontiers in Marine Science, 1927.1 conducted in the Pacific and Indian Oceans have revealed the key role played by these seabird populations in keeping the reefs healthy.
Many species of tern that visit coral islands have highly mobile colonies. These species may breed on different sites from one year to the next, so monitoring and managing them is a complex challenge.
© Tristan Berr
"When the seabirds' droppings, called guano, are deposited on the islands, this adds nutrients to the surrounding coral ecosystems, which is good for their health," explains Tristan Berr. Tristan is a PhD student in the Joint Research Unit comprising ENTROPIE (Tropical Marine Ecology of the Pacific and Indian Oceans) in Nouméa, New Caledonia, and IMBE (Mediterranean Institute of Marine and Terrestrial Biodiversity and Ecology) in Aix-en-Provence, France.
A virtuous yet fragile circle
On coral islands, these benefits are reciprocated: reefs provide the foundations on which the islands form, and the islands are used by seabirds as breeding sites. "Consequently, we realise that a decline in seabird numbers and/or a deterioration in the reefs or coral islands can weaken all the components of this ecosystem through a vicious circle effect: a shrinking bird colony will affect the health of the neighbouring reefs, which may cause the corals to deteriorate more quickly and interrupt the sedimentary mechanisms that maintain the islands… meaning that the birds will no longer be able to breed on them", the scientist continues.
The white tern, in this image captured on the small island of Nukufetau (Wallis and Futuna) spends time on rocky islands as well as coral ones. It is likely that this versatility protects the tern from some effects of climate heating.
© Tristan Berr
Despite their ecological importance, the seabird communities of coral islands have not yet been studied on a large scale. "We know little about the size and geographical distribution of these bird populations, and without reliable estimates, it is very difficult to make a practical assessment of how they contribute to the reef ecosystems, i.e. whether it's through the supply of nutrients, regulation of the food chains, etc. However, this contribution is already threatened by some of the consequences of climate change, particularly rising sea levels coupled with ocean warming and acidification. To sum up, we know that something will be lost to us if these species and environments disappear, but we don't know exactly what this loss means in ecological and socioeconomic terms," says Tristan Berr.
Insufficient sharing of data
There are multiple reasons for this knowledge gap: teams monitoring these bird colonies in the tropical belt do not share the data they gather; isolation of the small islands, which renders them difficult to access for scientific trips; limited research infrastructure and logistical resources in many tropical island states, where a number of these islands are located, etc.
Coconut trees on Solitaire, Poindimié, New Caledonia, in 2020. Over the last ten years, this small island and its two neighbours have been subjected to severe erosion, which has damaged their vegetation and reduced their capacity to host seabird colonies.
© Tristan Berr
In response to this situation, a team of scientists, coordinated by members of the IRD (the French National Research Institute for Sustainable Development) in Nouméa, New-Caledonia, has just published an appeal asking the scientific community to conduct more sustained and coordinated work on these issues. "We urgently need to remedy this lack of information, to find out how best to direct future efforts and strategies for stewardship of tropical seabirds," explains Éric Vidal, IRD research director within the ENTROPIE Joint Research Unit. The Coral Sea, which includes the waters off the east coast of Australia and around New Caledonia, offers an example of the current administrative shortcomings: there is still insufficient collaboration between the conservation managers here, even though many bird species are common to both regions and some colonies are probably demographically connected.
Steps toward a global information database
Thanks to projects they are already running, the ENTROPIE and IMBE Joint Research Units can take action at the heart of these research issues. Over the last five years a database has been created. At the moment it mainly comprises information gathered during field trips to France's Pacific Overseas Territories (New Caledonia, French Polynesia, Wallis and Futuna, and Clipperton). This kind of tool could absolutely be used to centralise worldwide data, in a standardised format, and subsequently share the information, for large-scale scientific analyses and/or to enlighten and provide inputs for managers of Marine Protected Areas. "Through different collaborations that we are setting up gradually, we should very soon be able to link data from sites in the Indian Ocean, and also from locations off Australia's East and West coasts. This 'movement', dubbed CORIS (COral Reef Island Seabirds), is an example of the kind of initiative we are keen to promote and, in the coming months, make known to the scientific community and managers of coral islands. No database of this kind – with this ambition and such a high level of accuracy – currently exists at global level," the ecologist enthuses. This ambitious project is part of the global movement to share and open access data for better use: to inform decision-makers and public actors and work together towards the sustainability of ecosystems.
What future for Clipperton Atoll (Western Pacific) and the colony of masked boobies it hosts? Isolated, it is partly protected from human-made disturbance, but it is not shielded from overfishing, plastic pollution and global changes.
© Tristan Berr
Seeking predictive insight
Researchers envisage developing another project: to predict the future of coral islands and their biodiversity on a scale of 30, 50 and 100 years. Are these islands, and the colonies of seabirds they host, doomed to disappear? In this case, does it make sense to try and save them, in spite of everything? Conversely, will they at least partly withstand the changing climate, and if so, what policies and strategies could be deployed for their stewardship and conservation? "Seabirds are known as long-lived species, because they have a long lifespan and they reach sexual maturity several years after birth. Consequently, it often takes a long time before the impact of a stewardship measure can be seen: it is therefore urgent to identify the appropriate method! " concludes Tristan Berr.
