Coral ‘clumpiness’ is a superior indicator of stress effects on coral
Coral reef ecosystems – often called ‘the oceans’ rainforest’ – are home to at least 25 % of marine life despite covering only about 1 % of the global seafloor. They provide ecosystem services valued up to EUR nine trillion annually. Their degradation has been documented since the 1980s, mostly by measuring a decreasing percentage of coral cover. New approaches providing deeper insight are urgently needed. With the support of the Marie Skłodowska-Curie Actions (MSCA) programme and collaboration between the University of Utrecht and the Pontifical Catholic University (ECIM) in Chile, the INDECOSTAB project developed a mathematical model of coral ecosystems to get a closer look. MSCA fellow Alexandre Génin investigated observed changes in spatial organisation as an indicator of stress and predictor of large, irreversible change.
Building on experience, expanding data collection
Génin studied corals at Easter Island (Rapa Nui), leveraging the tremendous experience and historical data of Chilean colleagues on the island as well as the crystal-clear water. Together, these factors enabled simple and fast survey of large areas using cameras from a boat. In fact, without his Chilean colleagues, it would not have been possible to quickly conduct his only possible field campaign near project end as a result of COVID restrictions. “We surveyed three quarters of the coastal areas of Easter Island in a week, something that would have taken experienced divers months to do not long ago,” Génin says.
Coral reefs: resilience in a fragile ecosystem
The corals at Easter Island are among those that can show a ‘positive feedback’ loop in response to overgrowth by algae. They can survive for decades by creating cracks that harbour herbivores like fish and sea urchins, which eat the algae and create space for new corals to form. Recently, it was shown that this results in the creation of new colonies mostly near existing ones, creating ‘clumps’ on the seafloor. Génin set out to model this patchiness and determine if the model could predict large, abrupt transitions based on these clumps. He also investigated the impact of herbivores on these clumps.
Modelling change in spatial indicators
INDECOSTAB’s mathematical model confirmed that the patchiness of corals can be used as a spatial indicator of stress and of potentially irreversible degradation. Interestingly, the model’s spatial indicators were not affected by the number of herbivores – mostly sea urchins at Easter Island –, a result confirmed by the field observation data. Similar support for spatial indicators of stress emerged from INDECOSTAB’s field studies. “Of the two species of corals on Easter Island, our field surveys revealed that one species’ abundance decreased significantly with wave exposure while the other remained the same. However, they both showed changes in patchiness, so it is not enough to measure conventional change in coral cover,” notes Génin. “Our work stems from very theoretical ideas about the spatial structure of coral reefs yet conclusions correspond to those of applied ecologists who use population models to forecast the impact of climate change on reefs,” he adds. INDECOSTAB plans to release its data soon. This will support continued surveys and improved models to track and predict the state of these fragile ecosystems, leading to improved sustainable management.
Keywords
INDECOSTAB, coral, corals, spatial indicators, algae, ecosystems, coral reefs, mathematical model, population models, stressors
