Article: Benkwitt, C. E., Wilson, S. K., & Graham, N. A. (2019). Seabird nutrient subsidies alter patterns of algal abundance and fish biomass on coral reefs following a bleaching event. Global change biology. https://doi.org/10.1111/gcb.14643
It’s getting hot in here, so take off all your algae
Climate change is a serious threat to many ecosystems across the globe. Coral reefs are one ecosystem that is particularly threatened by climate change, however. One way that climate change impacts coral reefs is through bleaching events, which occur when water temperatures get too high. Under normal conditions, there is a mutually beneficial relationship between corals and algae that live on them. The algae get a place to live and in return they provide the coral with food that the algae make through photosynthesis. These algae are also what give coral their bright and beautiful colors. When the water gets too warm, however, the algae leave the coral. When this happens, the coral lose their food source and if the water stays warm enough for long enough, the coral can die. With the algae gone, the coral turns white, which is why this process is called coral bleaching, even though it has nothing to do with the bleach you have in your cleaning supplies at home.
One bird’s poop is a coral reef’s nutrients
Knowing the severity of the threats facing coral reefs, it’s even more important to know what sorts of things might be able to help coral reefs. Having seabirds that nest nearby is one of these things that scientists have found can be beneficial to reefs. Birds can act as a fertilizer for the plants and fish in coral reefs. Birds go out into the open ocean and eat fish. They then come back to the islands next to coral reefs where they not only nest, but also poop a lot. This poop is full of nutrients from the food that the birds ate out in the ocean. The plants and fish that live in the coral reefs can then benefit from these nutrients that end up in the water around the reef.
An opportunity for the birds?
While scientists already know that reefs can benefit from nearby seabirds, the question remains of whether these benefits of the added nutrients from nearby seabirds could help coral reefs to resist and recover from bleaching events. In order to do this, a team of scientists led by Cassandra Benkwitt studied coral reefs around a group of islands in the Indian Ocean before and after a bleaching event that occurred in 2015-2016. They looked at the number of different types of fish and assessed how much of the surface of the reef was covered by coral, algae, or was bare. They compared these numbers at different sites around the islands. In some spots, there were lots of seabirds on the islands next to the reefs. In other spots, there were invasive rats on the island that had driven away the birds. They compared these sites where there were birds to where there were rats to see if the reefs with birds nearby responded differently to the bleaching event than those with without birds nearby.
In the two years following the bleaching event, the scientists observed big changes in both the coral reefs that were near seabirds, as well as the reefs that were near rats. There were big losses of coral in all the reefs, which lost 32% of their coral cover on average. Fish species that eat coral and plankton (which rely on the coral habitat) also saw big drops in their numbers. This suggests that even with birds nearby supplying extra nutrients, the reefs were still greatly impacted by the bleaching events. However, there were also some encouraging results. In the reefs with birds nearby, there was more growth of some types of algae. These bird-adjacent reefs also experienced smaller decreases in the populations of fish whose diets consisted of plants and/or other fish.
Protecting our environment: We cannot waste this waste opportunity
While there are still a lot of questions about some of the complex interactions and feedbacks that happen in these ecosystems, studies like these help to give us an idea of some of the ways that we might better protect them moving into the future. This study found that while the birds don’t necessarily make the reefs more resistant to bleaching, they might help them recover. After a previous bleaching event on the islands in 1998, it took the reefs 10 years to recover, so this study only looked at the first bit of the process. It also means that any action to help grow the bird populations in the coming years, such as programs to get rid of the rats, could also help the reefs to recover at the same time.