In our Research Highlights blog series, we debut newly published fisheries research by our women of fisheries colleagues. If you have research you would like to highlight and share with our readers, submit a nomination form here!

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This Month’s Research Highlight:

Gunn, Rachel L., Cassandra E. Benkwitt, Nicholas A. J. Graham, Ian R. Hartley, Adam C. Algar, and Sally A. Keith. 2023. Terrestrial invasive species alter marine vertebrate behaviour. Nature Ecology & Evolution 7:82–91.

This month’s research highlight takes us to the Chagos Archipelago, a group of islands located 500 km south of the Maldives. Picture beautiful remote tropical islands surrounded by coral reefs in the middle of the Indian Ocean.

And rats.

Lots of them.

Black rats Rattus rattus call more than half of these islands home. They were brought here by ship in the 1700s and have wreaked havoc on the islands’ terrestrial ecosystem ever since. But as you will soon learn, these effects have even broader implications.

As a PhD student at Lancaster University in the U.K., Dr. Rachel Gunn was conducting research on reef fish behavior in response to environmental changes. It was through this work that she came up with the hypothesis that nutrient input changes resulting from the presence of black rats (more on that in a second) may change reef fish behavior. The story goes something like this. When black rats established on these islands, the seabird populations – and all the nutrients they provided to the surrounding waters – declined so much that the food quality on the reef diminished. With less quality food resources available, reef fishes, like the jewel damselfish Plectroglyphidodon lacrymatus had to weigh the cost of defending their territories.

Dr. Gunn explains this in terms of the economic defendability model. “If food resources are low, then it is too energetically costly to invest in aggressive defense. If food resources are very high, then individuals can access all the food they need without having to aggressively defend their resources from intruders. The model predicts that aggressive defense will take place only where the benefits of defending a territory are higher than the costs, or where the benefits are higher than the benefits of not being territorial.” In this case, it was hypothesized that it was energetically advantageous for fish on reefs with higher quality resources to aggressively defend smaller territories; whereas with poorer quality resources, the cost was too high and so they must resort to larger, less defended territories instead.

For this study, Dr. Gunn and her colleagues used underwater cameras to document the behavior of the jewel damselfish inhabiting reefs around ten islands in the Chago Archipelago, five with rats and five without rats. In this system, seabirds on rat-free islands provide over 250 times more nitrogen to the surrounding reef systems than on rat-infested islands. Their research yielded some amazing findings that supported their hypotheses. Dr. Gunn explains, “…it is the first time that the behavior of reef fish has shown to be affected by the presence of a terrestrial species in a different ecosystem.” Even though the amount of turf algae did not differ between the islands, the quality of that food resource did. With a more nutritious food supply, fish in the rat-free island reefs could have smaller territories and had the energy available to defend these territories. Ultimately, it was a case of quality over quantity. 

What makes the jewel damselfish an ideal study species?  Dr. Gunn explains that it is very abundant on these reefs, has been the subject of previous island seabird studies, and has territories small enough to record behavior with stationary underwater cameras. It is considered a “farmer” that not only feeds on turf algae within their territory but actually takes care of it. “Similar to how we would take care of a houseplant!” states Dr. Gunn.

Although less visible beneath the surface, this study illustrates nature’s rippling effect. The introduction of black rats completely transformed not just the island ecoscape but its effect rippled out to the adjacent reef systems and changed them in ways scientists are only now beginning to understand. As Johann Wolfgang von Goethe writes, “In nature we never see anything isolated, but everything in connection with something else which is before it, beside it, under it and over it.” Still, these jewel damselfish keep on “farming” – albeit in a completely different way now.

As a post-doc at Tuebingen University, Germany, Dr. Rachel Gunn will be continuing her work on the behavior of reef fishes in the face of environmental changes.

The full manuscript can be found and downloaded here:
doi.org/10.1038/s41559-022-01931-8 (open access)