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Invasive rats topple ecological domino that affects reef fish behavior

Fishes in a coral reef.

Fishes in a coral reef. Image by Renata Romeo / Ocean Image Bank.

  • A recent study reveals that the presence of invasive rats on islands can lead to behavioral changes in fish living on coral reefs offshore. A team of scientists found that damselfish have larger territories that they defend less aggressively on reefs near rat-infested islands.
  • Rats and other rodents often tag along on ships. For hundreds of years, they’ve colonized islands around the world, where they feast on seabird chicks and eggs, decimating local populations.
  • Seabirds deposit nutrient-rich guano on islands, some of which flows out to reefs and fertilizes the growth of algae.
  • Smaller seabird numbers on rat-infested islands mean that fewer nutrients end up on reefs, and the algae there has lower nutritional value than off rat-free, seabird-rich islands. The study’s authors concluded that damselfish were less aggressive near islands with rats because it wasn’t worth the energy to defend a less valuable resource.

Damselfish can be fastidious farmers. They prune and cultivate small patches of algae, often no more than half a square meter (5 square feet) in area, ensuring they have a steady food source. One species in the Caribbean even herds tiny mysid shrimp around their underwater “gardens,” which the shrimp dose with their fecal fertilizer.

It might not come as a surprise, then, that damselfish would relentlessly chase any intruders from their territory looking to snatch a bit of their algae. Behavioral ecologist Rachel Gunn and her colleagues witnessed that sort of “hyper-aggressive” behavior in jewel damselfish (Plectroglyphidodon lacrymatus) living on reefs in the Chagos Archipelago of the Indian Ocean — but only off the coast of islands that didn’t have invasive rats. Around islands infested with rats, aggression toward interlopers waned, and the damselfish had larger territories.

In a study published Jan. 5 in the journal Nature Ecology & Evolution, the team demonstrated for the first time that the presence of invasive rats on islands affects the behavior of fish living on nearby reefs.

In her earlier research, Gunn had been studying how other reef fish respond to the availability of their food. Butterflyfish, for example, spend more time foraging for the live coral they like to eat when it’s not particularly abundant, rather than expend a lot of energy defending a territory large enough to supply them with sufficient food.

A jewel damselfish (Plectroglyphidodon lacrymatus) among the coral reef of a rat-free island. Image by Rachel Gunn.

At the same time, Gunn, who was completing her doctorate at Lancaster University in the U.K., was also studying how invasive rats affect the ecology of islands. The story is complex, and it starts with the raucous colonies of seabirds that roost and raise their young on islands around the world. Seabirds returning to those islands from their fishing forays far out at sea bring back nutrients in their guano. Those nutrients are vital to ecosystems on land — and at sea.

Some of those nutrients wash back onto coral reefs, infusing the system with fertilizer that helps algae grow and boosts its nutritional value. But when invasive species like black rats (Rattus rattus) take up residence on islands after hitching a ride on ships as they’ve done for hundreds of years, they feast on seabird eggs and chicks and decimate island populations.

A 2018 study led by Gunn’s co-author, Nicholas Graham, revealed that rat-free islands have seabird populations around them that are up to 760 times greater than rat-infested islands. With so few seabirds catching fish at sea and returning to deposit their guano on islands, the ecosystem is missing a primary source of nutrients. Graham and his colleagues also showed in 2018 that jewel damselfish grow faster on reefs next to rat-free islands than on rat-infested islands, likely because the bird guano delivers higher-quality nutrients and more of them to the reefs.

Gunn wondered if these dynamics — what a recent paper published in the journal Restoration Ecology calls “a circular seabird economy” — might extend even further, affecting the behavior of fish.

“It made logical sense” that such disruptions could ripple through the ecosystem and change fish behavior, said Gunn, currently an academic research associate at the University of Tübingen in Germany.

To test her hunch, Gunn chose 10 islands in the Indian Ocean’s Chagos Archipelago. Five were rat-infested with few seabirds, and five were rat-free and supported vibrant seabird communities. The team then set up GoPro cameras over damselfish hangouts to get an idea of how aggressively the fish defended their territories.

The difference was clear from the outset. Just the presence of a camera fixed to a PVC structure was enough to trigger a damselfish into attack mode near rat-free islands. Gunn said the first few minutes of footage on reefs off rat-free islands was unusable because the damselfish are so protective of their algal farms. Soon enough, though, they’d figure out the camera wasn’t a threat, and the recordings then showed how unwelcome other passing intruders were.

Another jewel damselfish at a rat-free island. Around rat-free islands, scientists witnessed a “hyper-aggressive” behavior in jewel damselfish. Image by Rachel Gunn.

On reefs ringing rat-infested islands, damselfish were much more docile, even though the algal mats covering parts of the reef appeared similar to those near rat-free islands. That clued Gunn and her colleagues in that damselfish were responding not just to the quantity of the algae, but to its nutritional value.

They then tested the algae for the presence of a specific form of nitrogen called delta-15-N, which is both an indicator that nutrients are coming from seabird guano and linked to higher growth rates in fish. Sure enough, they found more of this nitrogen on reefs near rat-free islands than near rat-infested islands. This evidence helps explain why, off the seabird-crowded islands, it pays off energetically for damselfish to tend and defend a small, nutritionally rich algae farm, Gunn said.

“If you’re a fish next to a bird island, you get more for your money when you’re foraging and feeding,” she said. Conversely, fish have to spend more time grazing over a larger area when the nutritional value isn’t as high, such as on reefs near the rat-infested islands in Gunn’s study.

“It doesn’t make sense for that fish to invest energy in defending something that’s poor,” she added.

“I think this paper makes a really important contribution,” said Dena Spatz, a senior conservation scientist with the Honolulu-based nonprofit Pacific Rim Conservation, who was not involved in the study. Spatz was the lead author of a study published in August 2022 that found that attempts to remove invasive vertebrates from islands between 1872 and 2020 had an 88% success rate.

“I definitely think [this study] bolsters evidence of the importance of removing highly damaging invasive species like rats from islands,” Spatz told Mongabay.

The findings also point to potential changes in the reef’s ecology, she added.

“Once you change the behavior of how an animal interacts with its environment, it opens up that space to lots of other things that can happen,” Spatz said. In the natural systems identified in this study, seabirds provide nutrient subsidies that lead to damselfish holding and defending territories on the reef. “When they no longer are doing that, other fish can come in and change that ecosystem significantly,” she added.

Considering the possible implications of these hidden links between land and sea for the health of reef fisheries, which are important sources of protein for humans, adds support for eradicating harmful invasive species, Spatz said.

“To know that those fisheries are going to be impacted by the mere presence of an invasive species and that is going to subsequently impact your ability to live off of the sea?” she said. “That’s a big deal.”

Banner image: Fishes in a coral reef. Image by Renata Romeo / Ocean Image Bank.

A jewel damselfish (Plectroglyphidodon lacrymatus) among the coral reef of a rat-free island. Image by Rachel Gunn.

John Cannon is a staff features writer with Mongabay. Find him on Twitter: @johnccannon

Island conservation should focus on land-sea links for most impact, paper says


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