A study shows that nature is characterized by sometimes surprisingly complex connections: invasive rats on tropical islands can therefore indirectly change the behavior of fish in the surrounding reefs. The rodents reduce seabird populations, which means fewer nutrients are released into the water. The example of the algae-eating jewel damselfish shows that this effect can in turn influence the territorial behavior of inhabitants of the underwater world. The scientists say that this may have further impacts on the ecosystem.
With many "machinations" of humans, the consequences for nature are obvious: Certain interventions or environmental changes have a direct negative effect on specific living beings or fundamentally damage ecosystems. However, domino effects can also occur if, for example, other living beings are dependent on an acutely damaged species. Such complex consequences can also occur in the case of animal and plant species being introduced into new habitats. A notorious example is the black rat invasion of many tropical islands. The omnivores arrived there as stowaways from seafaring and then attacked many creatures in the sensitive ecosystems. Among other things, the seabird species there are severely affected: the invaders eat their eggs, which means that the population densities on infested islands can be up to 720 times lower than on rat-free islands.
Rodent invasion with far-reaching effects
The scientists led by Rachel Gunn from Lancaster University have now investigated whether this could even lead to a domino effect that reaches from the rats to the coral reef. There was a specific assumption about this. It is based on the premise that rats can disrupt the ancestral nutrient cycle around the islands by damaging bird populations. This is because seabirds fly out into the open ocean to feed and return to the islands to nest. There they then deposit nutrients with their droppings, some of which are washed into the sea and fertilize the surrounding coral reef ecosystems, the scientists explain.
As part of their study, they compared five infested and five rat-free islands in a remote archipelago in the Indian Ocean. As they report, it was initially shown that the mineral nutrient content in the seawater around the islands infested with rats is actually comparatively low. This in turn clearly characterizes the growth of fine, grass-like algae in the reefs: the researchers found that they develop a lower energy content. Using the example of the jeweled damselfish species Plectroglyphidodon lacrymatus, they then investigated how this can affect algae-eating fish. These fish maintain their own small meadows of algae in the reef and usually attack other fish species that come too close to their "farms".
Saved on defense
Through their underwater research and the evaluation of video recordings of numerous jeweled damselfish, the scientists were able to show that the fish on the reefs adjacent to rat-infested islands have significantly larger territories than the comparative specimens. Interestingly, as the video footage documented, they defend them much less fiercely than the jewel damselfish on the reefs of the rat-free islands. As the researchers explain, this is apparently due to the fact that the effort to defend themselves is no longer energetically worthwhile for the fish due to the lower nutrient content of the algae food. "We're showing that invasive rats can have an impact on both terrestrial and ultimately marine ecosystems," says Gunn.
The domino effect may even go further, say the scientists: "Algae breeding by the damselfish influences the balance of corals and algae in the reef," emphasizes Gunn. “In addition, your aggressiveness towards other fish can affect the way reef inhabitants move around and use the ecosystem. While we can't say exactly what the consequences of behavioral changes are, ecosystems develop delicate balances over long periods of time, so any disruption could impact the entire network," says Gunn.
In addition to the fundamental insight into the complexity of ecological connections, the study also shows again the importance of the fight against invasive rodents on tropical islands, say the scientists. "According to the study results, eliminating the rats has the potential to bring multiple, ecosystem-wide benefits," Gunn concludes.
Source: Lancaster University, professional article: Nature Ecology & Evolution, doi: 10.1038/s41559-022-01931-8