By: Jasmine Haskell
Edited by: Danielle Moloney
Have Pinky and the Brain finally succeeded in their plan of taking over the world? Not quite yet, but invasive rats originating from Southeast Asia have successfully decimated native wildlife on many islands around the world. In fact, invasive rats have been implicated as the cause of ecological and sociological demise of Rapa Nui (Easter Island). While this theory is still widely debated, rat invasions in recent centuries have proven to be quite the nuisance, with some countries initiating rat eradication programs to help alleviate the problem and restore populations of native species. Tropical islands are particularly susceptible to ecological damage through rats as many tropical islands have endemic species (species only found within these islands) and/or do not have native mammal species, thereby allowing the rat population to explode in this seemingly untapped niche.
Over many hundreds of years humans have sailed the world’s oceans to explore and trade, stopping from port to port and by doing so, inadvertently bringing travelling rats with them in the hulls of ships or canoes. Rats would interfere with their new ecosystem by disrupting nutrient pathways, pollination routes and by consuming an enormous amount of tree seeds, birds and their eggs. It is through the disruption of these nutrient pathways, rapid breeding rates, and voracious appetites that these invasive rat species are quite possibly capable of influencing the behaviour of reef fish.
A prized menu option for these invasive rats are seabirds and seabird eggs. Seabird densities are nearly 760 times larger on rat-free islands when compared to rat-infested islands. Seabirds are critical vectors for nutrient transport as their guano (droppings) contain high levels of nitrogen and other key nutrients. These nutrients then wash out to sea and get transported to nearby coral reefs, acting as a natural fertiliser. This is a critical process for these coral islands as coral reefs require nitrogen and it is a limiting nutrient in these environments. With the presence of rats, this cycle gets disrupted as seabird populations rapidly decline.
A recent study by Gunn et al. (2023) compared the behaviour of jewel damselfish surrounding five rat-infested islands to five rat-free islands in the Chagos Archipelago in the Indian Ocean. All 10 islands in this study displayed relatively low human impact, eliminating compounding factors to rat populations. The jewel damselfish was selected for this study due to their well-studied territorial nature and prevalence on coral reefs. Their research demonstrated that jewel damselfish inhabiting waters nearby to rat-infested islands were less interested in protecting their food source and patrolled larger territories than their rat-free neighbours. This could be explained by the quality of their food source, algae. Due to the seabird-enhanced nutrient recycling on rat-free islands, algae surrounding rat-free islands was nutritionally more dense, and thus of higher quality, than the algae surrounding rat-infested islands. Therefore, jewel damselfish were less aggressive around their food source around rat-infested islands because they were earning less nutritional yield per unit of foraging effort (in other words, not worth the effort).
This has several effects for jewel damselfish themselves and the wider ecosystem. As their food becomes less nutritionally valuable around rat-infested islands, jewel damselfish demonstrate slower growth rates when compared to jewel damselfish surrounding rat-free islands. Algal patches “farmed” and heavily guarded by these jewel damselfish display higher rates of primary productivity than those not aggressively protected. This could have wider implications when it comes to photosynthetic efficiency of the algae, and thus carbon capture. Jewel damselfish patches have also been shown to influence coral larvae settlement, with juvenile coral recruitment being lower inside these “farmed” patches when compared to outside of the guarded patches. With the jewel damselfish guarding larger areas due to decreased nutritional value of their food source next to rat-infested islands, this could potentially negatively affect the settlement of coral larvae. The reduced aggression by jewel damselfish also influences the social behaviour of other coral reef fish species, by either recruiting or encouraging certain species to cohabitate in the same area. For example, the foraging behaviour of the butterflyfish has been shown to be heavily influenced by damselfish aggression. Additionally, surgeonfish density around damselfish territory has been shown to have a negative correlation. This is not all to say that dominance over one fish species is “preferred” than the other but more to say that behaviour between fish species is interconnected and complex and an alteration of the behaviour of one species can have an effect on others.
While the invasive rat problem persists on many of the world’s islands, successful eradication programs have seen great success. Lord Howe island is one such example. Prior to the eradication program in 2019, there were approximately 1000 rats for every person on the island. The rats had caused 5 species of birds, 13 invertebrates and 2 plant species to go extinct. Once the rats were eradicated, populations of endangered birds began to flourish, dormant plant species emerged once again and the sound of crickets returned to the island. Continued persistence into rat eradication has the potential to re-establish ecosystem balance by rekindling species interactions. With such impactful downstream effects demonstrated by the rat-invaded islands described in the Gunn et al. (2023) study, rat-eradication could lead to healthier reefs. With more research into animal behaviour and ecosystem processes we may discover more seemingly strange downstream ecological effects.
Featured Image: (Reefcause Conservation, https://conservation.reefcause.com/rats-alter-coral-reef-populations-the-chagos-islands-story/)
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