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Eliminating invasive rats may restore nutrient flow across food chain networks in Seychelles

Ecosystems are characterized by interconnected structure and functions. A study published in the open-access journal by Casey Benkwitt at Lancaster University, United Kingdom and colleagues suggests that restoring seabird populations via eradication of rats may help coral reefs by restoring nutrient connectivity in disrupted food chains.

Invasive rats in the Seychelles have decreased native seabird populations. However, the mechanism by which seabirds may impact coral reef ecosystem structure and function across multiple food chain links is unknown.

In order to quantify the ecosystem-wide food chain disruptions from species decline, researchers analyzed the movement of nutrients from seabirds through a marine ecosystem in five marine study sites across four islands in the inner Seychelles.

One of the islands had invasive rats that predate seabirds, one island had rats eradicated, and two of the islands were historically rat-free. They accessed breeding bird survey data to estimate seabird biomass and measured seabird-derived nutrients in turf algae, as well as turf algae growth and cover, and herbivorous fish biomass and productivity across all five study sites.

The researchers then used a causal modeling framework to test the effect of seabird nutrient inputs on structure and function across two different food chain levels.

The researchers found that seabird guano increased turf algae growth on coral reefs, but not algal cover. Through seabird-derived nutrient (guano) uptake, algal turf grew more quickly, which led to an increase in both the mass and productivity of herbivorous fishes beneficial to reefs. This feedback loop between producers and consumers limited the amount of turf algal cover on reefs.

The study was limited by its small sample size of four islands, and future studies are needed to replicate the results in different sites.

According to the authors, "These findings could be used to predict the benefits of removing introduced rats from islands, which can increase seabird populations and restore nutrient connectivity, thus potentially enhancing ecosystem function across multiple trophic levels on coral reefs.

"In terms of conservation, these findings add to the body of evidence that herbivores are key to preventing coral to algae regime shifts on coral reefs, and additionally suggest that restoring ecosystem connectivity, combined with effective fisheries management, is another route to help achieve this goal."

Casey Benkwitt says, "We show that natural nutrient inputs, provided by seabirds, cause turf algae on coral reefs to grow faster. This boost in primary productivity, in turn, enhances herbivorous fish biomass and productivity, and stimulates top-down controls that limit turf algal cover."

Benkwitt notes, "It was amazing to see how much faster turf algae grew where seabirds are nearby, because this is something that you normally can't see with your naked eye. That seabird nutrients increased algal growth, but not how much of a reef it covers, shows the hidden pathways by which seabirds can influence coral reefs."

Co-author Anna Zora (Conservation and Sustainability Manager on Fregate Island, Seychelles), adds, "Living and working on a small island—observing its wildlife on land and beneath the sea—you witness firsthand the profound connection between a healthy terrestrial ecosystem and the vibrant reef that surrounds it.

"You can't help but think: the land and sea are deeply intertwined. This study confirms that protecting the ocean begins on land. A thriving island fosters a thriving reef."