Large school of herbivorous fish, which eat turf algae enriched by nutrients from seabirds. Credit: Casey Benkwitt (CC-BY 4.0)
According to a new study, the key to restoring food chain networks across some coral reefs could be eradicating invasive rat populations.
The East African country of Seychelles is home to 1,690km2 of coral reef. Across its archipelago of 115 islands, there are 13 species of amphibians, 30 reptiles and 220 bird species.
However, invasive rats have caused dramatic reductions in seabird populations by preying on eggs and chicks.
The study, published in PLOS Biology, suggests that eradicating these invasive rats to restore seabird population can help rehabilitate nutrient connectivity in the once-disrupted coral reef food chain.
“These findings could be used to predict the benefits of removing introduced rats from islands, which can increase seabird populations and restore nutrient connectivity … potentially enhancing ecosystem function across multiple trophic levels on coral reefs,” say the authors.
The study focused on 4 islands in the inner Seychelles.
One island had invasive rats present and another had eradicated the rats. The other 2 islands were historically rat-free.
“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,” says co-author Anna Zora, a conservation and sustainability manager on Fregate Island, Seychelles.
The researchers used breeding bird survey data to estimate seabird biomass. They also measured seabird-derived nutrient inputs, known as guano, in turf algae, as well as the algae’s growth and cover and herbivorous fish biomass.
The researchers accessed breeding bird survey data which they used to estimate seabird biomass. They also measured seabird-derived nutrients, known as guano, in turf algae, as well as the algae’s growth and cover, and herbivorous fish biomass.
Colony of frigatebirds, which provide nutrient inputs to nearby coral reefs. Credit: Casey Benkwitt (CC-BY 4.0)
“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,” says Casey Benkwitt from Lancaster University, UK.
Allowing the turf algae to grow more quickly also increased the mass and productivity of herbivorous fish.
“This boost in primary productivity in turn enhances herbivorous fish biomass and productivity and stimulates top-down controls that limit turf algal cover,” says Benkwitt, who is first author of the study.
While the study was limited by the small sample size, it does leave promising room for future studies.
“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,” says Benkwitt.
The Seychelles coral reefs were impacted greatly by a mass-bleaching event in 2016 which resulted in a 50% decrease in live hard coral cover, making reef restoration a major focus for conservation groups in Seychelles.
Coral reef restoration is also important to protect the economy of Seychelles, which relies on the reefs for tourism and fisheries.
“This study confirms that protecting the ocean begins on land. A thriving island fosters a thriving reef,” says Zora.
“You can’t help but think: the land and sea are deeply intertwined.”