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Riled Up is a journal of science, the environment, exploration, new technology, and related commentary.  Contributors include scientists, explorers, engineers, and others who provide perspectives and context not typically offered in general news circulation.  For interested readers, additional resources are included.

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Of Rats & Reefs

Of Rats & Reefs

Booby Chick Nesting Over Lagoon, Chagos Archipelago (credit: Nick Graham, Nature)

Stable ecosystems are a web of interconnected physical, chemical, and biological relationships. The complexity of a tropical rainforest is hard to completely comprehend from the myriad connections. However, on islands it can be easier to see the direct relationships. This is one reason islands have played such a pivotal role in biogeograpahy, ecology, and evolution. A new study on two coral atolls in the Indian Ocean are textbook examples. It also offers a clear way to build resilience and restore damaged coral reefs.

Publishing in Nature, ecologists with the UK's Lancaster University, connected three ecosystem features: nesting sea birds, coral reefs, and invasive rats to resolve the web of mutual relationships. Lancaster researcher Nick Graham working in the Chagos Archipelago comparing two adjacent coral atolls: one covered in tropical tree species inhabited by thousands of sea birds and the other infested by invasive rats. The rats escaped in 17th-18th centuries from sailing ships. The voracious predators quickly eliminated the birds on the one atoll by eating their eggs, chicks, and even attacking adult birds. The island's vegetation was highly degraded as well. Globally, it has been estimated rats have discriminated nesting bird populations on 90% of the world’s temperate and tropical islands.

From the study: the still bird-populated atoll being used as the control base for the study. The bird droppings helped to fertilize not only the island's vegetation but also the reefs surrounding the shores. On the rat-infested island this natural nutrient resource was lacking. Graham noted:

“Seabirds are crucial to these islands because they are able to fly to highly productive areas of open ocean to feed. The birds return to their island homes where they roost and breed, depositing guano (droppings) on the soil. This guano is rich in the nutrients, nitrogen and phosphorus. Until now, we didn’t know to what extent this made a difference compared to adjacent coral reefs.”

The ecologists discovered rat-free islands had significantly more nesting birds and nitrogen in their soils. The higher nitrogen level ran off into the ocean, benefiting algae, sponges, and reef fish in the offshore corals. Fish were far more abundant with their numbers estimated to be 50% greater than on the rat-infested island. The researchers also observed fish grazing where algae and dead coral was consumed, providing a stable basis for new corals to settle and grow. Coral regrowth was more than three times higher than on the rat-infested island. These discoveries show that ecological restoration and coral recovery requires removal of the feral rats.

Graham's message is clear:

“The results of our study are clear. Rat eradication should be a high conservation priority on oceanic islands.

An animation explains the island and reef differences determined by the study. Ecological restoration simply requires the will and direction actions necessary to eliminate the invasive rats. WHB

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