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The natural world. Looking pretty for 3.5b years.

Of Rats & Reefs

Of Rats & Reefs

Author: Hugh Bollinger/Sunday, July 15, 2018/Categories: natural history, wildlife conservation, marine life, sustainability, environment, climate change

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

Stable ecosystems are a web of interconnected physical, chemcial, and biological relationships. The complexity of a tropical rainforest is hard to completely comprehend because of all the myriad connections. However, on islands it can be more simple to see the interactions. This is one reason islands have played such a key role in biogeograpahy, ecology, and evolution. A new study on two coral atolls in the Indian Ocean is a textbook example. It also offers a clear way to build resilience and recovery of damaged coral reefs.

Publishing in Nature, ecologists with the UK's Lancaster University, connected three ecosystem components: nesting sea birds, coral reefs, and invasive rats to resolve the web of mutual relationships. According to the announcement, Nick Graham, working in the Chagos Archipelago, compared two adjacent coral atolls: one covered in tropical trees and inhabited by thousands of sea birds and the other infested by escaped rats from 17-18th Century ships. The invasive rats quickly eliminated all 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. As an global example, it has been estimated that rats have desciminated nesting bird populations on 90% of the world’s temperate and tropical islands.

From the new study: on the bird-populated atoll, bird droppings helped to fertilize both the island's vegetation but also the reefs surrounding the shores while on the rat-infested island this natural source of nutrient was lacking. Nick Graham said:

“Seabirds are crucial to these kinds of islands because they are able to fly to highly productive areas of open ocean to feed. They then return to their island homes where they roost and breed, depositing guano – or bird 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 to adjacent coral reefs.”

The ecologists discovered that rat-free islands had significantly more nesting birds and nitrogen in their soils. This higher nitrogen source ran into the sea, benefiting algae, sponges, and fish in the ringing coral reefs. Fish life was far more abundant with the number of reef fish estimated to be 50% greater than on the rat-infested island. They also observed fish grazing where algae and dead coral was consumed, providing a stable basis for new coral growth. Coral regrowth was more than three times higher than on the rat-infested island. This island discovery shows the direction needed for ecological recovery and coral restoration by removing feral rats.

A short animation explains the land and sea differences observed.

Nick Graham's message couldn't be more clear: “The results of this study are clear. Rat eradication should be a high conservation priority on oceanic islands.

Ecological restoration simply requires the will and simple actions to eliminate the feral rats.

WHB

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