Saturday, January 7, 2023

Removing rats from islands has big impacts on nearshore ocean productivity

 New research shows that small islands without rats have more productive nearshore environments than those with rats.

It’s a fascinating bit of data that seems to confirm Leonardo Da Vinci’s observation: “Learn how to see.  Realize that everything connects to everything else.”

But how do rats and fish connect?

Well, through seabirds.

The birds catch fish at sea. They nest on islands. They poop on those islands. The guano runs off to the nearshore water, where it fertilizes the reef. That means reef fish have more to eat. (And then, of course, the seabirds eat the fish, closing a great circle.)

If you put invasive rats on such an island, they eat the birds and eggs, the seabird population collapses, there’s less guano, and reef fish populations do less well.

The details of this dynamic are described in a new 2023 paper in the journal Nature Ecology and Evolution. It is entitled, Terrestrial invasive species alter marine vertebrate behavior. 

Ultimately, it’s a natural nutrient cycle, and the rats break it. Across the globe conservation organizations have been trying to heal the cycle by eradicating alien rats from small islands. It’s a huge task.

The removal of rats from Lehua Island, north of Ni’ihau, required a concerted effort by a broad consortium that included the organization Island Conservation, along with the state Division of Forestry and Wildlife and its parent agency the State Department of Land and Natural Resources, U.S. Fish and Wildlife Service, U.S. Department of Agriculture’s National Wildlife Research Center in Hilo, Niihau Ranch, U.S. Coast Guard, National Tropical Botanical Garden and several other associated organizations and agencies. 

At Lehua, they were able to remove the rats, and have recorded recovering bird populations. Whether fish populations around the island have changed or increased has not been reported.

Most of the research to date on rat removal has been used to document the recovery of bird populations, like this report on Anacapa Island, off California. 

And this report on Hawadax Island in the Aleutians, formerly known as Rat Island. 

Guano has been mined off small bird islands around the world for many years, for use as a natural fertilizer for land-based agriculture. Guano has similar fertilizing impacts on aquatic systems, says the Nature Ecology and Evolution study.

“The movement of naturally occurring nutrients across habitats and ecosystems is a strong driver of productivity and can influence community dynamics,” the authors wrote.

They found that fish in rat-free environment have to spend less time fighting for food, and are able to cruise larger territories.

The research was done on 10 islands, 5 without rats and 5 with rats, in the Chagos Archipelago, which is in the Indian Ocean south of the Maldives. One finding: “Seabird densities on rat-free islands are up to 720 times higher, and the nitrogen input provided by seabirds is 251 times greater, than around rat-infested islands.”

One interesting finding is that there wasn not necessarily more algae growing on rat-free islands, but that the algae there was more nutritious, so fish didn’t have to eat as much.

The Nature study’s English and Canadian authors are Rachel L. Gunn, Cassandra E. Benkwitt, Nicholas A. J. Graham, Ian R. Hartley, Adam C. Algar and Sally A. Keith. Most are from the Lancaster environment Centre at Lancaster University.

An earlier study by some of the same researchers confirmed that rat-free islands had higher nutrient levels in nearshore waters. 

And this study suggests that while there aren’t necessarily more fish around rat-free islands, the fish grow faster and are significantly heavier.   “Overall mean body size was 16% larger around rat-free islands.”

© Jan TenBruggencate 2023

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