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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