This island in the Indian Ocean supports thriving seabird colonies and rich coral reefs
but nearby other islands are eerily silent.
The cause of this difference lies in a complex web of birds, rats, coral...and poop.
The Chagos archipelago in the Indian Ocean is home to over 60 tropical islands
nearly a third of which are completely free from invasive species like rats or foxes.
This safe haven for birds creates quite the cacophony.
So here we are in the Chagos Archipelago
We're on a rat-free island,
there's never been rats on this particular island
There's a huge abundance and diversity of seabirds here
Terns, shearwaters, boobies, frigatebirds.
On neighboring islands, however, the arrival of sailing ships in the 1800s
brought an unwanted visitor - rats.
The rodents bred like, well, rodents.
They devoured seabird eggs and chicks until they all but wiped out the bird populations.
In contrast to this island where there's rats,
there's very, very few seabirds.
The skies are empty and it's very quiet.
But the story doesn't stop there, because the birds play a critical role in the wider ecosystem
all thanks to their poop.
Now these birds return to the islands where they roost
and they breed and they deposit huge amounts
of guano, or bird poo, on to the islands.
And this guano is highly rich in nitrogen
So they're effectively fertilising the islands.
This nitrogen is great for plants, which thrive on the rat-free islands.
But scientists wondered whether this aerial nitrogen delivery system
was also impacting the surrounding coral reefs.
So we then went out into the shallow waters there
and sampled sponges that filter feed and macroalgae.
And those fish that are feeding on the algae out on the
reef crest, we were able to look at their growth rates.
The team found that rat-free islands had two-and-a-half times more nitrogen across the entire ecosystem
and fish living on the nearby reefs grew faster and bigger than those around rat-infested islands.
These fish graze on algae that otherwise covers the coral, slowing its growth.
The scientists observed three times more grazing on the reefs surrounding rat-free islands.
These healthier reefs may be more resilient to climate change.
When it comes to interwoven ecosystems like those of the Chagos Archipelago
it seems invasive species like rats have a bigger impact than we thought.
So de-ratting oceanic islands in archipelagos such as this
should really be a very high priority for conservation.
It's not difficult to de-rat these islands.
and it would have a huge impact on both the terrestrial
environment and the adjacent marine environment.
The team is now urging governments to consider investing in rat extermination programmes
as a way to save coral reefs.
This unusual solution could be a quick and cost-effective tool in the fight against coral decline
all due to a complex chain linking rats, birds, fish, coral, and poop.