Predator-free islands rich in sea life

It’s like fertilising the sea. When seabirds return to islands that are free of mammal predators, their guano runs off into the sea and enriches the seaweeds and kelp living there.

That might seem predictable, but the wider effects of predator eradication on islands haven’t been much studied or quantified.

Two American biologists published a study earlier this month that looked at four islands in the Mercury archipelago off the Coromandel.

They provided a strong study group because each island has a different invasion and eradication history.

Atiu/Middle and Green Islands were never invaded by mammals, Korapuki Island was cleansed of cats and rats in 1986, while Ahuahu/Great Mercury was cleared in 2014.

There are eight groundnesting seabird species on the Mercury Islands, with varying breeding periods, resulting in steady guano supplies being deposited on the land throughout the year.

Heavy rains, typically from June to September, transport the guano into the sea.

Guano, or bird poop, is rich in nutrients, especially nitrogen that kelp and seaweed need to flourish.

‘‘We expected never-invaded islands and those cleared of invasive mammals earlier to have higher species richness/ diversity [in the near-shore environment],’’ wrote Lyndsay Rankin and Holly Jones of the Department of Biological Sciences at Northern Illinois University.

And that’s what they found, with some complications.

Islands such as the Mercury group are called ‘‘seabird islands’’ because the birds transform the ecology. Thousands of birds over many years burrow nests, deposit guano and leave their dead to decompose.

Most seabirds forage at sea and bring nutrients from the water to the land. These processes change the soil and physically alter the landscape. To humans, they are smelly.

Rats and cats devastate seabird colonies because most birds dwell on the ground and are easily killed. That slows or stops the flow of nutrients back into the sea.

When the predators are removed, there’s hope that seabird islands can resume functioning. This study used the two never-invaded islands as baselines and the other two islands as measuring posts to see how quickly and fully the flow of nutrients into the sea occurs.

The scientists found lots of nitrogen in the sea, especially after the rainy season. They also found lots of macroalgae diversity, especially near the never-invaded islands.

The complication was that the recently eradicated island had higher nitrogen enrichment than the island eradicated more than 30 years earlier.

That needed explaining. They wondered if lower wave exposure at the recently eradicated island contributed.

And because this island is by far the largest in the group, it has stream catchments, forests and a farm – all of which could contribute nitrogen to the sea.

Pied shags also nest year round on tree branches above the sampling sites and contribute their guano to the near shore. This wasn’t the case on the other, much smaller islands.

For this study, Rankin and Jones didn’t report on the invertebrate, fish and other populations that typically live in and feed on seaweed. But other scientists have done this elsewhere, including in the Chagos Archipelago in the Indian Ocean and on the west coast of South Africa.

They found higher biomass in a range of species in near-shore environments akin to those in the Mercury Islands. The Chagos study included islands with and without rat populations.