New study provides further evidence supporting the connection between predator-free islands and richer sea life.

Researchers publish the latest evidence connecting terrestrial and ocean health in a study of four islands in the Mercury Archipelago off the Coromandel. The study analyzes the effects of seabird guano on terrestrial and marine health.

Seabird populations are unique, in that, they are some of the few species who live on both the land and sea. Their guano introduces valuable nutrients to plants on land and into the coastal environment. Seabirds are often known as ecosystem engineers, acting as significant drivers of biodiversity by impacting both terrestrial and marine ecosystems. Seabirds bring nutrients from the marine environment by eating fish and deposit that nutrients onto the island through their guano or poop, which is then washed back out to the ocean.

Predators including invasive rodents, who prey on their eggs, pose a significant threat to seabirds and thus biodiversity on islands. As a result, there is evidence supporting the correlation between predator-free islands that have flourishing seabird populations and richer life at sea.

Lyndsay Rankin and Holly Jones of the Department of Biological Sciences at Northern Illinois University chose the Mercury Archipelago off the coast of New Zealand as a case study, because the islands have alternative histories involving the invasion and removal of invasive rodent predators. Two of the islands, Aitu or Middle and Green Island, have no history of invasive predators. Whereas Korapuki and Ahuahu Island had invasive rats and cats which were removed from Korapuki in 1986, and Ahuahu in 2014.

We expected never-invaded islands and those cleared of invasive mammals earlier to have higher species richness/diversity [in the near-shore environment],” wrote Rankin​ and Jones.

In total, there are eight ground-nesting seabird species throughout the Mercury Archipelago. Rankin and Jones hypothesized that the islands without invasive species present would have more birds and higher biodiversity. This is because invasive species such as rats and cats prey on seabirds and their eggs. So, in theory, islands free of predators should support greater populations of seabirds. By using the never-invaded islands as baselines, Jones and Rankin were able to compare the other two islands to see how quickly the populations and the flow of nutrients rebounds without predators.

In particular, it is the presence of nitrogen in guano that allows seaweed and kelp to flourish. Typically, from June to September, there is the highest transport of their guano to the sea, made possible by heavy rains. Rankin and Jones were able to detect lots of nitrogen in the sea of the islands, in addition to high quantities of macroalgae, but these findings were not as clear cut as they anticipated. On Ahuahu Island, where invasive species were removed only a few years ago researchers detected more nitrogen in the water than Korapuki Island which had decades to recover. The researchers are not entirely sure why this is the case.

Still, their findings provide additional evidence of the influence of seabirds on island ecology and add to accumulating scientific evidence that shows how the removal of invasive predators can play a critical role in not only the recovery of island biodiversity but also in ocean health and climate change resilience.

Featured Photo: Mercury Islands, New Zealand. Credit: Ariel Ophelia

Source: Science