Rats are considered to be the primary driver of the local extinction of seabird populations across the Pacific. They are also known to modify island vegetation, suppressing plant growth through consumption.
The predation on seabirds in particular has recently been found to disrupt the ongoing transfer of nitrogen between land and sea. Such processes have gained recent attention from researchers who have tested for correlations between increased nesting seabird populations and improved coral reef ecosystem recovery.
Similar to much of the world’s wildlife, seabirds can connect ecosystems by transferring organic matter and nutrients between them. Seabirds are globally important drivers of nutrient cycling, transferring nutrients from their pelagic feeding grounds to islands which they roost and breed on.
This input of nutrient-rich guano (seabird poo) increases plant biomass, and has been discovered to assist in coral reef productivity. For instance, nutrients leached from guano to adjacent marine systems may increase plankton densities and influence feeding behaviour of coral reef species.
You may be thinking, “but doesn’t rat poo contribute to nitrogen levels?”
The thing with rat poo is that it is made up of recycled nutrients already present on the island.
The majority of seabirds, however, feed on the open ocean, at substantial distances from the reef.
By foraging offshore, seabirds feed from vastly different food webs to those found on land and on adjacent coral reef ecosystems. Their capacity to access these oceanic prey resources then results in greater deposits of oceanic nitrogen that would otherwise be unavailable or at lesser quantities on rat invested islands.
Recent studies conducted on the northern atolls of the Chagos Archipelago have found that rat-free islands had nitrogen levels that were 3.8 times higher than on rat-infested islands.
More nitrogen was also evident in new growth of leaves of coastal plants on rat free islands, highlighting the uptake of oceanic nutrients by island vegetation. Nitrogen is also expected to leach off islands to nearshore marine environments through periods of rain.
The leaching of nitrogen from rat-free islands has been found to contribute to higher beneficial nitrogen values in corals closer to seabird colonies.
As for those ecosystems found on the reef crest (approximately 3 metres deep) nitrogen levels were substantially higher in turf algae and the muscles of herbivorous damselfish adjacent to rat-free islands.
The damselfish adjacent to rat-free islands were also found to be growing significantly faster towards their maximum expected size compared to those found on rat infested islands. Such findings suggest higher fish production is likely to occur adjacent to islands with significant seabird colonies.
The removal of predators is therefore a key step to stimulating seabird nutrient subsidies for coral reef ecosystems, by reflecting natural productivity and function in the absence of introduced rats.
Such recent findings add further value towards the removal of rats on islands as the return of seabird populations would not only benefit island ecosystems but also adjacent nearshore marine ecosystems.
Improved island ecosystems and functions would then further contribute to climate change adaptation as well as improving resilient communities.
· This article is based on one by Nicholas Graham and others, “Seabirds enhance coral reef productivity and functioning in the absence of invasive rats”, July 2018).