While islands occupy 5.5 percent of the terrestrial surface area, they contain more than 15 percent of all terrestrial species. According to new work published in Proceedings of the National Academy of Sciences, invasive mammal eradication is an important tool for protecting threatened island-dwellers.

At least $21.5 billion is spent every year on global biodiversity conservation. Despite their importance for preventing (or at least slowing) local extinctions, these actions are rarely assessed in a systematic way. Compared with mainlands, islands are home to disproportionately high amounts of biodiversity, and island species make up nearly two-thirds of all recent extinctions and 37 percent of all critically endangered species on the International Union of the Conservation of Nature Red List. Eradicating invasive nonnative mammals has been attempted on more than 700 islands, and the most commonly eliminated groups are rodents (57 percent), goats (11 percent), and cats (8 percent). 

To examine the benefits to native island fauna of removing invasive mammals, a large international team led by Holly Jones of Northern Illinois University paired a literature and database review with expert interviews. After identifying all islands with successful mammal eradications using the Database of Island Invasive Species Eradication, they then limited their analysis to the eight countries with the most eradications: New Zealand, Australia, Ecuador, Seychelles, U.S., U.K., France, and Mexico. These countries and their overseas territories represent 82 percent of the world’s invasive mammal eradications. 

Across 251 eradications of invasive mammals on 181 islands, the researchers found 236 native species that benefited and seven native species that were negatively affected (mostly because of toxicant ingestion meant for invasive mammals and reduction in prey). 

The native island species that benefited included invertebrates, landbirds and seabirds, mammals, and reptiles – with birds being the most frequent beneficiaries at 69 percent. These benefits mostly involve resident population recovery, new settlements and recolonization, and reintroductions of animals that had become locally extinct (or extirpated). 

Four threatened species – island fox, Seychelles magpie robin, Cook’s petrel, and black-vented shearwater – were even downlisted to a reduced extinction risk category, and not one was moved to a higher extinction risk category.

"The tools to completely eradicate island invasive mammals are so successful because islands are contained, and once you get rid of all the mammals, preventing them from getting back is easier than on the mainland, where there's almost constant reinvasion," Jones explains to IFLScience. "Most mainland invasive mammal efforts end up being constant control methods instead of complete eradication as a result."

^{Image in the text: 16 populations of tuatara benefitted from invasive mammal removal in New Zealand. Holly Jones}