Besides being the primary carriers of diseases that have killed millions of humans, rats have had damaging impact on other fauna. In oceanic islands, such as Polynesia and Micronesia, Pacific, brown and black rats brought by vessels over the past several millennia, more particularly in the last two centuries have single-handedly driven more bird species to extinction than in any other region in the world.
This is still a clear and present danger. A few years ago, an Asian fishing vessel went aground on a reef at McKean Island, part of Kiribati’s Phoenix Islands. It introduced Asian rats ashore which found millions of seabirds from about 15 species. Within a few years, rat numbers soared and the bird populations crashed. If active intervention werenot made rats would have consumed every last egg and chick on the island. Thanks to a 2006 eradication campaign on McKean Island, funded by the New Zealand Agency for International Development and the support of the New Zealand Department of Conservation and the Kiribati Ministry for the Environment this tragedy was averted. Despite the numerous logistical issues involved, this mission demonstrated the feasibility of island restoration through protection of endemic biodiversity, even in such a remote location
Those Dirty Rats: Removing Invasive Species in the Pacific Islands Conservation International Blog