I learned a new term this weekend "rat spill."
Think of an oil spill and then replace the oil with introduced black or brown rats flowing onto an island and threatening the entire natural habitat: animals and plants.
It's quite an image and appropriate for the environmental damage that can occur when these large rat species are introduced to an island ecosystem. California's Channel Islands are constantly threatened by the potential of rat spill.
Black rats (Rattus rattus) and brown rats (Rattus norvegicus) are the number one cause of extinction for reptile and bird species on islands. These large rodents reproduce rapidly and eat everything, including eggs and hatchlings.
Because islands are primary nesting locations for seabirds, rat spill can take out entire populations. Brown pelicans, oyster catchers, western gulls, and rare species–like ashy storm petrels and Scripp's murrelets–all nest on California's Anacapa Island. Prior to 1940, however, a rat spill introduced hungry rodents to the island with devastating impacts. Some of the bird species were headed toward extinction.
Beginning with efforts in 2001, Channel Islands National Park spent millions of dollars eradicating the rat spill. The effort was successful, but the process wouldn't be feasible on larger neighboring islands with small endemic mammals–like the island fox and island spotted skunk live.
While you might think an island fox could prey on a large rat, the fox's small size and the rat's aggressive self-protection, make this rodent a difficult catch for the diminutive island fox. Additionally, rats are a disease vector and can bring viruses, which threaten the fox's survival.
Across the Channel Islands prevention is the goal. Just like with oil, it is much more cost effective to prevent a rat spill, than to clean up the damage and hope you can restore a habitat.