As people have moved around the world, they have taken many species with them. From domestic dogs and horses to wild sparrows and dandelions, non-native plants and animals are traveling too. Since the days of the early explorers, one problem species has stowed away many times – the rat.
The damage caused by rats has been well-documented for islands in many regions. Ground-nesting bird species are particularly vulnerable to rat predation on eggs and chicks. In particular, the hardest hit are those birds that use small holes as nest sites, such as auklets and petrels.
A new study published in this week’s Proceedings of the National Academy of Sciences has examined the effect of rat populations on the Aleutian Islands. By comparing islands with and without rats, researchers found that rats not only affect nesting seabirds, but they also seem to have an impact on the intertidal zone. Where rats are present, seaweed coverage is lower and invertebrate populations larger.
The proposed mechanism starts with the fact that rats prey on seabird nests, particularly when adult birds are absent to feed. The result is a reduced bird population that requires less food prey, such as snails, barnacles, and shellfish. Increased numbers of herbivorous invertebrates then graze large quantities of algae, denuding the intertidal zone and providing areas for new invertebrate settlement.
Not only are seabird populations affected, but rats also influence songbirds, as well as eating vegetation. On islands in other areas, reptiles and small mammals have fallen victim. On most affected islands, ground-nesting birds have few, if any, terrestrial predators. Similarly, rats then have no predators to keep down their populations. In some past cases, cats and foxes were introduced to eradicate rat populations. Unfortunately, these larger predators also proved detrimental to the natural balance on islands.
There have been successful rat eradication programs across more than 274 islands globally. On Rat Island, the Aleutian island targeted for the initial control measures, rat poison (brodifacoum) in pellets would be dropped from helicopters. With no other viable wildlife on the island, only rats should be affected. In addition, there are increased fines for ships found harbouring rats; new brochures have been produced to educate people about rat control on board vessels. With fewer rats at sea, there is less chance of repopulation after control efforts are finished. In addition, the proposed scheme attempts to reduce unwanted effects on birds, marine mammals, and fish species.
Recent eradication measures on California’s Anacapa Island had rapid results, with seabirds rebounding within months. In British Columbia, Langara Island has also demonstrated successful removal. In addition, Campbell Island, off New Zealand, has been cleared of rats; other introduced predators have also been removed from mainland sites.