Introduced Rats
Background

Few vertebrates have had greater impact on the biodiversity of islands than introduced rats. Three species  
widespread throughout the Pacific are all originally from Asia, the Pacific rat; introduced by the earliest  
Pacific voyagers, and the other two species unwittingly transported on ships in the last two-hundred or so  
years. As a consequence rats are now found on almost every Pacific island. The impacts of these more recent  
arrivals and especially the Black Rat (Rattus rattus) are particularly acute. In addition to their well-
documented impacts on island biodiversity, rats represent an on-going threat to human health spreading  
bacteria such as leptospirosis and result in millions of dollars of damage to agricultural crops and food supplies  
wherever they occur.

Rats prey on a wide range of plants, insects and larger animals, such as seabirds. Globally, the Black Rat is one  
of the chief causes bird extinctions on Pacific Islands. Five species of birds disappeared from Taukihepa Island,  
New Zealand, after the arrival of the Black Rat. On Lord Howe Island, Australia, five endemic bird species  
became extinct following the arrival of rats from a shipwreck. In the Pacific a number of bird species survive  
only where intensive rat control operations occur, or on islands free of Black Rats.

The Black Rat is also a threat to island people and economies. Throughout the world, vast quantities of  
agricultural crops and food supplies are consumed or despoiled by rats each year. In Tonga it was estimated in  
one study that 20% of the coconut crop was lost due to rats.

Case Studies

The Tuamotu Archipelago in French Polynesia is the home of many bird species such as the Polynesian  
ground-dove and Tuamotu Sandpiper (Kivi Kivi).  The continued existence of these species is threatened by  
rats that predate on the birds, chicks and eggs.  In 2015 Birdlife International and its local partner Société  
d'Ornithologie de Polynésie (MANU), together with Island Conservation plan to eradicate rats from 6 atolls  
and small islets in the Acteon and Gambier island groups of the Tuamotu Archipelago.  

The Malau (Polynesian Megapode) is a critically endangered bird native to the Kingdom of Tonga.  Ongoing  
survival of this species is severely threatened by rats, and although it is not feasible to eradicate rats from the  
entire Kingdom, it may be possible to eradicate them from some of their critical habitat such as Late Island.   
This planned eradication, coupled with translocation of breeding birds from nearby island, can potentially  
save this iconic species from extinction.

In cases where eradication is currently not deemed feasible for reasons such as cost or the presence of  
vulnerable non-target species, specific actions can be taken to protect native species at risk. These may include  
intensive control of rats and other predators.  The Raratonga Monarch, for example, has been brought back  
from the brink of extinction through rat and cat control, and the transfer of 30 young birds to Atiu Island  
(known locally as the Land of Birds) in 2001-2003.
Solutions
There are many projects in the Pacific region ready to be funded that would reduce the impact of rats on island  
biodiversity and people. Fortunately, the tools and techniques necessary to reduce the impact of rats on  
islands have advanced considerably in the past 20 years. Since the 1960s, rats have been successfully  
eradicated from over 300 islands, and the number increases every year.  The impetus of these past successful  
eradications must be maintained to continue the battle to save the unique Pacific fauna and flora.  An essential  
part of the eradication process is ongoing biosecurity which is paramount in the fight to ensure that rat-free  
islands are not re-invaded after successful eradication.


 
 
ON THE VERGE OF A BIOLOGICAL CRISIS:
the state of invasive species in the Pacific
The survival of the Tuamotu Sandpiper (top) and the Polynesian Megapode (bottom) are both threatened by rats.
Image Peter Morris, Birdquest (top) and SPREP (bottom)