The answer is self-evident: because we didn’t kill all the rats. However, the answer to the question “why didn’t we kill all the rats” is more complex. Tropical rat eradications currently fail more often than those in temperate or polar regions (16.1% vs 6.3%). If we discount operational reasons (i.e. the eradication wasn’t undertaken properly), the two prevailing biological hypotheses are that either with rats constantly breeding some pups may be able to survive and re-populate the island, or food is so abundant that not all adult rats diet switch to the poison bait.
Experimental rat eradications have proven very profitable in the past for advancing the science of rat eradications, but not everyone wants to allow their rat eradication for conservation to be an experiment, particularly when this increases the risk of failure. This week a team of scientists from University of Auckland (Araceli Samaniego, Markus Gronwald, James Russell) have been undertaking an experiment in association with a tropical rat eradication on Reiono Island in French Polynesia. The 22 hectare island will be treated with poison to eradicate the rats which are widespread across the otherwise relatively pristine island dominated by Pisonia forest and native seabirds and reptiles.
The team have radio-collared over 60 female rats and will track them throughout the course of the eradication. They will monitor their nests to determine the likelihood of any baby rats surviving over the two weeks of the eradication. This intensive monitoring effort will reveal the most detailed data yet on the behaviour of rats during a tropical eradication campaign, and hopefully inform future rat eradications on tropical islands so that they may be as successful as those undertaken in temperate and polar regions around the world.
For more information see the special issue of Biological Conservation on tropical rat eradication.