Last week I had the pleasure to finally visit an island I had long wanted to survey. My perverse reward was finding not one but two species of rats inhabiting the island. Rakitu lies off the east coast of Aotea (Great Barrier Island) – a remote island off the coast of a remote island. Its inverted shape; a valley in the centre surrounded by towering cliffs and tors, lends it English name of Arid Island. With delays to our expedition I was left wondering if the island ever wanted me to visit, but eventually we arrived and I had the pleasure of meeting the descendants of the previous owners also staying out there.
The 328 hectare island was a major centre of Māori activity, and in colonial times was farmed through its central valley and slopes. In 1993 the island was purchased to be managed by the New Zealand Department of Conservation where today it is an open access scenic reserve. Black rats (Rattus rattus) were long known to be present, and our team wanted to assess their abundance, and their impacts on the abundance of native birds, reptiles and invertebrates.
Using an intensive 25 metre trapping grid in the remaining rich native forest in Bush creek, we studied the rat population. Checking only 37 live traps took the entire day, every day. To our surprise, on the first day, we caught a small number of Pacific rats (Rattus exulans) in the forest. This species would have been introduced during Māori occupation, and so it was a surprise that it had never been recorded when its presence was likely, but the abundance of the much larger black rats masked the presence of Pacific rats (locally known as kiore) for a long time, as known from elsewhere. Indeed, the black rats on Rakitu are among the largest ever recorded in New Zealand, averaging over 200 grams weight, and the most dense, at over 20 per hectare. The Department of Conservation intends to eradicate the rats from the island this winter, which is urgent given the declines in bird species and reptiles we were also observing on the island.
Given the extremely challenging terrain (with locations inaccessible to humans where rats could evade trapping) and size of the island (over 4,000 traps would be required if it was even possible), the only way rat eradication (every last rat removed) can be guaranteed would be by aerial distribution of brodifacoum. Some of the local community on Aotea are fundamentally against poison, which as a principle is laudable (poisoning an environment or species is never good). However, evidence from over 100 rat species eradications on islands around New Zealand for over 50 years shows no long-term effects of one-off brodifacoum use for rat eradications. The evidence does show, however, many, many long-term benefits to the resident native species on all those islands. The dilemma is thus not poison or not poison, but poison or rats?