Clearing an area of introduced pest species is a huge job. The goal of making New Zealand PredatorFree by 2050 is as aspirational as the moon-shot was in the 60s. Is it achievable? Of course it is. However, it require a LOT of resources, better ways of doing things that have worked so far, new ideas to do things that we haven’t done in the past even better in the future, lots of people, lots of buy-in from the general public. In short, a steadfast commitment for 30 years. Even if we don’t get there in the timeframe, reducing the range of predators by 50%, or the total numbers of predators by 80%, say, is still a tremendously good and useful outcome.
How hard could it be? Let’s examine a small island in Lyttelton Harbour. Quail Island (Otamahua) is an 85ha island several hundred metres from the surrounding mainland shores. The island has a focus of a long-term attempt to eradicate introduced mammal pests and restore native vegetation and animal species for the last three decades. There have been a lot of successes with about 100 000 trees planted, and the eradication of cats, rabbits, hedgehogs, rats, possums, stoats, ferrets and weasels. So eradication can be achieved on a small island!
Well, almost. There are still plenty of mice. But mice are tiny and meek, surely they are not a problem? In the absence of other, larger rodents, mice tend to become more rat-like in their behaviour. On some islands they also increase in size and aggression and cause problems to large nesting bird species that they would normally keep away from. To make Quail Island all that it could be, the mice also need to be removed.
Mike Bowie has spent the last 25 years working towards the goal of restoring Quail Island. In a paper in the New Zealand Journal of Zoology he, and other colleagues, looks at the various attempts to remove mice from the island. Mice were almost removed accidentally in the late 90s. A large poisoning campaign using the toxin Pindone in cereal baits, was aimed at removing rabbits, which it did very successfully. As a side-effect the mice also ate the baits and their population was reduced to almost zero for three years before bouncing back.
An attempt was made to remove all rodents in 2002. This time the toxin brodifacoum was delivered in rodent baits from bait stations placed in a 40 m grid across the island. This was very successful and eradicated rats from Quail Island. Mice were initially thought to have been eradicated but were detected again after just three months and their population quickly recovered.
Another attempt was made in 2009 to remove the mice. Even more planning and effort went into this event. A helicopter was used to spread brodifacoum baits over the entire island. This happened over two days, late July (where the helicopter flew in GIS programmed NE to SW transects of the island) and early August (where it flew the same course and then NW to SE). Intensive monitoring with tracking tunnels was then undertaken. By December of the following year the mice were back in good numbers.
One of the questions that Mike was interested in was whether the mice survived on the island or whether they recolonised from the mainland. King Billy Island lies between Quail Island and the mainland and might provide a stepping stone for mice to reinvade. Mike was able to take samples of mice from before and after the control operation, as well as from populations in the wider Christchurch area. He found that the mice on the island after the operation were genetically indistinguishable from before (but different to mice on the mainland). So it seemed that at least a handful of mice had survived the poison drop and that was enough to build up the population again.
Many lessons have been learned from the Quail Island control operation (and many others over the last two decades). Not least is that eradication requires a huge effort from a lot of people. The experience earned has allowed recent control operations to be even more meticulously planned and, ultimately, successful in eradicating mice from islands (see the recent removal of mice from the Antipodes). If the resources allow, then the next attempt on Quail Island is likely to be successful.
Removing mice from larger mainland areas is something that will remain out of our reach for quite some time. It is no coincidence that mice are not listed as one of the predators that we want to remove from New Zealand by 2050. If we are able to remove possums, stoats and rats by this time, the mice will indeed inherit this piece of the Earth.