Stewart Island/ Rakiura has hit the news with the visit of Prince Harry. It is a truly unique part of the world, as he is discovering first hand. Amongst other things Harry has visited the predator free sanctuary of Ulva Island which is off the coast of Oban, the island’s main town. Many Kiwis think of Stewart Island/ Rakiura as a refuge for our native wildlife, but a new study paints a stark difference between refuges like Ulva Island and the rest of the island, which is not predator free. This shows a glimpse what bird life could be like on the rest of the island if it were also predator free. That would make Stewart Island/ Rakiura a destination fit for a King.
What difference does being Predator Free make to bird life?
With such a small part of our country free from invasive mammal predators, it is difficult for most Kiwis to understand what our country should be like. Only a visit to places like Zealandia or Tiritiri Matangi can afford us a glimpse of what our native wildlife was like before we got here, and even then it is only a glimpse.
New technology has recently been deployed at various spots on Stewart Island/ Rakiura to help illustrate this very point. The results were interesting – showing a huge difference between predator free areas and the rest of the island.
Stewart Island/ Rakiura Background
This work was done as part of the Predator Free Rakiura project, which is working with the community to determine whether it is viable to remove mammal predators from the island. At the moment the main problems are rats, possums and cats. Yes – there are no mustelids (stoats or ferrets) on the island so feral cats are top of the food chain. No one knows how the population got established, but they probably started from some domestic moggies that went bush. They ended up decimating the kakapo population and are now the major threat to the island’s kiwi population.
Stewart Island/ Rakiura already has some predator free sites – a relatively small peninsula (known as Dancing Star) which has a predator fence, and Ulva Island just off the coast which has been clear of predators for 20 years (barring a minor incursion). There is also quite a lot of trapping done around the town, which also benefits from the exodus of birds from Dancing Star and Ulva Island.
The experiment
In the past, we have had to rely on birdcall counts to give us data on bird populations, an approach that is expensive and pretty unreliable. Thankfully digital recorders can now identify bird calls and sophisticated software can tell us whether the calls are native or exotic.
Five digital recorders were placed at various spots around the island:
- North Arm – an area in the forest with no predator control
- Port William – an area in the forest with no predator control
- Dancing Star – the small peninsula with a predator fence
- Ackers Point – the area with predator trapping near the town
- Ulva Island – a predator-free island just off the coast.
The aim was to see if areas that had received long-term pest control had higher numbers of birds than other areas. It would also give an indication of the change in bird numbers that we could expect if we removed predators from around the township.
The Results
The following graph really speaks for itself.
MEAN CALL COUNTS FOR NATIVE BIRD SPECIES PER 5 MINUTE SAMPLE INTERVAL. SITES: 1 = NORTH ARM, 2 = PORT WILLIAM, 3 = DANCING STAR, 4 = ACKERS POINT, AND 5 = ULVA ISLAND.
Ulva Island is setting the standard for what we would expect from an island that has been predator free for almost 20 years, with around 5 times the level of native bird calls than other non controlled sites. The rat invasion and eradication a few years ago has obviously had little long-term negative impact. The other outstanding feature is the high level of birdlife at the Dancing Star site, with about twice as many birds as the other sites. This clearly shows the benefit of the predator fence and seven years of ensuring that any invading predators are removed.
What does this tell us about a Predator Free New Zealand?
If we set Ulva Island as the level that birds should be at in our forests, then you can see how poorly these native birds are doing in sites without any control of rats, cats and possums. Perhaps also it suggests how much we have to gain should we follow through with the predator free Stewart Island concept.
Comparisons with the mainland are likely to show an even starker picture. Stewart Island/ Rakiura has higher levels of birdlife compared to the rest of the country, largely thanks to the lack of mustelids (stoats and ferrets). So the benefits of removing predators on the mainland are likely to be even higher.
Photo by Tim Rooke/REX Shutterstock
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