Mohua in trouble from stoats and rats
DOC biodiversity manager Barry Lawrence knew his precious mohua were in trouble at the head of Lake Wakatipu when his ground control operations to control rats in the Dart failed comprehensively in 2006.
Barry is very passionate about the mohua, a species in grave danger of extinction on the mainland, clinging to a few valley sanctuaries, and one of the focal points of DOC’s Operation Ark programme to protect some key indicator species.
“It’s an incredible little bird, one of the real characters of the bush. It has adapted itself to be able to live off bark, and has developed a really strong little beak and stout legs,” says Barry. And it has a great warble.
But it certainly isn’t custom built to survive rats and stoats and the unfortunate habit of roosting, as well as nesting, in small holes. There is no way out when predators appear at the door.
DOC biodiversity manager Barry
Threat from stoats
“The stoats had been well dealt to, through the use of extensive trapping and poisoned eggs over 105 kilometres of the valley. We found with stoats that ground control proved reasonable and easy. We set out long lines, wide spacing, and infrequent trap checks, and found we could reduce their number by 80%,” says Barry.
So much for stoats, but the challenge is that if you take out stoats, which are natural predator of rats, then the rat numbers may rocket.
Threat from rats
There had been a massive beech masting in 2006, which means that in the right, warmish, climatic conditions, there is an explosion of beech seeds. These can total 500 kilograms per hectare which means 33,000 rat feeds in a rat’s backyard of one hectare!
So with this kind of spare silage in winter, the rats can feed and breed, producing around 6 young every 32 days. And when the seed germinates or rots, then rats move up the trees to dine out on the native birds.
“We had done our best with ground control for rats, putting bait stations every 100 metres and covering 30 square kilometres of ground, but the rat rates continued to rocket, rising from 20% in July to 50% in September.
“There is some tough country in there, and the trappers were told to keep going until they ran out of red beech or they got bluffed!” (i.e. couldn’t get any further). Despite best efforts, the ground control didn’t work because of the volume of rats.
1080 aerial control
"So it was time to haul out the 1080, put it in a bucket and fly it on to save this population. We put it on at 500 baits per hectare and this meant there was a 500 times greater chance for a rat to encounter a 1080 bait than a ground based bait station.
“It has worked really well. In the areas where there was no rat control we had 90% of mohua disappearing, and the population can’t survive that sort of damage. But using the aerial control, less than 20% of mohua disappeared,” Barry says.
One of the really big winners was actually the robin population which increased by 180%, a figure that is quite significant because some believe robins can be susceptible to aerial 1080. But in this case the benefits overwhelmed any risks. A number of other bird species also benefitted but these two were the big winners.
The aerial operation was repeated in 2009, trialling much less bait (only 1kg per hectare) and that has proved equally effective in protecting these populations.
Future rat control
There are still enormous challenges. It is very difficult to eliminate rats completely from an area, and it is suspected that surviving rats are subservient individuals which do not get to baits because the stronger rats predominate. How to solve this and get a longer lasting knockdown is the next big thing.
Critics often charge that the problem with using 1080 is that you have to keep doing it indefinitely. ut that is true of every method of pest control including trapping and it applies to virtually every field of human endeavour. Doing nothing is not an option.
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