Drought conditions and weka giving Abel Tasman's native snails a hard time

Saturday, 1 August 2020

The large Powelliphanta hochstetteri is one of two native snails found in the Abel Tasman National Park. (File photo)

There may be fewer tourists walking the Abel Tasman track, but a group of carnivorous snails that call the park home are getting a hard time from a booming weka population.

Both the critically endangered Rhytida o’connori and nationally declining Powelliphanta hochstetteri snails are found in the Abel Tasman National Park, near Nelson.

Project Janszoon have been monitoring the snails at two sites for the past four years and have found while numbers are increasing in the Canaan area, their numbers are falling in the upper Wainui Valley.

Scientific advisor Ruth Bollongino said snails were facing pressure from predators and the hot and dry summers which resulted in changes in the forest.

* Sailors delighted by critically endangered leatherback turtle sighting in Tasman Bay

* Radio tracking reveals more rare snails in the Abel Tasman National Park

While it was “nothing new” that weka predated on snails, the recent survey showed in 11 months a growing weka population in the upper Wainui Valley had reduced the population of the Rhytida snails by 46 per cent, with more dead snails found than in previous years.

Weka are known predators of carnivorous snails, like the Rhytida o’connori and Powelliphanta hochstetteri. (File photo)

“To see such a quick change in a short time that it influences the snail population to such an extent is quite a surprise to us,” Bollongino said.

Rhytida are one of the rarest species in the Abel Tasman and are only found at two sites in New Zealand; on the Takaka Hill and at Parapara Peak.

The number of Powelliphanta in the valley had also fallen by 30 per cent.

The snails are surveyed at night and are captured each year to gauge the population and growth rates. A total of 857 Powelliphanta and 509 Rhytida snails were estimated to be on the two 4900 square metre sites in the most recent survey.

Bollongino said it was good news the numbers were higher than expected in the Canaan area, but it was concerning to see the sharp decrease at the Wainui Valley site.

The critically endangered Rhytida o’connori snail is found at only two sites in New Zealand. (File photo)

Researchers were able to tell how snails were killed by the way their shells were damaged and most showed predation by weka.

“The weka damage is quite distinguished because they just stab the snails with their beaks.”

It was thought a drop in numbers did not occur at the Canaan plot because the rocky karst habitat gave the snails places to hide from the inquisitive birds.

The Western weka all but disappeared from the Abel Tasman National Park in the 1980s and 90s, with one theory being that they were wiped out by a virus.

In 2006, the species was reintroduced at Totaranui using birds captured in the Marlborough Sounds and their numbers were now strong throughout certain parts of the park, with a population having re-established in the Wainui Valley over the past three to four years.

Bollongino said last season’s monitoring also showed when there were drought conditions in the park, snails didn’t grow as much as previous years.

“Snails only forage during humid nights, so a drier climate means there were fewer nights that the snails can be active and thus they have less time to feed.”

Project Janszoon Director Bruce Vander Lee said they knew snails were vulnerable to introduced predators such as rats and possums, but it was challenging to have a native bird as a major predator.

Monitoring would continue and the data would be watched closely to understand the impact of weka on the snails.

“If monitoring shows snail populations continue their decline we will look at other ways to protect them like weka-proof fencing in population strongholds.”