Wellington has its very own snail species, Potamopyrgus oppidanus, found nowhere else in the world – and it’s smaller than a grain of rice. But their numbers are alarmingly decreasing due to bikers and weeds.
Science Researcher Lara Shepherd tells us where they are found and what we can do to help save this rare species.
Te Ahumairangi Hill
Most people in Wellington appreciate the town belt for providing a scenic backdrop to the city and opportunities for recreation and connecting with nature.
But not everyone realises that the town belt is also important as habitat for native species. For example, the freshwater snail, Potamopyrgus oppidanus, is only known from Te Ahumairangi Hill (formerly known as Tinakori Hill), less than 1km from the Beehive.
Where is it found?
This small snail, which is only 3mm long, is found in damp gullies and seepages with bush cover and undisturbed leaf litter.
It was first described in 2008 by Dr Martin Haase and the type specimen (the specimen on which the name is based) is held by Te Papa.
Potamopyrgus oppidanus was classified as Nationally Critical in 2018, the most threatened ranking available.
However, it’s now in even more trouble. Bronwen Shepherd (Te Ahumairangi Hill Ecological Restoration) and Dr Karin Mahlfeld (Wellington Open Science Lab) recently surveyed the snail population and found it is now rare in areas where it was common a few years ago.
The most significant threats to the snail come from the construction of illegal mountain bike tracks on the hillside.
Track building is harmful to snails because it removes vegetation cover and leaf litter, increasing run-off and sediment.
Weeds such as wandering willy, which smother snail habitat, can also be spread by track building.
What can you do to help this unique snail?
If you live in Wellington you could attend a planting working bee to restore the damaged forest on Te Ahumairangi Hill.
If you’re a mountain biker then only ride on authorized mountain bike tracks (there are four such tracks on Te Ahumairangi Hill).
There is a valuable lesson to be learned from Potamopyrgus oppidanus – even small patches of indigenous vegetation can be home to unique plants and animals. It is therefore important to conserve as much of this diversity as possible, even in the middle of our capital city.
Hi Lara, can you provide links to the data that proves the tracks have directly impacted the snail population please? Many of the tracks have been there since even before the snail was discovered, and while there are some new ones, blaming the tracks for a sudden decline seems to be conjecture only at this stage, without real evidence. This sort of opinion shouldn’t be on a Te Papa blog page without something to back it up.
Has anyone studied other possible factors? What about the drier summers? What about the walkers and dogs disturbing the environment and spreading weeds too? What about the other changes in biodiversity up there in as the pine trees age and are felled? Seems to be lots of possibilities, but the blog is blaming just one small group.
Note that I don’t agree with the illegal trail building, but so far I’ve not seen any evidence that they have caused a single snail death. Happy to change my view if you have the evidence.
If there is no direct evidence then it might be worth retracting this blog and issuing an apology.
Hi John, the TA Hill site is part of an ongoing study into the impact of the illegal mtn bike tracks on this snail habitat. In order to truly test the the effect of the tracks the first step is stop their use to see if the snails increase in number (if it isn’t too late – their aren’t many left). Attempts to date to block off the illegal tracks have been unsuccessful so far. Note that we aren’t criticising mountain bikers in general (I’m a mtn biker myself), just those that construct and ride illegal tracks.
Hi Lara. The statement I have an issue with is “The most significant threats to the snail come from the construction of illegal mountain bike tracks on the hillside.” I think you should retract this from the blog unless you have clear evidence. At the moment this statement still seems conjecture as you are only gathering the evidence.
Previous information on those snails seemed to point to other risks such as walker and dogs too, but now it seems the tracks (some of which predate the discovery of the snails) are being blamed as the most significant threat without any attempt to differentiate from long standing tracks and the newly built ones, or to analyse other causes/risks. This is not science and does not belong on a Te Papa blog site without eveidence to back it.
Disclaimer- I regularly mountainbiker in the town belt (and pest trap and tree plant and walk). I’d love to be able to ride some of the tracks up there but am avoiding them until we can work though getting a few of the long-standing tracks approved for riding by the council. I’m against any new unapproved trail building, and was part of the group that planted out one of the illegal tracks the other weekend.
Unsolicited downhill tracks have been built adjacent and through the known snail habitat area during 2018/19, which makes up a significant portion of total known snail habitat. Observations of the damaged area reveal bank impaction/erosion and significant removal of trees. This species and other freshwater species are known to be highly susceptible to physical habitat damage as they rely on leaf litter, and low sedimentation load. Weeds quickly inhabit light tunnels opened up by tree removal which we assume also negatively impact habitat. The species has reduced in population in the damaged habitat area.
The snail habitat in question is not otherwise accessible to the public. No other cause of tree removal or steam bank damage can be found. Macroinvertebrate and water quality data continues to be collected at the site. No other reason for the population decline has been found.
Specialist freshwater scientists, ecologists and malacologists from
DOC, Te Papa, GWRC, WCC, Zealandia are continuing to monitor the area to better understand population distribution and take action to minimise risk to remaining population, whilst WCC Rangers are continuing to attempt to close access to the illegally built tracks that are impacting the area.
If you want more details I suggest you contact Brony, who is leading this study – firstname.lastname@example.org
Its good to see that the Wellington Mountain bike community has recently been out to plant over 300 seedlings in this location to help protect the snails and close the unsanctioned trail.
Seems to be void of quantifiable details. Is this really science?
See above reply to John
Thanks, who would have thought!