Everything you ever wanted to know about snails (on Lord Howe Island)
Dr Isabel Hyman and Dr Frank Köhler might look unassuming enough, but this dynamic duo are anything – they’ve just released a field guide, the first of its kind, to the land snails of Lord Howe Island.
The World Heritage-listed island, lolling in the Tasman Sea between Australia and New Zealand, is renowned for its high numbers of endemic plants and animals, and has Australia’s highest diversity of land snails, with around 70 species found nowhere else. Frank and Isobel spent several years studying these unique and beautiful creatures.
© Craig Stehn
“Our journey to this point has been eventful and exciting – and more intrepid than you might expect, considering that we are chasing such slow-moving creatures!” Isobel says. “Lord Howe Island has diverse habitat types, ranging from towering basalt mountains topped with cloud rainforest to coastal grasses and heath. Three guesses as to where most of the snails are… yes! The most inaccessible and hard-to-reach mountaintops, dripping with moisture from the clouds that sit around them, are perfect sites for an explosion of snail diversity. This means that on expeditions to Lord Howe Island, we often spend large portions of our time climbing to remote sites, sometimes lugging heavy packs and camping equipment, in order to track down elusive species.”
So why are these land snails so deserving of all this time and effort?
According to Isobel they represent a large radiation of unique but poorly understood species found nowhere else, and a number of them are under threat and at risk of extinction.
“The predominant threat has long been rodent predation,” Isobel says. “Rats are a huge problem on oceanic islands, where the fauna are very vulnerable to disturbance. Black rats were introduced to Lord Howe Island in 1918, and immediately there were clear and well-documented species declines and extinctions. Five species are currently listed as Endangered or Critically Endangered due to rodent predation, and more are likely to have suffered.
“However, the good news is that after many years of planning and preparation, an island-wide rodent eradication was carried out last year, and the last rat was seen in October 2019. Since then, there have been remarkable improvements in seed germination and plant succession and increases in the numbers of many animal species. On our next survey, planned for November 2020, we hope to see an increase in the abundance of the endemic snail species as a result.”
Another current threat is climate change. Our warming climate has led to a decrease in the number of days of cloud cover on the mountaintops, leading to the oceanic cloud forest being listed as a Critically Endangered Community and a decline in some of the summit snail species. Drier summers are also causing a contraction of some lowland snail species as well. “By studying the past and present ranges of these species, we can determine if a species is in decline and if it is necessary to implement protective measures,” Isobel says.
“Several of our trips to Lord Howe Island in the last four years have been to assess the status of four Critically Endangered species, in the lead-up to the rodent eradication. These species are all from the southern mountains, and all are extremely rare, probably primarily due to rodent predation. They are ground-dwellers, ranging in size from 7mm to 35mm in shell diameter, and can generally be found by patient searching under leaves, palm fronds, rocks and logs on the ground. The three smaller species have been collected in small numbers in recent years, but the largest, the Magnificent Helicarionid Land Snail, has not been seen alive since 2002.
“The southern part of the island has two large basalt peaks, Mt Gower (875m) and Mt Lidgbird (777m). The lower slopes of Mt Lidgbird have some marked trails, but around 500m a series of steep cliffs begins, and so the summit is best reached by helicopter and has only been surveyed very infrequently. We have undergone training to allow us to be winched on to the summit, but unfortunately the weather conditions have never been suitable and to date we have had to content ourselves with forays to the base of the cliffs, at different points around the mountain. Over four years of surveys, we have identified four sites where one of our target species can be found on the upper slopes of Mt Lidgbird (Mt Lidgbird Pinwheel Snail, pictured). It lives on basalt rock surfaces, sheltering in crevices during dry weather.
Image: Lachlan Copeland © Lachlan Copeland
“Our second destination, Mt Gower, has a well-marked track and can be climbed with a guide, but with a 3-hour climb each way to reach the summit it is necessary to camp overnight to allow sufficient time for sampling. We have developed a series of 24 survey sites evenly spaced across the summit. Over four years, we have found two Critically Endangered species at three different sites either on or just below the summit – Whitelegge’s Pinwheel Snail, Slug-like Pinwheel Snail [pictured below].
Image: Adnan Moussalli © Museum of Victoria
Image: Adnan Moussalli © Museums Victoria
“In all three cases, we have found no more than 5 specimens at any one site (and usually we only find one!). These species are clearly very rare, but with the removal of rodents we hope to document a slow increase in their numbers. The final species, the Magnificent Helicarionid Land Snail, has not been found now in many years and it is looking more likely that this species is extinct. However, given that their main predator has now disappeared, I live in hope that one day, I will turn over a leaf and find a Magnificent surprise, and you will hear my cries of joy echoing from the mountaintop.”
Image: Abram Powell © Australian Museum
