Recovery and discovery: rare snails on Lord Howe Island

After more than a year rodent-free, two of the Critically Endangered land snails on Lord Howe Island are showing strong signs of recovery – and a closely related mystery species has also reappeared!

In November 2020, I travelled to Lord Howe Island with Dr Frank Köhler to survey the Lord Howe Island land snails. This tiny island harbours Australia’s highest diversity of land snails, with around 70 endemic species that are found nowhere else. Frank and I have recently documented these in a new book, A Field Guide to the Land Snails of Lord Howe Island. The snail populations have suffered heavily from rodent predation since rats were accidentally introduced in 1918, particularly the larger species. As a result, several species are considered to be extinct and five are listed as Endangered or Critically Endangered. An island-wide rodent eradication program was carried out in 2019, and we had high hopes of seeing increased abundance after more than a year without any rodent predation.

On Lord Howe Island, we met up with other members of Team Snail, including Craig Stehn and Melissa Geise (Biodiversity and Conservation, Department of Industry, Planning and Environment) and Caitlin Woods (LHI Marine Parks). Our aim was to record and collect representatives of all the endemic species, from sites across the whole island. We had a particular focus on the Critically Endangered species, which are primarily found on the upper slopes and summits of the southern mountains. These include a genus called Pseudocharopa that contains three species, P. whiteleggei (Whitelegge’s Pinwheel Snail), P. ledgbirdi (Mt Lidgbird Pinwheel Snail) and P. exquisita (Exquisite Pinwheel Snail).

The three known species of Pseudocharopa were described in the period 1880-1930. One, P. exquisita, has not been seen since 1914 and is considered extinct, a victim of rodent predation. The other two both underwent significant declines after rats were introduced in 1918. Several other species were also described but they are now regarded as synonyms of the three valid species – that is, multiple names for the same entity.

When we first began searching for these rare species in 2016, our searches revealed just a single live specimen each of P. ledgbirdi and P. whiteleggei, in two weeks of surveying. Over the next few years, we often didn’t find any, or if we did, it was no more than one or two. Every single find was thrilling, and was greeted with shrieks of excitement. Then with the rodent eradication in 2019, things began to change. The summits and slopes of Mt Gower and Mt Lidgbird were free of rodents by July 2019 (although the eradication took longer around the settlement area). When we surveyed in October 2019, we saw the greatest numbers ever – seven live specimens of P. whiteleggei, and 11 specimens of P. ledgbirdi.

So, it was with great anticipation that we climbed up Mt Gower with our packs in November 2020, 15 months after the southern mountains were rodent-free – and we were not disappointed! We were able to observe an increase in both species, finding 12 live specimens of P. whiteleggei (and some eggs) and counting an astounding 73 live specimens of P. ledgbirdi!

We also came across two highly unusual specimens that we couldn’t easily identify. They clearly belonged to the genus Pseudocharopa, but could not be assigned to any of the three existing species. Upon our return to Sydney, I searched through all the early descriptions and the holotype specimens at the Australian Museum and discovered that there is a fourth Pseudocharopa species that should also be formally recognised. After checking every Pseudocharopa specimen in our collections, I found several more specimens of this enigmatic species, and pieced together its past history.

This species, Pseudocharopa balli (Ball’s Pinwheel Snail), was first collected in 1887 and appears to have been relatively abundant until 1915. After that, it was not seen again, apart from a single shell found in a very remote locality in 2002, until now. So this species shows a very similar pattern to the other members of Pseudocharopa, showing a significant drop in abundance after the introduction of rats, and is likely to have suffered heavily from rodent predation. It is a wonderful sign that this overlooked and unrecognised species has been rediscovered, and offers hope for other species that have not been seen for many years and are believed extinct, such as Pseudocharopa exquisita. It also highlights the extremely low detectability of land snails, which can hamper attempts to locate rare species or estimate their population size.

We hope to return to Lord Howe Island in November 2021 to continue to document the recovery of these rare and fascinating creatures, and to try to learn more about the poorly understood Pseudocharopa balli. And next time, who knows? We may find that yet another species presumed extinct has survived the rats, and is only now emerging from hiding. We live in hope…

Dr Isabel Hyman, Scientific Officer, Malacology, Australian Museum Research Institute.

Acknowledgements

This research was supported by the Lord Howe Island Board.

• Documenting the Land Snails of Lord Howe Island and Norfolk Island was funded by the Australian Biological Resources Study, the Graeme Wood Foundation and the NSW Department of Planning, Industry and Environment.
• A Field guide to the Land Snails of Lord Howe Island was funded by a grant from the Australian Museum Foundation.