Snail “wiped out by climate change” still alive after all

The Tuatara’s Tale (q.v.) exemplified a pervasive problem in conservation these days: that of wrongly blaming climate change for the troubles of threatened species. The main reason why this is a problem is that pointing the finger at climate change means overlooking all the real reasons behind the decline of a species, reasons that it is actually feasible to address in cost-effective ways. You cannot save the tuatara by reducing carbon dioxide emissions, but you might be able to save it by eliminating rats from its island homes.

Rhachistia aldabrae, the Aldabra banded snail, went one stage further than the tuatara: it wasn’t just threatened, it actually “became extinct” due to climate change. The fact that it was subsequently re-found alive ought to give pause to anyone itching to declare a species extinct, and to those considering publishing their declaration; maybe it should also give pause to those wishing to blame climate change for everything bad that happens in the world.

This pretty little snail is endemic to the Seychelles atoll of Aldabra, hence its specific name. Although the atoll is relatively large (>30 km long), the snail is still left with quite a small area of available habitat since the islands surround a large central lagoon. Such a small area of habitat, as you might guess, makes a species particularly prone to extinction.

In 2007 came the news that R. aldabrae wasn’t just prone to going extinct. It was extinct, and the culprit was climate change (Gerlach, 2007). Gerlach’s article in the Royal Society’s Biology Letters was titled “Short-term climate change and the extinction of the snail Rhachistia aldabrae (Gastropoda: Pulmonata),” leaving little room for doubt about the cause. The author attributed the loss of the snail to decreasing rainfall leading to increasing mortality of juveniles; based on limited meteorological data showing that recent years were drier than in the 1970s, together with a correlation between rainfall and the number of snails collected, this seemed a plausible connection. It just so happened that the driest years in the record happened in the 1950s, but as isolated data from two adjacent years this did not give a clear indication about the climate at the time. No juveniles had been seen since 1976, and no adults since 1997, despite targeted searches in 2005 and 2006: it seemed unrealistic to suppose that the snail was not extinct.

When I heard that this snail was extinct, I was inclined to believe it. A lot of land snails, particularly island endemics, have become extinct. Régnier et al (2009) report 400 species of them. Many had met their fate via a deliberately introduced species called Euglandina rosea:

Of the 400 extinct species we listed from oceanic islands, 234 lived on islands to which Euglandina rosea had been introduced, and it is highly probable that of these 234 extinctions, 134 of them were ultimately caused by the introduction of E. rosea.

Régnier et al

What is this charmingly named entity Euglandina rosea, you ask? Doesn’t sound like a species of rat. Well, no, it’s a snail, one that eats other snails. It’s the rosy wolfsnail. It was introduced all over the place in a stupid attempt to control another introduced snail, the giant African one so beloved of primary schools.

Other threats to endemic snails include a flatworm – again introduced to control the giant African snails – rats (again), goats (stripping out palatable plants) and of course humans (over-collecting). So to hear that an island snail had gone extinct was completely unsurprising.

(If you like, you can replace that list of threats above with humans, humans, humans, humans and humans, since we are directly responsible for all the introduced species too.)

What I did not believe at the time was that said extinction had anything to do with climate change. I might have been inclined to accept it if the rainfall record was complete and showed an obvious change. But the rainfall record was woefully inadequate to the task of proving anything. Also, it felt as if the world had been waiting for just such a declaration. Treehugger certainly seemed to be:

Is the Aldabra Banded Snail the First Global Warming Related Extinction?

Title of an article at Treehugger

News of the extinction and the role of global warming in it informed a 2013 paper that was then cited 9 times in the fifth IPCC report (WGII Chapter 4).

However, there was an immediate pushback by authors familiar with the atoll and the snail. Hambler et al (2007) submitted a reply to Biology Letters, stating that the gap in records of the snail was not enough to show it was extinct, that it was infrequent in the 1980s, and that various impacts other than climate change were at play: vegetation changes due to goats, tortoises* and rainfall, and the potential impact of rats as predators. The reply was rejected. Hambler et al (2007) ends with the portentous “We predict “rediscovery” when resources permit.”

Now before our snail comes back from the dead, let me just digress into the rest of its genus for a moment. According to WoRMS**, there are 37 species of Rhachistia, one of which is the hero of our tale. What do its relatives tell us of its ecology? Not very much, in truth. They are concentrated in coastal East Africa, with a few species further east in places like Bengal, Thailand and a supposed introduction in Australia. I would characterise the whole genus as poorly known. Most importantly the ecology is poorly known. The affinities of the “species” are poorly known too, to the extent that some of the 37 species are likely to be synonyms of others. They are often identified based on empty shells: the shapes of the shells are more-or-less identical except perhaps to the eyes of specialists in conchology. Our hero seems to be fairly distinctive in that regard at least, although that is an entirely amateurish analysis by me based solely on its colour (its shell may be more sturdy than many of the others as well). The relative abundances of the species are unclear. These days we expect biodiversity to be well understood, but it usually isn’t. If these species were European, you can be sure that we would know everything there is to know about them. Instead we know next to naught.

Anyway, the snail was rediscovered on the second-largest island at Aldabra (Malabar) in August 2014, thus fulfilling Hambler et al’s prophecy. Those authors asked Biology Letters for a retraction of the original paper declaring the snail extinct: the journal refused. There is thus an unretracted journal article declaring the extinction, via climate change, of a snail that is not actually extinct. This may seem extraordinary and wrong, but it really isn’t. That a paper is wrong is not in fact grounds for its retraction. In this case it might though be cause for reflection by the journal’s editors re: what standard of evidence is required before declaring a species extinct.

The hero of our tale is not dead – at least, it wasn’t 7 years ago. But its future is far from secure. If I were part of the Seychelles Islands Foundation, I’d start a captive population going just in case; the known population occupies an area of perhaps only 3 square kilometres. But I wouldn’t try feeding the snails on the mature leaves of woody perennials as appears to be the case in the featured image. This is in fact a great opportunity to find out more about their ecology: a good start would be to find out its food preferences. I suspect this is a naïve hope on my part. The SIF has enough to be getting along with. Their priorities are likely to be, perhaps understandably, the vertebrates in their charge like the giant tortoises. Invertebrates are likely to be well down the pecking order.

On the face of it, you might expect droughts to be bad news for terrestrial snails, since they travel on a layer of mucus. And for snails restricted to one coral atoll, and with a known range about a third of the area of Heathrow Airport, it wouldn’t be surprising if a couple of bad years (stochastic effects rather than climate change) were enough to wipe the sole population out. Unfortunately the rainfall data are too sparse to show whether it is actually getting drier on Aldabra. The snails are hanging on by a foot: but again it is a case of if humans had never found Aldabra… they’d probably be getting along just fine.

Has this episode taught us anything about the threats to the Aldabra banded snail? Apparently not. I leave it to the reader to spot the logical fallacy in this statement in Wilson & Primack (2019):

The impacts of climate change on Africa’s dispersal-limited species can already be seen. For example, the once abundant Aldabra banded snail (Rhachistia aldabrae, CR) is today so rare that this Lazarus species was once believed to be extinct due to climate change (Battarbee, 2014).

Wilson & Primack

Seychelles Snail—One of Global Warming’s First Victims—Found Alive and Well

National Review, 2014

NOTES

This tale was first told in abbreviated form in Denierland.

Featured image: I got the photograph from Times of India or somewhere but it is apparently credited to Catherina Onezia/Seychelles Islands Foundation. I tried to go to their website but all sorts of alarms went off on my PC claiming it to be a phishing site. I think we’re OK to use the pic here with the attribution attached. It’s used by Wilson & Primack in their book and tagged CC, so maybe it is. I wanted to show a photograph of a living snail: so often a snail is depicted as just an empty shell. Newsflash guys: that’s like depicting a human as just a skeleton. A snail is what’s in the shell.

*Let’s add another threat to the list. Hang on, I hear you cry: how can tortoises be a problem? They belong there, right? Yes. But consider the population fluctuations of Aldabra’s herbivores. In the idyllic deep past, we had a high density of giant tortoises as the sole large plant eater. Then along came European sailors who started collecting the tortoises as handy little fresh food parcels for long trips. Then they let some goats go on the atoll with the hope that when they returned there would be more. So the flora underwent marked changes up until tortoises stopped being harvested in large numbers: with the advent of engine-powered ships? At that stage the tortoise population went back up again. Then the goats got culled in large numbers. In short, it’s likely that the selection pressure on the plant community has been all over the place over the past two centuries, which will inevitably affect our little snail. So I guess we can chalk tortoises up as yet another threat to relabel as humans by proxy.

**The World Register of Marine Species. But our snail is not marine. No, don’t ask.

REFERENCES

Gerlach, J. (2007). Short-term climate change and the extinction of the snail Rhachistia aldabrae (Gastropoda: Pulmonata). Biology Letters, 3(5), 581-585.

Hambler, C., Coe, M.J., Gibson, C.W.D. & P.A. Henderson (2007). “Extinction” of Rhachistia aldabrae contested. Unpublished manuscript. Available at: https://www.hertford.ox.ac.uk/wp-content/uploads/2019/02/Hambler-et-al.-2007-manuscript.doc

Régnier, C., Fontaine, B., & Bouchet, P. (2009). Not knowing, not recording, not listing: numerous unnoticed mollusk extinctions. Conservation Biology, 23(5), 1214-1221.

Wilson, J.W. & R.B. Primack (2019). Conservation Biology in Sub-Saharan Africa. Open Book Publishers. Available at Archive.org.