Speciation and extinction: the land snails of Lord Howe Island and Norfolk Island

I'm going to be talking to you today about speciation and extinction on Lord Howe Island and Norfolk Island. So three years ago Frank and I started an ABRS project on the land snails of Lord Howe Island and Norfolk Island and our aims in this were to do a complete taxonomic revision of all of the land snails of both islands and also conservation assessments.

The current state of affairs on both islands was that the most recent taxonomic revision was from the 1940's, all based on shells and there were no live photographs. There were, what really existed for each species was usually a drawing of the shell, in some cases there was not even a drawing of the shell and a brief description, so there's been a lot to do in terms of taxonomic revisions. But also islands are really fascinating places to study evolution. So today, rather than putting up phylogenetic trees and you know and talking about detailed revisions of obscure species, I'd like to talk to you a little bit about speciation and sort of the evolution of this amazing biodiversity on these islands.

So Lord Howe and Norfolk Island are both volcanic islands lying between Australia, New Zealand and New Caledonia and they have Australia's highest diversity of land snails. So just briefly about each island, Lord Howe was formed around 7 million years ago and is quite small, the main island is only fourteen and a half square kilometres. It's got two basalt mountains on the southern end of the island, reaching up to 875 metres and lower hills in the northern part of the island. And unusually for a Pacific island, large amounts of it have never been cleared and 75 percent is protected as a permanent park preserve. There are at least 60 endemic land snails from 10 different families on the island and we're also in the process of describing several more and five of the species are listed as threatened.

So in contrast, the Norfolk Island group is much younger being formed only 2.3 to 3 million years ago and also much larger. The main island is 35 square kilometres. It's also got a much larger permanent population. So the national park is 6.5 square kilometres, a smaller protected area. But most of the rest of the island has been cleared and even the national park was only fenced in the mid 80's. So before that you know cattle roamed through it freely and there was not much in the way of pest control or anything like that. There are around 57 endemic land snails from eight families described from the island. However, some of those were described as sub-fossils, so species that were extinct when already, when they were first described and so not quite as many extant species. And there are 5 species listed as critically endangered but 3 of these are probably extinct.

So I wanted to talk a little bit about Oceanic Islands. A pattern that's been seen on many islands and this was first observed in mammals, is that once founders arrive on an island there's an initial rapid diversification. So high, a fast rate of evolution which gradually slows after this initial period, this founding period. And we can see some examples of this on these islands and one example is in the Microcystidae, which is a family that make up about half of the diversity on Norfolk Island. So in this group, the land snail taxonomists who've worked in this group before, have described a large number of genera that are all really quite morphologically distinct and you can see from the shell photos here, that you know they've got different shell shapes and sculpture and that sort of thing. But within each genus the species are really quite, really very similar and they're distinguished on very small features of you know slight differences in shell shape or coilings, you know spire height that sort of thing.

And what this seems to reflect is that arrival and rapid diversification into different morphological types, you know filling different niches potentially. And then thereafter a slower rate of evolution and smaller differences, you know much smaller differences between these recently diverged species. So we have this really high diversity on these two islands, we know that for land snails which are not you know, they're subject to passive dispersal but they don't move great distances on their own. And so they're subject to, they tend to have large endemic radiations on islands. We wanted to look a little bit about what some of these drivers of speciation might be on these two islands that's caused such huge biodiversity. And so one of the things that we've seen is some signs of ecological niche differentiation on both islands but I wanted to give an example here from Lord Howe Island.

On Lord Howe Island, on the two southern mountains, Mount Gower and Mount Lidgbird, there are these, there are lots of these basalt, exposed basalt cliffs. These sort of sheer cliff faces and quite a lot of species across the different families on Lord Howe Island have adapted to be specialised rock face dwellers. So on the photographs here you can see on the left, one of these basalt cliffs and on the right members of team snail searching through the leaf litter. And so what we tend to have, so here, we've got five different families and in each in each of these cases the top row is a leaf litter or arboreal species and the lower row is a closely related sister species that's become a rock face dweller. So you can see here that there does seem to be this speciation being driven by ecological niche differentiation, where species will be sitting in different niches and feeding in different ways.

Another factor behind speciation on these islands is geographic separation. So another example from Lord Howe Island, there are three genera I've depicted there from the family Helicarionidae. And in this family there are three extant genera and each one has two species and these species pairs are always one from the northern part of the island, from the northern lowlands and then a second closer related species from the southern part of the island. Another factor is sexual selection and this is an interesting one because most of the species that we're looking at are simultaneous hermaphrodites and sexual selection usually involves conflict of interest between male and female partners, which can lead to a kind of arms race where you can get extreme mating behaviours that are sometimes even quite costly. It has been shown to occur even in simultaneous amphetamines and one of the prime examples of this, is the use of love darts in some land snail groups.

And so I'd like to talk here about an example of sexual selection that we can see on Lord Howe Island. So the species depicted on this slide here is Parmellops etheridgei and you can see this is the usual sort of mating behaviour that we have in land snails. It's a simultaneous transfer of sperm, so exchange of sperm masses and and its internal fertilisation. So one of the groups that we have been studying on Lord Howe recently has a species complex, where there are a number of really closely related species that are very similar externally. In fact we can only distinguish them by dissection or by sequencing.

But what they really differ in is penis length. And so you can see these are sympatric species. The green dots and the green bordered diagrams show the long penis species, which has a very long coiled penis and the blue dots show a separate species with a very short penis. And this is a primary difference between the two species. There's also a, it's also supported by genetic data as well. And we were aware that penis length is very important in this group and not just in these two species but the other members of the species complex but it wasn't until recently that we discovered the reason. When Ian Hutton from Lord Howe Island sent us this photograph of the long penis species mating. And you can see from this that in this case, there's external fertilisation.

So what you can see here are the two penes intertwined and a sperm transfer will happen at the tips of the two penes. So what we know from other species of land snails with external fertilisation is that in cases like this penis length is really important because if they if the penis length differs, they're unable to make a successful sperm transfer. So this seems to be a case where penis length is really driving speciation and if we have deviated into a short penis species and a long penis species then hybrids would be likely to be unable to mate because they would have some sort of intermediate length penis that wouldn't be able to have a successful copulation. So we would have reinforcement driving this reducing the fitness of hybrids. I should say that we actually don't know very much about the mating behaviour of most of these species so this is an area that's really interesting to investigate. This is the first photograph as far as we know for this family of land snails at all, so it's something that we'll be really interested in looking into a little bit further if we get the opportunity.

I also wanted to talk a little bit about conservation on Lord Howe and Norfolk Island because while we initially started with this taxonomic grant to fund taxonomic revisions, we've ended up on both islands becoming involved in additional projects to do with conservation on these islands. So land snails you may not know this, but land snails are the species worldwide with the highest number of extinctions and a lot of this is driven by extinctions on Oceanic Islands due to introduced predators.

So again I'd like to just briefly talk through the situation on each island and tell you about some of the things that we've encountered and some of the work that we're doing in the conservation area. So as I mentioned before, Lord Howe Island has had relatively minimal clearing and there's still a large proportion of the original vegetation remaining. But there are introduced predators and I'm only really focusing here on the predators that are a problem for the snails. There have been lots of other species as well that are you know predators on or introduced species on Lord Howe Island. So the two key ones for the snails are the rodents mice introduced in the mid-1800's and rats introduced when there was a shipwreck in 1918. However, an island-wide rodent eradication was carried out in 2020, which I'll talk about a little bit more in a minute.

Another threat on Lord Howe Island is climate change, so the two southern mountains, Mount Gower and Mount Lidgbird both have cloud rainforest on the tops of the mountains and there's a clear record of a decreasing number of cloud cover days and so it means that those areas are gradually drying out. And in fact the entire the Mossy Gnarled Cloud Forest on Mount Gower was listed as a threatened ecological community in 2012 because of this. So any of those species that are only found in that particular environment are potentially at risk.

On Lord Howe Island the first extinctions of land snails were recorded after rats were introduced in 1918. So this picture here is of Epiglypta howeinsulae, one of the largest land snails on the island and it was first recorded by you know the early collections of the first 50 years I guess, of museum expeditions that was recorded and we've got records in the collections from 1970 up to 1920. So it was last collected in 1920, 2 years after the rats were introduced and hasn't been seen since. This is a species that's limited to the summits of the southern mountains and it seems to, it seems that all of the species that have been really heavily affected are the larger ones, which might be more likely to be preyed on by rats but also they tend to be from the southern mountains. So we have the largest species on the island, Placostylus bivaricosus, used to have subspecies on the southern part of the island and the northern part and the southern ones have disappeared and it only remains in the northern part of the island now. So whether this is a combination of rodent predation on the larger species and climate change may be having a factor as well, we're not completely sure. But all the critically endangered species are from the southern mountains as well.

Before we started our targeted surveys in 2016, there were also a number of other species that hadn't been collected or observed live for very many years and they were also, you know suspected to be extinct. So since the rodent eradication in 2020, it seems to have been very successful. There has been one pocket of rats found last year, which have since been eradicated and apart from that so far, the medications is appearing to be very successful. And we have certainly found it's a little, it's early days really to tell the impact on the snails but we certainly have found since eradication several species that were thought to be extinct were found alive.

So these are four of them, which is very exciting. And also the critically endangered species all appear to have been increasing in numbers, one of them quite spectacularly. It's worth saying that around the same time the eradication, Lord Howe Island's drought broke and so there's been better weather in those recent years and so we hope to keep on carrying out these targeted surveys to really try and unpack you know what the differences are and what might be a result of the eradication. But we're certainly seeing more of the critically endangered species and actually more of the larger snails in general. There are a couple of species that haven't never been listed and which we didn't suspect were being heavily impacted by the rats but once the rats disappeared there was this population explosion among this species Parmellops perspicuous on the summit of Mount Gower in particular.

Norfolk Island in contrast, has been much more extensively cleared and so these two maps show plant communities in 1750 on the left and in 2020 on the right. And you can see that a huge amount of the vegetation has been cleared and in fact some of the vegetation types have almost completely disappeared and are only left in little remnants, some of which are on, quite a few of which I think are on private land. You can also see in the picture on the right, the purple sections are non-native vegetation so there's also problems with weeds. As well as this, there are also introduced predators and again I'm only mentioning the ones that are really relevant to snails. And so, as well as the black rat, which was introduced in the 1940's is also the Polynesian rat, which has been there for a lot longer and also chickens. So feral chickens run wild on the island, they're found all through the national park and also the reserves and they're really a problem for the snails and they also seem to be increasing as a problem in the last 20 years or so since I was first there. Climate change is also an issue on Norfolk Island there certainly seems to have been you know, drier weather that in recent years, they've had their several driest years on record and you know so fewer wet days. And we're seeing the results particularly some of the coastal habitat types that are quite exposed, we're really seeing a drying out of some of those habitats.

Norfolk Island hadn't had very many targeted land snail surveys in recent years and so possibly just as a result of spending a lot of time there and doing repeated surveys, we have once again had the excitement of finding some species that were thought to be extinct alive in our recent surveys. So of the three species on this page, two of them are listed as extinct by the IUCN and the third the lovely hairy snail there, hasn't been listed as extinct but hadn't been seen alive for a great many years, so we're very excited to find one little pocket remaining. In fact on our first trip to Norfolk Island in 2020, the two critically endangered species Advena campbellii and Mathewsoconcha suteri, really looked like they were on the brink of extinction. Each of them was reduced to a single population with a small number of specimens and also in a very limited area and there were signs in one of them that there'd been a big recent mass mortality event, probably linked to dry weather so there were lots of freshly dead shells adult, you know not predated just freshly dead shells littering the ground and we found very few that were alive.

So upon seeing that we sought out a collaboration with Taronga Zoo and also Parks Australia and the Norfolk Island Regional Council who are the land managers and we've managed to establish a conservation program supported so far by three small conservation grants, which is allowing us to to run a next situ breeding program at Taronga Zoo and also to increase pest control on the island particularly around the two known populations, to continue surveys and to look for additional populations and monitor the health of the existing wild populations. And we also have funding now to begin some trials of exclusion fencing so that we can make some plans for how to return our captive bred snails to the island once we get to that stage. It's been a really interesting area to work in, challenging to set up a capture breeding program for snails that are very poorly understood but we've learned a lot about them in the process and we're looking forward to extending this if we can and potentially even extending to other species. Because there are other species on the island that are not listed that really are also in in quite dire straits, so there's a lot to be done but very exciting work to be involved with.

So I think I'm out of time so I'd like to finish by thanking all of those who provided photographs for this talk and all of the Malacology Department who supported the field work and the research that we're doing on Lord Howe and Norfolk Island. Thanks very much for listening.