The remarkable tree lobster

I can’t remember the first time I heard about the Lord Howe Island stick insect, and I can’t say my interest has been much more than in passing. They would pique my interest when I saw them in the news, only to fade back into the vast and ever growing pile of ‘oh, interesting’. Then, at the beginning of November, I actually got to come face to face with one of these creatures and it was so incredible I decided to find out more. This is what I’ve learned…

Lord Howe Island stick insects are - I’m sure you’ll be surprised to learn - found on Lord Howe Island. This is a small crescent-shaped island in the Tasman Sea between Australia and New Zealand. I didn’t realise until researching this piece that the island is actually inhabited. It has a population of 382 people according to the 2011 census.

The Lord Howe Island stick insect (Dryococelus australis), or LHISI as it’s known to friends, was first described by Xavier Montrouzier in 1855 from specimens collected by the HMS Herald several years before. It’s thought that mice reached the island some time in the latter part of the 19th century but it was the grounding of the Makambo that is believed to have led to the arrival of ships rats on 14th June 1918 and it didn’t take long for them to spread across the island, leaving a trail of devastation in their wake. As many as 15 species of vertebrates, and countless invertebrates, have gone extinct as a result of their introduction, and the LHISI is just the most notable casualty.

But why is the LHISI so famous? The main reason is its size: it’s huge! (for a stick insect). It’s also a rare conservation success story, and one which I think deserves a wider audience.

photo by Sarah Hearne
Lord Howe Island stick insect at Melbourne Zoo

The LHISI was thought to have gone extinct within a few years of the ship rats arrival and was recorded as presumed extinct in the IUCN Red Data List 1983. However, Lord Howe Island is surrounded by other, smaller, islands and there were a few people who held out hope that a relict population may be hiding on one of these. These weren’t pipe dreams – in 1964 a rock climber named David Roots was climbing Balls Pyramid, an extinct volcano located 23km to the southeast of Lord Howe Island, when he came across the remains of a recently deceased female LHISI. Then in 1969 two further individuals were found, suggesting a breeding population was present.

Balls Pyramid is a terrifying-looking place. The sheer rock rises out of the ocean almost vertically to a point. There’s hardly any vegetation and no soil and seems to be the most unlikely place to find a tree-, shrub- and moisture-loving stick insect, but they were clearly there. Over the years attempts were made to find living LHISI on this unforgiving island, but in vain.

By Fanny Schertzer - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=7947729
Balls Pyramid

In vain, that is, until 2001, when a group of researchers led by David Priddel surveyed Balls Pyramid in order to find this illusive insect. They were limited to areas that didn’t need ropes or specialised climbing equipment which couldn’t have given them much scope. Luckily these accessible areas accounted for most of the islands vegetation and so were the main areas where the LHISI might be found. They searched during the day and found evidence of their presence including frass (which seems to be the name for stick insect poo) and eggs, but still no live insects. A night survey was conducted and there in the torchlight, for the first time in more than 70 years, they saw two live LHISIs munching away on the new leaves of an endemic tea-tree bush!

They found two female adults and one female nymph that night and, not knowing how big the population was, took only photos, and left only footprints. 

One of the first rediscovered Lord Howe Island stick insects (from Priddel et al. 2003)



The researchers returned the following year to conduct another night survey. This trip was much more successful, finding 24 live LHISI. While a much greater number than the previous year, this still made them the world’s rarest known invertebrate. They were able to sex 10 of them and found that all but 2 were female. This skewed sex ratio isn’t unusual in stick insects and it agrees with historical records of the species which indicate that males were much rarer than females.

One big question remained though. Were these actually LHISI or were they a sub-species, genetically distinct from the main island’s population? They certainly looked different to the main island population – they were a different colour and had a distinctive stripe on the abdomen, not present in the museum collections. However, this could have been down to deterioration of the specimens. Or it could be a sign they were distinct.

Another question was how on earth did they reach Balls Pyramid? It’s never been attached to Lord Howe Island, even during the last ice age when sea levels were much lower than they are today. Several explanations have been proposed, but my favourite (and arguably the most plausible) is that they were mistaken for sticks in an otherwise rather stickless environment by common noddies and incorporated into nests. Dead or dying stick insects can still harbour viable eggs which would hatch on this inhospitable island and a few would find a home amongst the tea-tree bushes. This may sound implausible but it’s really not – one of the 1969 stick insect finds was as part of a nest!

So now that the stick insects had been rediscovered the challenge was to decide what to do with them. Their home on Balls Pyramid was tiny and vulnerable. There was no water source beyond rain and it was clear that the population grew and shrunk with the rainfall. A couple of years of drought and they could potentially be gone.

A captive breeding programme was needed and in 2003 two adult pairs were taken from Balls Pyramid and transported to mainland Australia. One pair went to Sydney, the other to Melbourne Zoo. Things did not start off well. The Melbourne Zoo female soon became listless and stopped eating. Vets examined her and x-rayed her to see if they could identify a cause but there was nothing physiologically wrong with her. After 5 days of not eating,
… she became completely immobile and did not react to touch or light, and a solution of [tea tree] leaves (mashed with a mortar and pestle), glucose and calcium in distilled water was administered to her with an eyedropper on her mouthparts. Within a few hours, she became active again and resumed normal activity, subsequently living for another year. 
[Honan (2008), p407-408]

Once she was back on her feet she began laying eggs, but things still weren’t right. The resulting nymphs were smaller and had a much lower chance of survival than expected and it was feared that this was a result of inbreeding. This problem continued in subsequent generations and in the end four male adults were switched between Melbourne and Sydney. Things almost immediately improved. The nymphs were larger and survived better and the population exploded reaching almost 700 individuals in Melbourne by 2007.

The people involved in the captive breeding programme are true pioneers. Nothing was known about these animals. There was no information about their behaviour, diet, or lifestyle from the records of the 19th and early 20th century, and it was clear they were living in a marginal and probably sub-optimal habitat on Balls Island. They were known to eat the endemic tea-tree shrub but that was it. And failure wasn’t an option. Somehow, these people managed to not only keep their LHISIs alive, but got them to thrive, so that by 2007 two glass houses in Melbourne Zoo housed more than 10 times the known wild population!

They worked on a miniscule budget which was only sufficient to maintain the captive population but not effectively study their biology and genetics, a situation sadly common for invertebrate conservation. But somehow they managed to do trials to test for the best material for incubating their eggs, to work out what food they like, and to work out whether a reintroduction to Lord Howe Island was feasible. And along the way they were able to get some insights into the unique behaviour of these insects, the most remarkable of which is that they are highly gregarious – they like to sleep together in large groups – and they can form pair bonds between males and females. 

Male (bottom) and female (top) Lord Howe Island stick insects in their daytime retreat (from Honan, 2008)

So is it possible to reintroduce LHISI back onto Lord Howe Island? Potentially, yes. But that big question remained – are the Balls Pyramid stick insects actually LHISI or are they a distinct sub-species? The original paper raised concerns over colouration, and subsequent work has also found discrepancies in the size of various body parts. The problem is that none of this is conclusive as the researchers are comparing with museum collections and there’s no way to know how representative these collections are. It could have been that those original collectors took the exceptional specimens, rather than the bog-standard ones, and this would lead to a skewed sample that wouldn’t be representative.

To try and get to the bottom of this a group of researchers led by Alexander Mikheyev did gene sequencing on the Balls Pyramid stick insects and those in museum collections. This is simple to write but not so simple to do. Obtaining genetic sequences has got much quicker and cheaper in recent years but when you’re dealing with new species it’s still incredibly complex and the genome of the LHISI turned out to be massive! They think that it’s hexaploid (human are diploid – we have two copies of each chromosome. Hexaploid means that they have six sets of chromosomes!). More importantly, they found that the two populations were genetically identical, meaning the differences between them are most likely due to unrepresentative museum samples. And this means that they can be used to form the basis of a reintroduction programme.

The one thing standing in the way of this is the rats. They’re still there. And any stick insect released onto Lord Howe Island would have a very short life expectancy while that’s still the case. But luckily, plans are afoot to do something about this. Rodent eradication programmes have been highly successful over the last decade or so and while Lord Howe Island is large, it’s still well within the realms of possibility to completely eradicate rodents from the island within a few years. For islands like Lord Howe where mammals aren’t native, baiting can be highly effective as there’s no fear of collateral damage. The first bait spreading is scheduled to take place in austral winter next year. And, if/when successful, it shouldn’t be long before Lord Howe Island once again is home to its eponymous stick insect. 

Me with the Lord Howe Island stick insect at Melbourne Zoo

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Descriptions of history, rediscovery and research on LHISI comes from:
Priddel, David et al. (2003). Rediscovery of the ‘Extinct’ Lord Howe Island Stick-Insect (Dryococelus Australis (Montrouzier)) (Phasmatodea) and Recommendations for Its Conservation. Biodiversity and Conservation 12(7): 1391–1403.
Honan, Patrick. (2007). The Lord Howe Island Stick Insect: An Example of the Benefits of Captive Management. The Victorian Naturalist 124(4): 258–61.
Honan, Patrick. (2008). Notes on the Biology, Captive Management and Conservation Status of the Lord Howe Island Stick Insect (Dryococelus Australis) (Phasmatodea). Insect Conservation and Islands: 205–19.
Magrath, Michael J L. (2017). Determining Host Plant Preferences for the Critically Endangered Lord Howe Island Stick Insect ( Dryococelus Australis ) to Assist Reintroduction. Journal of Insect Conservation 21(5): 791–99.
Mikheyev, Alexander S et al. (2017). Museum Genomics Confirms That the Lord Howe Island Stick Insect Survived Extinction. Current Biology 27(20): 3157–3161.e4. https://doi.org/10.1016/j.cub.2017.08.058.

Comments

Head*desk said…
I am not hugely interested in invertebrates in general, but the LHISI absolutely fascinates me and I so want to see the ones at Bristol Zoo!

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