Isles of the Tasman Sea – Part I: Lord Howe
Introduction
600km off the east coast of Australia, amidst the Tasman Sea, sits a tiny archipelago – the only specks of land for a hundred leagues. The Lord Howe Island Group. The island chain consists of the central ‘Lord Howe Island’ which spans a mere 15 km2, along with a collection of smaller isles and volcanic stacks. Despite their tiny area these landmasses sport high jagged peaks, hundreds of meters tall, towering above the seascape. The size of the Islands would have waxed and waned with the eons, as sea levels are higher during interglacials and lower during glacial periods. The expanse of Lord Howe Island at its zenith is controversial, but it would never have constituted a large island. On the other hand, the coastline lay 2-4m higher during the last interglacial when compared to the modern day (16). Today the entire archipelago is considered UNESCO world heritage due to its interesting collection of flora and fauna with high rates of endemism. Unfortunately, as is also often the case with islands, this diversity has become much diminished in historical times.
Lord Howe Island offers an interesting perspective in these island extinctions as it stands above most others in its stellar fossil and historical records, which allow a detailed documentation of extinction. Herein the focus of this exploration will be on the diversity of the native avifauna prior to human arrival as well as on the turtle Meiolania platyceps, and their demise in the Holocene. Nevertheless, extinctions also occurred amongst other groups, including gastropods, insects, and plants (8).
Fig 1. Map of the Lord Howe Island Group, including the Ball’s Pyramid (Located SE of Lord Howe & not shown to scale).
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The Faunal Overview
Meiolania platyceps stood as the sole species of megafauna on Lord Howe Island, to which it is endemic. The clade to which it belongs, the Meiolaniformes, extends back to the cretaceous and has no living representatives (15). These would have been a remarkable sight to behold, exhibiting both horned heads and spiked tails (4, 5) and reaching immense sizes. Despite fossils of this genus being known from Sahul, New Caledonia and possibly Vanuatu and Fiji (6, 15) it was discovered on Lord Howe Island and M. platyceps remains the type specimen, with an abundance of well-preserved fossils, including eggs (10). M. platyceps was likely a slow terrestrial grazer akin to modern tortoises (3, 9, 15) with an estimated length of over 3m – a giant in the context of such a small island (3). In addition, two extant species of lizard round out the roster of endemic herpetofauna on Lord Howe Island (though also found on Norfolk Island): the Lord Howe Island Skink (Oligosoma lichenigera) and the Lord Howe Island Southern Gecko (Christinus guentheri) (8). No native amphibians exist on the island. Only a single mammal was native to the island, the Lord Howe Long-eared Bat (Nyctophilus howensis) – and this known only from sub-fossil remains (12)
In contrast to the impoverished mammal and herpetofauna, birds are widely represented on the island. 182 species have been recorded on the island in historical times – 15 of which are native taxa (8, 11). Perhaps most striking amongst the endemic avifauna is the extinct White Gallinule (Porphyrio albus), a species of swamphen characterized by its almost entirely white appearance with bright red beak and forehead (11) and likely flightless (11). Another flightless rail, the Lord Howe Woodhen Hypotaenidia sylvestris (8, 11) managed to survive to the modern day (11). A third species, the buff-banded rail (Hypotaenidia philippensis), extends its range to Lord Howe Island but is known from Australia, New Guinea and parts of Southeast Asia (8, 11).
About half the islands birds are Passerines. Two endemic taxa of white-eye are known from the island – the extinct Robust White-eye (Zosterops strenuous) (11) and the extant Lord Howe Silvereye (Zosterops lateralis tephropleurus). Lord Howe was also home to the only extinct species of Gerygone, the Lord Howe Gerygone (Gerygone insularis), though some treat this as a subspecies of the Grey Gerygone (Gerygone igata) (11). Local subspecies of the Island thrush (Turdus poliocephalus vinitinctus), New Zealand Fantail (Rhipidura fuliginosa cervina) and the local subspecies of the Tasman starling (Aplonis fusca hulliana) once inhabited Lord Howe but are now extinct (11). An endemic subspecies of the golden whistler (Pachycephala pectoralis contempta) still inhabits the island (11) as does a distinct subspecies of Pied Currawong (Strepera graculina crissalis) though the latter species is highly threatened (11) .
Two species of pigeon are recorded: an extinct local subspecies of the White-throated pigeon (Columba vitensis godmanae) and the common emerald dove (Chalcophaps indica), which still subsides on the island.
An endemic parrot is now extinct but used to be common on the island at the time of European arrival. This taxon is considered a sub-species of the Red-crowned parakeet (Cyanoramphus novaezelandiae subflavescens), though populations from New Caledonia and Norfolk Island have recently been reclassified as distinct species and this may extend to the Lord Howe subspecies (11).
The endemic apex predator of the island was the Southern boobook, which had a local subspecies (Ninox novaeseelandiae albaria) (11), though the swamp harrier (Circus approximans) is also known to visit the islands on occasion (8, 11).
The only extinct seabird recorded is that of Pycroft’s petrel (Pterodroma pycrofti) known purely from sub-fossil remains (11), though many other seabird species breed on the island, including the Providence Petrel (Pterodroma solandri) which is endemic to the Lord Howe Island Group (as well as Norfolk Island Group).
Though, outside the scope of this article it should be noted that Lord Howe is home to a wide array of plant species, half of which are endemic (8) and one - Solanum bauerianum - extinct (8). A rich ensemble of endemic invertebrates also inhabits the islands. Famously, the the Lord Howe Stick Insect (Dryococelus australis) was rediscovered on the island of Balls Pyramid in 2001 after having been thought extinct in the 1920s (8, 13). Another Lazarus taxon, Lord Howe Island wood-feeding cockroach (Panesthia lata) was recently rediscovered in 2022 (14). Many other invertebrates were less lucky and particularly affected were terrestrial gastropods (8).
Fig 2. Two male Meiolania platyceps fighting for territory on the Jungle slopes of Mt Gower on Lord Howe Island. A Lord Howe Woodhen (Hypotaenidia sylvestris) flees the scene and two Lord Howe Red-crowned parrots (Cyanoramphus subflavescens) fly overhead. In the foreground a Lord Howe Stick Insect (Dryococelus australis) forages in the branches of the forest.
Terms of use: Artwork by Hodari Nundu and Commissioned by The Extinctions
Human Arrival & Extinctions
To best understand the depletion of biodiversity on Lord Howe Island we must look towards the history of human settlement. Despite extensive surveys for archaeological remains there is currently no evidence of pre-European occupation on Lord Howe (1, 11). This is peculiar as many other islands in the South Pacific have clear evidence of settlement by Polynesians. In addition to a lack of archaeological evidence, the conspicuous absence of the accompanying Polynesian rat (Rattus exulans) suggests Polynesians never established themselves on the island (1). That the adept seafarers of Polynesia were incapable of locating Lord Howe is doubtful as soaring mountains make it visible for over a 100km in any direction (1). It is certainly possible that the island was infrequently visited but never settled, but in the absence of evidence this is purely conjecture. Furthermore, there is evidence of Polynesians on Norfolk Island, which supports a very similar ecosystem to Lord Howe and is similarly remote. What prompted Polynesians to settle Norfolk Island whilst missing or foregoing Lord Howe? It remains one of the great mysteries in the history of Lord Howe Island….
Consequently, the first recorded discovery of the island was by the HMS Supply in 1788 en route to the British penal colony on Norfolk Island. In the decades to come, the island was visited by additional ships stopping by for provisioning – mostly whalers (8, 11). The White Gallinule is known only from these initial stopovers, and it is likely that given its flightless nature, that human predation during these stopovers were to blame for its extinction (11). Accounts suggest that the White Gallinule, White-throated Pigeon, and Lord Howe Woodhen were all captured and consumed in large quantities by sailors, especially easy as they were tame – presumably due to an absence of native predators (11).
Settlement of the island was initiated only in 1834 (11). Alongside settlers came livestock introduction of domestic cats (Felis catus) in the 1850s (8) and House Mouse (Mus musculus) in 1868 (1). Pig (Sus scrofa), goat (Capra hircus) and cattle (Bos taurus) were also introduced sometime during the 19th and 20th centuries (8, 11). The White-throated Pigeon and Red-crowned Parakeet both are reported extinct in the aftermath of human settlement, with the last sightings in 1853 and 1869 respectively. the pigeons likely were overexploited for food, but the Parakeets were described as agricultural pests and actively discriminated against by early settlers (11), though an added impact from hunting by cats is possible.
Black Rats (Rattus rattus) arrived with the grounding of the SS Makambo in 1918, soon booming in population and causing havoc (11). Almost immediately a local collector observed crashes in the native songbird populations and the extinctions of the Robust White-eye, Fantail, Island Thrush, and Tasman Starling within a decade (11). Extinctions of land snails and other invertebrates are also noted from this period (8). To combat the rats, three owl species were introduced in the 1920s: Barn Owl (Tyto alba), Southern Boobook (Ninox novaseelandiae) and Masked Owl (Tyto novahollandiae), all but the last of which have since been extirpated. These introductions further exacerbated predation pressures on local birds, herein possibly the Lord Howe Gerygone, which was still considered common a decade after the arrival of the rats but thought extinct by 1936 (11). The introduced owls probably also outcompeted the local sub-species of Southern Boobook (Ninox novaseelandiae alberia). The last Southern Boobook call was recorded in the 1950s, marking the final avian extinction on the island (11). Fossils of the Lord Howe Long-eared bat indicate it also went extinct sometime in the early 20th century (8), probably also because of rodent or owl introductions.
Though the avifauna presents a relatively clear picture of extinction, Meiolania platyceps is quite a different matter. The species is known solely from fossil remains from the Pleistocene and Holocene, but a lack of direct dating makes it difficult to determine when it disappeared from the island (8). It may well have survived into historic times, but it was certainly extinct by the arrival of the HMS Supply, as there are no accounts of terrestrial turtles on the island. Several hypotheses may be entertained. It is important to note that given the miniscule size of the island and the considerable size of Meiolania, the population was probably small at any given time making the species vulnerable to extinction. The level of exploitation needed to wipe out M. platyceps was probably very low. Whilst evidence of pre-European settlement is lacking, the presence of early seafarers passing through can’t be dismissed altogether and hunting of Meiolaniids elsewhere in the pacific has been demonstrated (6). This could also explain the puzzling disappearance of Pycroft’s petrel (11) which was a favored prey by Polynesian settlers on Norfolk Island prior to European arrival (7) though also likely consumed by Europeans (7).
Nevertheless, without any evidence of human presence prior to 1788 this remains conjecture. Alternatively, non-human explanations may also be considered. Firstly, the rise in sea levels during the Holocene might have reduced the island to a size incapable of sustaining a population of large terrestrial tortoises. Indeed, at earlier stages of the Holocene Sea levels were even higher than today (16). It should be noted however that M. platyceps is also known from the previous interglacial period (8, 16) which had even higher sea levels and lasted longer than the Holocene (16). Then perhaps a change in climatic conditions could have prompted an extinction? The extinction of M. platyceps occurred during the Holocene, which had relatively small fluctuations in climate compared with the glacial cycles of the Pleistocene. Unfortunately, we lack the vegetation history of the island prior to settlement by Europeans and it is difficult to test whether there was a marked shift in local conditions. Another hypothesis could be a stochastic event – such as a tsunami or extreme drought. Such a hypothesis is only relevant where population sizes are miniscule. The long occurrence of Meiolania on the island does speak to a level of resilience but the idea can’t be entirely dismissed. Nor can disease in a small – potentially genetically homogenous – population be ruled out as a root cause. With evidence lacking for any of these scenarios the extinction of Meiolania platyceps remains a mystery.
Fig 3. One of only two museums specimens of the White Gallinule (Porphyrio stanleyi), on exhibit at Worlds Museum in Liverpool.
Terms of use: This image is licensed under a Creative Commons Attribution-Share Alike 4.0 International. It is attributed to the vertebrate Zoology Curator. The image is unedited and can be found here.
Conclusion, Conservation, The Future
Despite lacking an answer to the Meiolania question, the general pattern of extinction on Lord Howe seems quite clear. Certainly, the demise of the avifauna was human mediated, but the chronology also shows two clear mechanisms: Exploitations/Discrimination and Invasions. Human exploitation caused the first wave of extinction with White Gallinule and White-throated Pigeon, as well as a significant reduction in the Lord Howe Woodhen all naïve ground dwelling birds. Invasions are a little more difficult to disentangle, though cats and mice do not seem to have caused immediate extinction they may well have functioned as pressures. Furthermore, the uncertain arrival date of feral livestock shrouds their impact. The devastating loss in the wake of the Black Rat however appears highly suspicious. Likely all these invasives had an additive impact upon the local fauna. An interesting comparison can be made with Norfolk Island, which sports very similar fauna but where extinctions played out somewhat differently. This will be topic of part two of Isles of the Tasman Sea.
The future of Lord Howe appears brighter than its tragic past. Most livestock were successfully eliminated in the 1970s, except for a small goat population (11), as a recovery plan for the Lord Howe Woodhen which was especially threatened by pigs. The population at the time was reduced to 20 individuals (11) and has since recovered to over 200 (2). Following this success, the Black Rats and House Mouse (Mus musculus) were also successfully eradicated by 2021 (8) paving the way for potential reintroduction efforts of species once present on the island. Though some species such as the White Gallinule or Tasman Starling are irretrievably lost, most of the other endemics were distinct subspecies with extant populations of the same species found on Norfolk Island, New Zealand or Australia, allowing for a potential revival of a lot of the island’s ecosystem.
Fig 4. The Lord Howe Woodhen (Gallirallus sylvestris), one of the surviving endemic birds of Lord Howe Island. The species is a key conservation and is on the rebound after extirpation of pigs (Sus domesticus).
Terms of use: This image is licensed under an Attribution 2.0 Generic. It is attributed to David Cook. The image is unedited and can be found here.
References
1. Anderson, A. (2003). Investigating Early Settlement on Lord How Island. Australian Archaeology. 57, 98-102.
2. BirdLife International. (2016). Hypotaenidia sylvestries. The IUCN Red List of Threatened Species, e.T22692395A93352096
3. Brown, L. E., Moll, D. (2019). The enigmatic palaeoecology and palaeobiogeography of the giant, horned, fossil turtles of Australasia: a review and reanalysis of the data. Herpetological Journal 29, 252-263.
4. Gaffney, E. S. (1983). The Cranial Morphology of the Extinct Horned Turtle Meiolania platyceps, from the Pleistocene of Lord Howe Island, Australia. Bulletin of the American Museum of Natural History 175(4).
5. Gaffney, E. S. (1996). The Postcranial Morphology of Meiolania platyceps and a Review of the Meiolaniidae. Bulletin of the American Museum of Natural History 229.
6. Hawkins, S., Worthy, T. H., Bedford, S., Spriggs, M., Clark, G., Irwin, G., Best. S., Kirch, P. (2016). Ancient tortoise hunting in the Southwest Pacific. Scientific Reports 6: 38317.
7. Holdaway, R. N., Anderson, A. (2001) Avifauna from the Emily Bay Settlement Site, Norfolk Island: A Preliminary Account. Australian Museum, Supplement 27, 85-100.
8. Hume, J. P., Hutton, I., Middleton, G., Nguyen, J. M. T., Wylie, J. (2021). A Terrestrial Vertebrate Palaeontological Reconnaissance of Lord Howe Island, Australia. Pacific Science 75 (1), 43-73.
9. Jannel, A. (2015). Neck mobility, grazing habits, and intraspecific combat behaviour in the giant Pleistocene horned turtle Meiolania platyceps. Examensarbete vid Institutionen för geovetenskaper, Uppsala Universitet
10. Lawver, D. R., Jackson, F. D. (2016). A fossil egg clutch from the stem turtle Meiolania platyceps: implications for the evolution of turtle reproductive biology. Journal of Vertebrate Paleontology 36 (6): e1223685.
11. McAllen, I. A. W., Hutton, I., Cooper, R. M. (2004). The Birds of the Lord Howe Island Group: a Review of Records. Australian Field Ornithology 21.
12. McKean, J. L. (1975). The bats of Lord Howe Island with the description of a new Nyctophiline bat. Australian Mammalogy 1(4), 329-332.
13. Priddel, D., Carlile, N., Humphrey, M., Fellenberg, S., Hiscox, D. (2003). Rediscovery of the ‘extinct’ Lord Howe Island stick-insect (Dryococelus australis (Montrouzier))(Phasmatodea) and recommendations for its conservation. Biodiversity and Conservation 12, 1391-1401.
14. Siossian, E., Kirton, M. (2022). ’Extinct’ wood-feeding cockroach rediscovered on Lord Howe Island. ABC News.
15. Sterli, J. (2015). A Review of the Fossil Record of Gondwanan Turtles of the Clade Meiolaniformes. Bulletin of the Peabody Museum of Natural History 56(19), 21-45.
16. Woodroffe, C. D., Murray-Wallace, C. V., Bryant, E. A., Brooke, B., Heijins, H., Price, D. M. (1995). Late Quarternary sea-level highstands in the Tasman Sea: Evidence from Lord Howe Island. Marine Geology 125, 61-72.
