Back from the brink but still divided: enhancing resilience of genetic islands of the Lord Howe Woodhen

Although restricted to a 15 km² island in the Southwest Pacific Ocean, populations of the endangered Lord Howe Island Woodhen are still maintaining social distancing.

It is easy to see how the flightless Lord Howe Woodhen became the rarest of Australian birds:

“We had nothing more to do than stand still a minute or two & knock down as many as we pleas’d wt. a short stick” wrote the ship’s surgeon, Arthur Bowes-Smyth, when the Lady Penhryn visited the island in 1788.

Human exploitation to supply fresh food for sailors, and later predation by introduced pigs and cats, reduced the population of this endemic species to just eight pairs by 1980. Then the most successful conservation program for an Australian vertebrate kicked in. Feral-animal removal and a captive-breeding and release program saved the species from extinction, restoring the population to at least 245 individuals today.

But what does the future hold for this Lazarus bird, locked-down on an isolated volcanic island, 600 km off the NSW coast, that is highly exposed to climate change? We know that genetic variation is essential to allow animals to evolve, and with a gene-pool bottlenecked to just 16 individuals, we would expect variability to be in short supply in the Lord Howe Woodhen. The aim of our research was to understand the current genetic landscape of the species.

The Lord Howe Island Board has been catching and releasing Woodhens ever since the captive breeding program, as part of an annual monitoring program. Analysis of recapture data revealed that the species is not particularly dispersive, with little evidence of interchange between birds living on the mountain summit and birds on the lowlands. If the population is fragmented, further threats confront the species’ survival and genetic variability.

To measure this genetic variability, the Woodhen survey team collected blood samples for genetic analysis during their 2018 survey. The Woodhen DNA team was also fortunate to be able to access a large sample of historic DNA from museum specimens. Even after the species was known to be in trouble, an over-zealous collector obtained 82 Woodhen specimens in 1915 which eventually ended up in the American Museum of Natural History, New York. Our team successfully extracted DNA from these 100-year-old specimens, enabling us to measure genetic diversity prior to the more recent major population bottleneck.

The results were unambiguous. There was a pronounced reduction in genetic diversity over the last 100 years, and consistent with the survey observations, birds living on the mountain summit today are genetically distinct from birds inhabiting the lowlands. This was surprising, as the mountain Woodhens were the original source of the captive bred birds that re-established the lowland population in the 1980s. Interestingly, the mountain birds have greater genetic variability, despite comprising a relatively small proportion of the current total population. This supports anecdotal observations that most of the lowland birds were derived from a single pair.

A totally unexpected result was that the historic Woodhen population was also divided. Roy Bell, the overzealous collector, obtained around half his specimens from the mountain summit, and half from an isolated pocket of habitat at the base of the mountain that was isolated by cliffs from the main lowland population. This pocket had therefore escaped the ravages of the pigs that had eliminated the main lowland population by the time of Bell’s collections. Divergent genetic signatures between summit and pocket birds clearly showed that historically, movements of Woodhens were restricted by the island’s rugged topography – perhaps understandable for a flightless bird.

So how can we use this new information to enhance the species’ long-term survival? Population numbers and reproductive success are in good shape – and likely to see further improvement since the eradication of rodents from Lord Howe Island this year. There is therefore plenty of scope to translocate birds without risking population numbers. We recommend periodic translocation of birds from the mountain summit to the lowlands to ensure that genes currently quarantined on the mountain are shared with the larger lowland population. This would bolster its genetic variability and guard against further loss of potentially valuable genes in the event that a catastrophe were to strike the mountain population.

This study illustrates the value of museum collections in allowing us to travel through time to unravel the ecological history of populations, as well as assessing the genetic impact of threatening processes and management interventions.

Dr Richard Major, Senior Principal Research Scientist, Terrestrial Vertebrates, Australian Museum Research Institute

Dr Mark Eldridge, Principal Research Scientist, Terrestrial Vertebrates, Australian Museum Research Institute.

Acknowledgements

We are grateful to Memocorp Australia Pty Ltd for supporting the Lord Howe Island Woodhen project through a donation to the Australian Museum Foundation.

Further Information

Major, R.E., Ewart, K.M., Portelli, D.J., King, A., Tsang, L.R., O’Dwyer, T., Carlile, N., Haselden, C., Bower, H., Alquezar-Planas, D. E., Johnson, R.N. & Eldridge, M.D.B. (2020) Islands within islands: genetic structuring at small spatial scales has implications for long-term persistence of a threatened species. Animal Conservation. https://doi.org/10.1111/acv.12603