National recovery plan for the Lord Howe Woodhen (Gallirallus sylvestris)

NSW National Parks and Wildlife Service, 2002

8. Management Issues

8.1 Threats and Reasons for Decline

The decline of the Lord Howe Woodhen started with the discovery of the island in 1788. Mariners visiting the island described the species as abundant and a good source of food (Miller and Kingston 1980, Miller and Mullette 1985, Fullagar 1985).

Observations reported in Miller and Kingston (1980) indicate that the Woodhen underwent its most dramatic decline between 1833 and 1853. The Australian Museum expedition in 1887 as reported the Woodhen as 'soon to become extinct' (in Miller and Kingston 1980).

It is clear that by the end of the 19th century they were restricted to the area now known as Little Slope, the higher slopes of Mounts Lidgbird and Gower, and the Erskine Valley (Figure 1). The decline continued until about 1940, by which time the species was restricted to small populations on the tops of Mounts Lidgbird and Gower, and on the south-eastern flank of Mount Gower.

The introduction of exotic fauna is considered to be the principal reason for the decline of the Woodhen. Permanent settlement of the island in 1833-34 resulted in the release of Cats and Dogs. The reasons for the dramatic decline in Woodhens between settlement and 1853 were thought at the time to be the 'ravages' of Cats and hunting by people for food (Miller and Kingston 1980). Table 3 summarises the history, status and impacts of key exotic species.

Table 3. Status of selected exotic fauna on Lord Howe Island and their impacts on the Lord Howe Woodhen. (Sources are provided in brackets).
Earliest Date Status Impacts on Woodhen
Pigs
1800 (Miller and Mullette 1985)
before 1839 (Fullagar 1985)
Spread through all but the inaccessible areas of the island isolated by the sea and cliffs (eg. Mount Gower Plateau, Little Slope). Almost eradicated from 1979 - 85 (200 killed in this period). By 1985, one boar thought to remain on Mt Lidgbird. Pigs now considered to have been eradicated from the island.
  • Eat Woodhens and their nest contents.
  • Eat earthworms so possible food competitor with Woodhen.
  • Disturb soil invertebrate communities.
  • Thin dense vegetation and arrest regeneration.
  • Cause erosion and downslope sedimentation and stream turbidity.
  • Probably responsible for the decline of the Woodhen in the southern part of the island.
Goats
before 1851 (Miller and Mullette 1985;
Fullagar 1985)
Spread through all accessible parts of the island. Introduced to Big Slope around 1900 and to Little Slope in 1920s. Eradicated from Little Slope (300 animals) in 1955. In 1998, 150 - 200 were thought to occur in the southern mountains. In 1999, a goat control program was undertaken which reduced the known population to three. Follow up programs are required to achieve eradication.
  • Thin dense vegetation and arrest regeneration.
  • Cause erosion and downslope sedimentation and stream turbidity.
  • Contribute to habitat modification by weeds through soil disturbance and dispersal of propagules.
Cats
before 1845 (Miller and Mullette 1985;
Fullagar 1985)
c. 1837 (Miller and Kingston 1980)
Spread through much of island by 1850s. Probably not in high numbers in the wetter, southern and mountainous parts of the is land. Cats probably not present at Little Slope after 1950. Cats are now banned from importation and one old, desexed domestic cat now remains. Feral Cats have been eradicated.
  • Eat Woodhens.
  • Cat predation probably partly responsible for the initial disappearance of the Woodhen from the northern and central parts of the island in the early years of settlement.
  • Probably not responsible for the decline in the southern part of the island.
Rats
1918 (Fullagar 1985) Came off shipwreck in 1918 and increased dramatically, spreading throughout island. In 1927, in response to a bounty, 13,771 rat tails were handed in. Numbers kept low through poisoning in settlement area and kentia palm seeding areas (eg. Little Slope). Numbers at higher densities on Mount Gower. Ongoing rat control programs (poisoning with warfarin) are in place.
Predation by the Ship Rat (Rattus rattus) on Lord Howe Island was listed as a Key Threatening Process under the TSC Act in 2000.
LHIB is considering a rodent eradication proposal.
  • Probably have had a limited effect on Woodhen.
  • Woodhen can kill Rats and defend themselves from attack.
  • Co-existed with Woodhens for many years on Mount Gower.
  • May predate eggs and small chicks.
  • Woodhens consuming rat bait may be poisoned.
  • A Threat Abatement Plan must be prepared for Ship Rats on Lord Howe Island.
  • The LHIB rodent eradiciation proposal would likely require Woodhens to be removed and held in captivity during the baiting program.
Masked Owls
1920-1930 (Fullagar and Disney 1975) Masked Owls and Barn Owls introduced to control Rats, without success. Barn Owls thought to be extinct. Masked Owl persists today in most parts of the island but more commonly in the southern mountains (D. Hiscox, LHIB, pers. comm.).
  • Initially not thought to have caused decline.
  • Woodhens have been killed by owls and a captive bird was taken once from its enclosure.
  • Possibly implicated in the decline of the Woodhens on Little Slope.
  • Potentially a significant factor limiting population growth in some parts of the island. (D.Hiscox, LHIB, pers. comm.).
  • If Rats are eradicated, the potential threat to Woodhens is likely to increase.
Humans
first visited 1788 then settled permanently 1833-34
(Miller and Mullette 1985;
Fullagar 1985)
Small community established in northern and central part of island. Current population 320 plus a maximum permissible 400 tourists at a time.
  • Human predation probably largely responsible for the initial disappearance of the Woodhen from the northern and central parts of the island in the early years of settlement.
  • Clearance of 10% and disturbance of further 10% of island's forests, some of which was Woodhen habitat.
  • Woodhens are occasionally run over by vehicles.
  • Woodhens consume rat bait (some types of which are lethal) in the settlement area.
  • Some members of the community protect Woodhens in their gardens and provide supplementary food to increase survival and breeding output.

The current and potential threats to the Woodhen population are discussed below.

  • The species occurs only at Lord Howe Island and, as such, is vulnerable to disease or natural disaster.
  • Introduced Masked Owls may be impacting on juvenile Woodhen survival rates (D. Hiscox, LHIB, pers. comm.).
  • Increased rat control is likely to cause an increase in predation of Woodhens by Masked Owls (B. Harden, BRMD, pers comm.).
  • A proposal is being considered by the LHIB to eradicate rodents from the island using aerial baiting and bait stations. A Risk Assessment Report is currently being prepared for the LHIB which will consider the potential impacts of the program on the Woodhens and other species and the mitigation measures required. The Woodhens are likely to be significantly at risk from the baits.
  • In the settlement area, introduced Blackbirds and Song Thrushes, together with apparently increasing numbers of Buff-banded Rails and Purple Swamphens, may be competing with the Woodhen for food (D. Hiscox, LHIB, pers. comm.).
  • The introduction to the island of additional exotic species (animal or plant) could threaten the species or the integrity of its habitat.
  • Existing weed species may encroach into areas of key habitat and could threaten habitat integrity.
  • Quarantine controls on the island are not well developed. There is a risk that a soil, plant or animal pathogen could reach the island and significantly affect the habitat, food or health of Woodhens.
  • Loss of preferred habitat through clearing for agriculture or development.
  • Consumption of rat bait by Woodhens.
  • Impacts of domestic Dogs.
  • Reduction in amount of supplementary feeding may affect populations numbers in the settlement.

The bulk of the population is descended from a very small number of captive-released birds derived from three pairs taken from the wild at Mount Gower. Inbreeding and a lack of genetic diversity may theoretically cause future inbreeding depression and associated problems (eg. decreased reproduction success or decreased resistance to disease). However, this is not considered a priority issue at this stage.

A population viability analysis (PVA) of the species using the VORTEX program by Brook et al. (1996) revealed that, in the absence of further changes or catastrophes, there was a 2% chance of the species going extinct in the coming 100 years. When the time frame was increased to 1,000 years, the probability of extinction rose significantly to 21%.

The impacts of a range of factors on the Woodhen's future survival were assessed using the model developed for the PVA. A 10% increase in mortality resulted in the Woodhen's demise within 100 years. Juvenile mortality was more important than adult mortality in raising extinction probability. Doubling the variance in fecundity (eg. due to climatic change) resulted in a lower population size but did not significantly increase the probability of extinction. Halving of carrying capacity quickly reduced population size, did not significantly affect the probability of extinction within 100 years but increased the probability of extinction within 1000 years.

Harden and Robertshaw (1988) suggest that habitat availability may be limiting the expansion of the species on the lowlands. There is some evidence to support this proposal in that the Woodhens habitat preferences appear to be very narrow and most of the available habitat has existing territories within it. There has been little increase in numbers or range extension beyond these areas since their initial re-occupation after captive-release. If the preferred habitat on the island is fully occupied, this has implications for management (see Sections 10 - 12).

8.2 Social and Economic Consequences

The total cost of implementing the recovery actions will be $476,500 over the five year period covered by this plan. Responsibility for implementing the plan lies with the LHIB and the NSW National Parks and Wildlife Service. Some funds will be provided from existing resources within the NPWS and LHIB. The balance of the costs identified in Table 3 ($452,000) are unsecured. External funding options including State and Commonwealth Government funding will be investigated.

The Plan will be forwarded to the Commonwealth Minister for consideration for adoption, once it has been approved by the NSW Minister for the Environment.

Social Benefits

The Woodhen has been a focus for educating local, mainland and international communities about the unique and vulnerable environmental values of Lord Howe Island and about the conservation of Endangered Species.

The local community has been very involved in Woodhen conservation. It participated in the captive breeding program and many residents have contributed to the successful re-establishment of birds in the settlement through supplementary feeding and protection of the birds on their properties.

Commercial Value

Ecotourism is a growing business activity in Australia. Lord Howe Island has particular values as an ecotourism destination. The Lord Howe Woodhen is one of a number of bird species for which the island is famous among tourists with an interest in natural history. The Woodhen thus contributes to the image of the island as a high value ecotourism destination.