Emerald Dove (Christmas Island) (Chalcophaps indica natalis)

Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the list of Threatened Species under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)
12 April 2005

1. Scientific name, common name (where appropriate), major taxon group

Chalcophaps indica natalis (Emerald Dove (Christmas Island))

2. Description

The Emerald Dove (Christmas Island) is a small, plump short-tailed pigeon. It is mostly purplish brown, with iridescent dark-green wings and a pale shoulder-patch. Females are generally duller than males and lack a pale shoulder-patch.

The Emerald Dove (Christmas Island) occupies most forested habitats on Christmas Island, including secondary regrowth dominated by the introduced Japanese Cherry (Muntingia calabura). It can also occur in the settled areas, sometimes foraging on lawns. It feeds mostly on fruit, and usually forages on the ground beneath trees near the edge of forest, along creeks and tracks, or in clearings. It builds low, flimsy stick nests.

3. National context

This subspecies of Emerald Dove (Christmas Island) is endemic to Christmas Island. It is not listed as threatened under any State or Territory legislation, nor is it currently listed under the EPBC Act.

EPBC Act criteria.

TSSC judges the species to be eligible for listing as critically endangered under the EPBC Act. The justification against the criteria is as follows:

Criterion 1 - It has undergone, is suspected to have undergone or is likely to undergo in the immediate future a very severe, severe or substantial reduction in numbers.

The most serious threat to the Emerald Dove (Christmas Island) is the spread of Yellow Crazy Ants (hereafter called Crazy Ants). Crazy Ants are an aggressive invasive species that were accidentally introduced to Christmas Island between 1915 and 1934 and are now widespread throughout the rainforest and settled areas on the island. They can build up into colonies with extremely high numbers called supercolonies. Before recent abatement programs, Crazy Ant supercolonies had formed in 24.4% of island's rainforest.

Crazy Ants have both direct and indirect effects on this species. Direct attacks on injured Emerald Doves (Christmas Island) have been observed in supercolony areas, though it was unclear whether attack by Crazy Ants was the initial cause of injury, as healthy birds appeared to be able to dislodge the ants before serious injury occurred. A report that Crazy Ants were responsible for killing Emerald Dove (Christmas Island) nestlings (Garnett & Crowley 2000) has not been verified. In areas where supercolonies occurred, there was a 80-90% reduction in numbers of sightings of Emerald Doves (Christmas Island), compared to uninvaded forest (Davis 2002). This may have been because doves normally feed on fruit on the ground, where the Crazy Ants occur at their highest densities. Within supercolonies, the doves would be exposed to competition for food and aggressive attacks by Crazy Ants.

Although Emerald Doves (Christmas Island) are attempting to nest in ant-invaded areas, many attempts are failing (Davis 2002). Nesting doves may be particularly vulnerable to ant predation as their nests are often low in the vegetation where the threat of interference by ants is high (Davis 2002). It is possible that the reproductive success of the Emerald Dove (Christmas Island) could be reduced by Crazy Ant's harassing nesting adults and predating on nestlings and juvenile birds. Therefore, areas occupied by Crazy Ant supercolonies are likely to provide much poorer breeding habitat compared to unoccupied areas.

Crazy Ants also impact the Emerald Dove (Christmas Island) indirectly through the removal of the keystone forest species, the Red Land Crab (Gecarcoidea natalis). In areas of forest where Crazy Ants have eliminated Red Crabs, it was found that the seeds that were previously consumed by the crabs had germinated, and the forest structure had altered, with dense layers of seedlings covering the forest floor. This makes foraging on the ground more difficult for the Emerald Dove (Christmas Island) and may result in a possible long-term alteration of forest structure and composition.

Predation of adult doves by cats (Felis cattus) and predation of eggs and young by Black Rats (Rattus rattus) is also a potential threat to Emerald Doves (Christmas Island). While rats and cats do not commonly occur in the rainforest possibly due to the presence of the Red Land Crabs, reduction of the crabs may facilitate rainforest invasion by feral mammals, including cats and rats, and the introduced Wolf Snake (Lycodon capucinus) and may lead to an increased threat.

The impact caused by the mutualistic relationship between Crazy Ants and scale insects may also have an indirect effect on the Emerald Dove (Christmas Island). Infestations of scale insects associated with Crazy Ants, have detrimental effects on the health of the canopy and may also lead to long-term decline in the habitat.

Although, an island-wide aerial baiting campaign in September 2002 has abated the threat of Crazy Ants resulting in a 98-100% reduction in ant activity within two weeks, the baiting program was based on control rather than eradication and new invasions and the reformation of a number of new, small supercolonies has been reported.

In areas where supercolonies occurred, there was a 80-90% reduction in numbers of sightings of Emerald Doves (Christmas Island), compared to uninvaded forest (Davis 2002). As supercolonies covered 24.4% of the Emerald Doves' (Christmas Island) habitat, the island's rainforest, prior to baiting, it could be inferred that the species has lost approximately 25% of its preferred habitat for foraging and nesting and has undergone a considerable reduction in numbers, although this is not great enough to be considered a substantial reduction in numbers. It is unclear what the future rate of decline may be in response to the abatement of the threat. Therefore it is not eligible for listing under this criterion.

Criterion 2 - Its geographic distribution is precarious for the survival of the species and is very restricted, restricted or limited.

This subspecies of Emerald Dove (Christmas Island) is endemic to Christmas Island. The species has a restricted geographic distribution occurring predominantly in the forested area of the island, an extent of occurrence of approximately 100 km².

The restricted geographic distribution is considered precarious for the survival of the species as the Emerald Dove (Christmas Island) occurs in only one location where its habitat is seriously threatened by the spread of Crazy Ant supercolonies. Crazy Ants have both direct and indirect effects on this species (see Criterion 1).

In areas where supercolonies occurred, there was a 80-90% reduction in numbers of sightings of Emerald Doves (Christmas Island), compared to uninvaded forest (Davis 2002). As supercolonies covered 24.4% of the habitat of the Emerald Dove (Christmas Island), the island's rainforest, prior to baiting, it could be inferred that the species has undergone a fairly significant reduction in numbers as a result of the spread of the Crazy Ants.

There has been some abatement of the threat as a result of the baiting programs. However, although baiting appeared successful in the short term, its success in the longer term is not clear and the threat may increase yet again.

The future impact of the threat of Crazy Ants on Emerald Doves (Christmas Island) is also not clear. The longer term effects of the change in species composition and structure of the rainforest may result in more habitat becoming unsuitable or unavailable for the species causing a further decline.

The Emerald Dove (Christmas Island) has a restricted geographic distribution which is considered to be precarious for the survival of the species as long as Crazy Ants are present on Christmas Island in supercolonies. Therefore the species is eligible for listing as endangered under this criterion.

Criterion 3 - The estimated total number of mature individuals is limited to a particular degree and: (a) evidence suggests that the number will continue to decline at a particular rate; or (b) the number is likely to continue to decline and its geographic distribution is precarious for its survival.

The total number of mature individuals is estimated as 2000 breeding birds based on field surveys on Christmas Island (Stokes 1988). As the species is endemic to the rainforest of Christmas Island, it is considered that 100% of mature individuals occur in one population.

In areas where supercolonies occurred, there was a 80-90% reduction in numbers of sightings of Emerald Doves (Christmas Island), compared to uninvaded forest (Davis 2002). As supercolonies covered 24.4% of the Emerald Doves' (Christmas Island) habitat, the island's rainforest, prior to baiting, it could be inferred that the species has undergone a fairly significant reduction in numbers as a result of the spread of the Crazy Ants.

There has been some abatement of the threat as a result of the baiting programs. However, although baiting appeared successful in the short term, its success in the longer term is not clear and the threat may increase yet again.

The future impact of the threat of Crazy Ants on Emerald Doves (Christmas Island) is also not clear. The longer term effects of the change in species composition and structure of the rainforest may result in more habitat becoming unsuitable or unavailable for the species.

The estimated total number of mature individuals is limited, it occurs in a single population, and appears to have undergone a decline. Therefore this subspecies is eligible for listing as endangered under this criterion.

Criterion 4 - The estimated total number of mature individuals is extremely low, very low or low.

The estimated population of the Emerald Dove (Christmas Island) is 2,000 breeding birds. The subspecies is not eligible for listing under this criterion.

Criterion 5 - Probability of extinction in the wild

There are no quantitative data available against this criterion. Therefore the subspecies is not eligible for listing under this criterion.

5. Conclusion

The Emerald Dove (Christmas Island) is endemic to Christmas Island. The species has a restricted geographic distribution occurring predominantly in the forested area of the island, an extent of occurrence of approximately 100 km². The estimated population is 2000. There has been an inferred significant decline in the abundance of this species due to the spread of Crazy Ant supercolonies. The direct threat of Crazy Ants has not yet been eliminated and the long term effects of the change in habitat are yet unknown. The species is therefore eligible for listing as endangered under criteria 2 and 3.

6. Recommendation

TSSC recommends that the list referred to in section 178 of the EPBC Act be amended by including in the list in the endangered category:

  • Chalcophaps indica natalis (Emerald Dove (Christmas Island))

Conservation Advice

The Emerald Dove (Christmas Island) is a small, plump short-tailed pigeon endemic to Christmas Island. It occupies forested habitats on Christmas Island most of which are contained within the Christmas Island National Park. The estimated population is 2000 birds. The key threat to the subspecies is the invasion of its habitat by Yellow Crazy Ant supercolonies. Christmas Island is part of the national NHT funding process and is not subject to a separate NRM regional plan.

The Christmas Island National Park Management Plan, the Action Plan for Invasive Ants on Christmas Island, and the planned Threat Abatement Plan (TAP) for Tramp Ants (including Crazy Ants), identify threat abatement actions needed to address the Crazy Ant invasion on Christmas Island.

The priority recovery and threat abatement actions required for this species are:

  • To continue effective and comprehensive Crazy Ant threat abatement actions

This list does not encompass all actions that may be of benefit to this species, but highlights those that are considered to be of the highest priority at the time of listing.

There is no recovery plan in place for the Emerald Dove (Christmas Island).

Priority for the development of recovery plan: Low.

Publications used to assess the nomination

Davis, N. (2002) The Invasive Yellow Crazy Ant (Anoplolepis gracilipes) on Christmas Island, Indian Ocean: Impacts on the Frugivorous Bird Fauna. Honours thesis, Monash University.

Garnett, S.T., & G.M. Crowley. 2000. The Action Plan for Australian Birds 2000. Environment Australia, Canberra.

Higgins, P.J. & Davies, S.J.J.F. (eds) (1996) Handbook of Australian, New Zealand and Antarctic Birds. Vol. 3, Snipe to Pigeons. Oxford University Press, Melbourne.

O'Dowd, D.J., Green, P.T. & Lake, P.S. 1999. Status, impact, and recommendations for research and management of exotic invasive ants in Christmas Island National Park. Darwin, Northern Territory, Environment Australia.

Stokes, T. (1988) A review of the birds of Christmas Island, Indian Ocean. Australian National Parks and Wildlife Service Occasional Paper No. 16. Australian National Parks and Wildlife Service, Canberra.