Richard Hill and Andrew Dunn, 2004
ISBN 0 642 55008 5
Threats to the taxon
- Habitat loss
- Crazy Ants
- Dust from phosphate dryers
- Other Potential Threats
Since early settlement, the nesting distribution of Christmas Island Frigatebirds has been fragmented by human development resulting in the three colonies that remain today. Early accounts suggest that the shore terrace of Flying Fish Cove may once have been their main breeding area (Andrews 1900, Gibson-Hill 1947, 1949, Stokes 1988) and they probably had an almost continuous nesting distribution along the north-eastern coast from Margaret Beaches to North East Point, and a separate colony in the sheltered area where the golf course colony is now situated (Figure 1). During the early years of human settlement, much of the habitat of Christmas Island Frigatebirds in Flying Fish Cove and Settlement was cleared. Stokes (1988) estimated that approximately 90ha of breeding habitat had been cleared since settlement.
The exotic invasive yellow crazy ant (Anoplolepis gracilipes) arrived on Christmas Island more than 70 years ago, and is now widespread throughout rainforest on the island (Orchard et al. 2002). These ants have the ability to form multi-queened 'super-colonies', in which the ants occur at very high densities. Supercolony formation has apparently been a relatively recent phenomenon; the first supercolony was discovered in 1989, with further dramatic increases in supercolony formation probably beginning around the mid-1990s.
At supercolony densities, this invasive, alien ant is having a devastating impact on the island's ecosystems. Red crabs, robber crabs, blue crabs and most other ground dwelling animals such as reptiles have been and are continuing to be severely impacted, sometimes to the point of local extinction in heavily infested areas. By eliminating local populations of red crabs, the ants are also having a marked effect on forest composition and structure, and litter dynamics in infested areas. Further, the feeding activities of these ants and their mutualistic scale insects can stress large trees to the point of death, and appear to be causing canopy dieback in areas of supercolony formation (Orchard et al. 2002). In addition it should be noted that groundings of birds in crazy ant supercolonies would most probably result in the death of the bird. In the recent Risk Watch List compiled for the Christmas Island National Park and Conservancy, the crazy ant invasion was rated as an Extreme Risk to biodiversity and conservation values, with catastrophic consequences of failure to implement effective control measures.
Parks Australia North field workers and assistants conducted an island-wide ant survey during the months May-August, 2001. This survey was designed by CAMBI (Centre for the Analysis and Management of Biological Invasions, Monash University). Of 972 sites surveyed, 741 surveyed points fell in natural forest. Crazy ants were recorded at 46.7% (346/741) of sites in natural forest. Super-colony densities were recorded at 22.7% (168/741) of sites in natural forest. Using these survey points as a representative sample of the forest, it was estimated that 2,379 hectares of the estimated 10,492 hectares of forest on the island was infested with crazy ants at super-colony densities (Orchard et al. 2002).
Crazy Ants are not evenly distributed throughout natural forest but are more commonly found on terrace forests and less commonly found in deep-soil tall-closed forest, which make up most of the natural forests on the island plateau. Deep-soil tall-closed forest made up 31% of census sites in natural forest but only 6.5% of supercolony records (Orchard et al. 2002). Christmas Island Frigatebirds are especially at risk from Crazy Ants because of this concentration of supercolonies on terrace forests where all Christmas Island Frigatebirds nesting habitat occurs. The distribution of Crazy Ant colonies in relation to Christmas Island Frigatebird nesting areas is shown in Figure 1.
In September 2002 an aerial baiting program was undertaken, with all known supercolonies treated with insecticide. Results so far indicate that the program was successful in controlling supercolonies over 2500ha of Christmas Island. Crazy ants are still present in low densities on Christmas Island. However it must be noted that despite this action, further high densities of crazy ants may establish in the terrace forests without warning. PAN staff will continue to monitor any new supercolony formation and treat by hand baiting over the next few years.
Part of the phosphate mine, the "Dryers" chimneys were built in the early 1970's and emitted very large amounts of phosphate dust which settled on the downwind terrace forests causing dieback and death to some trees. A major refit has since reduced dust output from around 30kg/cubic metre of exhaust to well below 50mg/cubic metre of exhaust (M. Bennett, CIP, pers. comm).
An island-wide survey of terrace nesting seabirds in May/June 1984 (Stokes 1984) estimated that 100 pairs nested in the dryers colony. A decade later, a brief reconnoitre of the area found fewer than 30 Frigatebird nests between Smith point and Margaret beaches and it was estimated that there would have been at most 50 nests at the peak breeding time (A. Dunn unpub. data).
In 1967, Nelson (pers. comm to Stokes 1988) reported that the dryers and golf course colonies were the two main Christmas Island Frigatebirds nesting areas. Nelson did not note birds nesting in the area now supporting the cemetery colony. This prompted Stokes (1988) to suggest that the cemetery colony may have formed relatively recently from birds displaced from the dryers colony.
Since January 2000 Christmas Island Phosphates have carried out basic monitoring of the utilisation of the "dryers" breeding colony area by all birds including Christmas Island Frigatebirds and there was some suggestion that use of the area by birds generally had increased since the dust emissions were controlled (M. Bennett pers. comm.). However, as of May 2003 there are no Christmas Island Frigatebirds nesting in the area where the old dryers colony was.
Christmas Island tends to be affected by severe storms every five to ten years. The small population size and limited breeding distribution mean that chance events such as severe storms could have marked effects on breeding success and possibly survival of juveniles and adults. It is known that many eggs can be destroyed in strong winds (Marchant & Higgins 1990).
Although fires are uncommon on Christmas Island, they did occur in terrace forests during a long dry season in 1994, and again in terrace forest in September 1997. Terrace forests can become extremely dry during the dry season and a fire could have devastating effects on breeding areas. The limited area of the nesting colonies makes them especially vulnerable to fire. The Cemetery and Golf Course colonies are close to human activity, which substantially increases the risk of wildfire in those areas.
A serious threat to all island birds is the introduction of new disease, particularly when the populations are small. Island birds have often evolved in the absence of diseases common in continental bird faunas and the introduction of such diseases to island birds can be disastrous. An example of this is the introduction of avian malaria to Hawaii which caused the extinction of almost the entire endemic bird fauna from below 600 m altitude, and was probably the main cause of the total extinction of several bird species (Hay 1986). The range of many surviving species was severely reduced and fragmented which in turn markedly increased their chances of extinction. Avian malaria arrived with the accidental introduction of a new species of mosquito (Hay 1986). Christmas Island has been very vulnerable to the accidental introduction of new disease. In 1994 a quarantine barrier was established between the island and Indonesia and Australia and this has reduced that risk.
Weeds, especially newly introduced invasive species, could impact on Christmas Island Frigatebird nest sites, for example by forming vine towers over nesting trees.
Sea-surface temperature was strongly correlated with average annual breeding success of Abbott's Boobies. It was hypothesised that cold water upwellings provided rich food resources (Reville et al. 1990). Higher sea-surface temperatures predicted as a result of global warming could reduce food availability in marine areas adjacent to Christmas Island.
Currently there is very little information on the areas in which Christmas Island Frigatebirds forage. It is likely that over-fishing is a potential problem in some of the foraging habitat of Christmas Island Frigatebirds.
Domestic cats could also pose a potential threat to grounded birds, especially in settled areas.