National recovery plan for the Christmas Island Hawk-Owl Ninox natalis
Prepared by Richard Hill
Department of the Environment and Heritage, 2004
ISBN0 642 55010 7
Threats to the species
Island birds are particularly vulnerable to extinction and a very high number of island birds relative to continental species have become extinct in the past century (Diamond 1985, King 1985). This is because: (i) island birds tend to have smaller population sizes than continental species and small populations are more vulnerable to extinction (Lacy 1987); (ii) island birds have often evolved in the absence of many predators, diseases and competitors and they can be catastrophically affected by the introduction of one of these (King 1985); (iii) there are often no opportunities for dispersal and thus recolonisation on islands, and this factor also predisposes a population to extinction (Lacy 1987); and (iv) there is often less opportunity to make reserves on islands because land area is limited (Diamond 1985). Predators in general are often naturally rare because their prey, other animals, are less abundant than other food resources. Thus island raptors are a particularly vulnerable group of island birds.
The exotic invasive yellow crazy ant (Anoplolepis gracilipes) arrived on Christmas Island more than 70 years ago, and is now widespread throughout rainforest (Orchard et al. 2002). The ants can form multi-queened 'super-colonies', in which they occur at very high densities. This has apparently been a relatively recent phenomenon, with the first supercolony discovered in 1989, with further dramatic increases probably beginning around the mid-1990s.
At supercolony densities, the 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. The feeding activities of the 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 groundings of birds in crazy ant supercolonies would most probably result in the death of the bird. Although research has shown that crazy ants have little impact on canopy insects, it is possible that the ants may reduce insect populations, the primary prey of the Hawk-Owl. 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 Centre for the Analysis and Management of Biological Invasions, Monash University (CAMBI). 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 more commonly found in Marginal Rainforests and less commonly found in Primary Rainforests. Primary Rainforests make up most of the natural forests on the island plateau and 31% of census sites in natural forest, but contained only 6.5% of supercolony records (Orchard et al. 2002).
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 2500 hectares 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.
A serious threat to all island birds is the introduction of new disease. 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 is 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.
Approximately 25% of the island’s original forests have been cleared and replaced by shrublands of ferns on minefields, regrowth vegetation on stockpiles, and roads and housing (Environment Australia 1994). This has meant a loss of their preferred forested habitat and thus a decrease in the total population size of the Christmas Island Hawk-Owl. Regrowth vegetation growing along roadsides and on stockpiles may provide good feeding habitat for Christmas Island Hawk-Owls at least at some times of the year. These small areas of regrowth vegetation may have slightly offset the effect of habitat clearance.
Rainforests on Christmas Island have been fragmented in places by clearing and dissected by roads and old mining 'grid lines'. The grid line system, a series of parallel lines 120 m apart and a bulldozer blade in width, was installed in the 1970's to explore the island's phosphate resources. After 25 years these are mostly overgrown with dense stands of regrowth vegetation. Numerous large trees were undoubtedly knocked over when the grid lines were being put in, and this may have reduced the number of suitable nest sites for owls. It seems unlikely that any clearings are large enough to be a barrier to dispersal.
There are some major development proposals and associated infrastructure in particular the Immigration Centre, Satellite launching station, airport upgrade, and expansion of phosphate mining that have the potential to reduce the area of Hawk-Owl habitat. One development condition for the satellite launching facility is to develop and implement an environment management plan (EMP), which is currently underway. The EMP will address threatened fauna and their habitat on the sites. The immigration centre is exempt from assessment under the EPBC Act, however, the DEH is being consulted on environmental aspects of the development, including monitoring of habitat for potential impacts. The airport upgrade and phosphate mining expansion proposals are undergoing assessment for environmental impact under the EPBC Act. Survey and monitoring of the Christmas Island Hawk-owl will aid in determining potential impacts of these and future developments on Hawk-Owl habitat.
The effect of climatic catastrophes such as cyclones are more severe on small populations. Cyclone frequency may increase with Greenhouse-induced climate change and it would be useful to understand the effects of cyclones on owl populations to help predict the impact of any increase.
Weeds, especially newly introduced invasive species, could impact on Christmas Island Hawk- Owl nest sites, for example by forming vine towers over nesting trees.
Road killed Christmas Island Hawk-Owls are occasionally reported. A substantial increase in vehicular traffic will be associated with the proposed satellite launching station and the new immigration, reception and processing centre. This will likely increase the number of roadkills in high traffic areas. Thus it is possible that collisions with cars will become an issue for the conservation of populations in high traffic areas.
Feral Cats Felis catus are widespread throughout the original and modified vegetation on the island. Owls are unaccustomed to predators and may roost very low to the ground and consequently may occasionally be at risk of being caught by cats. However, cats do not appear to be a significant threat to owls at present (Tidemann et al. 1994, Van der Lee and Jarman 1996).