Prepared by Richard Hill
Department of the Environment and Heritage, 2004
ISBN 0 642 55009 3
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 in infested areas. 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. 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.
PAN field workers and assistants conducted an island-wide ant survey during May-August, 2001. The 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% of sites in natural forest. Super-colony densities were recorded at 22.7% of sites in natural forest. Using these survey points as a representative sample of the forest, it was estimated that 2,379 ha of the estimated 10,492 ha of forest on the island was infested with crazy ants at supercolony 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).
In September 2002 an aerial baiting program was undertaken, and all known supercolonies treated with insecticide. Results so far indicate the program was successful in controlling supercolonies over 2500ha of Christmas Island. Crazy ants are still present in low densities, however, 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 diseases. 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.
Approximately 25% of the island's original forests have been cleared and replaced by shrublands of ferns on minefields, Secondary Vegetation on stockpiles, and roads and housing (Environment Australia 1994). This has meant a loss of available forested habitat, although secondary vegetation growing along roadsides and on stockpiles contributes to the feeding habitat for Christmas Island Goshawks at least at some times of the year. Nonetheless, based on the decrease in available habitat, the total Christmas Island Goshawk population has probably decreased by at least 25% since settlement (Stokes 1988).
Primary forests on Christmas Island have been fragmented in places by clearing and most Primary Rainforest is 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 Secondary Vegetation.
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 goshawk habitat. One development condition for the satellite launching facility is to develop and implement an environment management plan, 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 Goshawk will aid in determining potential impacts of these and future developments on goshawk habitat.
The Christmas Island Rainforest Rehabilitation Program (CIRRP) is a long term program to revegetate old minefields. The CIRRP is funded by a conservation levy that is collected from Phosphate Resources Limited at an indexed rate per tonne. Rehabilitation priorities are primarily determined to protect Abbott's Booby sites, although other threatened species are also considered in determining the priority of sites. A significant additional result of CIRRP will be the reestablishment of habitat that supports other species such as the goshawk.
Although goshawks, as generalist raptors, are likely to cope well with habitat modifying natural catastrophes, the effect of events such as cyclones are more severe on small populations. There is a possibility of an increase in severe storms and cyclones occurring as a result of Greenhouse gas-induced climate change. It would be useful to understand the effects of cyclones and severe storms on goshawk populations to help predict the impact of any increases.
The risk of extinction varies inversely with population size (Lacy 1987) due primarily to demographic and environmental variability the effects of which can become marked in small populations. Demographic variability is the individual variation in reproductive success, which is masked when population size is large. Very small populations (< 30 individuals) can easily become extinct as a result of random demographic variation between individuals (Caughley & Sinclair 1994).
The reproductive success of a population is also likely to vary with fluctuations in the environment. The degree of this fluctuation is determined by how much seasonal and annual variation in habitat quality there is. The influence of this on the risk of extinction increases markedly with decreasing population size. For example, a cyclone, an extreme example of environmental variation, causing the death of half the population of 10000 animals has an insignificant affect on the probability of extinction of that population. However, if half a population of 200 animals dies, the risk of extinction of that population increases significantly (Caughley & Sinclair 1994).
The risks of inbreeding depression increases with decreasing population size due to the increase in frequency of homozygous alleles. Populations at risk of inbreeding depression are probably a couple of dozen individuals or smaller in size which have been at that size for several generations, as a population contraction must last for several generations to lead to a significant loss of heterozygosity (Caughley & Sinclair 1994). Any action on the island which might permanently, rather than temporarily, reduce the number of mature individual Christmas Island Goshawks (such as permanent forest clearance rather than temporary storm damage) would raise the possibility of inbreeding depression as a concern in their conservation.
Killing of Christmas Island Goshawks, especially by poultry owners, has been considered to be a significant threat to goshawks in the past (Stokes 1988). Given the small population size, any killing of either juveniles or adult Christmas Island Goshawks would be considered a significant threat to this taxon. Another possible threat is posed by cats, especially near settled areas.
Road killed Christmas Island Goshawks have been reported. A substantial increase in vehicular traffic will be associated with the proposed satellite launching station and the new immigration, reception and processing centre (IRPC). 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.
Weeds, especially newly introduced invasive species, could potentially impact on Christmas Island Goshawk nest sites, for example by forming vine towers over nesting trees.