Identification of threatsIdentification of threats

Identification of threats

Known Threats: Not known.

Potential Threats:

Disease

The Christmas Island Shrew was considered to be probably extinct by 1908 (Andrews 1909), suggesting exposure to threats other than broadscale habitat destruction. About this time (1901 to 1904), the endemic Maclear’s Rat Rattus macleari became extinct, proposed to be the result of the introduction of trypanosome-infected Black rat Rattus rattus (Pickering and Norris 1996). Isolated island species that have evolved in the absence of diseases common to continental faunas are more susceptible due to loss of population heterogeneity (Caughley and Sinclair 1994). Introduction of a new disease is the most likely cause of the initial decline.

Yellow Crazy Ant Anoplolepis gracilipes

This ant was accidentally introduced to the island between 1915 and 1934. Crazy ants form multi-queened supercolonies, and dramatic increases in supercolony formation began in the mid to late 1990s at several widespread locations, including The Dales area. Prior to aerial baiting in September 2002, supercolonies infested over 2500ha of rainforest.

Crazy ants have the potential to alter the whole ecology of the island. For example, the Red Crab Gecaroidea natalis population has declined by at least 30% due to ant attack, resulting in dramatically altered plant community dynamics (Garnett & Crowley 2000; M. Jeffery, PAN, pers. comm.). Additionally, the ants farm scale insects, causing damage to trees resulting in dieback and canopy thinning (Garnett & Crowley 2000; M. Jeffery, PAN, pers. comm.). Flow on effects could include spread of the introduced Black Rat into areas formerly occupied by the crabs, alteration in both ground- and tree-dwelling invertebrate diversity and abundance, changes in ground layer vegetation structure, invasion of weeds and introduced Giant African Land Snail Achatina fulica in die-back affected forest, and alteration in leaf litter depth affecting soil moisture. The direct effects on the shrew are unknown, but it is likely that breeding, shelter and foraging sites would be severely effected. It is also likely the ants kill young animals in the nest, and possibly adults in severely affected areas (as recorded for a Christmas Island Pipistrelle in a harp trap in The Dales area by Lumsden et al. 1999).

A priority conservation management objective of PAN is to control crazy ant supercolonies by aerial baiting. The impact of this on the shrew, both through contact with baits and flow on impacts on prey species, is unknown. In September 2002 all known supercolonies were baited. Results indicate this was successful in controlling supercolonies over 2500ha of Christmas Island. Crazy ants are still present in low densities and PAN staff will continue to monitor any new supercolony formation and treat by hand baiting over the next few years.

Habitat Loss

Although the dramatic decline of the shrews occurred before extensive clearing and prior to the network of bulldozed drill lines, these factors could have contributed to the population not recovering from the initial decline. The unconfirmed 1958 reports were the result of habitat clearance for phosphate mining (Meek 2000). This suggests that, similar to many of the island’s endemic avifauna, remaining shrew populations would have been adversely affected by the destruction of about a third of the rainforest for phosphate mining. They may also have been affected by the drill lines resulting in increased predation risk and localised alteration in microhabitat characteristics, such as the loss of ground cover.

New proposals to clear primary rainforest on vacant crown land may apply additional pressure on any remaining shrew populations. These proposals include phosphate mining at sites in the eastern section of the island, and activities associated with developments such as the siting of a mobile phone tower on Limestone Hill, South Point; the Christmas Island airport upgrade; road re-alignment and new port facilities north of the former Christmas Island Resort area.

Habitat Alteration

Alteration of the rainforest habitat due to the extinction of Maclear’s Rat and Bulldog Rat R. nativitatis and theorised alteration in crab populations (particularly Red Crab), may have resulted in an adverse change in microhabitats within rainforest critical to the maintenance of shrew populations (Meek 2000).

Predation

No instances of predation have been recorded. However, it is likely that introduced predators (e.g. Feral Cat, Black Rat and Wolf Snake), the naturalised Nankeen Kestrel Falco cenchroides, and endemic predators, such as the Christmas Island Goshawk Accipiter fasciatus natalis and Christmas Island Hawk-Owl Ninox natalis, may opportunistically predate on the Christmas Island Shrew.

Small Population Size

Current evidence strongly suggests that the Christmas Island Shrew occurs in very low densities, compared to early observations made of the species. A small population size increases the risk of extinction through inbreeding depression and stochastic events (Caughley & Sinclair 1994). The network of wide mining haulage roads may have further isolated populations due to a possible reluctance of individuals to cross wide open spaces due to elevated predation risk.

Other Potential Threats

The two individuals recorded in 1985 were both located on park roads or walking tracks within rainforest in the western section of the island. There is a possibility some shrews may be road killed, and such a threat is likely to increase with a substantial growth in vehicular traffic associated with developments, such as the Asia Pacific Space Centre and the Immigration, Reception and Processing Centre.

Although forest fires are uncommon on the island, during recent extended dry periods in 1994 and 1997, fires occurred in terrace forest. The effects of forest fire on the Christmas Island Shrew is unknown, but may result in adverse impacts due to the loss of ground cover and leaf litter, and by affecting invertebrate populations.