Survey for Feral Black Rats (Rattus rattus) in the Victoria River Area in the Northern Territory of Australia
About
Photo: Black rat (Rattus rattus) JCU website
The Department of Natural Resources, Environment, The Arts and Sport was commissioned by the Environment Management Unit of the Department of Planning and Infrastructure to survey and prepare a management plan for the introduced Black rat (Rattus rattus). The survey was completed in cane grass and Eucalyptus woodland habitat along the Victoria River from the Victoria River Bridge to Sandy Creek.
Anecdotal evidence has suggested that Black rats may prey on the eggs of the nationally and Northern Territory listed threatened species the Purple-crowned Fairy-wren which inhabits this cane-grass.
Black rat predation as well as disturbance from the Victoria Highway Upgrade Project may have detrimental impacts on the Fairy-wrens.
Male and female Purple crowned fairy wren
Male Purple crowned fairy wren
Objectives of the Study
The objectives of the fauna survey were to:
- Determine the abundance of Black rats and non-target species in the cane grass habitat and surrounding vegetation.
- Where possible, determine the size of the area occupied by Black rats; and
- Identify predators of the Purple-crowned Fairy-wren eggs by using plasticine eggs at artificial nests in the Purple-crowned Fairy-wren habitat.
Methodology
Three sample sites were selected:
- Victoria River Roadhouse
- Coolibah Crocodile Farm
- Victoria River Boat ramp
And three trap grids of two major habitat types were selected for sampling at each site:
- Cane grass and
- Eucalyptus woodland
Resulting in eighteen trap grids.
Map showing trapping sites
Trapping
Live capture/release methods were used to conduct small mammal surveys at each site on four separate occasions throughout 2008 (May, July, October and November).
Small and large aluminium Elliot box traps and wire cage traps were used.
Traps were checked each day in the early morning and all sites were trapped for a minimum of 10 nights, giving a total of 3780 trap nights for the entire study.
Results
A total of seven terrestrial vertebrate species were captured during this study. Of these, five species were native and two were introduced species. Species included the:
Northern brown bandicoot (Isoodon macrourus)
Grassland melomys (Melomys burtoni)
Delicate mouse (Pseudomys delicatulus)
Western chestnut mouse (Pseudomys nanus)
Pale field rat (Rattus tunneyi)
Black rat (Rattus rattus)
Cane toad (Bufo marinus)
Out of seventy individual mammal captures for the entire survey, the most individuals were caught at the Coolibah Crocodile farm and the fewest at the Victoria River roadhouse.
Two Black rats were caught; one at a cane-grass site at the Victoria River boat ramp; and one at a eucalyptus woodland site at the Victoria River Roadhouse .
The lack of Black rat captures may be attributed to the species having a particular aversion to changes to their environment, such as traps, alternative foods and the presence of people.
Nest searches and Purple-crowned fairy-wren surveys
Nest searches and surveys were conducted during the May, July and October trapping sessions in an attempt to locate and estimate populations of Purple-crowned Fairy-wrens. Each site was surveyed for two hours, either during mid-morning or late afternoon.
Results
No purple-crowned Fairy-wren nests were located during the study, however an adult pair was observed frequenting a specific clump of cane-grass at the Victoria River Boat Ramp site during both May and June surveys and incidental sightings were recorded during the November trapping session. Purple-crowned Fairy wrens were observed to inhabit both cane-grass and woodland habitat during the study.
Plasticine eggs
Artificial Purple-crowned Fairy-wren nests with plasticine eggs were set up in the cane-grass sample sites in an attempt to identify predators at nests via teeth marks left in the eggs.
Four nests with three eggs each were placed at each cane-grass trap grid and left out for three to four consecutive nights during the November trapping session.
Results
No signs of bite or teeth marks were observed on any of the eggs across the sample sites. However at the Coolibah Crocodile farm cane-grass sites, all four eggs from one nest and one egg from another site were completely taken.
Increased grazing and trampling of this cane grass site by pigs, cattle and horses may have made the artificial nests and eggs more visible from the air for avian predators which may explain why the eggs were taken in their entirety.
Joint management staff member making a nest.
Discussion
Due to the small sample size of Black rat captures and absence of teeth mark evidence from the plasticine eggs, the abundance and distribution of Black Rats in the cane-grass habitat and surrounding vegetation was unable to be calculated. Furthermore, no trends in seasonal abundance for the species could be determined.
While no Black rats were caught at the Coolibah Crocodile Farm site, Black rats were caught at nearby Coolibah Station in an earlier study by Archibald and Horner (undated). It is quite feasible that a Black rat population at Coolibah Station could travel the short distance to the Coolibah Crocodile Farm site which is likely to be a good food source for the Black rat. It is also possible that Black rats from Victoria River Roadhouse and the Victoria River Boat Ramp would be able to colonise other cane grass areas along the Victoria River.
The Future
- Any control or eradication of Black rats in the Victoria River area must consider the presence of non-target native rodents, specifically the Pale field rat and Western chestnut mouse which are classified as near threatened in the Northern Territory (Northern Territory Government 2007).
- Aside from their physical appearance, the Black rat can be distinguished from native rodents by their good climbing ability. This fact suggests that a method of baiting, using elevated baiting stations where non-target species can’t access them would be preferred.
- Education on the identification of Black rats and similar native species targeted towards landholders, traditional owners and other stakeholders would enable accurate reporting of the presence of Black rats and targeted control of the species in the Victoria River area.
- Inadequate management of waste materials in the Victoria River area could encourage or increase Black Rat populations. Therefore waste management activities including regular collection and burning are encouraged and considered to be adequate.
Acknowledgments
The Northern Territory Department of Planning and Infrastructure funded this project. David Hooper developed the Operational Plan and Methodologies for the study. Anne-Marie Van Doorn provided valuable comments and information regarding Purple-crowned fairy-wrens and the Victoria River area. Roy and Teresa at Victoria River Roadhouse and Bluey and Janelle at Coolibah Crocodile Farm kindly allowed NRETAS access to their properties for sampling. Parks and Wildlife service staff, including Joint Management staff and the rangers at Gregory National Park assisted with background information, participation and resources for field trips.
All information cited from:
Foate, M., Farrelly, M., and Walters, A. (2008). Survey for feral Black rats (Rattus rattus) in the Victoria River area in the Northern Territory of Australia. December 2008. Northern Territory Government.
Joint management staff; Rangers and Wildlife management staff.