Rat monitor trial provides eradication insights

11 May 2017

A pest monitoring trial undertaken on the Miramar Peninsula is a first step to understanding the scale of the predator issue facing the peninsula as the community gears up to eradicate pests and bring back birds and other native animals to the region.

Map of Council pest-controlled areas.

The peninsula was surveyed in March using 281 chew cards placed by volunteers to record the presence of rats and mustelids ahead of Predator Free Wellington’s eradication campaign to make the peninsula predator free. It is already free of possums.

“The monitoring was really useful to understanding the job in front of us, with recordings of both target species (rats and mustelids) being made,” says Dr Philippa Crisp, Team Leader, Terrestrial Ecosystems and Quality, Greater Wellington Regional Council.

“It is interesting that the greatest numbers of cards chewed by rats were found on the peninsula coastline. Some of these sites are areas frequented by people (such as near carparks or picnic areas). Rats are attracted to food sources such as rubbish bins and dumping areas.

“There were only a few rat-chewed cards found in the urban areas, but this may be where the cards were placed, rather than because the rats weren’t present. For practical reasons, the cards were placed on street verges, but there may have been more rat chews recorded if they were positioned in back yards.”

Project director for Predator Free Wellington James Willcocks says the research will provide a benchmark against which populations will be monitored. “Local trapping groups are already out there doing a great job knocking back rat numbers. Data showing the impact of their activity on rat populations will help target eradication efforts.

Dr Crisp says that more research may be needed to ensure the eradication campaign is well targeted, including using cameras at some sites to properly identify the chews seen on cards and to encourage people to place cards in their backyards next time the monitor is completed

Another possible avenue of investigation could be of the distribution of ship versus Norway rats as it is suspected that many of the rats recorded by the chew cards around the coast may have been Norway rats. This is important for bird conservation as Norway rats show less of a willingness to climb and so are believed to have less of an impact on tree-roosting bird populations than ship rats.

The results of the survey were as follows:

  • Rat chews were found on 12% of the cards (31 out of 257). By far the most chewed cards were found around the coastline.
  • Six of the 257 retrieved cards (2%) were identified as mustelid. These were found in areas of natural bush.
  • Additionally chew marks of other species such as hedgehogs and mice were identified on the chew cards.

Predator Free Wellington is a partnership between Greater Wellington Regional Council, Wellington City Council and the NEXT Foundation, the objective of which is to eradicate predators in Wellington City to promote biodiversity within its borders.