Understanding rat habits in different habitats

Becoming the world’s first predator-free urban island is no small feat. There are many questions still to be answered about how to remove rats from Waiheke and the rat pilot operational trials are designed to do just that.

With abundant food sources, shelter, and temperate winter months, Waiheke has been described as a rat’s dream and a manager’s nightmare, but on the flip side, if eradication can be done here, it can be done anywhere. 

Mangroves

So, what makes Waiheke different to other island eradications? Well, the biggest difference is us, humans that is. The biggest inhabited island rat eradication to date was on Lord Howe Island with a population of 400 people on 1,455 hectares of land. Waiheke by contrast has around 9,500 residents on 9,200 hectares of land. While Waiheke is more than six times larger than Lord Howe, it’s the human element that poses the biggest challenge.  

The techniques for eradication on open land and forested areas have been tried and tested, but complete removal from a highly urbanised environment hasn’t yet happened at this scale. Little is known about how rats use different urban habitats and so eradication techniques must be tested and refined before island-wide eradication can be pursued. Our sister project Predator Free Wellington has started the mahi of eradicating rats from the capital and regularly share their learnings with us.

Other habitats that little is known about in New Zealand include coastal cliffs and dense mangrove areas. These are both complicated habitats to manage in eradications. Access and ability to service the traps or stations have continually proved to be a challenge elsewhere, so the operational trials are carving a new path for these hard-to-reach places. Learnings from the rat pilot operational trials and these key habitats will be keenly watched by other eradication programmes around the country and around the world.

Let’s have a closer look at some of the habitat types and how the rat pilot operational trials will address their unique challenges.

Habitat: Industrial 

Area in trials: Ostend

Challenges: Heavy machinery, moving landscape, and day to day operations of industrial areas.

Outline: Industrial habitat has what’s called a high three-dimensional complexity. When rats are foraging, they are constantly moving. They climb, burrow, jump and run so their movements aren’t measured as the crow flies. They could spend their whole evening exploring the inside of an abandoned car, a container, a building or a skip, always moving but not going far. This talks to the three-dimensionality of multi-layered industrial areas. 

There can be a lot of things in industrial areas that will also provide shelter to rats, containing lots of cracks, crevices and holes where they can build nests and forage within a small ground area. 

To date, there hasn’t been a successful eradication in such an industrial/urban area, and little is known about rat behaviour in this habitat. A key learning from the trials will be if the bait station density used will be sufficient to remove all rats from complex industrial areas. 

Habitat: Mangroves 

Area in trials: Ostend

Challenges: Accessing the bait stations, preventing the bait from entering the water.

Industrial

Outline: Mangroves are prone to be a difficult habitat to remove rats from. In other international eradications, different methods have been tested. One, for example, is a bola net with bait inside, thrown into mangrove trees but if the rat chews the net, the bait falls into the water. The second is using big block baits below the high tide line that will last longer when submerged. Neither of these options are viable for Waiheke as all bait must be in secured bait stations and kept out of waterways. Tackling Waiheke’s mangrove habitat required a little innovation, a lot of patience and design expertise. 

Rat Team Leader, Phil Salisbury, has worked hard over the past 6 months, designing, and refining the floating mangrove bait station which will be used in the trial. The small rafts float on recycled milk bottles that are attached to bamboo poles that will slide up and down with the tide. They sit high enough out of the water for the bait to be secured, but low enough that rats can jump onto them from the mud. They also have some wire mesh attached to help swimming rats climbing onto the raft.

Monitoring will be needed to ensure they are working effectively, but that also presents a challenge as tracking tunnels will likely not work in this habitat. Wax blocks and chew cards, alongside trail cameras, will be the main detection tools used when it comes time to see if the bespoke bait stations have worked. Watch this space. 

Habitat: Commercial 

Area in trials: Ostend

Challenges: Alternative food sources and small home ranges.

Outline: In commercial areas, one of the biggest challenges is alternative food sources and potentially small home ranges. Unlike industrial areas where there is lots of shelter but potentially not as much food, commercial areas that include food services or food production could unintentionally provide an alternative food source for rats. All food premises in the rat pilot area do currently have robust rat control programmes in place for their premises, although it’s outdoors that poses a challenge. 

If people are consuming food outdoors in these areas, dropped crumbs and food scraps are an attractive meal for rats in the evenings after the humans have left. As in other urbanised areas, little is known about how the rats utilise this habitat. A key learning from commercial habitats is if the rat bait is attractive enough for the rats to compete with delicious free snacks that can be found out in the open.

Habitat: Residential 

Area in trials: Ostend, Kennedy Point, and Rocky Bay

Challenges: Alternative food sources, lots of shelter, and warmth

Clifftops

Outline: Perhaps the most complex urban habitat is the residential area as there are ample opportunities for shelter, warmth, and food sources. As rats are more than happy to consume the same foods as we humans, the byproducts of our meals in turn attract the rats. Whether it’s the rubbish bin and compost or even fruit trees and vegetable gardens, rats will happily enjoy a meal from any source. To mitigate the risk of other food sources, the operational trials are being carried out over winter when natural food sources are low, gardens are relatively empty and trees are not carrying fruits. 

Rats also share a human’s general desire to stay dry through the cold winter months and will seek out dry spaces around properties. Garages, beneath upturned kayaks, under balconies, anywhere that is dry and preferably warm is a perfect place for rats to wait out the winter. 

Lucky for Te Korowai o Waiheke, access challenges were few and far between because the landowners and businesses in the pilot are so supportive. 

Habitat: Coastal Cliffs

Area in trials: Kennedy Point

Challenges: Accessing traps. 

Outline: This is a habitat that has been ignored in other eradication attempts and has subsequently led to failure. It is clear that cliffs provide important habitat for rats, so in the Kennedy Point trial, traps have been installed on cliff faces as part of the peninsula’s trapping network. The devices can only be accessed by an experienced ropes team, so one of the key learnings will be the feasibility of device servicing on cliffs. 

Cliffs make up a big section of Waiheke’s coastline and before an island-wide plan can be made for eradication, appropriate techniques and solutions are needed for this key habitat. 

The learnings from these five key habitats will be invaluable. Not only will they inform the rat eradication plan for Waiheke, but they will offer a foundation for any future urban, mangrove, and cliff eradication projects.