Tackling four unique rat habitats on Waiheke Island

Department of Conservation —  21/07/2023 — Leave a comment

Imagine a Waiheke Island that is a haven for unique and ancient species of birds, lizards, insects and plants. Unfortunately, harmful introduced rats pose a threat to wildlife all over the island – from mangroves and cliffs to commercial and industrial areas. One Waiheke conservation project is tackling the problem. Helped by Jobs for Nature funding, Te Korowai o Waiheke share the successes and challenges of trials to remove rats from all these environments.

Bait consumption in the trial area for industrial, commercial and mangrove areas in a) Week 1, b) Week 4 and c) Week 7. 📷: Te Korowai o Waiheke

Te Korowai o Waiheke is a community-led conservation project based on the beautiful island of Waiheke in Tīkapa Moana/ the Hauraki Gulf. Their long-term aim is for Waiheke to become the world’s first predator-free urban island.

Predator-free islands are havens for unique and ancient native species to thrive. For example, since introduced predators were eradicated from Kapiti Island in 1996, some lizard populations grew 28-fold and around 1200 kiwi pukuku now call the island home. The dawn chorus is deafening with songs of kākā, tūī and kākāriki.

Waiheke is a 9,300-hectare island with a permanent population of over 9,000 people, which swells to around 50,000 with summer holiday residents, plus they welcome around one million visitors per year.

And where people go, rats are not far behind.

Te Korowai o Waiheke secured Jobs for Nature funding through Predator Free 2050 Limited to fund 7 full-time roles over 1 year to start rat operational trials.

The Jobs for Nature programme provides nature-based employment to communities and industries in response to the COVID-19 pandemic.

Te Korowai o Waiheke were funded as part of a wider strategy to learn more about elimination techniques in different landscapes – when faced with different challenges. DOC has supported the group with technical advice on predator control from our leading scientists.

Te Korowai o Waiheke shared what they have learned so far about rat control in four unique habitats found on Waiheke Island:

Mangrove habitat

Challenges: Accessing the bait stations for servicing, preventing the bait from entering the water, enabling rats to access bait stations.

Mangroves are woody shrubs that grow in coastal estuary areas. Tackling Waiheke’s mangrove habitat required a little innovation, a lot of patience, and design expertise.

It’s challenging to access the bait stations in muddy, dense mangrove areas. And bait must be in secured stations and kept out of waterways.

To meet the mangrove challenge, Phil Salisbury, Rat Team Leader, designed a floating bait station. The bait is secured onto small rafts, which float on recycled milk bottles. This is attached to bamboo poles to allow the raft to slide up and down with the tide and also rotate around the bamboo pole with the current. The rafts 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 wire mesh attached to help swimming rats climb onto the raft.

Wax blocks and chew cards, alongside trail cameras, were the main detection tools to see if the bespoke bait stations worked.

Results: These floating stations were a great success. They can be made from easy-to-source or even recycled materials. Over four months, only one of the 74 floating stations got stuck in the mud.

A 50 m x 50 m bait station grid was used in the mangroves and it appeared to work effectively in removing rats, similar to the nearby dry land traps set on a 25m x 25m grid.

Rats were present right across the mangrove area – this wet environment did not deter them, so it needs to be included in future plans to remove rats from the whole island.

Industrial habitat

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

Industrial habitat has a three-dimensional complexity. When rats are foraging, they are constantly moving. They could spend their whole evening exploring the inside of an abandoned car or a skip. They climb, burrow, jump, and run so their movements aren’t measured in flat metres on the ground. Therefore will a standard 25m x 25m grid still work?

Results: The 25m x 25m grid of bait stations, serviced weekly over 17 weeks, was successful in removing rats. Interestingly, they were all placed on the ground – and this didn’t affect the success.

However, the industrial area did see some lone nibbles on chew blocks and rats caught on camera after several weeks of no activity. Based on timings and locations, these rats appear to have been transported into the area through waste disposal on trucks or trailers.

Picking up this high-risk avenue means the team knows where to focus their efforts once they’re close to elimination.

Commercial habitat

Challenges: Alternative food sources and small home ranges.

Commercial areas that include food services or production could unintentionally provide an alternative food source for rats. Food premises have robust rat control programmes in place, however, it is the outdoors that poses a challenge. 

If people are eating al fresco or takeaways, dropped crumbs and food scraps are an attractive meal for rats after the humans have left.

A key objective was to learn if the rat bait is attractive enough for the rats to compete with delicious free snacks that can be found out in the open.

Results: Move over mince pie! The cereal-based bait, with a chocolate flavour, was just as attractive to rats, and the 25m x 25m grid worked to remove rats as effectively as in the industrial area.

It was important to really get the business community on board and share some key details with their customers on the trial. Along with posters, the team engaged with business owners to build their understanding. 

Coastal Cliff habitat

Challenges: Accessing traps. 

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

It’s possible to access cliffs but it is very time and resource intensive, with devices in this trial installed and checked by an experienced ropes team.

Ropes teams placed and serviced traps on the cliffs. 📷: Te Korowai o Waiheke

No bait was used in this trial area – traps only. Te Korowai o Waiheke installed 499 traps across 30ha of the peninsula trial area. There were also 133 bait stations in the adjacent interception zone, to stop rats migrating in from outside the trial area.

Results: The traps gradually caught fewer and fewer rodents over the first five weeks, but they were never completely removed – with some rats still being caught every week during the six-month trial.

Traps alone don’t appear to work in elimination but are a useful control tool.

As for those steep cliffs, the team is now planning more trial work for the summer to see how devices could be serviced without qualified rope workers.

Looking ahead

The learnings from these four key habitats will inform the rat eradication plan for Waiheke, plus offer a foundation for future urban, mangrove, and cliff eradication projects. 

Their work is part of a much wider Predator Free 2050 goal that is working towards an Aotearoa where our native species are safe from extinction and thrive alongside us. It aims to eradicate our three most damaging predators (rats, stoats, and possums). You can learn more about Te Korowai o Waiheke’s incredible conservation mission on their website.

No Comments

Be the first to start the conversation!

Leave a Reply