You dirty rat!

Taking out the introduced predators

December 2024

4. Need to identify and remove pest animals. Possums and mice will feed on leaves and flowers and seeds of moth host plants, removing these from the ecosystem. Wasps will reduce caterpillar numbers substantially.

When I undertook the bioblitz of the garden, I saw a ship rat up in the trees at the top end of the garden. I failed to get a photo, so you'll have to take my word for it. I ordered chew cards the following morning to help identify what predators were in the garden and where they liked to be.

I realise that the action point above doesn't mention rats or mustelids and that's because they are the really bad guys and it goes without saying. Unless you are writing a blog about it. Then you should spell it out.

Okay, I have to admit I didn't really think about mustelids. There was no evidence of them at the North Shore reserve I used to work in, and where they had been caught locally, they were a rarity. In writing this I looked up sightings on iNaturalist - have I told you how much I love that app? Yes, yes, I have - and there has been a sighting in Chelsea Estate Heritage Park, not far from here. Stoats, weasels and ferrets were all introduced to New Zealand and decided that instead of chasing the rabbits which had also been introduced and were over-running the place, they would eat the stationery prey, like ground birds, nesting birds, chicks, eggs and bats (because our short-tailed bat likes walking on the ground).

Rats are much, much more common in urban and suburban areas and they'll pretty much eat anything they come across - birds, eggs, lizards, insects, chicks, seeds - and none of it is good for the local environment. If they aren't eating the native animals, they are directly competing for their food sources.

Rat at Auckland Wintergardens looking for food

Then there are possums and Argentine ants and hedgehogs - the list goes on. If you want to know more, read the Department of Conservation webpage on pests. There's a butterfly on there too, just to whet your appetite. That's why New Zealand has set itself the audacious and inspiring target of being predator free by 2050 and the government is investing in technology to help achieve the goal. This doesn't include cats, because - controversial - but if Australia can put restrictions on our favourite furry pets and their incredible hunting abilities, so can New Zealand.

If you don't like the idea of animals being killed, stop reading and move onto another post. I deal with it by making the kills as humane as possible and if that means I have to dispose of the remains, so be it. Auckland focuses on using poisons, but my take is that it is not the fault of the animal for hunting naturally, so a quick death is the least it deserves.

Mapping potential trap locations at the new house

I'm so keen on the removal of introduced predators that I had already planned where my rat traps were going to be before we moved. I looked up the aerial shots of this property from Auckland Council's geomaps software, measured 50 metres - the recommended gap between traps in reserves - and marked the possible spots for the rat traps. However, chew cards take the guesswork out of it.

A chew card by the house

Chew cards are corflute with bait, usually peanut butter, stuffed in the gaps. The animals bite the cards and the bite marks identify the cheeky snacker. You put them every 20m - mice have small territories, so you have to allow for that - and leave them out for three days in good weather. Then you analyse them.

The chew cards from the garden

This is what the chew cards looked like once they had been out in the garden for three days in October. There was clear rat chewing on the cards where the rat had been seen (card #1) and the nearby compost heap (card #2). You can't see from the photo above, but there had been some delicate nibbling of the other side of card #5, which showed mouse teeth marks.

So I put the first rat trap where the rat was spotted and chew card 1 had been placed. I didn't realise how difficult access to the stream would be when I mapped the potential spots, so the southerly trap moved closer to the house (card 4's location). My rat traps are snap traps, usually baited with peanut butter, that are housed inside wooden tunnels. Rats are nervous about entering the tunnels, but without them, birds, pets and little humans would all be at risk of injury from the traps. I also have an automatic re-setting rat trap that I've yet to put out and I've ordered a possum trap. Whilst there was no sign of possums on the chew cards and no visible scratch marks on the trees in the garden, I'm planning on planting some of their favourite trees to eat and I want to protect them as food sources for the birds.

Sure enough, as soon as I put the traps out, I caught two ship rats in the north trap within a week of each other. I'll now follow Pest Free Kaipatiki's suggested trapping programme, that targets rats and possums at key times of the year.

Will it make a difference? It has been shown that if a rat population density is under 5%, the local bird life flourishes. The density is calculated using chew card results so the rat density in my garden would be 2/5 cards eaten = 40%, mice 20%, possums 0%. This is my starting point. Chew card monitoring is usually done annually, so I'll need to decide if I do it again in October, or fall in with the Pest Free Kaipātiki chew card cycle during winter. Probably the latter because the lack of food in the environment will make the chew cards a more valuable source of energy and the results more accurate.

Predator Free Miramar is an example of a highly successful trapping programme. Possums had already been eradicted from the peninsula, which is surrounded on three sides by the sea, and now the area is officially rat and mustelid free. Their Facebook page used to be filled with people's trapping successes, but is now filled with pictures of the rare native birds that frequent their gardens. Indeed, with so many active community trapping programmes across Wellington, they introduced kiwi back into the city in 2022, which is truly remarkable.

Okay, you say, that's great for Wellington, but you are in Auckland, and what about the wasps in your action point? You haven't mentioned them yet. Well, Auckland is definitely behind Wellington, but work in my lovely area of the North Shore, Kaipātiki, which has 450 hectares of indigenous forest and some critically threatened ecosystems, is very active. Pest Free Kaipātiki's monitoring since 2017 has shown a reduction in predator species, which is the result of many amazing volunteers doing their bit. Unfortunately, one of the consistently troublesome areas is at the end of my road and round the coast. The rats are so numerous they are often visible and there are ground nesting birds like banded rails, a species of wetland bird that is in decline, also living there. So it's even more important for me to be trapping, even if I only get the rats that have come for a holiday up the hill through the sweep of bush that runs through the gardens here.

Wasps are another matter and they'll get their own blog post. Probably full of swear words. You have been warned.