Ridding boxthorn from predator free Motunau Island

Department of Conservation —  19/07/2018

Motunau Island is the only offshore predator free island in Canterbury. Community ranger Vanessa Mander tells us why ridding the island of boxthorn weeds is important for the sea birds survival.

“Hey Vanessa, do you want to do some weeding on Motunau Island?”

Most people would read that sentence and think “weeding? No thanks.” But all I heard at the time was “Motunau Island” and suddenly the job preceding it sounded glamorous.

Motunau Island holds such a special place in the Canterbury district as our only offshore predator free island, and although it doesn’t have the same profile of it’s more famous cousins like Little Barrier, Codfish or Kapiti Island, its importance isn’t any less diminished. It’s just the fame we need to work on.

Predator free Motunau Island off the coast of Canterbury. 📷: Vanessa Mander

So, what’s so special about Motunau Island? If you take a close look at the vegetation, the first thing you notice is the lack of native or endemic cover. In fact, it has the most amazing crop of mallow I’ve ever seen and if there was a market for it then it would bank-roll our work for ages. Sadly, it is not a desired species but better than the boxthorn that used to be in its place. Cook’s scurvy grass has been planted in places and appears to be doing well. However, it’s not the vegetation that makes this place special (yet).

(Left) A seal in the mallow (Right) Senior Biodiversity Ranger Anita Spencer holds a Canterbury Gecko. 📷: Vanessa Mander

Motunau Island is distinct because of its importance to the local sea bird and New Zealand seal populations. Due to the soft earth found on the island, it is covered in burrows created by several species of sea bird. The burrows are so numerous that it takes immense concentration to come off the island without a rolled ankle or a troubled conscience about how many you accidentally collapsed. Bird species include fairy prion, sooty shearwater, white-faced storm petrel and the little blue penguin (who are barely birds by my reckoning but easily make the list under “adorable”). Nothing is cuter than seeing a pair of penguin butts hanging out the entrance to a burrow, except maybe if they faced forward but I’m just not that lucky. For those fortunate enough to lack a sense of smell then the seals resting on the beach are also worth a photo.

While much of the boxthorn was removed over the years (hence healthy mallow stock) there are still the odd plants that need to be removed over time and that’s the justification I have for my seat on the helicopter – I have a healthy obsession around killing boxthorn. It’s not just an invasive weed, but deadly to our smaller sea birds using the island. In clumps, birds get tangled up as they land or take off and they are left to die a slow death from exposure and starvation. Therefore, the targeted destruction of boxthorn continues to be important core work on this island.

Landing on pest free Motunau Island is by permit only. 📷: Vanessa Mander

This island isn’t just home to the charismatic sea birds. For those who know where to look, it is also a safe bastion for several gecko and skink species. This was somewhat evident without the need to spot a single lizard on opening the hut door on arrival. I’ve never seen such a mass of shed skins in all my life. It honestly looked like the entire population of skinks and gecko everywhere turn up in this one spot for a spectacular party and left the rubbish behind.

After a good days work, the arrival of the helicopter to bring us home is usually a very welcome sight, but I can’t help but feel a little bit despondent. I know it’s going to be a while before I return and I’m starting to get a bit of FOMO already. Seeing a large boxthorn bush on the cliff (out of any sensible reach) as we take off does little to dispel the feeling. Your time will come boxthorn, I’m watching you…

3 responses to Ridding boxthorn from predator free Motunau Island

    John Parkes 21/07/2018 at 7:02 pm

    Years ago Chris Challies, Graham Nugent and I were out on Motunau banding birds -sometime after a boxthorn cull. The dead boxthorn had been thrown into the tide and merely washed back up to jam between the rocks at high tide, thereby blocking many penguin access routes.

    A good deed was done as we hauled it out and down to the low tide platform in the east. A match and bonfire was the solution but bloody hell dry boxthorn burns. Thank God the wind was in the right direction otherwise …

    John Parkes

      Irene Shepherd 11/10/2018 at 1:04 pm

      I remember when the boxthorn was tossed into the tide as it also got washed up the river


    Great article. There was never a problem with boxthorn or mallow when rabbits were present. Boxthorn seeds brought from the mainland by roosting starlings and the seedlings then grazed by rabbits. The wisdom of hindsight?