Rare plants and trees of the Hauraki Gulf islands

Sticky seed pods, seabird poo, food webs and a message in a bottle…

The Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa is a biological hotspot, home to a multitude of species from the microscopic to the mighty. An often-overlooked group of species that are fantastically diverse are the plants of the Gulf. To find out a bit more about the Gulf’s rare flora, we asked Bec Stanley from Auckland Botanical Gardens to share with us her experience undertaking a threatened plant survey on Motukino Island.

The smell of a rat-free island is one of the clearest memories I have of a threatened plant survey on Motukino (Fanal Island) in the Mokohinau group. Why? Because the seabirds on Motukino are thriving, and likewise so is their guano (excrement)! Guano gives the soil a distinctive soft texture and acidic aroma, the smell of which is near impossible to remove from your hands. This soil gives life to some interesting native plant-life, and we were camped under an odd bunch of plants including parapara (Pisonia brunoniana), and a stand of coastal maire (Nestegis apetala) with mistletoe pirita (white mistletoe, Tupeia antarctica) attached to it.

While it’s well-known that controlling rats helps protect native animals, it is less well-known that rats also threaten plants. They do this directly by eating their flowers, fruits and seeds, and indirectly by predating native animals that help create plant habitats, pollinate flowers and disperse seeds. The flora of Motukino is a snapshot of what all coastal mainland areas in the north would be like without rats.

The parapara is a tree that has all but disappeared from places where you find rats. Its sticky seed pods evolved to attach to a seabird, a bird large enough to preen the pods off its feathers. This disperses the seeds to new locations, making it a great example of a plant that has adapted to its coastal habitat.

The flipside of this however, is that in cultivation, smaller (non-sea) birds can get stuck in the bunches of pods and they are not large enough to preen them off – hence its colloquial name, ‘the bird-catching tree’. I don’t recommend this tree for a home gardens because of this, but this sticky habit should not diminish the value of this rare tree. While camping under parapara for five nights I didn’t see any forest birds caught in the pods on Motukino. Perhaps smaller native birds are savvy to the risks and know to stay away? And let’s not forget, native birds on predator-free offshore islands can take the time to preen the seeds off, something that a bird in the city can’t safely do with neighbourhood cats around!

The coastal maire has olive-like fruit, eaten by rats and this is likely the main reason why this tree has declined from northern coastal mainland areas, as well as any islands with rats. It is a smaller, compact tree (grows to 6 metres) with glossy foliage and grows in a winding fashion giving it a bonsai-like appearance – all of which make it a beautiful addition to the home garden.

The pirita (or white mistletoe) popped up on Motukino three years after rats were eradicated. It was noticed by a ranger when he saw ‘sprouts’ coming out of a coastal maire trunk.  Memorably he sent me a sample of the sprouts to identify, poked into an empty plastic milk bottle to protect it on its journey! My kind of message-in-a-bottle.

For a botanist, one of the most wonderful things about the Mokohinau Island group are its populations of nau or coastal cress (Lepidium oleraceum).  Seabirds on these islands and islets create ground disturbance when they dig their burrows allowing for herbs like nau (coastal cress or Lepidium oleraceum) and native watercress (Rorippa divaricata) to flourish. Birds also fertilise the soil with their guano which makes this habitat tough for other plants, but cresses can handle the high nutrient levels.

While predator-free islands are wonderful places to see how native plants flourish without rats, there is a catch. On one islet in the Hauraki Gulf a population of nau (Lepidium oleracuem) declined markedly after rat eradication. A possibility is that rats may have been keeping the exotic slugs down, most likely from the vegetable garden present when the island was once inhabited, which, once the rats were gone, ate the nau seedlings. Perhaps rats were also eating seeds of an exotic grass which spread rapidly into nau habitat after rat removal. 

There is no ‘one size fts all’ with conservation management and while I am no friend of the rat, sometimes exotic animals may perform a function in a food web which needs to be understood for plant conservation to be assured.

A food web is a tool to help understand the relationships and interactions among species in a community. Think of a food web like the game Jenga. In this game players construct a tower by removing other blocks and the tower gets more unstable as the lower blocks are removed. If a place has other exotic species in the food web, we need to understand how removing one predator will impact the whole system. It’s ‘ecological Jenga’ and if we really understand what every ‘block’ in this ecological system is doing we can start to remove them without destabilising the ‘tower’. It doesn’t mean keeping rats, it means planning eradications with all the values, including plants, being considered.

I am very fortunate and feel very privileged to have visited most islands and islets in the Gulf. One way we can all share in this experience is to visit the Auckland Botanic Gardens to get the sense of an offshore island habitat. In the Threatened Native Plant garden, you can touch the sticky seeds of parapara and see nau up close. In the Urban Trees garden, you can see for yourself why we recommend coastal maire as an ideal tree for an urban garden. It would be wonderful to see you at the Gardens sometime.

The Auckland Botanic Gardens is a 64-hectare botanical garden located in Manurewa, South Auckland. The Threatened Native Plant garden features plants from the Auckland and Northland regions in gardens designed to mimic the wetlands, offshore islands, lowland forests, lava fields, saltmarsh and dunes they originate from. From April to September, the gardens are open from 8am – 6pm, while from October to March they are open from 8am – 8pm.

Love it, Restore it, Protect it.