As island-dwellers, we are also familiar with animal pests. I don’t mean whiny little sisters, I mean animals that shouldn’t be in Aotearoa New Zealand. You’ll already know about rats and mice, stoats and goats, pigs and possums. They cause damage mainly by eating — they eat our flowers, they eat our baby birds, they eat our grain and vegetables. Many experts think that our biggest conservation issue is weeds and pests (even more than pollution, habitat loss or climate). And on land, that’s probably true.
Aotearoa New Zealand is far from other lands, so how do creatures from other lands arrive? Some appear to have come on their own, such as the white-faced heron, which flew and/or was blown in. Small ones might hitch a ride in bags and clothing. Others were carefully imported and released, including useful animals such as sheep and cows, but also not-so-useful ones, such as starlings and bell-magpies. Not all of these animals are pests, of course.
We have a detailed list of who is what in the Wildlife Act, listing all the animals you can hunt (both game and unprotected wildlife) or shoot if they are annoying (partially protected wildlife) or overly plentiful (culling). Most of our special native species are protected from human hunters, but we can’t easily protect them from pests and weeds.
And then the ocean is, as always, a different place. For one thing, it’s all connected, so swimming and floating organisms can get pretty far. And then there are thousands of ships travelling around from port to port. In 2025 so far (January to September), Maritime NZ has counted 750 cargo and vehicle carriers, 583 container ships, 227 tankers of various kinds, 94 passenger ships and 16 research vessels arriving in our country. Most of them stop at more than one port.
Ships can carry small organisms or larvae or eggs on their hulls, in their ballast water, or among their fishing gear. When they land in a new part of the world, these hitchhikers might find conditions to be hostile, but sometimes they find places where they can thrive and spread out. Just like on land, their growth can stifle or damage native species.
At present the marine waters of Aotearoa New Zealand host invasive seaweeds, crabs, barnacles, corals, bryozoans, worms, molluscs — more than 200 species, with more arriving all the time. Some of them are especially harmful, known as "fouling" species because they grow all over surfaces, blocking pipes and even waterways. Freshwater examples include water hyacinth and didymo. On the coastline, the North Island hosts a rampant tube worm called Ficopomatus enigmatus. Down south, we are contending with the invasion of a large brown kelp called Undaria pinnatifida, which is particularly harmful to our taonga pāua.
We don’t like to think about it, but invasion can work both ways. One abundant and widespread freshwater invader is the small New Zealand mud snail. It’s found in lakes and streams across Aotearoa but, when it gets out into the world, it can overpopulate lake bottoms. And it does, in America, Europe, Australia and Asia. Wikipedia calls it the 42nd-worst alien species in Europe.
Ships can also carry viruses and bacteria, some of which can be very harmful. There are plenty of invasives we don’t see yet, which we really don’t want (for example, green crabs). We have laws and policies to prevent introduction by regulating hull-scrubbing and insisting on offshore ballast-water exchange. This is certainly one of the areas where prevention is easier than the cure, and education can also make a big difference. We "check, clean and dry" our gumboots and boating gear, we spray our boots to save the kauri trees, we declare what’s in our luggage at the airport and we are vigilant when we visit pest-free islands. It is ordinary New Zealanders who spot something unusual and contact Biosecurity NZ. Our gardeners, boaties, hunters, hikers and snorkellers are our first line of defence, noticing plants and animals who are in the wrong place.
To report a weed, pest, or disease, you can contact the MPI free 24-hour emergency hotline on 080080-99-66.
Emeritus Prof Abby Smith is recently retired from the University of Otago’s marine science department. Each week in this column, writers address issues of sustainability.
