Learn About Introduced Browsers

An example of the different stages of horoeka (lancewood). It changes as it grows to over moa height and develops much more lush green leaves. Photo: Rachel Thompson

The forests of Aotearoa have always had browsing animals (animals that eat plants). They are an important part of the ecosystem, spreading seeds and spores, providing nutrients for plants through their poo, and helping keep the plants in balance. Over millions of years, our native plants evolved with moa and other large birds as their main browsing animals, as we do not have any native land mammals, apart from tiny bats. Many of them adapted to be able to survive moa by having small leaves on divaricating (tangly) branches when young and then changing to lush green leaves as they reach a height that was safe from moa. Horoeka (pictured) has tough spiky leaves that point down until it is above moa height. Then the leaves soften, become shorter and greener, and point upwards. In the Chatham Islands horoeka doesn’t have this juvenile (young) phase and they did not have moa there. Want to know more? You can read this awesome article about all about our unique plants here.

Our colourful pouch fungi, pictured below, looked like berries to moa and other large birds. These birds would eat them and disperse their spores (like seeds of fungi) through their poo. Unfortunately, these spores don’t survive traveling through the gut of mammals such as deer and possums. Read more about the relationship between moa and pouch fungi here.

Sadly, after the arrival of humans and the introduced mammals that came with them, many native species became extinct, including all species of moa. Others only survive on predator free islands or in sanctuaries, such as kākāpō. The new browsing animals that arrived had no natural predators here to keep their numbers in control and they spread quickly through the country. For example, it is estimated that we have 70 million possums in Aotearoa, 15 per person! Our plants and fungi that had evolved over millions of years to survive moa had no time to adapt to survive these browsers.

These 70 million pairs of jaws consume something like 21,000 tonnes of green matter every 24 hours—the equivalent of a big container ship full of leaves, young shoots, berries, flowers and grasses departing from our shores every night. -New Zealand National Geographic

Deer, pigs, goats, rabbits, wallabies, and possums have had a huge impact on our forests and the biodiversity that lives within them. In many forests in Aotearoa the understory has been destroyed. Young trees are eaten before they have a chance to grow. From the outside, these forests of large trees look healthy, but as the older trees reach the end of their lives and die, we will begin to notice that there are no trees coming through to replace them. If you can see nothing but horopito and crown ferns (two species of plants that are not liked by deer) growing on a native forest floor, that is a sign of mammalian browsers impacting the ecosystem. Deer ringbark trees to get sugar from the bark, possums will eat all of the leaves on a tree until it dies then move on, pigs dig up the ground destroying the understory. The pictures below, taken in forests around Taupō, show this damage. Have you noticed any of these things?

Possum browse - Whauwhaupaku (Five Finger) leaves are pulled off by possums that chew the stems and eat the fruit. Look how many leaves were on the ground from one night’s browsing! You can see the damage to the tree.

Deer browse- trees ringbarked by deer and areas of nothing but horopito and crown ferns

Pig rooting - pigs have dug up the understory. The cage that you can see in the middle photo was protecting a rare plant called dactylanthus. The cage has been pushed off and the threatened plant destroyed. Pigs love warm geothermal areas and can destroy the rare plants that live in them too.