environment

Deer are not like moa

Fossilised and fresh dung has busted the myth introduced deer replaced the role of moa in New Zealand ecosystems.

The study completed by Manaaki Whenua - Landcare Research’s Jamie Wood and Janet Wilmshurst analysed the pollen in fossilised poo of moa and fresh poo of deer from the same location in the South Island.

The results have been described as a "final nail in the coffin" for the argument sometimes used in the 1080 debate that deer play an essential role in forest eco-systems.

The study finds that since the introduction of deer, plants that were present when moa roamed the country are now absent from all but the most inaccessible of places.

Deer and moa are both herbivores and graze on forest foliage. There has been speculation deer replaced the role of moa as forest browsers, and originally dense forest which thinned-out after the introduction of deer was due to a "moa-gap".

The theory was that forests were originally sparse, without many plants in the understory, but the extinction of moa allowed them to grow out of control.

The study found a wider variety of pollen in the moa poo than deer poo, which points to a wide range of plants existing in forest, even when moa roamed.

“There is this growing body of evidence now that, the effects of the moa and the deer on New Zealand's ecosystems is quite different to each other,” said study co-author Wood.

Daley’s Flat in West Otago's Dart River Valley, where the samples of moa and deer poo were found, has been largely untouched by humans. There’s no evidence bush fires occurred and changed vegetation. The only vegetation impact is the change from moa to deer.

"The study site is a giant rock avalanche, which came down off the mountain about a thousand years ago we think. It's left this jumble of house-sized, boulders, through the forest. As you're walking through there you're clambering over these giant rocks, trying to avoid deep holes that go down between them."

In dry nooks and crannies where humidity is low, moa poo can last for thousands of years. Wood said one find of moa poo in a cave in Nelson is thought to be 8000 years old. 

"The stuff we find from moa is really just dried. It's not turned to stone or anything. It's like a really dried out sheep dropping that you might find in a paddock in the middle of summer."

The moa droppings found in Daley's Flat were soaked in water, sometimes for two to three weeks, to soften them up enough to be able to tease them apart and examine them under a microscope.

Dried moa droppings, also known as coprolites. Photo: Jamie Wood

It’s thought four species of moa lived in the area. The bush moa, the heavy-footed moa, described as a “40-gallon drum walking on toddler’s gumboots”, the upland moa and the South Island giant moa. There’s evidence each of these species grazed on different plant types within an area.

The study noted while the understory of the forest was sparse in areas deer would be able to graze easily, in other areas such as on top of boulders, some up to two-storey’s high, and in places deer couldn’t access, it was dense, with a wider range of plants.

"If you go and walk around you can find every single plant that turned up in moa droppings. So there really hasn't been any losses of plants from the area. But what you do find is that most of them are mainly restricted to places deer can't get to," said Wood.

The differences between deer and moa

The cause of the sparser understory where deer have access could be down to several factors. 

New Zealand had nine different species of moa, with different sizes and preferred habitat. Flightless bird expert Mike Dickison points out all nine lacked something which deer have.

“Deer have teeth, not beaks, so can feed in a much more efficient and destructive way than moa, including eating bark if they need to.”

In fact, the skull of the moa is so unique there are no other birds which have similar feeding habits. A 2016 study which used 3D scanning to replicate their skulls found even between moa species it was likely each one evolved eating styles to suit the plants which were present.

Their feet are also different. Deer have small hooves, which can churn up the forest floor. Moa had splayed feet, which dispersed the weight of even the heftiest species.

The sparseness of the forest understory could also be due to numbers. Moa weren’t as prolific as deer.

“Partly that’s because they [deer] only have one inefficient predator in NZ: deer hunters,” said Dickison.

Introduced deer were set free in New Zealand in 1851 to establish deer hunting as a sport.

The head of the Department of Tourism and Health, Thomas Donne, imported a range of animals, including deer, saying:

“Nature neglected New Zealand in providing game animals. Man has remedied the omissions.”

Without non-human predators, and with vegetation which hadn’t evolved to evade grazing the numbers grew.

It’s estimated biomass, a measure of density in an area, for wild deer is a hundred times greater for deer than it was for moa. 

The Department of Conservation (DOC) considers deer a pest and is eradicating them from Northland and some areas of Auckland. In other areas it is controlling deer. It has also attempted to remove deer illegally released in the Egmont National Park.

DOC encourages hunting and issues free permits. A 2010 survey estimated hunters kill approximately 135,000 deer a year. The deer provide food for some, while others are shot for sport.

The final nail

Dr Nic Rawlence, the University of Otago's director of the Otago Palaeogenetics Laboratory, has worked with the study co-author Wood in the past. He calls this study a final nail in the coffin for the deer-as-moa argument.

"A long running and often vitriolic debate in New Zealand has been whether introduced deer fill the same job vacancy as the extinct moa in what remains of our unique ecosystem – an ecological surrogate to re-wild New Zealand.”

Some argue without deer grazing, ecosystem balance could be thrown out and plants would grow out of control.

The debate has spilled out to become a common point raised by the anti 1080 movement. If deer eat 1080 they die a horrible death. It’s been suggested the deer are essential to replace the extinct moa, so 1080 should not be used. 

“Ever wondered why our native forests are relatively open under the canopy. Now you know why.”

“Deer are a pest damaging our precious remaining ecosystems and should be treated as a pest,” said Rawlence.

“I would say this study is pretty much the final nail on the coffin that deer are like moa. As Mythbusters would say, completely busted.”

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