Are deer the new moa: Ecosystem re-wilding or a flight of fancy?

It’s the depths of winter and I’m squatting in the snow, surrounded by southern beech forest, using a pair of tweezers to pick up fresh steaming deer poo.

My wife Maria, and palaeoecologist Jamie Wood, from Landcare Research, are doubled over in laughter, having just given me the official job title of pooper scooper.

We’re helping Jamie collect deer poo as part of a project investigating whether introduced deer fill the same job vacancy as the extinct moa in what remains of our unique ecosystems – an ecological surrogate to re-wild New Zealand.

This long-running and often vitriolic debate has become closely associated with the anti-1080 movement. Hunters have illegally re-introduced deer into national parks and other protected conservation areas, sometimes where they had previously been eradicated. An uproar ensues whenever a potential deer cull is floated by DOC.

So how did this feathers to fur debate start and can the latest Time Lord science help shed some light on it?

From feathers to fur

In the late 19^th and early 20^th Century conservationists became increasingly concerned about the damage deer were doing to our indigenous forests. Visit any forest with deer in it, and the understory that would have greeted time travellers to prehistoric New Zealand, is now almost non-existent. Deer have eaten to death what they liked most. What little remains they either don’t like or can’t eat.

Hunters, worried that their favourite animal was going to be controlled or eradicated by the burgeoning conservation movement, came up with an overly simplistic theory to justify the release of more deer and continued hunting; that deer were ecological surrogates of moa.

Their argument was that moa, as the dominant herbivore in the pre-human ecosystem, had a pronounced impact on our unique plants. With the extinction of moa, forests were left unchecked, poised and ready to take over Aotearoa, like a scene from Day of the Triffids, or an army of ents.

Rather than a pest, the argument is that deer are just filling the same ecosystem function as moa, keeping the botanical forces at bay. Why then eradicate them, hunters would say? Deer simply replaced moa and everything is fine. Nothing to see here, so the argument goes.

More recently, the debate has reared its head again with some scientists advocating for introduced species to be embraced as part of a new hybrid post-apocalyptic ecosystem.

Ecosystem re-wilding

For re-wilding to be effective, an ecological surrogate needs to be a closely-related species. Even that is fraught with danger, as our work on the poūwa showed. This extinct mega-swan spent more time on the ground, had elongated legs, reduced wings, and was built like a rugby player. In contrast, its close cousin, the Australian black swan, spends more time on the water and is built like a lithe soccer player – not a good ecological surrogate by any means.

But deer are nothing like moa, I hear you say. This is the equivalent of saying that if I can’t find a nice single malt scotch, rather than settling for an Irish whisky, I’ll have a beer instead. The only similarity is that scotch and beer are alcohol, just as the only similarity between deer and moa is that they are both herbivores.

In all this debate, hunters, and their supporters, have forgotten one key thing that all self-respecting scientists know: the past is the key to the present. Aotearoa is blessed with an exceptionally well-preserved recent fossil record. This includes the skeletal ghosts of moa, allowing scientists to look at how and what they ate, but also their preserved poo, called coprolites. These Rosetta stones have revolutionised our knowledge of moa diet and what role they played in the prehistoric ecosystem.

A big ole pile a them bones

So what do them bones tell us? We once had nine species of moa, ranging in size from dwarfs, one metre high, to true giants. Despite their size, their weight was dispersed through large feet and widely splayed toes. Deer, on the other hand, walk around on small hooves that cut and trample the forest floor into something resembling a rugby field after a particularly brutal match.

The growth rings in some moa leg bones, much the same as those on a tree, indicate moa lived life in the slow lane, compared to the more boom-bust deer in the fast lane, which eat themselves out of house and home. With only around 1-2 moa/km², competition for resources was reduced, compared to at least an order of magnitude higher numbers of deer (~ 10-20/km²), crammed into what’s left of Aotearoa’s indigenous forest.

Look at a moa and deer skull, and other obvious differences crop up. Deer have teeth and a prehensile tongue that can twist, and pull, leaves and twigs into the mouth, whereas the skulls and beaks of each moa species are uniquely shaped for cutting, allowing different species to minimise dietary competition. Moa skulls are so unique that not even other big birds like ostrich, rhea, emu or cassowary would get a job interview in the new world order.

Rosetta stones

Moa coprolites, (preserved poo), which superficially look similar to a half-melted Cadbury Picnic bar, contain a biological treasure trove of dietary information including leaves, twigs, small seeds, pollen, and ancient DNA. These nuggets of ancient history are the final nail in the coffin of the feathers to fur theory.

Moa had a significantly more diverse diet than deer, in the types of plants preferred by these big birds, including those with anti-browsing defences thought to have evolved in response to moa. Lancewood is a classic example. Once it reaches three meters in height, the limit giant moa could reach, this tree stops resembling an unhealthy dead individual and develops a bushy green canopy and starts flowering. Not only that, but moa were not able to disperse the fleshy fruits and large seeds of some of our iconic plants like miro and mataī. In a classic example of Kiwi scientific ingenuity involving a concrete mixer and acid to mimic a moa gizzard, Jo Carpenter and colleagues showed these seeds did not survive passage through the gut to germinate. Once thought to have evolved to be dispersed by moa, it now looks like these feathery giants were only capable of transporting small seeds.

Some of the plants on the menu, capable of surviving in a world ruled by herbivorous birds, underwent a form of ecological release after the extinction of moa, only to be hammered when deer arrived on the scene. The Garibaldi Plateau in northwest Nelson is a perfect example of this differential browsing pressure. Coprolites from Euphrates Cave frequently contained small herbs that are now largely restricted to refugia inaccessible to deer, like steep rock outcrops or the walls of the numerous sinkholes that dot the plateau.

Overall, the coprolites and skulls tell us that moa utilised a wide variety of habitats. Some species, like the little bush moa were forest specialists, while others, such as the heavy-footed moa, (think Cartman out of South Park), were open habitat specialists. The biggest moa of all, the South Island giant moa, was quite cosmopolitan. There is even evidence for migratory behaviour during different seasons in upland moa.

Hot off the press in the last few years is evidence that moa, and other large herbivorous birds, may have played a key role in the dispersal of fungi and the spread of native forest. Alex Boast and colleagues recently found ancient DNA of brightly coloured mycorrhizal fungi, (a classic adaptation for bird dispersal), in moa coprolites. These fungi are key to the survival and spread of southern beech forest, that may now be curtailed in this post-extinction world.

Remember those fresh steaming deer poo I was picking up with tweezers? Jamie used them to show that while deer eat both native and introduced mycorrhizal fungi, only those associated with invasive tree species, like wildling pines, survive passage through the deer gut and germinate. Time Lord science is illuminating not only the unintended consequences of introduced alien species to fragile ecosystems, but also that deer are promoting the spread of invaders that have never been present in New Zealand until recently.

So what now?

Moa were truly unique. No suite of feathery or furry pretenders could replace the broad range of feeding types exhibited by moa. Their extinction was a sucker punch to the way ecosystems worked in Aotearoa. Rather than deer being a case of ecosystem re-wilding, they are more a flight of fancy by people desperate to save their trophy heads and sport.

The stage is also set for the full power of the ancient DNA revolution to illuminate a more nuanced picture of how moa functioned in the pre-human New Zealand ecosystem, as well as the consequent impacts of introduced deer and a detailed comparison of the diet of these heavy weights from the same sites.

So now that we have thrown deer out with the bathwater, what do we do about this pest given the damage they do?

Eradication is one possibility. Kiwis are an are an ingenious bunch, so I don’t envisage many issues here. If this is not possible, then serious consideration must be given to removing deer from all but the areas of least conservation concern. Better still, confine them to deer fenced country stations where people pay for the privilege of hunting these trophy animals, with the majority of revenue going back into conservation management and ecosystem restoration projects.