Are deer the new moa revisited: the MythBusters episode

In a rockshelter at the base of a giant two-storey house-sized boulder, Jamie and Janet strike pay dirt. A few centimetres under the floor of this dry overhang are the tell tale signs of a prehistoric megafaunal latrine.

Jamie Wood and Janet Wilmshurst, from Maanaki Whenua Landcare Research, are deep within an ancient Fangorn-like forest at Daley’s Flat in the upper reaches of the Dart River Valley. Snow-capped mountains, tall enough to make you feel quite insignificant in the geological timescale, surround this U-shaped glacial valley.

The floor of this goblin forest, dominated by red and mountain beech, is carpeted in a thick blanket of moss. Put a foot wrong and you’re likely to fall down a crevasse into the dark unknown. Starting life as an epic rock avalanche brought down by an Alpine Fault rupture at least 1000 years ago (in what turns out to be the only case in the world of using prehistoric bird poo to date an earthquake), the area is now home to some of Aotearoa New Zealand’s precious and picturesque indigenous forest, relatively untouched by humans. It has escaped Polynesian and European burning, climate change, forestry and agriculture. The biggest risk are the pesky deer, which leave distinct browse lines in the forest understory – everything palatable below the line has been eaten out.

Four species of moa once called this valley home: The South Island giant moa (Dinornis robustus); heavy-footed moa (Pachyornis elephantopus), which kiwi archaeologist Beverley McCulloch described as a 40-gallon drum walking round on toddlers’ gumboots; one of the smallest moa, the little bush moa (Anomalopteryx didiformis); and the mountain goat of the moa world, the upland moa (Megalapteryx didinus). These avian megastars lived in the area after the forest developed, and flourished for around 400 years until their extinction in the late 14^th Century. Now all that remains are the ghostly bones of these lost avian giants, and lots of little presents.

Under these supersized geological high-rises is an expansive system of dry, cool rockshelters, in what can otherwise be very wet rainforest. As well as the bones of moa, the intrepid team of palaeoecologists discovered hundreds of desiccated moa faeces, (termed coprolites), and the recent droppings of introduced deer.

These big nuggets of prehistoric dietary gold represented Jamie’s and my first forays into reconstructing moa diet. We would revisit these coprolites several times over the years, using tools such as ancient DNA, pollen, (the lovely stuff that gives you hay fever), plant macrofossils (i.e. leaves, twigs, seeds), and stable dietary isotopes, (think you are what you eat), to learn more about how the different moa functioned within this ancient ecosystem.

But why does all this matter? Deer are currently wreaking havoc on what remains of our precious ecosystems. A long running and often vitriolic debate in Aotearoa has been whether introduced deer fill the same position description as the extinct moa – an ecological surrogate to re-wild New Zealand. This argument has been pushed by some hunters, the anti-1080 movement, and their supporters. Robust scientific research, however, has shown that 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.

For a crash course in this debate, you can check out my prequel to this blog ‘Are deer the new moa: ecosystem re-wilding or a flight of fancy’.

We already know 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. These feathery giants, and other large herbivorous birds, may also have played a key role in the dispersal of fungi and the spread of native forest, in contrast to deer and possums, which promote the spread of exotic fungi and wildling pines.

However, until now it has been difficult to test for differences in the diet of moa and deer from the same locality but at different times. Why, you ask, is this important? The answer: to ensure that reconstructions reflect local habitat and dietary preferences.

Jamie and Janet have done just that by peering through the mists of time to see how the forest at Daley’s Flat has changed as a result of moa and deer browsing. Remember those deer droppings they found with the prehistoric moa poo. Pollen from these was used to reconstruct deer diet and was compared to the diet we had previously reconstructed for moa.

The diets of these herbivorous heavyweights differed markedly. Moa had a much richer and varied palate, in turn indicating a richer and varied prehistoric forest understory, particularly of shrubs and small trees, compared to today. This is the equivalent of moa having a Michelin Star dining experience compared to deer having to settle for the food university students can usually afford.

So why the difference? It’s likely the higher population densities and browsing pressure of deer, compared to the more ecologically friendly moa, (at least in the context of Aotearoa), has driven the loss of many understory plant species that could survive moa browsing, but not deer. Ever wondered why our native forests are quite open under the canopy, devoid of shrubs and small trees? Now you know. Moreover, the increased abundance of understory ferns can also be attributed to deer browsing.

Like in any good movie where it looks as if all hope is lost, hope remains. The highly palatable understory plants survive atop the giant boulders that dot the beech forest, like trendy rooftop gardens, inaccessible to deer. Further north, at the opposite end of the South Island, moa coprolites from Euphrates Cave on the Garibaldi Plateau frequently contained small herbs that are now largely restricted to geological refugia inaccessible to deer. These oases in a sea of plant predators are the final nail in the coffin for the theory that deer are filling the same job vacancy as the extinct moa.

I’m going to say something bold that some folks may not want to hear. Deer are a pest and should be treated as a pest. A pest that is having a profound impact on our environment. The economic benefits of sustainable deer hunting should not outweigh the conservation concerns. Sorry to those scientists advocating for introduced species to be embraced as part of a new hybrid post-apocalyptic ecosystem: we can either have lush native forest with a diverse understory, and botanically rich alpine zones, or we can’t. It’s not a case of having our cake and eating it too.

The use of 1080 deer repellents by DoC, and its conservation partners, to preserve the viability of ‘ecologically important’ (whatever that means) deer herds to satisfy the New Zealand Deerstalkers Association, should rightly fade into history. 1080, combined with hunting, can be effective against deer, as the latest 1080 drop on the alpine Molesworth Station has shown. Local deer populations were decimated by 90%, no doubt giving our endemic alpine flora a fighting chance of survival. In my view, the age-old argument pushed by some hunters that they are ‘conservationists’, yet want to preserve deer herds, does not pass the pub test. If they were true conservationists, hunters should be giddy with delight about decreasing or eradicated deer populations, making their quarry harder to hunt.

We can’t bring moa back, but we can damn well conserve what remaining ecosystems we have left, and maybe, just maybe, turn back the clock for some aspects of prehistoric New Zealand.

So are deer the new moa?

Busted.