NZ’s Biodiversity 5: Browsers And Forests

Introduced by Leslie Clague

Jim Hilton’s talk to the Nelson Science Society continues with the role of the Moa and other browsing fauna.

(To see the earlier installments, refer to the New Zealand’s Biodiversity section at the top.)

The impact of browsers before human settlement

By Jim Hilton

Moa browsingQuestion: What effect did Moa and the other browsing birds have on NZ’s vegetation and forests?

They ate palatable native species in a similar way to the introduced wild animals in our forests today, and our domestic animals like cattle and horses. They also left wide tracks and trampled vegetation. They stomped up and down river banks, produced greenhouse gasses and defecated in our rivers.

They did all this without killing our native fishes or exterminating rare native plants, snails, frogs, lizards and insects, bringing our forests to a state of collapse, like some people would have you believe about domestic animals and introduced wild life today.

The diet of the Giant Moa

Native snailsThe last meals of Giant Moa, conveniently preserved in swamps, were analysed in the 1980’s, revealing lots of leaves and twigs. “Moa were habitual forest-dwelling species.” Their closest living relatives are the Cassowaries, rainforest dwellers of Northern Australia and New Guinea. They eat a lot of fruit, as well as pruning trees. Moas had huge feet and their diet would have included frogs, lizards, snails and insects.

Can you imagine the damage 200 kg of giant bird is going to do to the forest floor when it is scratching about with its feet? Combined with Kakapo, Takahe and Weka, Moa would have eaten thousands of native snails. That’s probably why our native snails are nocturnal, to avoid predation. Are possums and rats going to make our snails extinct? I doubt it. Especially in the low population numbers we now have in our forests.

Browsing wildlife and the environment

Moa browsing 2Our forest vegetation has evolved with wildlife disturbing the soil for millions of years. There are countless large native trees in farmers’ paddocks in New Zealand which survive very well with their roots trampled and their bark rubbed as animals scratch themselves.

Large browsing animals and bird feed along contours, they leave wide tracks, natural terraces which slow down water as it runs down our hillsides. Yes, they loosen soils sometimes, but they compact them, too. The idea that browsing wildlife will turn our high country into giant shingle slides, which will ruin our farms and swamp our cities was put to rest by hydrologist in the 1970’s and by botanists in the 1980’s and 1990’s.

New Zealand’s vegetation has more to worry about than being eaten to death by introduced animals.

Natural hazards cause forest damage

Patrick J. Grant, hydrologist and botanist, wrote a book in 1996 called “Hawke’s Bay Forests of Yesteryear.” He studied the diaries of missionary William Colenso, the first European to enter the Ruahine Mountains in 1845 and 1847. Dead trees, canopy collapse and accelerated erosion were described and sketched in detail, years before deer and possum were released. Early photos taken in 1917 show high numbers of standing dead trees. They correlate with severe droughts.

Pat Grant identified seven possible factors in forest damage:

~ wind,

~ droughts

~ heavy snowfalls (glacier ice)

~ heavy rainfalls (floods)

~ fire (volcanic eruptions)

~ earthquakes

~ wildlife (like Moa and introduced animals).

I remember the Wahine storm (1968). Others will remember Cyclone Alison (March, 1975), Cyclone Bola (March 1988) and Cyclone Ita (17 April 2014). The devastation to our forests was horrific.


The impact of Cyclone Bola
The impact of Cyclone Bola .

Beech forest expert Peter Wardle tells us “natural physical disturbances” have been of far greater impact on forests that browsing by Moas or deer. “No vegetation in the North Island has escaped the impact of repeated volcanic eruptions.”

He also notes: “Sooner or later most tall forest trees are in the path of destructive gales. Damage from drought, wind, snow and frost may lead to lethal plagues of insects.”

Vegetation protects itself to survive

New Zealand vegetation is full of plants which have evolved elaborate defences against extinction by Moa and other plant eating birds. “The most common anti-moa adaption is ‘divarication’ where plants protect their delicate leaves and new growth by ‘turning themselves inside out.”

Forest defencesAt least 50 species of NZ native plants protect themselves in this way. Wild Spaniard will be known to most serious bush walkers. They have sharp spines and Moa would have cursed them as much as I do.

New Zealand’s native pines – Kauri, Totara, Rimu, and White and Black pine, all have spikey, browse resistant juvenile forms. (See photo alongside.) Clearly they have evolved to withstand browse. Without browse our forests will eventually be dominated by faster growing broad-leaved Taraire, Tawa and Kamahi, instead of the giant podocarps.

The Lancewood is probably the best example of a tree which has adaptations to avoid being eaten by Moa. The seedlings camouflage with the forest floor from where they sprout. The sharp dagger-like leaves have backwards facing barbs, which make them difficult for a Moa to swallow. The leaves of younger trees have bright spots by each barb, like a warning signal.

Once a Lancewood grows beyond three metres in height, the leaf shape “relaxes and morphs back into something which resembles a normal tree leaf.” The Chatham Island Lance wood (800 kilometres east of New Zealand) does not have this variable leaf shape. There were no Moa on the Chatham Islands.

Part Six of this series will look at where and how science first got biodiversity wrong.