by Tony Orman
The headlines in the “Dominion/Post” of 27 October 2006, screamed “Hungry caterpillars strip beech forests.”
Underneath the big words was a photo of the d’Urville Valley in the Nelson Lakes National Park and further below, the report that “Millions of hungry caterpillars have stripped beech forests in Nelson Lakes National Park, leaving hundreds of hectares of bare trees.”
The article went on that “Nelson-Marlborough conservancy entomologist Ian Millar said two species – Thiotricha tetraphala and Thiotricha lindsayi, both commonly known as beech case moths could be responsible for the defoliation.”
“The trees were not dying and the department expected them to return to normal next year.”
It wasn’t the first time in recent years for in 1997 Thiotricha lindsayi caused beech defoliation in the Travers Valley and most trees recovered.
A Forest and Bird regional field officer said the defoliation was devastating to look at but conservationists were confident the trees would bounce back. “It’s just one of those things,” she said. “Something just gets radically out of kilter.”
Yes she was correct in recognising New Zealand’s vegetation will recover because it’s regarded as a very aggressive one with strong power to regenerate.
But at the time, something made me paus e and ponder that Forest and Bird saying everything’s fine despite the stripping of beech foliage.
Would Forest and Bird have murmured similar sentiments if there had been stripping – or even browsing – of beech foliage by possums or a deer had browsed the tips of a juvenile beech tree?
If a possum or deer been involved there would have been an outrage declared and a call for extermination blitzes from helicopter shooting to 1080 poison drops. Indeed had it not been for the DOC entomologist’s analysis, then possums and/or deer would probably have been blamed for the defoliation. Preposterous? Hardly, that’s what Forest and Bird and DOC have done in the past.
And full marks to DOC entomologist Ian Millar for explaining clearly, without emotion what the probable cause of the defoliation was, namely insects.
However insect defoliation is nothing new. When I researched material for my book “About Deer and Deerstalking” back in 2002, through advice from a scientist friend, I discovered that back in 1986, a symposium was held entitled “Moas, Mammals and Climate Change in the Ecological History of New Zealand.”
Two scientists M N Clout and J R Hay presented a paper on “The Importance of Birds as Browsers, Pollinators and Seed Dispersers in New Zealand Forests.”
In their introduction the duo said “contrary to the earlier view that introduced mammals (e.g. deer) entered and devastated a naturally unbrowsed vegetation (Howard 1965) that New Zealand flora evolved in the presence of at least some ver terbrate browsing pressure “ (e.g. moas)
Clout and Hay then stated “in addition to moas, other bird species (e.g. pigeon and kokaho) may have been (and in some cases still are) significant browsers of forest vegetation.”
Then the scientists because their paper focused primarily on birds, briefly dealt with invertebrates, i.e. insects.
Here are their exact words, “The most widespread, abundant and important browsers of vegetation in New Zealand forests are invertebrates. Trees can be almost totally defoliated by them over a wide geographic area.”
The two cited browsing of red beech by the beech leaf-roller and of mountain beech by the mountain beech moth.
Within the booklet of the proceedings of that symposium, I found a wealth of information which I was amazed at. For instance on other insects But why, why, the thought nagged, wasn’t this story told? Why did people not know of this?
I recalled from an issue of “NZ Hunting and Wildlife” many years ago about a mid -1960s, Forest Research Institute study of the role of alpine grasshoppers in the Cupola Basin, a tributary of the Travers Valley in the Nelson Lakes National Park – the same area where the 2006 beech defoliation by caterpillars had taken place.
C L Batcheler in his scientific paper wrote that during the study of deer and chamois, an opportunity arose for a study of the role played by alpine grasshoppers as “fairly large numbers of grasshoppers observed close to scree-vegetation ecotones – which in many places are eroding rapidly – suggested they might have a significant effect on this critical vegetation.”
Four species of grasshoppers were present. The results were both fascinating and amazing.
The grasshopper biomass was calculated from careful counts over “convenient sample areas” that ranged from dense snowgrass swards to unstable bare screes. Less than 1 lb per acre (biomass of grasshoppers) was recorded for two dense snowgrass sites. However on other sites which ranged from dense snowgrass to rock screes, estimates went as high as 22.8 lb per acre in 1965 and 28.7 lb per acre in 1966.
By comparison complementary studies of deer and chamois populations showed a maximum biomass of deer and chamois near the timberline at about 10 lb per acre. Above 5,000 feet the deer and chamois biomass decreased to 0.5 lb per acre.
“The inference of the above comparison,” said the study, “ is that biomass of grasshoppers is much greater than that of deer and chamois in the greater part of the alpine basin above 5,000 feet.”
The study suggested that grasshoppers are arguably the most important herbivore (browser) of the alpine zone, i.e. three times more than wild animals.
Batcheler wrote that the findings suggested one species of grasshopper (Brachaspis) may be “a critically important herbivore associated with much of the eroded ground” and suggested that higher – “above 6,000 feet.” the same could be said of another grasshopper species Sigaus.
So New Zealand over millions of years has had browsing of vegetation by insects and by moas and other birds. Moas of course were the obvious, most visible browser. The same scientist Les Batcheler who carried out the grasshopper browsing study, at the symposium “Moas, Mammals and Climate Change in the Ecological History of New Zealand” bravely estimated the moa population at between 6 and 12 million birds.
Other scientific views poured out of that symposium’s paper as I delved into it. A trio of scientists concluded “that the feeding effects of moas may not have been very different from those of browsing animals.”
Why then have some within the Department of Conservation and Forest and Bird continued to hold that New Zealand’s vegetation evolved in the absence of browsing. Well it might be like prising a stone out of a moa’s gizzard to get them to explain.
There might be something in the character of a bureaucrat. Myths have been propagated in the past to give rise to jobs and little bureaucratic empires, no matter that taxpayer’s money was used to fund policy staffing and operations.
Then there might be the human nature flaw that few have the courage to admit their earlier sermons were totally wrong and that the reverse is true, i.e. New Zealand’s vegetation did evolve in the presence of browsing, and heavy browsing pressure at that, far more than the numbers of wild deer do in New Zealand today.
Hark back to Les Batcheler’s estimate of 6 to 12 million moas. In 2002 the Department of Conservation commissioned Landcare Research to estimate the wild deer population.
The figure was 250,000 animals – a far cry from 6 to 12 million moas.
So if we take the average of 6 million and 12 million, we get 8 million. That means by scientist Les Batchelar’s estimate – generally – there were 36 times more moas in New Zealand than there are deer today.
So don’t be deluded by departmental or extreme green claims. The facts are that New Zealand’s forests are adapted to browsing whether it be by moas, deer or insects. Indeed browsing is an important vital aspect of the functioning of the ecosystem.
In short the ecosystem needs browsers and wild deer is one.
A fallow deer – photo by the late Matt Winter