Sunday 21 April 2013

Twenty Three and a Half Reasons

We live on an eccentric planet, thanks to the number twenty three and a half. That’s the axial tilt of our dear old planet, and the reason for the seasons. Which in turn is a grounds for both rejoicing and lament.

Here in the higher latitudes of the southern hemisphere, April brings a noticeable attenuation of days, a lessening of sunlight, a return of colder weather and frost. Slowly the plump warm air that has coddled us for half a year is receding, and the sun’s strength is gradually diminishing.

Everywhere we respond to this with a mixture of alarm and alacrity. No sooner are we relieved that wild fires are over, than we start gathering in our firewood. We start warming our houses, burning the dust off our heaters. We wash our woollens and put them to everyday use. Our vegetable harvest gradually slows, and salads make way for soups.

[Tasmania's green rosella: detail of head and shoulders] 

In the wider world the signs are there too. With this week’s first dusting of snow, currawongs and cockatoos congregate in our forest, suddenly more earnest in their search for food. Beside our house a flock of green rosellas (Platycercus caledonicus) lands on a kunzea bush. Without a sound – not even their soft, bell-like contact call – they work away at the seed heads.

They are normally shy, but they are so intent on eating that I can study them carefully without disturbing them. Their markings are subtly spectacular, from a fluoro lime-yellow head and chest, to a striking crimson frontal band; from a back that's an olive-blue plaid to a tail that's a rhapsody in blue-green.

[A green rosella displaying its tail feathers] 
I have observed our resident rosellas often over the decades, once peering through binoculars as several of them de-berried a cotoneaster bush. At first I thought them sloppy, as they discarded great gobs of berry pulp beneath the bush, like gormless diners talking with their mouths full.

Slowly it dawned that the pulp was of no interest to them. By rolling the berries between their tongue and beak, they were able to break up the fruit, discard the pulp and ingest the seeds. I later learned that cotoneaster berries contain cyanogenic glycosides, which are toxic. Doubtless the rosellas had learned this long before me.

Our native plants, with the spectacular exception of Tasmania's deciduous beech (see link), don’t respond to autumn via leaf colouration and deciduousness. But this doesn’t mean they don’t respond to the shortening of the days, and the drop in temperature and light levels. Particularly in colder areas, a number of our eucalypts undergo their own response to lowering photosynthesis, resulting in brilliant colouration of their peeling and fresh bark.

[A yellow alpine gum in autumn colours]

On recent alpine walks I have come across yellow alpine gums (Eucalyptus subcrenulata), snow gums (E. coccifera) and yellow gums (E. johnstonii), all putting on brilliant bark colours in the face of cold, wet weather.

[A Tasmanian snow gum] 

It appears that the withdrawal of chlorophyll allows the pigment anthocyanin to exert a stronger influence over the bark, colouring some of it red or orange – just as it does in deciduous leaves. As this older bark peels it contrasts strongly with the paler new bark that is revealed beneath. The process varies from species to species, even from tree to tree. But in the end it’s all in response to that amazing 23.5 degree tilt.

Sunday 7 April 2013

Oh Possum!

[A common brushtail possum:
Illustration courtesy Tasmania Parks & Wildlife Service]
I don’t always sleep well in a bushwalking tent. Certainly my Thermarest is good, but it’s not a mattress. My pillow is a lumpy, makeshift bundle of things I’ll be wearing in a few hours. And a sleeping bag takes its own getting used to.

But at the end of a day, hard walking has its effect. It joins with the occupied silence beyond – calling birds, stridulating crickets, shushing or rushing wind and water – to lead me by the hand into that strange land that is sleep … Until the sounds of tearing, growling and crunching yank me awake. Something is invading our tents!

We’re on camping platforms at Wild Dog Creek in Tasmania’s Walls of Jerusalem National Park. But this is no wild dog. It’s a pair of common brushtail possums (Trichosurus vulpecula) under Jim and Jeff’s tent flap, squabbling over some carelessly hidden morsels.

[The "boys" at Wild Dog Creek - all smiles before the raid] 
The neighbours show no signs of attending to their marsupial visitors, so I struggle out of the tent – not in the best of moods – and hoosh the beasts away. I do a bit of my own growling at the neighbours, shove their raided food scraps back into their tent, and struggle back into my sleeping bag. Ten minutes later, just as La La Land is welcoming me back, the possums return. This time Jim and Jeff respond, and have a better go at hiding their food.

About every ten to thirty minutes thereafter, our furry friends return, eventually invading our tent too. At one point I tetchily shove my hand towards a possum that’s snuffling in our vestibule. I connect direct with its nose – not hard – but we’re both equally surprised. It’s still no deterrent. The possums keep coming back until Tim and I, and Jim and Jeff next door, haul our packs inside our tents with us.

They finally give up, and leave us to find the long road back to slumber. I travel there via the path of pondering. My initial thoughts are grumpy ones. These possums are just plain nuisances that have “invaded” our space. Of course on reflection, we are the interlopers. And as I think about my wider experience of them, I can’t help admiring their amazing adaptability.

[Watching and waiting: A brushtail possum ready to raid as evening approaches]

True omnivores, they supplement their mainly vegetarian diet with almost anything else they can find. In New Zealand, where they were introduced in the 1830s for the fur trade, they have been observed eating eggs, birds and even reptiles. But their incredible versatility isn’t confined to diet. They are native to virtually every part of Australia, varying their diet, size, fur colour and breeding behaviour to suit local conditions. With a gestation period of less than three weeks, they can respond to favourable conditions by breeding rapidly.

They are one of the few marsupials to have adapted well to the introduction of humans. Crosbie Morrison, an Australian naturalist and broadcaster of the 1940 and 50s, made this apt, biblical-sounding pronouncement about brushtail possums.

When man came on the scene and destroyed the bush and made gardens, and the other animals went away, the possum stayed and tasted the new things that man had brought, and found them good.

Morrison, a man of his time, used words like “primitive” to describe marsupials like the brushtail. Although he greatly admired them, he couldn’t help emphasising what they “lacked” in comparison with placental mammals such as cats and dogs.

[Naturalist Crosbie Morrison. Photo courtesy Australian National Botanic Gardens] 
As I think back to an earlier possum-punctuated sleep in the bush, I’m not sure I agree. On that occasion a “brushy” had slipped under my tent vestibule, found my pack, unzipped the top pocket, and carried off a number of zip-lock plastic bags. One of those bags contained a book, Christobel Mattingly’s biography of Deny King, called “King of the Wilderness”. I found it under a bush the next morning. I don’t know of any cat or dog that could have mounted that raid, especially one involving such a dexterous unzipping. To this day the book bears possum bite marks. They prompt me to ask: who is the real king of the wilderness?