The greater glider

Come back to the gum forest after dark and you find it far busier than the daylit one. By day most of its mammals are asleep, waiting for the light to go; after sunset the canopy fills with the night shift. High in the beam of a torch you may catch the animal itself: a slow, wide-eyed shape the size of a cat that leans off a branch and sails away in a long swooping arc, steering on a membrane of skin stretched between wrist and ankle, and never once touching the ground. This is the greater glider, in this region the southern species, Petauroides volans, split out in 2020 from what was long treated as one wide-ranging animal. Its life runs on almost exactly the koala’s terms. It eats little but eucalypt leaves, browses only a favoured handful of gum species, and pays for that thin, poisonous, low-grade diet by living slowly and quietly, a folivore running the koala’s own low-energy economy, awake instead of asleep.

The difference is a second dependence the koala does not carry — the very thing that undoes the glider. By day it dens in the hollow of an old tree, and rarely just one; an animal may rotate through several across its range. So it needs the two slowest things the forest makes, and it needs them together: the right feed trees, and a century-old hollow to sleep in. No animal can dig that hollow. Decades of fungi, termites, fire and rot excavate it, working through the heartwood of a veteran. In this region’s eucalypts the process only begins at roughly a century, and the large hollows big fauna can use come at something like two-and-a-half centuries (see hollow dependence). And because a glider cannot cross open ground, but must launch from height and glide, the leaf and the hollow have to lie within gliding reach of each other, in forest that has not been cut apart. It cannot shop around. Clear between the trees and you strand it as surely as felling them.

That double bind has made the greater glider one of the quiet casualties of the age. Once common down the eastern forests, it has fallen away sharply, to land-clearing, to the loss of the old hollow-bearing trees, and above all to a hotter, drier climate that its thick fur and meagre diet leave it badly fitted for. Heat hits it hardest. With only evaporative cooling to fall back on, a greater glider on a night that stays warm struggles to shed enough body heat. A severe heatwave can empty a whole forest of them in a stroke, as can a hot fire running through the very canopy it cannot flee. In 2022 it was uplisted to Endangered under national environmental law. Where the koala’s decline is famous, the greater glider’s is barely known, and it may be the more precarious of the two.

Here is the calibration a tidy-minded manager gets wrong: you can keep every hectare of a forest as forest, thick with green leaf, and still lose the greater glider, if you have logged out the old, half-dead, hollow-bearing trees. The leaf is rarely the limit. The century-old hollow is, and no amount of young regrowth replaces it within the animal’s lifetime. So read a stand of neat, even-aged young poles, however green, as a forest a glider cannot yet live in; read the old, half-dead, hollow-bearing giants as the thing it cannot live without. Leaf and hollow, both slow, both needed — and the slower of the two falls first.