Mudjimba Island (Old Woman Island)

A couple of kilometres off the Maroochy shore, low and rocky and easy to lose between the surf and the horizon, sits Mudjimba Island, or Old Woman Island on the older charts. It is continental, a knuckle of rock rather than a heap of sand, and a boulder reef rings it. Duck under with a mask and it is a two-oceans reef, warm-current coral keeping company with the cool-water sponges and weed of the temperate south, a story told in full on other pages. Mudjimba’s other lesson is up out of the water, in the thin soil on top of the rock, and it turns on an absence.

The island is a seabird island. Wedge-tailed shearwaters come ashore here to breed, and like most of their family they nest neither in a tree nor on a ledge but underground, digging burrows into whatever soil the place can spare and raising a single chick out of sight. They come and go at dusk and dawn, and spend the rest of their lives out over open ocean. It is a beautiful arrangement with one glaring weakness: a bird that lays its egg in a hole in the ground has no answer at all to a mammal that hunts by nose.

On the mainland just across the strait, two imported hunters have been making exactly that point for a century. The European fox and the feral cat, loosed on a fauna that had no defence ready for them, fell hardest on the ground-dwellers and the low-nesters, the animals with nowhere to go when something came for them at ground level. Neither predator swims willingly, and neither travels by anything but land. A strip of open sea mostly stops them.

And so an island becomes a fortress by doing nothing at all. The water is a moat. The island itself is much the poorer address — rock and a thin skin of soil, offering less of everything than the coast it faces. What makes it worth having is the gap of water. Birds that need somewhere undisturbed to dig, and to sit out a long breeding season in the dark, hang on out here for one reason: the thing that would find them ashore cannot reach them across the sea.

That is worth keeping in your head as you read the coast, because here the dependence runs backwards. What holds these birds is the absence of an enemy, and a geography that happens to supply it. Mudjimba does not feed them better. It keeps the fox and the cat out, and for a bird that nests in a burrow that turns out to be the whole of the argument.

Which is also why the fortress is more fragile than it looks. It holds by geography alone, and geography can be undone in an afternoon. Carry a fox or a cat across in a boat, into a colony that has kept no defences ready, and the moat stops being a moat; the thing could be over in a season, and nothing about the island itself would slow it down. Even a well-meaning landing at the wrong time of year does damage, because a burrow roof is a few centimetres of sandy soil and it will not carry a boot.

So Mudjimba reads two ways. Under a mask, a boulder reef where the warm current lays tropical life against the cool south. Up on the rock, a colony of ocean birds that persists because a moat of sea keeps the hunters off. Reef below, burrows above. Two reasons to leave the little island, and the water round it, alone.