The dugong

Somewhere in the sheltered green shallows off this coast, out of sight of everyone on the beach, a large grey animal moves slowly over a meadow, cropping it. The dugong is one of the strangest secrets the region keeps: a warm-blooded, air-breathing mammal the size of a small cow that spends its whole life underwater eating a flowering plant, cutting slow furrows through the seagrass like a plough through a paddock. Its family tree is nothing you would guess: manatees for cousins, elephants for distant kin. Most people who live here have never laid eyes on one. They would be astonished to learn that the sheltered waters on the coast’s southern doorstep carry a nationally significant number of them, grazing a few hundred metres offshore in the broader Moreton Bay region and the Pumicestone Passage that opens off it.

Nearly everything about the dugong comes back to the meadow. Like the koala, it has staked its whole living on one narrow resource. The seagrass it depends on is a true flowering plant, not a weed, returned to the sea and rooted in the soft sand of the shallows wherever the water stays clear enough for light to reach bottom. That last requirement is also the dugong’s weakness. Seagrass needs light; light needs clear water; so anything that clouds the water shades the meadow out and kills it: a flood dumping silt off the land, run-off, dredging, a run of hot still days. When the meadow dies, the animal that eats almost nothing else has nothing left. You do not lose one plant. You pull the floor out from under the grazer standing on it.

It is tempting to pin a herd like this to a tidy figure, and you will hear the bay’s dugong called the largest gathering living this close to any major city. Treat the superlative gently, and the count more gently still, because the answer is a range rather than a number. When Janet Lanyon flew her aerial surveys over Moreton Bay in 1995, she found the population swinging with the seasons — of the order of five hundred animals in the cold months, climbing toward a thousand in the warm, as the dugong wandered the bay after the seagrass and the temperature. So the safe reading is nationally significant, strongly seasonal, several hundred to around a thousand, and never a single confident tally. The animals move; the meadows they follow shift; and a herd that looks fine in a good year can be hollowed out in a bad one.

Which is the sharp end of it. Because the dugong lives and dies by the meadow, it is hostage to everything the meadow is hostage to, and seagrass comes undone in a hurry. One wet summer can bury the beds in silt; a marine heatwave, a spell of hot still weather, can cook them where they stand. When that happens the dugong cannot switch to something else, because for a dugong there is nothing else. They lose condition, breeding stalls, animals quietly die. It plays out slowly: a die-off that surfaces months after the water has cleared and kilometres from where the harm was first done, so the cause and the corpse never seem to belong to the same event.

That is what makes the dugong worth reading closely. It ties the sea country to the land as directly as anything on the coast: cloud the water off a river mouth, and months later an animal starves in a bay downstream. Read the dugong and you are really reading the seagrass; read the seagrass and you are reading the clarity of the water above it, which is to say the health of the whole catchment draining into the sea. Keep the water clean and the light reaching the sand, and the improbable grazing giant goes on ploughing its furrows offshore, unseen and untroubled. Foul it, and the meadow goes, and the dugong goes with it.