The dugong

The dugong is one of those animals that sounds invented. It is a large, slow, air-breathing mammal that spends its whole life underwater grazing a flowering plant — a vegetarian the size of a small cow, related to nothing you would guess (its nearest living cousins are the manatees, and, further back, the elephants), ploughing slow furrows through the seagrass beds as it crops them. Most people who live on this coast have never seen one, and would be startled to learn that the sheltered waters on their southern doorstep hold a nationally significant population of them, grazing quietly a few hundred metres offshore.

Its whole existence is staked on one thing: the seagrass meadow (see seagrass-meadows). That is what makes the dugong worth reading closely — it is a specialist in exactly the koala's mould (see species-dependence), a large animal that has bet its entire living on a single, narrow resource, and is as fragile as that bet is bold. Seagrass is not seaweed but a true flowering plant returned to the sea, rooted in the soft sand of the sheltered shallows wherever the water is clear enough for light to reach the bottom. That last clause is the dugong's weakness written into the meadow's requirement. Seagrass needs light; light needs clear water; so anything that clouds the water — a flood dumping silt off the land, run-off, dredging, a run of hot still days — shades the meadow out and kills it. And when the meadow dies, the animal that eats nothing else simply starves. You do not lose one plant. You pull the floor out from under the grazer standing on it.

The numbers, kept honest. It is tempting to pin a population like this to a tidy figure, and you will hear the bay's dugong called the largest herd living this close to a major city. Treat the superlative gently and the count more gently still, because the honest answer is a range, not a number. Janet Lanyon's aerial surveys of Moreton Bay in 1995 found the population swinging with the seasons — of the order of several hundred animals in the colder months and climbing toward a thousand in the warmer ones — as the dugong move about the bay after the seagrass and the temperature (Lanyon 1995). So the safe reading is "nationally significant, strongly seasonal, several hundred to around a thousand," and never a single confident figure. The animals move; the meadows they follow shift; and a herd that looks healthy in a good year can be gutted in a bad one.

Which is the last and sharpest point. Because the dugong depends utterly on the meadow, it is hostage to whatever the meadow is hostage to, and seagrass is undone in a hurry by a big flood or a marine heatwave — a single wet summer that buries the beds in silt, or a hot still spell that cooks them, can strip a region's meadows in weeks. When that happens the dugong do not adapt or move on to something else; there is nothing else. They lose condition, breeding stalls, and animals die — a slow, quiet die-off that plays out months after the water cleared, kilometres from where the damage was done. Read the dugong and you are really reading the seagrass, and reading the seagrass you are reading the clarity of the water above it. Keep the water clean and the light getting down to the sand, and the improbable grazing giant offshore keeps its pasture; foul it, and the meadow goes, and the dugong goes with it (McPhee 2017).