The Fleurieu peat swamps

Most of the wet ranges drain the way you would expect: water arrives, water leaves. In a handful of high, spring-fed hollows on the southern ranges and the Fleurieu Peninsula, it does not. Groundwater seeps up into the same low ground year-round, the flat stays sodden through the driest February, and the dead plants there do not properly rot: waterlogged soil holds almost no oxygen, and without it the microbes that would otherwise break down leaves and roots slow to a crawl. So the dead vegetation piles up faster than it decays, one thin layer at a time, into a dark, spongy bed of half-rotted plant matter — peat — turning the trapped water sour and acid in the process, a chemistry few plants can stomach. Given centuries, and only centuries, a swamp like this becomes an accumulating archive of its own wet history, an acid sponge that holds water the way an ordinary sponge never could.

That harsh, acid chemistry is exactly why so few plants can live here, and exactly why the ones that do matter so disproportionately for their size. They were never common — something over two thousand hectares of them once dotted the high country — and a great deal less survives in good order today, something like a quarter of the original extent, still holding its water and its peat rather than having been drained and grazed away. For that rarity and that loss, the Fleurieu swamps are listed nationally as a critically endangered ecological community, among the most threatened habitats anywhere in the Adelaide region.

Their signature tenant is a bird you are more likely to hear rustle than see: the Mount Lofty southern emu-wren, a few grams of rust-and-grey feather trailing a wispy six-strand tail, that lives almost nowhere but in this sedgy, permanently wet cover (Donnellan 2009). It is a poor flier, unwilling to cross open ground, so where the swamps were drained and cut into islands the emu-wren populations were marooned with them — the largest surviving group, on the lower Finniss River, numbering perhaps only fifty to a hundred birds. That is an extinction debt made visible: a species can still be present in a fragment years after the fragmenting, running down a trajectory the clearing has already fixed, rather than vanishing the day the drain is cut (Szabo 2011).

Look for the giveaway a paddock keeps even after its swamp is drained: a black, peaty hollow that stays wet and rush-fringed longer than the ground around it, sometimes slumped where the exposed peat has shrunk and subsided. That low, dark patch is the last legible trace of an acid sponge that took centuries to build and can be undone in a single dry decade — the truest test, at the deepest, dampest floor of the whole gulf-to-range climb, of whether “still here” and “safe” are the same claim.