The acid frogs of the wallum

Follow a wallum track down off the sand ridges into one of the damp swales between them, and the heath changes under your feet. The shrubs give way to wiry sedges, the tall grass-trees to the low, swamp-loving kind, and the sand between the tussocks glistens with tea-dark standing water. This is acid blackwater, often around pH 3.5 to 4.5 — as sour as orange juice, and lethal to most freshwater life — and here the wallum’s strangest animals make their impossible living. For most freshwater creatures water this sour is a death trap; the acid strips the salts out of their bodies and burns delicate tissue. Yet a small, specialised club of frogs breeds here, and in some cases nowhere else on Earth: the wallum froglet, the wallum and Cooloola sedgefrogs, and the wallum rocketfrog.

Rather than avoiding the poisonous water, they have evolved the plumbing to breed in it, holding onto the salts their eggs and tadpoles would otherwise bleed away. Detailed work on the Cooloola sedgefrog shows how finely tuned that is: its larvae keep their salt balance down to about pH 3.5, right at the edge of what a vertebrate body can manage, and surveys have mapped the narrow, species-specific pH windows within which each frog can and cannot breed. Toughness would be the wrong word for it. Each animal carries a lock cut to a particular chemistry, which raises the obvious question of why a frog would evolve to live somewhere so hostile.

The answer is the wallum’s own logic, turned from a plant onto an animal: hostility made into opportunity. Water this sour is hard on almost everything else that would breed or hunt in it. Below about pH 5 it attacks the gills of ordinary fish and tadpoles and drains the salts out of them, and the recovery plan for these frogs records two things that follow. The acid, nutrient-starved pools seem to be unsuitable for the frogs’ own close relatives, which is why the acid frogs and the common sedgefrogs so seldom breed together; and most of the water bodies the acid frogs breed in hold no fish at all. That absence matters most for one fish in particular: the mosquitofish (Gambusia holbrooki), the little livebearer that has invaded so much of Australia’s fresh water and cleans out native tadpoles almost wherever it lands. So the water that looks like a death trap becomes, for the frogs, close to a refuge: a wall they hide behind. You can hear the result on a warm night after rain, a chorus with room in it, the sound of a community that has escaped most of its competition and has the sour water largely to itself.

That wall needs describing carefully, because the temptation is to call it a fortress. Nobody has shown that the sourness itself keeps the mosquitofish out. What is on record is that the fish is mostly absent from these small, isolated pools, and that it is a serious predator of eggs and tadpoles wherever it does get in, which is why the recovery plan treats it as a live threat to be kept out by every effort rather than a problem the acid has already solved. The refuge is real, and it is partial, and fragile in a way a true fortress would not be. A wall with a low door in it only keeps you safe while nobody widens the door.

And the frogs have no fallback, because their defence and their danger are the same thing. They cannot retreat to sweeter water, not because it would poison them, but because sweeter, better-connected water is exactly what lets their enemies in: the recovery plan warns that joining these pools up, or making them permanent, is what brings the mosquitofish. So anything that sweetens or pollutes the blackwater, run-off from a new estate, a slug of fertiliser, a drain that lets ordinary water seep in, lowers that one door in the wall and lets the fish march through. Anything that drops the hidden water table dries the breeding sites out altogether. The whole arrangement leans on a groundwater store you cannot see and a leached sand with nothing in it to buffer the water back toward neutral.

Which is why you cannot protect the acid frogs by managing the frogs. Each one is staked on a single condition it can neither replace nor carry away: the strange, sour, nutrient-starved water, and the sand and water table that keep it that way. Protect those, and the frogs look after themselves. Fail them, and the sourest water on the coast, the last thing you would think to defend, turns out to be the only wall between these frogs and the fish.