The loggerhead turtle

Of all the coast’s habitats, the open surf beach looks most like a dead one: sand, wind and water, scoured twice a day and apparently empty. On a warm summer night it becomes a nursery. The loggerhead comes ashore, a big, blunt-headed, hard-shelled turtle that spends its life at sea cracking crabs and shellfish with jaws built for the job, and the sea turtle that nests most often on the mainland beaches of south-east Queensland. The effort here is modest, a scatter of nests watched over by local volunteers, small against the great rookery up at Mon Repos near Bundaberg, but it is real. A female hauls out above the tideline, digs a chamber in the dry sand, lays a clutch of a hundred or so eggs, covers them over, and goes back to the water, leaving the sand to raise her young.

And the sand does something stranger than warm them. It decides what they become. A sea turtle has no sex chromosome. The temperature of the nest sets the sex of every hatchling during a critical window of its development. The rule holds right across the sea turtles: cooler sand runs to males, warmer sand to females, with only a narrow band of temperatures producing a healthy mix of both. This is not a measurement taken from this coast but a piece of standard turtle biology, and it turns a nesting beach into a thermometer with a sting in it. A warming climate, or a beach stripped of the vegetation that once shaded it, tilts clutch after clutch toward females; a rookery can drift toward too few males to sustain it long before any adult shows the slightest sign of trouble. It is one of the subtler ways a hotter world reaches an animal, unbalancing the next generation before it has even hatched.

Then the hatchling runs its gauntlet, and the odds are barely believable. Of the hundred eggs in a nest only a fraction hatch, and of those, a hatchling’s chance of ever returning as a breeding adult is often put at something like one in a thousand. The run begins the instant a hatchling digs free. Ghost crabs and foxes take them on the sand; seabirds and fish take them in the shallows and beyond. To that natural ordeal we have added one of our own. A newly hatched turtle finds the sea by crawling toward the brightest, lowest horizon, which on a dark natural beach is the pale sky over open water. A lit-up esplanade behind the dunes hijacks that ancient cue and draws the hatchlings inland instead, toward the roads and the exhaustion that kill them. It is called light pollution, and unlike almost every other threat to almost every other animal, it is embarrassingly easy to fix: turn the lights down. Vehicles compact the sand over nests and cut ruts a hatchling cannot climb; dogs dig; feral predators raid.

So the loggerhead is exposed at every step of a life half at sea and half on the beach. Its needs are short and specific. Sea healthy enough to feed in, a beach it can climb above the tideline, sand at the right temperature, and a dark clear run back to the water. It can be undone at any one of them. The loggerhead refuses to recognise the line we draw at the tide, and between the warming sand, the glare, the vehicles and the crabs, the most useful thing this coast can do for it is also the cheapest. Leave some of its beaches dark.