Disjunction (stranded on a wet island)

Stand under a stringybark in the Mount Lofty Ranges and you are, in a sense, looking at a castaway.

Messmate and brown stringybark, the two shaggy-barked eucalypts that roof these hills, belong by rights to the wet forests of Victoria and further east, where the bulk of their kind still grows thickly in reliably wet country.

The Mount Lofty trees are a separate, isolated population hundreds of kilometres from that main range, cut off by a stretch of country too dry for a stringybark seedling to cross, and clinging on here in a hotter, more summer-arid niche than their eastern relatives will put up with (Guerin et al. 2023).

Ecologists call this pattern disjunction, and once you know to look for it, the ranges are full of it.

It is not one species’ quirk but the fingerprint of a whole region’s climate history: a wetter, cooler chapter of the past joined this forest to the great eastern forests, and when the continent dried and the forest line pulled back, whatever could hang on in the one patch of high, wet ground that stayed livable was left behind, an outpost of the wet world marooned in a drying one.

Biological surveys of the southern ranges keep turning up the same story in birds and understorey plants as well as trees — populations that read, on the map, as small isolated dots a long way from where the textbook says their kind should be (Armstrong et al. 2003).

It is easy to mistake a disjunct population for merely a rare one, but the two are not the same thing.

A rare species may be scarce everywhere.

A disjunct population is scarce here specifically because here is nearly the edge of what it can tolerate — it is not thin on the ground so much as marooned, holding a shrinking foothold in a narrowing climate, with no easy way to reconnect to the main body of its kind should this last outpost fail.

That is what makes disjunction such a useful thing to read in the field: every stranded stringybark, every relict fern or bird tucked into a cool gully, is a small, legible clue to how much further a wetter climate once reached, and how precarious the wet island has become now that it does not.