The mysterious case of the Subantarctic Bedstraw

What is Macquarie Island’s rarest plant? One contender for the title is Subantarctic bedstraw or Antarctic bedstraw (Galium antarcticum), a small creeping herb with tiny delicate pinkish flowers. The species was first recorded in 1983, growing near Skua Lake around halfway along the 34 km long Subantarctic island.

It was not until 2013, following listing as Critically Endangered on the Commonwealth Environment Protection and Biodiversity Conservation Act, and after years of doubt about its persistence on Macquarie Island, that Subantarctic bedstraw was rediscovered in the same location. Where had it been hiding for 30 years?

Subantarctic bedstraw growing on a mossy bank at Skua Lake.
Subantarctic bedstraw growing on a mossy bank at Skua Lake.

The single known population of around 500 plants occurs on a mossy bank in short grassland vegetation. At around 140 metres above sea level on the plateau which covers most of Macquarie Island, the short grassland habitat in which the species occurs is typical of much of the island. Why it is not more widespread is a mystery. Perhaps it was preferentially grazed by the rabbits which have had a dramatic impact on the vegetation of Macquarie Island. It may be a formerly more widespread species nearly grazed to extinction by rabbits or, as appears to be the case with some other localised species such as Carex trifida, a recent immigrant yet to expand its range on the island.

Searching for Galium antarcticum at Skua Lake.
Searching for Galium antarcticum at Skua Lake.

Macquarie Island is a young and very remote landmass with an unusual cold maritime climate. Consequently its flora is the result of long-distance dispersal and composed largely of Subantarctic specialists. Galium antarcticum has a wide distribution around the Subantarctic, suggestive of dispersal by seabirds. It occurs in Patagonia, South Georgia, the Falklands, Crozet and Kerguelen islands. The species was first described from the Falkland Islands by J.D. Hooker in 1846.

Subantarctic bedstraw grows to a few centimetres in height and width, with creeping stems and solitary flowers. What appears to be whorls of four fleshy red- or purple-tinged leaves, are actually a pair of leaves and a pair of nearly identical stipules. Galium is a large and widespread genus in the Rubiaceae with several native and introduced species in Australia including the familiar garden weed Galium aparine (cleavers or sticky weed). Many species have distinctive bristly hairs whereas G. antarcticum is hairless.

Subantarctic bedstraw
Subantarctic bedstraw

The future prospects for Galium antarcticum on Macquarie Island are uncertain. It may benefit from the recent eradication of rabbits, expanding its range, or it may struggle to compete with taller growing plants as the short grassland transitions to a more closed vegetation community.

And the other rare vascular plants on Macca? The tiny filmy fern Hymenophyllum falklandicum is confined to a few boulders in mountain-top feldmark. The large sedge Carex trifida occurs in only one small population on the coastal terrace near Handspike Point.

An earlier version of this article first appeared in Australian Plants journal, issue 221.

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