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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A one-way ticket to a remote island

Where did Macquarie Island’s plants come from? Being hundreds of kilometres from its nearest neighbours, all of the plants must have been transported to this tiny speck of land in the ocean since it rose from the seabed less than a million years ago.

The chances that a seed might, for example, get stuck in mud on the feet of a bird in Tasmania, which is then blown into the Southern Ocean by a storm and eventually lands 1500 km away on Macquarie Island seem pretty low. But over thousands of years such unlikely scenarios probably will occur.

A giant petrel with several globular heads of Acaena seeds stuck to its feathers. Each Acaena seed has small barbs on it which hook firmly onto fur feathers (or socks!). The inventor of Velcro was inspired by plants with similar seed dispersal mechanisms.
A giant petrel with several globular heads of Acaena seeds stuck to its feathers (hundreds of seeds in total). Each Acaena seed has small barbs on it which hook firmly onto fur or feathers (or socks!). The inventor of Velcro was inspired by plants with similar seed dispersal mechanisms.

Not only does a propagule (a seed or spore or piece of plant capable of regenerating) have to find its way to the island, to successfully become established it needs to survive and reproduce.

Most of Macca’s vascular plant species are found elsewhere in the Subantarctic, which suggests that they are well adapted to long-distance dispersal. Few seeds survive in sea water for any length of time, so the obvious vectors for seed transport are wind and birds. Around half of Macquarie Island’s plants have seeds small enough to travel by wind. The strong westerly winds which predominate at these latitudes provide a mechanism for distributing small seeds and spores within the subantarctic.

Sea birds such as albatrosses, petrels and skuas travel widely, easily covering distances of hundreds or thousands of kilometres. Some seeds such as the hook sedges (Uncinia spp.) and ‘buzzies’ (Acaena spp.) are purpose designed for attaching to fur or feathers. Other seeds may be swallowed by birds, or stick to their feet or feathers in mud.

Uncinia species have distinctive seeds with a barb attached, hence the common name 'hook sedge'.
Uncinia species have seeds with a distinctive barb attached, hence the common name ‘hook sedge’.

Leptinella plumosa, a common coastal plant on Macquarie Island and many other subantarctic islands, was not known from Heard Island until 2004 when a single plant was found. This species has sticky seeds which can adhere to a bird, the most plausible explanation for this dispersal event. Similarly on Macquarie Island a large sedge, Carex trifida, is confined to a single location on the island and absent from other areas of similar habitat, suggesting it is a relatively recent arrival (though present since early botanical collections).

Leptinella plumosa is a common coastal plant on Macquarie Island and many other subantarctic islands yet only arrived on Heard Island this century.
Leptinella plumosa is a common coastal plant on Macquarie Island and many other subantarctic islands yet only arrived on Heard Island this century.

Rather than seeds, ferns and mosses produce spores which are far tinier and easily float on wind currents, or hide in mud attached to a bird. These types of plant are widespread in the Subantarctic and many species have extensive natural ranges. They are so well dispersed that very few species are endemic to a single location.

Exotic seeds occasionally turn up on Macca, transported by birds or ocean currents, but do not become established. A study of pollen in peat cores from a lake on the island reveals the presence of a diverse range of exotic pollen, from Australia and New Zealand, showing that strong northerly or northwesterly airflows can transport things to Macquarie Island. A small European bird, the redpoll, arrived on Macquarie Island in 1912, apparently having been carried by one of these weather systems from New Zealand where it is an introduced species. Similarly, starlings arrived in 1930.

Carex trifida, a large sedge known from only a single location on Macquarie Island, appears to be a relatively recent immigrant.
Carex trifida, a large sedge known from only a single location on Macquarie Island, appears to be a relatively recent immigrant.

However, the main vector for transporting plants to subantarctic islands now is humans. Seeds are frequently found in the clothing and equipment of researchers and tourists. Grass seeds are the most common stowaways and Velcro fastenings on clothing are the most likely place for them to hide. Procedures are in place to reduce the risk of importing these ‘subantarctic hitchhikers’ to the Macquarie Island Nature Reserve. Tiny grass seeds can easily evade detection. Two exotic species of grasses were discovered growing next to a track on Macquarie Island in 2014. Since rabbits and rodents have been exterminated, weeds now pose the number one threat to the naturalness of Macquarie Island’s ecosystem.