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 will occur occasionally.
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.
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).
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.
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.