Evidence for long-distance dispersal of Sophora microphylla to sub-Antarctic Macquarie Island

Abstract

A seed probably of kowhai (Sophora microphylla), was found on a lake shore at Macquarie Island. The species does not occur on the island, and the seed may have been carried there by a bird that found it at sea. It shows how two consecutively acting long-distance dispersal agents may have facilitated colonization of oceanic islands by the Edwardsia group of Sophora species.

Keywords:

• seed dispersal
• Fabaceae
• Macquarie Island
• marine drift
• seabirds

Macquarie Island (54.37°S, 158.53°E) in the Southern Pacific Province is the most southerly of the sub-Antarctic islands southwest of New Zealand. Its cool climate precludes survival of woody plants and its vegetation is variously dominated by tussock grasses, megaherbs, cushion plants, mosses and lichens. The island has been above sea level for less than a million years and has never had a terrestrial connection with any other land mass (Selkirk et al. 1990). Its native vascular flora (with low endemicity) probably has been derived entirely from seeds carried to the island by birds, wind and sea currents (Selkirk et al. 1990).

Two ‘exotic’ seeds have previously been found naturally dispersed to the island both being well-known for their capacity for marine drift. One belonging to the pantropical shrub Caesalpinia bonduc was collected from a beach (Hasselborough Bay, Costin 1965). The other, a species of Ipomoea (the seed size corresponding to I. pes-caprae) was collected from a lake shore (Waterfall Lake, Taylor 1955). Waterfall Lake is a large freshwater body beside which are breeding colonies of southern giant petrels (Macronectes giganteus), which commonly bathe in the lake as do kelp gulls (Larus dominicanus) and sub-Antarctic skuas (Stercorarius skua lonnbergi).

During winter 2010 searches for seeds were opportunistically conducted on downwind lake shores at Macquarie Island, to seek further examples of contemporary long-distance seed dispersal to the island. Due to accessibility, weather, snow cover and other constraints these searches were necessarily brief and partial. During searches on single occasions at Waterfall Lake, Major Lake and Lake Tiobunga no seeds were found. However on 14 June 2010 at the first of three searches at Island Lake, a single seed was discovered. Island Lake is constantly frequented by seabirds especially in summer, most conspicuously by sub-Antarctic skuas and kelp gulls.

The seed which floated when placed in water was larger than those of any native Macquarie Island plant. It was apparently leguminous although no legumes belong to the island's flora. It was badly weathered to a pale tan colour and clearly not viable, although it may have been intact and alive at the time of its ingestion and deposition by a seabird. Most parts of the testa had been weathered to form a reticulately pitted surface with the original smooth surface remaining over only small areas. The seed's shape, a flattened ovoid measuring 6×4.5×1.5 mm, is within the dimensions of kowhai (Sophora microphylla) seeds collected both from wild plants near Auckland and found on New Zealand beaches (see Figure 1). While the seed's identification cannot be certain given its weathered condition, it matches no other seed known to drift to ocean beaches in the Australasian region (Smith 1999). The seed of Sophora tomentosa, frequently drifting to tropical Pacific beaches, is larger and more spheroidal (Smith 1999).

Figure 1  The seed found at Island Lake, Macquarie Island (left), compared with a fresh seed of Sophora microphylla.

S. microphylla (Fabaceae) is a shrub native to New Zealand. Other taxa within the Edwardsia group of Sophora species occur on several temperate oceanic islands in the south Pacific, in Chile and on Gough Island. A proportion of S. microphylla seeds can float in sea water for months (e.g., Murray 1986; Smith 1999). At least some remain viable during years of flotation (Sykes & Godley 1968). The plant's wide insular and transcontinental distribution is thought to be due to such dispersal, a conclusion supported by molecular data indicating range expansion from New Zealand c. 2.5 million years ago (Hurr et al. 1999). Ocean currents in the region flow from west to east, endlessly circumnavigating Antarctica and bringing driftwood and flotsam to Macquarie Island from South America (Barber et al. 1959) and probably other southern land masses.

Seeds of many species are drifted to beaches (especially in the tropics) where germination may take place but where the dynamic and saline conditions preclude establishment for most of them (Smith 1999). Storms, king tides and tsunamis can be envisaged as being capable of occasionally moving seeds inland from beaches. Transport of seeds by birds directly from the sea surface to inland breeding or bathing sites provides an alternative route, one which might have been more important in the pre-human world when populations of seabirds may have been larger and more widespread than today. Many seabirds habitually swallow objects found floating at sea, generally depositing them later if they are indigestible including sometimes at their nests or elsewhere on land. Such objects today commonly include plastic artefacts (e.g., Harper & Fowler 1987; Gregory 2006; Ryan 2008). Gregory (2007) also reported the discovery of two S. microphylla seeds lying between albatross nests on a rocky islet north of Chatham Island (east of New Zealand) concluding they were picked up at sea by a foraging bird and later regurgitated. Although it is unlikely that S. microphylla would establish under present climatic conditions on Macquarie Island given the low temperatures, the present finding again demonstrates long-distance dispersal of its seed to an inland location beyond an ocean barrier.

In their recent review of dispersal in plants, Cousens et al. (2008) emphasized the considerable importance of rare long-distance dispersal events for plants’ invasions of previously unoccupied regions. Turner et al. (2006) suggest that Leptinella plumosa has recently become established on sub-Antarctic Heard Island following long-distance dispersal by a seabird. The present study indicates an avenue involving two vectors during dispersal, showing how S. microphylla and its related taxa may have achieved their wide distribution, and demonstrating that new propagules continue to arrive at Macquarie Island.

Acknowledgements

I thank the Australian Antarctic Division for employing me at Macquarie Island, and providing an opportunity to search for seeds there; Noel Carmichael (Tasmanian Parks and Wildlife Service) and Dana Bergstrom for allowing me to conduct the work under Scientific Permit FL10084 and my fellow expeditioners for their tolerance and support of their Station Leader's bizarre hobby.