Abstract
Birds are thought to be important vectors underlying the disjunct distribution patterns of some terrestrial biota. Here, we investigate the role of birds in the colonisation by Ochetophila trinervis (Rhamnaceae), a vascular plant from the southern Andes, of sub-Antarctic Marion Island. The location of O. trinervis on the island far from human activities, in combination with a reconstruction of island visitors’ travel history, precludes an anthropogenic introduction. Notably, three bird species occurring in the southern Andes inland have been observed as vagrants on Marion Island, with the barn swallow Hirundo rustica as the most common one. This vagrant displays long-distance migratory behaviour, eats seeds when insects are in short supply, and has started breeding in South America since the 1980s. Since naturalised O. trinervis has never been found outside the southern Andes and its diaspores are incapable of surviving in seawater or dispersing by wind, a natural avian dispersal event from the Andes to Marion Island, a distance of >7500 km, remains the only probable explanation. Although one self-incompatible shrub seems doomed to remain solitary, its mere establishment on a Southern Ocean island demonstrates the potential of vagrancy as a driver of extreme long-distance dispersal of terrestrial biota.
Highlights
Successful long-distance dispersal events are extremely rare, difficult to observe directly, and typically only reconstructed by phylogeographic means[1,2]
Since the DNA sequences and leaf characteristics of the unknown shrub on Marion Island were almost identical to the reference material (Figs 1 and 2), we identified the specimen with high confidence as Ochetophila trinervis (Rhamnaceae: Colletieae)
Since only a single individual of O. trinervis was found on Marion Island, no genetic diversity analysis could be conducted at the population level to estimate time since arrival
Summary
Successful long-distance dispersal events are extremely rare, difficult to observe directly, and typically only reconstructed by phylogeographic means[1,2]. Even though the terrestrial biota of these islands are relatively species-poor, they display a pattern of inter-island similarity that indicates long-distance dispersal in an eastward direction[14,15] This directional dispersal is likely the consequence of strong westerly winds and the West-wind Drift[7,16,17,18]. In spite of the remoteness of the Southern Ocean islands from major landmasses, historic and current anthropogenic activities have resulted in a steady rate of colonisation events of non-native species to the islands by means of an anthropogenic vector[23,24]. For non-wind-dispersed biota, pelagic and migratory birds are assumed to be the main vectors in natural long-distance dispersal events[9,12,13]. Recent discoveries of plant species that are known to other islands of the South Indian Ocean Province have been assumed to have been transported by birds travelling between the Southern Ocean islands[29,30]
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