Island colonisation: Do species jump, or are they pushed?

Abstract

Colonisation of species from one island to another is a fundamental process for the establishment of island biotas within oceanic archipelagos, but exactly how species arrive is a little understood question. Phylogenies can tell us approximately when a colonisation event may have occurred, but not how. Sexually reproducing arthropods, particularly flightless species are interesting in this context. Excluding secondary transport by birds, which is likely important for ectoparasitic and perhaps in some cases non-ectoparasitic taxa, colonisation must be either windborne or waterborne. Windborne colonisation most probably involves single gravid females, as it is unlikely that wind-transported individuals would arrive within the geographic proximity of each other required for subsequent mating. Even allowing for a multiply mated female, successful establishment of a windborne colonist will be challenging due to low genetic variation and inbreeding. On the other hand, colonisation by oceanic rafting can potentially result in multiple individuals arriving within geographic proximity of each other. In this case subsequent mating among unrelated individuals may limit the negative genetic consequences of a founder event. To date we are not aware of any study that has been able to distinguish between these two mechanisms to explain the origin of flightless island arthropod species. Here we present evidence from a weevil radiation in the Canary Islands for oceanic rafting between islands, mediated by a novel mechanism, and discuss the implications for island biogeographic theory