Mega-landslides as a vehicle for colonisation of distant islands

One of the great challenges of biology is to explain colonisation of distant islands, and indeed continents, by animals that are too heavy to be carried aloft. Various theories have been proposed, such as rafts of vegetation carried by floods, but none seem to answer the question adequately. In a paper just published in the Journal of Biogeography, Garcia-Olivares et al. (2017) have come up with a novel solution to the colonisation problem – mega-landslides. Using a study of weevils in various of the Canary Islands, they hypothesise that very large landslides (such as those from volcanic flank collapse) create large rafts of floating organic material that can carry not just single individuals but whole populations to distant islands, where they can establish a new colony of the animal. The authors even include a nice image of the sequence of events from a landslide perspective:

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The appealing thing about this theory is that it creates a testable hypothesis in terms of the population genetics. The authors have used DNA testing to demonstrate that for the weevils, the population and individual level genetics fit the hypothesis. They suggest therefore that the weevils that they examined on the island of La Palma are a part of the population located on on the island of Tenerife who were incorporated into the 500 cubic kilometre mega-landslide in La Orotrava, which occurred about 500,000 years ago. The weevils would then have been transported on rafts carried by the Canary Current between the two islands, before landing on La Palma to establish the new colony. Thus, argue the authors:

“mega-landslides are expected to promote the ocean deposition and rafting of significantly more organic material than that associated with the landslide itself, although the altitude of secondary deposition will be a function of the tsunami height. In the context of island biogeographical theory, for which colonization is a fundamental component, mega-landslides may be an important driver of colonization, mediated by ocean currents and archipelago geomorphology.”

Reference

García-Olivares, V., López, H., Patiño, J., Alvarez, N., Machado, A., Carracedo, J. C., Soler, V. and Emerson, B. C. (2017). Evidence for mega-landslides as drivers of island colonization. Journal of Biogeography. doi:10.1111/jbi.12961