Cryptic invasion and the interpretation of island biodiversity

Background/Question/Methods

Species with cryptic origins (i.e. those that cannot be reliably classed as native or non-native) present a particular challenge to our understanding of the generation and maintenance of biodiversity. Such species may be especially common on islands given that islands have had a relatively recent history of human colonization. It is likely that select island species considered native may have achieved their current distributions via direct or indirect human actions. As an example, we explore the origins of eastern bluebirds on the island of Bermuda. Considered native to the island and a distinct subspecies, this population has diverged in morphology relative to mainland North America. We used microsatellite markers and plumage reflectance data to evaluate the history and status of this species group.

Results/Conclusions

We show that the Bermuda population of bluebirds is the likely result of a single human-assisted colonization event that occurred during the 1600s; making this species a cryptic invader. To our knowledge this is the youngest example of a terrestrial vertebrate cryptic invader and the only one designated as a subspecies. Furthermore, there are differences in plumage brightness and color that run counter to most island patterns; including increased brightness and increased plumage dimorphism. We suggest that the eastern bluebird is not an isolated case of cryptic invader either on Bermuda or elsewhere and that extreme caution be exercised when studying present-day distributions of organisms. Such cases require biologists to reconsider their definitions of native and non-native species.