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Biogeography and Molecular Ecology of Ground Beetles on the Boston Harbor Islands

Stephanie Madden - Harvard College

My research focuses on the patterns of diversity and population genetics of ground beetles (Family Carabidae) on the Boston Harbor Islands. Carabidae is a large family of ground-dwelling beetles, generally dark-colored and nocturnal. Some species of these beetles have full flight capability (macropterous), some are completely incapable of flight (brachypterous), and some are wing-dimorphic, with both flight-capable and flightless individuals.

According to MacArthur and Wilson's theory of island biogeography, species richness on islands is expected to increase with island size and decrease with distance from the mainland. In addition, species with reduced dispersal capability should be less common on islands, particularly those that are distant from the mainland. In this study, I tested these two hypotheses by examining the patterns of diversity of both flight-capable and flightless species on one mainland site and 7 islands of varying sizes and distances from the mainland. Carabid beetles were collected in the summers of 2005, 2006, and 2007 as part of the All-Taxa Biodiversity Inventory (ATBI). In total, we collected 98 species of carabid beetles in 39 genera. Species richness increased with increasing island area as expected, but was not associated with island distance from the mainland. Flight ability did increase the ability of carabids to colonize more distant islands: relatively higher proportions of macropterous species were found on distant islands than on islands close to the mainland. However, flight-capable species were no more widely distributed across islands than flightless species, suggesting that flightless species actually have a survival or colonization advantage on islands where both forms compete.

Overall, 14.3% of carabid species collected were non-native to North America. Non-native species were no more widely distributed than native species or more successful at colonizing distant or disturbed islands. However, non-native species were proportionally more abundant, suggesting that they may have some survival advantage over native species. Because of the ecological importance of invasive species, further research on the colonization potential of these species is necessary.

Finally, I examined the molecular sequence diversity of the cytochrome oxidase I gene (COI) for three species on several islands of varying distance from the mainland. If distant islands and populations of flightless species are relatively isolated from immigrants, it is expected that these populations will show reduced genetic diversity relative to the mainland. However, I found that all of populations I examined were very genetically homogeneous. This suggests that carabid beetles are generally recent immigrants to all of the islands. In addition, habitat instability on the islands may cause repeated cycles of extinction and recolonization, resulting in a continuous turnover of populations.

I will be graduating Harvard in June, and am unfortunately leaving the Boston Harbor Islands ATBI behind. I have had a wonderful time working on this project, and would like to thank Brian Farrell, Jessica Rykken, and everyone else who has been involved with the ATBI for making this research possible.