Delivered at 2008 Boston Harbor Isla
nds Science Symposium
The role of native species diversity in providing biotic resistance to invasion remains controversial, with evidence supporting both negative and positive relationships that are often scale-dependent. Across larger spatial scales, positive relationships suggest that exotic and native species respond similarly to factors other than diversity such as island size and isolation. However, previous island studies examined only a few islands or islands separated by extreme distances. We surveyed exotic and native plant diversity on 23 islands separated by < 15 km in Boston Harbor
. Exotic and native species richness were positively correlated. Consistent with island biogeography theory, species richness of both groups was positively related to area and negatively related to isolation. However, the isolation effect was significantly stronger for native species. This differential effect of isolation translated into exotic species representing a higher proportion of all plant species on more distant islands. The community similarity of inner harbor islands versus outer harbor islands was greater for exotic species, indicating that isolation had a weaker influence on individual exotic species. These results contrast with recent reports focusing on similarities between exotic and native species and highlight the importance of studies that examine invasion ecology using an island biogeographic approach.