Geographical patterns of butterfly species diversity in the subtropical Ryukyu Islands: the importance of a unidirectional filter between two source islands

AIM: To examine the roles of geohistorical and ecological factors in the development of butterfly assemblages on continental islands with multiple source pools. LOCATION: The Ryukyu Islands, Japan. This arc of continental islands is located between two source islands, Kyushu and Taiwan, and contains two major straits, the Tokara and Kerama gaps. METHODS: A total of 489 butterfly species were identified on 59 islands, including the two source islands. The influences on species richness and rarity of latitude, area, elevation, and distances from the nearest source and from the nearest larger island were analysed using generalized linear models. The relationships between differences in area, elevation, and distance and dissimilarity between island butterfly faunas were evaluated by multiple regressions on distance matrices. The relationships between area, elevation, and distance from the source and dissimilarity to the source fauna were examined using linear models. The dissimilarity was based on the Sørensen index and its nestedness and turnover components. RESULTS: Latitude, area and isolation determined species richness and rarity, whereas differences in elevation and distance regulated species turnover between islands. The gaps between islands were associated with nestedness between island faunas. Area consistently had a negative relationship with the total dissimilarity to source fauna. The overall dissimilarity to Kyushu decreased with the distance from Kyushu, whereas dissimilarity to Taiwan increased with distance from Taiwan. MAIN CONCLUSIONS: Both environmental filtering and dispersal limitation determine the geographical patterns of butterfly assemblages in the Ryukyus. The present study focuses on the unique pattern wherein migration from Kyushu is facilitated on islands farther from Kyushu, while migration from Taiwan is inhibited on islands farther from Taiwan. The two source islands have contrasting roles that affect butterfly distributions unidirectionally. This study highlights the importance of resolving dissimilarity into nestedness and turnover components to elucidate the formation of island biota.