A multi-clade test supports the intermediate dispersal model of biogeography.
BACKGROUND
Biogeography models typically focus on explaining patterns through island properties, such as size, complexity, age, and isolation. Such models explain variation in the richness of island biotas. Properties of the organisms themselves, such as their size, age, and dispersal abilities, in turn may explain which organisms come to occupy, and diversify across island archipelagos. Here, we restate and test the intermediate dispersal model (IDM) predicting peak diversity in clades of relatively intermediate dispersers.


METHODS
We test the model through a review of terrestrial and freshwater organisms in the western Indian Ocean examining the correlation among species richness and three potential explanatory variables: dispersal ability quantified as the number of estimated dispersal events, average body size for animals, and clade age.


CONCLUSIONS
Our study supports the IDM with dispersal ability being the best predictor of regional diversity among the explored variables. We find a weaker relationship between diversity and clade age, but not body size. Principally, we find that richness strongly and positively correlates with dispersal ability in poor to good dispersers while a prior study found a strong decrease in richness with increased dispersal ability among excellent dispersers. Both studies therefore support the intermediate dispersal model, especially when considered together. We note that many additional variables not here considered are at play. For example, some taxa may lose dispersal ability subsequent to island colonization and some poor dispersers have reached high diversity through within island radiations. Nevertheless, our findings highlight the fundamental importance of dispersal ability in explaining patterns of biodiversity generation across islands.
Department of Biology, University of Vermont, Burlington, Vermont, United States of America ; Department of Entomology, National Museum of Natural History, Smithsonian Institution, Washington D.C., United States of America.
Ingi
I
Agnarsson
Institute of Biology, Scientific Research Centre, Slovenian Academy of Sciences and Arts, Ljubljana, Slovenia.
Ren-Chung
RC
Cheng
Department of Entomology, National Museum of Natural History, Smithsonian Institution, Washington D.C., United States of America ; Institute of Biology, Scientific Research Centre, Slovenian Academy of Sciences and Arts, Ljubljana, Slovenia ; College of Life Sciences, Hubei University, Wuhan, Hubei, China.
Matjaž
M
Kuntner
D000818:Animals; D058448:Biota; D049628:Body Size / Q000502:physiology; D005618:Fresh Water; D005843:Geography; D019156:Indian Ocean; D062312:Islands