Unraveling the determinants of body size shifts in a rodent species in the Zhoushan Archipelago, China.

Body size shifts of insular organisms have been widely observed, but few studies have simultaneously tested the combined and cascading effects of biotic and abiotic factors on morphological shifts. Here, we used two body size metrics (body length and body mass) of a small rodent (Rattus losea) to investigate its morphological shifts and the cascading mechanisms in the Zhoushan Archipelago, China. We used the live-trapping method to capture rodents and compared body size metrics between insular and mainland populations. We then constructed linear regression models to examine the relationships between the body size of rodents and three island attributes (island area, island isolation, and land use). Finally, we used structural equation modeling (SEM) to determine the cascading influences of three island attributes and four biotic factors (predator species richness, net primary productivity (NPP), capture rate, and rodent species richness) on body size. Island rodent populations had significantly larger body length and body mass than mainland counterparts. Both body length and body mass of insular rodents were negatively correlated with island area. SEMs showed that predator species richness had positive impacts on body length and body mass of insular rodents, while NPP had a negative effect on body mass. Moreover, SEMs revealed that island area positively influenced predator species richness, whereas land use negatively affected NPP. Body size shifts of the small rodent follow the prediction of the island rule (insular gigantism of small-sized species), but the determinants were strongly affected by which body size metrics were used. Therefore, both body length and body mass should be included in future studies to obtain a comprehensive understanding of the mechanisms underlying the body size shifts.