Community composition and size structure of murid rodents in relation to the biogeography of the Japanese archipelago

We investigated the geographical patterns of community composition and size structure of murid rodent assemblages in Japan. Rodent faunal composition showed three biogeographic zones in the studied area (Hokkaido, northern Ho nshu and southern Honshu), which are characterized by endemic species or ge nera. There was a large discrepancy between distribution patterns of murine species, which are generalist and widespread in Japan, and arvicoline spec ies, which are more specialized and locally restricted. We also found a str ong degree of nestedness of the murid rodent fauna, i.e. smaller faunas wer e subsets of larger ones, which is typical of relict Fauna. The structure o f murid rodent assemblages was studied using the size and shape of the lowe r incisor, in order to lest for the effect of interspecific competition on community-wide patterns. We used two different approaches: one tests for re gularity in the size structure of the community (Barton and David test), an d the other one tests for minimum mean overlap size in the community betwee n species (randomization procedure). There was no congruence between the re sults of the two tests: we did not find any case of regular size structure, whereas mean size overlaps were minimum or even zero in about half of the cases studied. Thus, the evolution of rodent communities on islands seems t o be characterized by minimization of size overlaps, perhaps as a result of interspecific competition. Also, the reduction of island area, which is co rrelated with a decrease in species richness, is accompanied by an increase in Hutchinsonian size ratios and a decrease in the total size range of the community. These patterns may be linked to the reduced diversity of enviro nmental resources on islands.