Local specialists among endangered populations of medaka, Oryzias latipes, harboring in fragmented patches

Habitat fragmentation seriously damages local biodiversity of widespread organisms, or so-called common species, in agricultural habitats. We hypothesized that specialists adaptive to local particular conditions makes a population of generalists vulnerable to habitat fragmentation. To evaluate the extinction-proneness of common rural species, we determined the extent of phenotypic divergence using paddy fish, medaka, Oryzias latipes. Despite its wide geographical range, a rapid population decline threatens its persistence, and remnants persist in fragmented patches. We studied niche profiling of populations from different habitats for a factor that possibly lies behind the species being abundant within particular areas. Measurements of behavioral and morphological characteristics provided comparable variables between populations. Principal component analysis summarized these variables into compounded elements relevant to foraging and predator avoidance. Detection of association between behavioral and morphological traits showed a limited number of phenotypes specific to a local habitat, through which individuals adapted to specific narrow niches. Medaka maintains its status by accumulating a variety of local specialists. Because of the limited-dispersal ability, specialized individuals are vulnerable to isolation in less suitable patches that are caused by the destruction of the habitat-network. From a conservation point of view, the results suggest that preservation of habitats that also serve as corridors is recommended for enhancing the richness of common species that are composed of adaptively diversified phenotypes.