Paddy-associated frog declines via urbanization: A test of the dispersal-dependent-decline hypothesis

Major drivers of amphibian declines via urbanization include land-use changes that cause loss, fragmentation, splits, and degradation of habitat. The effects of these changes in habitat conditions on the persistence of populations are expected to differ among species depending on their dispersal habits: species with strong site fidelity would likely be more affected by habitat loss and degradation, whereas species with highly dispersive habits would be more threatened by habitat fragmentation and split (the dispersal-dependent-decline hypothesis). To test this hypothesis, we examined the distribution patterns of two paddy-associated frog species (Pelophylax nigromaculatus and Hyla japonica) with different dispersal habits along a rural–urban gradient of the Osaka–Kobe metropolitan area, Japan. Our results partially support the dispersal-dependent-decline hypothesis in that the species with strong site fidelity, i.e., P. nigromaculatus, was threatened by habitat-area (agricultural field) decline and habitat-quality degradation (prevalence of concrete levees) rather than by habitat fragmentation. To the best of our knowledge, our study is the first report that suggests paddy-associated amphibian declines via urbanization in the Asian region. However, the second half of the hypothesis, i.e., that the dispersive species (H. japonica) would be more strongly affected by habitat fragmentation via roads and habitat split via declines in surrounding forests was not supported. The lack of support for this portion of the hypothesis may be due to the high adaptability of H. japonica to artificial landscapes. We discuss the value of the dispersal-dependent-decline hypothesis for conservation planning in agricultural lands of urban areas.

► We firstly report a paddy-associated frog decline in an Asian urban area.
► Pelophylax nigromaculatus is threatened by loss of paddy lands in urban areas.
► The prevalence of concrete levee in urban paddies also causes a decline in the species.
► In contrast, Hyla japonica was habitat generalist and could use artificial landscapes.
► Effects of urbanization on the presence differed among species with different dispersal ability.