Hydrological effects of paddy improvement and abandonment on amphibian populations; long-term trends of the Japanese brown frog, Rana japonica

In rice fields, the cultivation area itself can play an essential role as a habitat for wetland organisms. Many previous studies showed negative impact of agricultural intensification and abandonment on biodiversity in wet farmland ecosystems. However, verification of the direct impact of aquatic environmental change by the paddy improvement and abandonment still remains. Here, we investigated the effects of the intensification and abandonment on the area of wet fields remaining in paddies during the fallow season, as well as the factors driving the population decline of the Japanese brown frog (Rana japonica), using data of long-term monitoring numbers of egg masses at multiple sites. To quantitatively estimate the spatial and temporal variation in saturated areas with water in the paddies where the frogs spawn in early spring, we used infrared bands of Landsat images. Both paddy improvements and abandonment have affected R. japonica populations through the reduction of wet areas in the fields. Furthermore, the frog's population size was positively associated with the area of surrounding forest. Our findings suggest that conservation in wet farmland requires appropriate water management inside the cultivation area as well as in other landscape elements that serve as secondary habitats.