Life-Cycle Plasticity Underlying the Survival of Glandirana rugosa Tadpoles in Intensively Managed Rice Fields

Abstract

Rice field intensification in recent decades has negatively impacted wetland species, partly due to the shortened hydroperiods caused by effective water drainage during mid-summer, harvesting, and fallow seasons, which increase mortality risks for amphibian larvae. To investigate potentially adaptive responses to these altered conditions, we tested whether Glandirana rugosa, a species with long tadpole stages, accelerates metamorphosis in rice fields to avoid desiccation. We surveyed larval growth and adult breeding phenology across contrasting habitats and conducted a series of rearing experiments to identify factors determining the duration of the larval period. We found G. rugosa larvae often metamorphose before winter in certain rice fields and in a river, whereas larval overwintering commonly occurred in other sites. Frequent larval overwintering was associated with low water temperatures rather than permanent water bodies. In the experiments, larvae metamorphosed earlier at higher temperatures, while population origin, water level, photoperiod, and sex had no clear effects on the larval period. Because G. rugosa larvae possess traits associated with permanent water bodies (e.g., long larval period and no response to reduced water level), the observed developmental plasticity may not be a direct adaptation to temporal pools. Rather, it likely represents a coincidental adaptive expression of thermal reaction norms in rice fields with harvesting drainage. However, their development rate is not fast enough to complete metamorphosis before mid-summer drainage, possibly precluding their occurrence in many typical rice fields. This study highlights larval life-history plasticity as an understudied mechanism of population persistence during the nationwide land transformation in Japan.