Population model of Rana japonica crossing agricultural concrete canals: evaluating population conservation by improving the migration routes of frogs

Abstract

Agricultural canals with deep concrete walls are commonly installed in paddy field areas following land consolidation projects in Japan. Such canals create a critical migration barrier for frogs, as escape is not possible after falling in. Hence, countermeasures that allow migrating frogs to cross agricultural canals, such as lid structures to prevent frogs from falling in or the creation of partial slopes to allow escape, have been developed to conserve endangered frog populations. The objective of this study was to design a method for evaluating the population viability of the endangered Japanese Brown Frog, Rana japonica, following the implementation of countermeasures. Two types of model simulations were examined: (1) a “basic model,” based on an age-structured model and (2) a “crossing model,” to evaluate migrating frogs crossing canals. Parameters, such as survival rate and fecundity, were estimated from field observation data of age-dependent population fluctuations. Comparison of a basic model simulation with the field observation data suggests model validation. The crossing model simulation, which had an initial population of 1,000 individuals, showed that a crossing proportion of greater than 30–40% would achieve 95% population viability after 5 years. The simulation also showed that a crossing proportion of over 80% would also achieve 95% population viability after 40 years. These results indicate that frog survival during the migration period is insured, as long as no additional countermeasures are necessary after installation.