A possible mechanism to explain how deep concrete ditches diminish frog densities in paddy fields. April 9 2015

Frogs without finger disks like true frogs and common frogs could not climb up concrete walls when they fall into deep ditches. And if it's before the mid-season drainage of paddy fields these fallen frogs are likely to be died of the ditches because water levels are high and flows are fast there (caricatured on Keynote for Mac).