Native and non-native frogs responded differently to modernization at Japanese paddy fields

Abstract

Rice farming modernizations, especially changes in the irrigation system, have been accused as one of the reasons of frog population decline in Japan. Here, we examined responses of native frog, Daruma pond frog (Pelophylax porosus porosus) and non-native frog, Rice frog (Fejervarya kawamurai) population to the modernization in Japanese paddy field. We tested the following two hypotheses: 1) native frog’s density will be lower in the modernized paddy field, and 2) non-native frog density will not be affected by modernization. After two periods of field studies (2017 and 2018), we found that native frog’s density is indeed lower in modernized paddy fields, and the non-native frog’s density is lower in the non-modernized sites. Presumably, changes of shallow soil ditches to concrete ditches in modernized paddy fields act as a death trap for Daruma pond frog. Ditch depth and the distance from forest also significantly affecting the density of Daruma pond frog. On the other hand, the population of Rice frog did not show any negative response to the modernizations. The non-native frog population remained stable because they may have high fecundity.