Lower genetic diversity and hatchability in amphibian populations isolated by urbanization

Many amphibian populations worldwide have declined rapidly and been threatened with extinction in the past few decades because of human impacts on the environment. It is well known that urbanization reduces the genetic diversity of isolated populations. The concept that a reduction in genetic diversity leads to lower reproductive fitness has been predominantly supported by studies involving laboratory organisms, but has rarely been tested in wild populations. Here we examined whether genetic diversity affected hatchability, a population parameter related to reproductive success, in the populations of two pond-breeding amphibian species (Hynobius tokyoensis and Rana ornativentris) affected by urbanization. We surveyed 81 populations in the southwestern Kanto region of Japan. Mean hatchability of the populations was determined and genetic diversities were estimated via mitochondrial or microsatellite DNA analyses. Firstly, Random Forests (an ensemble machine learning method) models were applied to clarify the effects of environmental factors on both hatchability and genetic diversity. Subsequently, the relationships among environmental factors, genetic diversity, and mean hatchability were evaluated using path analysis. Mean hatchability was significantly affected by both urbanization and genetic diversity. This result shows that loss of genetic diversity may decrease a population’s reproductive success in the field.