All organisms are affected by variations in ecological variables, especially species breeding at the threshold of their physiological tolerance. This specifically applies to early breeding amphibians emerging from hibernation. Rana uenoi is the earliest breeding amphibian in the Republic of Korea and the species is distributed at most latitudes and altitudes on the peninsula. To investigate the relation between the breeding phenology of R. uenoi and ecological variables at their breeding sites, we conducted six to ten weekly surveys at 27 sites in 2016, 2017 and 2018. Sampling was conducted after drawing a 50 x 70 km grid on the map of the country, and randomly selecting one site per cell. We recorded the presence and absence of adult frogs, eggs and tadpoles, Calling Index (CI), the number of eggs, air temperature, humidity, air pressure, water temperature, conductivity, pH, Dissolved Oxygen value (DO), ice coverage, moonlight, sunset time, cloud coverage, date and time at each site.
Results/Conclusions
We first defined zonal variations for the first emergence of R. uenoi. Emergence started from the lowlands in the South-West of the peninsula, then followed by the southern and western coastal areas, followed by midlands, and finally the highlands in the North-East. The time gap between the first breeders at the two extremes was two months. Adult presence was significantly correlated with higher values for time of day, humidity and air pressure and low ice coverage. The CI significantly increased with time, humidity and sunset time but significantly decreased with high air pressure, ice coverage and moonlight. Specifically, humidity and sunset time had the strongest positive impact on CI while air pressure and ice coverage had the strongest negative impact. The first spawning and hatching were related to high air and water temperatures and late sunset time, however ice coverage had a significant negative impact on hatching. Egg numbers were significantly linked to high humidity, air pressure and pH and low water conductivity and ice coverage. The DO values were however not significantly related to any breeding phenology values for R. uenoi. Our results suggest that despite behavioral plasticity in response to ecological variations, the phenology of R. uenoi's breeding activity is likely to be strongly impacted by climate change.