Phenology of spawning for the common frog (Rana temporaria L.) in Finland from 1846 to 1986

Abstract

The onset and changes in the timing of spawning of the common frog (Rana tempo raria L.) in Finland was studied through the 3888 records received through enquiries car ried out by the Finnish scientific society Finska Vetenskaps Societeten-Suomen Tiedeseura in each year over the period 1846-1986. Spawning begins at the end of April in southwestern Finland and gradually proceeds towards northeastern parts of central Fin land and from there to the north. The mean date of the first spawning of the year is 18 April. The maximum delay between the mean dates of all the records of spawning from the southernmost (lat. 60°N) and northernmost Oat. 70°N) 100x100 km Finnish uniform grid squares is 35 days. The mean date of spawning correlates well with mean air temperatures (April in southern, and May in northern, Finland) and with the dates when the ground was no more permanently snow covered. A long-term skift towards earlier spawning (2-13 days de pending on the indice used and the area studied) during the observation period of about 150 years can be seen after smoothing the means of 10-year periods with a 50-year moving average. This long-term change is in accordance with the changes observed in spring temperatures in Finland during the same period. The greenhouse effect causing a worldwide increase in the mean air temperatures probably influences the onset of spawning in amphibian populations and accordingly its impacts should be monitored in this respect in other areas as well.