Factors Affecting Population Fluctuations in Larval and Adult Stages of the Wood Frog (Rana Sylvatica)

Abstract

During a 7—yr population on the wood frog, Rana sylvatica, the breeding population size fluctuated by a factor of 10 and juvenile production by a factor of 100. Variation in the adult population among years was largely due to variation in juvenile recruitment. Annual net replacement rates (R o) varied from 0.009 to 7.49. Survivorship curves (calculated using the number of eggs deposited as the initial point) showed that most variation in the proportion of individuals surviving to adulthood was due to variation on larval survival; juvenile and adult survival was relatively constant among years. Male and female survival did not differ. Because females matured a year later than males, on average 2.3 times as many males as females from a given clutch survived to breed. This difference accounted for the observed male—biased sex ratio in breeding choruses. Premetamorphic survival and size at metamorphosis were negatively correlated with the number of eggs deposited. Length of larval period was positively correlated with number of eggs deposited. Survival was higher among juveniles that metamorphosed early and were large at metamorphosis. Larger juveniles matured earlier and were also larger as adults. The population appeared to be regulated through density—dependent factors affecting larval survival, larval size, and time of metamorphosis. Adult population size also negatively affected total clutch volume. Mean monthly rainfall positively affected adult survival.