Density Dependence in the Terrestrial Stage of Wood Frogs: Evidence from a 21-Year Population Study

Population regulation in pond-breeding amphibians has generally been assumed to occur through density dependence in the aquatic larval stage. However, studies examining terrestrial stage population dynamics are comparatively rare. As a consequence, the relative importance of density dependence In the terrestrial stage remains largely unknown. Here I determine the density dependence of terrestrial stage vital rates, including juvenile and adult survival, age and size at first reproduction, and reproductive traits, for a population of Wood Frogs, Rana sylvatica, in Michigan. I also examine the carry-over effects of density-dependent variation in metamorphic traits on adult demographic traits. During the 21 years of this study, the number of breeding adult males and females varied by an order of magnitude, whereas the number of metamorphic juveniles leaving the pond varied by two orders of magnitude. Adult male and female annual survival was negatively correlated with the number of males and females, respectively. Partial correlation coefficients (holding population size of the opposite sex, number of juveniles, and precipitation levels constant) indicated that variation in adult male and female survival was largely explained by variation in the number of males and females, respectively. Juvenile survival (both males and females) was strongly negatively correlated with juvenile population size but not juvenile body size, adult population size, or total precipitation. Partial correlations revealed that variation in juvenile population size was the single most important factor accounting for variation in male and female juvenile survival, and female age and size at first reproduction. Clutch size and egg size also varied with juvenile population size independent of female body size. At lower juvenile population sizes, female juveniles matured earlier, at a larger body size, and produced larger numbers of smaller eggs than at high juvenile population sizes. Although juvenile body size was positively correlated with male and female reproductive body size and total clutch weight, juvenile fitness traits, in general, were more closely related to variation in juvenile population sizes. Larval traits did not affect adult fitness traits in the terrestrial stage. A nonlinear logistic model best described the functional relationship between juvenile population size and the number of surviving males, females, and total adult biomass. As number of juveniles produced increased, the number of surviving adult males, females, and total biomass plateaued, suggesting that the terrestrial environment limited adult population size. My results demonstrate that density-dependent effects operating in the terrestrial stage may be important in regulating Wood Frog population size.