Changes in phenological synchronization within populations alter demographic rates

Background/Question/Methods

Phenological events, such as migration, emergence from dormancy, and reproduction, exhibit a temporal distribution. The shape of this distribution is determined by the degree of synchronization in phenology by the individuals that comprise a population. Phenological synchronization can vary considerably among years due to interannual variation in seasonal weather patterns and differential responses by individuals to them. While variation in phenological synchronization is widespread in natural communities, we still have a poor understanding of how it affects the demographic rates of species. In this study, we take an experimental approach to determine the consequences of variation in the synchrony of offspring birth for subsequent offspring performance, using the Coastal-plain Toad (Bufo nebulifer) as a model system. Specifically, we used three treatments to manipulate variation in tadpole hatching around a mean hatching date to determine the effects on development time, mass at metamorphosis, and survival, which are attributes that generally correlate with future survival, growth, and reproduction. We executed each synchrony treatment at five different densities to determine if the effects of synchrony are density dependent.

Results/Conclusions

We found that hatching synchrony affected all three demographic rates and that these effects were density dependent. At low densities, low synchrony resulted in low survival, high mass, and similar development times relative to high synchrony, while at high densities, low synchrony resulted in similar survival, high mass, and short development times relative to high synchrony. These results suggest that the level of hatching synchrony that maximizes recruitment of juveniles into the adult population will ultimately depend upon the relative importance of these demographic rates. When tadpole densities are low, maximum recruitment likely will occur with high synchrony if survival is most important and with low synchrony if mass is most important. However, when densities are high, low synchrony should maximize recruitment because survival is similar, mass is high, and development time is short relative to high synchrony. Together, these results demonstrate that variation in the shape of a phenological distribution can have important consequences for demographic rates, but the specific outcome can depend on other population-level attributes such as density.