Demography matters: adult and juvenile survival rates will respond differently to a changing climate

Background/Question/Methods

Climate change may result in dramatic range shifts and contractions for many species within this century.  As a result, many local populations are expected to decline.  However, the typical species-distribution approach to climate change research cannot describe how changes in weather conditions can cause these declines, whether through a change in survival rates or reproductive success rates, or both. Recognizing the demographic links between climate change and population dynamics is critical to the effective management of populations at risk. I examined the direct and indirect effects of weather on adult and juvenile survival rates in a population of Song Sparrows (Melospiza melodia gouldii), for which daily local weather data and 30 years of year-round mark-recapture data is available (N=4,608). I constructed Cormack-Jolly-Seber mark-recapture models that incorporated weather covariates representing each of the hypothesized direct and indirect effects of weather on survival. I compared the fit of these models to a structural model, lacking any weather covariates, to identify those covariates that improved model fit. I then used the projected changes in local climate to estimate the expected change in average adult and juvenile survival rates. 

Results/Conclusions

Annual adult survival rates were best described by the direct effects of unusual breeding season rain and winter weather conditions. In contrast, annual juvenile survival rates were best described by the indirect, lagged effect of winter weather on conditions in the following breeding season, into which they hatch. Under projected future climate scenarios, these results suggest that the average adult survival rate may increase while the average juvenile survival rate may decrease, primarily due to warmer winter temperatures. In summary, adults and juveniles are sensitive to changing weather conditions in different ways, and may respond in opposite directions to a changing climate. Juveniles in a population may generally be more sensitive than adults to the indirect effects of weather on annual fluctuations in habitat conditions. As these indirect effects on habitat conditions could be mitigated through management actions, identifying the mechanisms through which weather affects demographic rates is essential to developing climate change adaptation plans that could slow or minimize the effects of climate change for a population of conservation concern.