The Demographic Basis of Population Growth: A 32-Year Transient Life Table Response Experiment

It has recently been recognised that populations are rarely in demographic equilibrium, but rather in a 'transient' state. To examine how transient dynamics influence our empirical understanding of the links between changes in demographic rates and population growth, we conducted a 32-year study of Columbian ground squirrels. The population increased rapidly for 10 years, followed by a 2-year crash, and a gradual 19-year recovery. Transient life table response experiment (LTRE) analysis showed that demographic stochasticity accounted for approximately one-fourth of the variation in population growth, leaving the majority to be explained by environmental influences. These relatively small rodents appeared to have a slow pace of life. But unlike the general pattern for large mammals with slow life histories, ground squirrel survival did not exhibit low variation associated with environmental 'buffering'; instead, survival varied substantially over time and contributed substantially (78%) to changes in abundance over the long-term study, with minor contributions from reproduction and unstable stage structure.