Estimating the contribution of survival and recruitment to large scale population dynamics

Abstract

At large spatial scales, variation in population abundance results from variation in the survival of reproducing adults and variation in the recruitment of new individuals. Which of these two parameters varies the most and how these parameters are correlated are fundamental questions if we want to understand the large-scale dynamics of such populations. I explore how Pradel’s seniority (complement of the proportion of new individuals in the population) may help to answer such questions. I show that the sign of the correlation between temporal variation in seniority and of an independent measure of population growth rate should indicate whether population growth rate is more influenced by variation in survival or by variation in recruitment. Various predictions are proposed for evaluating the degree of regulation in the population (i.e., the existence of a negative correlation between survival and recruitment). Data from the French integrated breeding bird monitoring programme, combining point count surveys, from which population growth rate is estimated, and standardized capture-recapture, allowing the estimation of survival and seniority variation, were used to evaluate the method. Patterns of variation were examined for the four most frequently captured species, using data from 32 trapping sites covering 13 years (1989-2001). For Blackcap and Chiffchaff, the pattern is consistent with population growth rate being under the additive influence of survival and recruitment. For the Reed Warbler, the population appears to be strongly regulated, but with recruitment unable to compensate entirely for survival variation. For the Blackbird, the pattern is more confused and may indicate complex population dynamics, with non-linear relationships between survival, recruitment and population growth rate. Altogether, the method appears extremely promising and is particularly suitable for large scale monitoring of breeding birds by means of ringing.