Survival in macaroni penguins and the relative importance of different drivers: individual traits, predation pressure and environmental variability

Abstract

Understanding the demographic response of free-living animal populations to different drivers is the first step towards reliable prediction of population trends.Penguins have exhibited dramatic declines in population size, and many studies have linked this to bottom-up processes altering the abundance of prey species. The effects of individual traits have been considered to a lesser extent, and top-down regulation through predation has been largely overlooked due to the difficulties in empirically measuring this at sea where it usually occurs.For 10 years (2003–2012), macaroni penguins (Eudyptes chrysolophus) were marked with subcutaneous electronic transponder tags and re-encountered using an automated gateway system fitted at the entrance to the colony. We used multistate mark–recapture modelling to identify the different drivers influencing survival rates and a sensitivity analysis to assess their relative importance across different life stages.Survival rates were low and variable during the fledging year (mean = 0·33), increasing to much higher levels from age 1 onwards (mean = 0·89). We show that survival of macaroni penguins is driven by a combination of individual quality, top-down predation pressure and bottom-up environmental forces. The relative importance of these covariates was age specific. During the fledging year, survival rates were most sensitive to top-down predation pressure, followed by individual fledging mass, and finally bottom-up environmental effects. In contrast, birds older than 1 year showed a similar response to bottom-up environmental effects and top-down predation pressure.We infer from our results that macaroni penguins will most likely be negatively impacted by an increase in the local population size of giant petrels. Furthermore, this population is, at least in the short term, likely to be positively influenced by local warming. More broadly, our results highlight the importance of considering multiple causal effects across different life stages when examining the survival rates of seabirds.

Unravelling the relative roles of top‐down and bottom‐up forces driving population change in an oceanic predator

“…Despite this, the estimates of survival from the state‐space model predominantly occurred within the confidence interval of the covariate model that was reported in our earlier study (Horswill et al. 2014), with lower values of survival estimated for the older age class during 2003 and 2004 (Fig. 1B,C).…”

“…In agreement with Horswill et al. (2014), survival rates were generally lower during the fledgling year, and increased from age one. However, under certain conditions (low predation pressure), the survival rates of the two age classes were comparable (Fig.…”

“…Survival data were obtained from a mark–recapture study conducted between 2003 and 2012 based on passive integrated transponders (PIT; Horswill et al. 2014). To remove any temporal correlation introduced by the covariates that were retained in this earlier study, the estimates of survival utilized in the current study were reestimated from a model without covariates, that incorporated age and time‐specific variation and year as a nominal factor variable (recapture and transition rates were specified in agreement with the best candidate model reported by Horswill et al.…”

“…The mean age of recruitment was therefore taken to correspond with the maximum age of first return for this population (4 yr; Horswill et al. 2014). In the absence of evidence for a significant sex difference in survival rates (Horswill 2015) we assumed a 1:1 sex ratio in the population.…”

“…2006), the study population did not fluctuate widely between years, and the PIT reader used in this study achieved very high recapture rates when it was fully operational (99%; Horswill et al. 2014). Consequently, it appears that intermittent breeding was not a common source of variation during the study period.…”