No evidence for fitness signatures consistent with increasing trophic mismatch over 30years in a population of European shag Phalacrocorax aristotelis.

Katharine Keogan,Francis Daunt,Sarah Burthe,Sarah Wanless,Richard A Phillips,Sue Lewis,Albert B Phillimore,Richard J Howells,Mark A Newell,Michael P Harris

doi:10.1111/1365-2656.13376

Abstract

As temperatures rise, timing of reproduction is changing at different rates across trophic levels, potentially resulting in asynchrony between consumers and their resources. The match–mismatch hypothesis (MMH) suggests that trophic asynchrony will have negative impacts on average productivity of consumers. It is also thought to lead to selection on timing of breeding, as the most asynchronous individuals will show the greatest reductions in fitness.Using a 30‐year individual‐level dataset of breeding phenology and success from a population of European shags on the Isle of May, Scotland, we tested a series of predictions consistent with the hypothesis that fitness impacts of trophic asynchrony are increasing.These predictions quantified changes in average annual breeding success and strength of selection on timing of breeding, over time and in relation to rising sea surface temperature (SST) and diet composition.Annual average (population) breeding success was negatively correlated with average lay date yet showed no trend over time, or in relation to increasing SST or the proportion of principal prey in the diet, as would be expected if trophic mismatch was increasing. At the individual level, we found evidence for stabilising selection and directional selection for earlier breeding, although the earliest birds were not the most productive. However, selection for earlier laying did not strengthen over time, or in relation to SST or slope of the seasonal shift in diet from principal to secondary prey. We found that the optimum lay date advanced by almost 4 weeks during the study, and that the population mean lay date tracked this shift.Our results indicate that average performance correlates with absolute timing of breeding of the population, and there is selection for earlier laying at the individual level. However, we found no fitness signatures of a change in the impact of climate‐induced trophic mismatch, and evidence that shags are tracking long‐term shifts in optimum timing. This suggests that if asynchrony is present in this system, breeding success is not impacted. Our approach highlights the advantages of examining variation at both population and individual levels when assessing evidence for fitness impacts of trophic asynchrony.

Highlights

In recent decades, surface temperatures around the globe have risen (Stocker et al, 2013), causing the timing of seasonally recurring life-history events, such as reproduction, to shift (Thackeray et al, 2010; Visser & Both, 2005)
We found that between-years mean breeding success declined significantly with mean lay date (BSp slope = −0.035, 95% Credible Interval [credible intervals (CIs)]: −0.052, −0.016; Figure 3a; Table S2 in Appendix 1), from close to two chicks fledging in the earliest years to about 0.5 in the latest
Our study supports the widespread finding that timing of breeding correlates with both breeding success at the population level and individual fitness, which is why it is extensively used to quantify the extent to which organisms respond to environmental change (Visser & Gienapp, 2019)

Summary

| INTRODUCTION

Surface temperatures around the globe have risen (Stocker et al, 2013), causing the timing of seasonally recurring life-history events, such as reproduction, to shift (Thackeray et al, 2010; Visser & Both, 2005). Fitness signatures consistent with the MMH include stronger directional selection on timing of breeding over time (Figure 1f), with increasing temperature (Figure 1f) and when average decline in proportion of principal prey within a season is steeper (Figure 1j). If mismatch has increased over time, the fitness signatures that would be consistent with this trend are that mean annual population-level breeding success will have declined, and strength of selection on relative lay date within a season will have increased (a) over the course of the study, (b) with increasing SST and (c) with a decline in the proportion of the principal prey, 1+ sandeels. The environmental sensitivity of selection is a key parameter for predicting the ability of populations to cope with climate change (Chevin et al, 2010; Gienapp et al, 2013; Vedder et al, 2013)

| MATERIALS AND METHODS

Findings

| DISCUSSION

| CONCLUSIONS