Episodes of opposing survival and reproductive selection cause strong fluctuating selection on seasonal migration versus residence

Abstract

Quantifying temporal variation in sex-specific selection on key ecologically relevant traits, and quantifying how such variation arises through synergistic or opposing components of survival and reproductive selection, is central to understanding eco-evolutionary dynamics, but rarely achieved. Seasonal migration versus residence is one key trait that directly shapes spatio-seasonal population dynamics in spatially and temporally varying environments, but temporal dynamics of sex-specific selection have not been fully quantified. We fitted multi-event capture–recapture models to year-round ring resightings and breeding success data from partially migratory European shags ( Phalacrocorax aristotelis ) to quantify temporal variation in annual sex-specific selection on seasonal migration versus residence arising through adult survival, reproduction and the combination of both (i.e. annual fitness). We demonstrate episodes of strong and strongly fluctuating selection through annual fitness that were broadly synchronized across females and males. These overall fluctuations arose because strong reproductive selection against migration in several years contrasted with strong survival selection against residence in years with extreme climatic events. These results indicate how substantial phenotypic and genetic variation in migration versus residence could be maintained, and highlight that biologically important fluctuations in selection may not be detected unless both survival selection and reproductive selection are appropriately quantified and combined.

Seasonal migration as a life history trait facilitating adaptation to climate change

“…In European shags (Phalacrocorax aristotelis), harsh climatic events in winter contributed to mortality and influenced natural selection on the migratory phenotype of individuals, supporting the idea that micro-evolutionary processes can influence the composition of populations with respect to migratory phenotype and affect the breeding distribution of species (Acker et al, 2021a;2021b). Our results underscore this idea by demonstrating thematic links between climatic, population demography, species distribution, and the evolution of seasonal migration in response to historic and ongoing climate change (Aitken et al, 2008;Sexton et al, 2009;Hendry et al, 2018;Bay et al, 2018).…”

“…Predicting how species distribution may shift in response to climate change will depend in part on how life-history traits such as seasonal migration versus residence influence species occupancy in response to seasonality and resource availability via their influence on individual fitness and population demography (e.g., Reid et al 2018, Visty et al 2018, Acker et al, 2021a2021b). Accordingly, we addressed uncertainties in the development and application of species distribution models, their empirical basis, and the reliability of their predictions whilst also exploring the potential for variation in migration to facilitate climate adaptation in a highly polytypic, mobile species.…”

“…Such transitions can elevate population growth rate in newly-established resident as compared to migrant subpopulations by enhancing over-winter survival and annual reproductive rate (Visty et al, 2018), and similar trade-offs involving individual fitness and climate can drive variation in migratory phenotype across populations. For example, Acker et al (2021a;2021b) reported context-dependent fitness of residents and migrants in European shags (Phalacrocorax aristotelis), wherein selection favored migrants in severe winters but residents in milder years.…”

Modelling the responses of partially migratory metapopulations to changing seasonal migration rates: From theory to data

“…As shags typically first breed aged 3 years (Aebischer et al, 1995), we formulate M life considering three stages (Figure 3): yearlings ($$$ a=1 $$$, 1–2 years), sub‐adults ($$$ a=2 $$$, 2–3 years) and breeding adults ($$$ a=3 $$$, $$$ \ge 3 $$$ years). We take previously estimated values of local breeding success for residents and migrants ($$$ {f}_k $$$) and seasonal survival probabilities ($$$ {s}_{abk} $$$; Acker, Burthe, et al, 2021; Acker, Daunt, et al, 2021; Grist et al, 2017; Reid et al, 2020). As seasonal movement survival probabilities ($$$ {w}_{abk} $$$) have not previously been explicitly estimated, we derive an estimate from the maximum observed difference in annual survival between residents and migrants (Appendix S7) and assume that $$$ w $$$ is season‐ and site‐independent but age‐dependent ($$$ {w}_a $$$).…”

Components of micro-evolutionary and phenotypic change in seasonal migration versus residence in a wild population

“…During breeding seasons (April–June, ‘summers’), intensive monitoring of all nests and adjacent roost sites on IoM generated very high overall summer resighting probabilities of adults (0.90–0.98, mean 0.95 across 2010–2018), including identification of most ringed nest owners (∼95% of all breeders in 2009–2021; Acker et al 2021a,b). Most adults were sexed through vocalizations and/or genotyping (Acker et al 2021a).…”