Full circumpolar migration ensures evolutionary unity in the Emperor penguin

Abstract

Defining reliable demographic models is essential to understand the threats of ongoing environmental change. Yet, in the most remote and threatened areas, models are often based on the survey of a single population, assuming stationarity and independence in population responses. This is the case for the Emperor penguin Aptenodytes forsteri, a flagship Antarctic species that may be at high risk continent-wide before 2100. Here, using genome-wide data from the whole Antarctic continent, we reveal that this top-predator is organized as one single global population with a shared demography since the late Quaternary. We refute the view of the local population as a relevant demographic unit, and highlight that (i) robust extinction risk estimations are only possible by including dispersal rates and (ii) colony-scaled population size is rather indicative of local stochastic events, whereas the species' response to global environmental change is likely to follow a shared evolutionary trajectory.

Figure 1. Drift and mixing concur to keep a high degree of worldwide homogeneity in the Emperor penguin.

All currently known Emperor penguin colonies are represented by orange dots; sampled colonies by coloured circles. (a) Estimated pairwise migration rates (outer coloured links) and Fst (inner dashed links) between sampled colonies. Migrations as estimated from joint allele-frequency spectra. Colours refer to the receiving populations (DDU, Pointe Géologie near Dumont D'Urville station; HAL, Halley Bay; MZW, Mertz West; MZE, Mertz East; NEU, Neumayer; WSH, Cape Washington). Dashed circle: Antarctic polar circle (S66°33'). (b) Neighbour-net based on pairwise identity-by-state between all sampled individuals. Branch lengths represent pairwise Hamming distances. Colours correspond to colonies on map a.

Figure 2. Demography is a matter of scale.

(a) Demographic reconstructions (extended bayesian skyline plots), for all samples, and per-colony (ALL, all sampled colonies pooled together; DDU, Pointe Géologie near Dumont D'Urville station; HAL, Halley Bay; MZW, Mertz West; MZE, Mertz East; NEU, Neumayer; WSH, Cape Washington). Solid line: mean population size. Shaded area: 95% confidence interval. Blue area: Last Glacial Maximum (LGM). (b) Census size for the six analysed colonies (from Fretwell et al. and Ancel et al.20). (c) Three different demographic timescales for DDU colony: I. coalescent-scale (EBSP reconstruction), II. monitoring-scale (from Barbraud et al. and Le Bohec, personal communication, 2016), III. recent catastrophic breeding failure: blue, eggs lost during brooding; grey, chicks found dead; orange, successfully fledged chicks (from Barbraud et al. and Le Bohec, personal communication, 2016).