title: Demographic responses of a site-faithful and territorial predator to its fluctuating prey: long-tailed skuas and arctic lemmings.
abstract: Environmental variability, through interannual variation in food availability or climatic variables, is usually detrimental to population growth. It can even select for constancy in key life-history traits, though some exceptions are known. Changes in the level of environmental variability are therefore important to predict population growth or life-history evolution. Recently, several cyclic vole and lemming populations have shown large dynamical changes that might affect the demography or life-histories of rodent predators. Skuas constitute an important case study among rodent predators, because of their strongly saturating breeding productivity (they lay only two eggs) and high degree of site fidelity, in which they differ from nomadic predators raising large broods in good rodent years. This suggests that they cannot capitalize on lemming peaks to the same extent as nomadic predators and might be more vulnerable to collapses of rodent cycles. We develop a model for the population dynamics of long-tailed skuas feeding on lemmings to assess the demographic consequences of such variable and non-stationary prey dynamics, based on data collected in NE Greenland. The model shows that populations of long-tailed skua sustain well changes in lemming dynamics, including temporary collapses (e.g. 10 years). A high floater-to-breeder ratio emerges from rigid territorial behaviour and a long-life expectancy, which buffers the impact of adult abundance's decrease on the population reproductive output. The size of the floater compartment is affected by changes in both mean and coefficient of variation of lemming densities (but not cycle amplitude and periodicity per se). In Greenland, the average lemming density is below the threshold density required for successful breeding (including during normally cyclic periods). Due to Jensen's inequality, skuas therefore benefit from lemming variability; a positive effect of environmental variation. Long-tailed skua populations are strongly adapted to fluctuating lemming populations, an instance of demographic lability in the reproduction rate. They are also little affected by poor lemming periods, if there are enough floaters, or juveniles disperse to neighbouring populations. The status of Greenland skua populations therefore strongly depends upon floater numbers and juvenile movements, which are not known. This reveals a need to intensify colour-ringing efforts on the long-tailed skua at a circumpolar scale.
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø, Tromsø, 9037, Norway.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Barraquand
authors.affiliation: Arctic Research Centre, Aarhus University, Aarhus, DK-8000, Denmark.|Department of Bioscience, Aarhus University, Rønde, DK-8410, Denmark.
authors.forename: Toke T
authors.initials: TT
authors.lastname: Høye
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø, Tromsø, 9037, Norway.
authors.forename: John-André
authors.initials: JA
authors.lastname: Henden
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø, Tromsø, 9037, Norway.
authors.forename: Nigel G
authors.initials: NG
authors.lastname: Yoccoz
authors.affiliation: Laboratoire Biogéosciences, UMR CNRS 5561, Université de Bourgogne, Dijon, 21000, France.|Groupe de Recherche en Ecologie Arctique, Francheville, 21440, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Gilg
authors.affiliation: Arctic Research Centre, Aarhus University, Aarhus, DK-8000, Denmark.|Department of Bioscience, Aarhus University, Roskilde, DK-4000, Denmark.
authors.forename: Niels M
authors.initials: NM
authors.lastname: Schmidt
authors.affiliation: Groupe de Recherche en Ecologie Arctique, Francheville, 21440, France.|Institut für Landespflege, University of Freiburg, Freiburg, 79106, Germany.
authors.forename: Benoît
authors.initials: B
authors.lastname: Sittler
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø, Tromsø, 9037, Norway.
authors.forename: Rolf A
authors.initials: RA
authors.lastname: Ims
mesh_terms: D000818:Animals; D003411:Arvicolinae / Q000502:physiology*; D046091:Charadriiformes / Q000502:physiology*; D003710:Demography; D020387:Food Chain; D006116:Greenland; D008954:Models, Biological; D011157:Population Dynamics; D011235:Predatory Behavior; D013731:Territoriality
keywords: demographic buffering; environmental variance; floaters; population cycles; territoriality