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RESEARCH PRODUCT

Demographic responses of a site-faithful and territorial predator to its fluctuating prey: long-tailed skuas and arctic lemmings.

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Summary1. Environmental variability, through interannual variation in food availability or climaticvariables, is usually detrimental to population growth. It can even select for constancy in keylife-history traits, though some exceptions are known. Changes in the level of environmentalvariability are therefore important to predict population growth or life-history evolution.Recently, several cyclic vole and lemming populations have shown large dynamical changesthat might affect the demography or life-histories of rodent predators.2. Skuas constitute an important case study among rodent predators, because of theirstrongly saturating breeding productivity (they lay only two eggs) and high degree of sitefidelity, in which they differ from nomadic predators raising large broods in good rodentyears. This suggests that they cannot capitalize on lemming peaks to the same extent asnomadic predators and might be more vulnerable to collapses of rodent cycles.3. We develop a model for the population dynamics of long-tailed skuas feeding on lemmingsto 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-tailedskua 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 expec-tancy, which buffers the impact of adult abundance’s decrease on the population reproductiveoutput.4. The size of the floater compartment is affected by changes in both mean and coefficient ofvariation 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 benefitfrom lemming variability; a positive effect of environmental variation.5. Long-tailed skua populations are strongly adapted to fluctuating lemming populations, aninstance of demographic lability in the reproduction rate. They are also little affected by poorlemming periods, if there are enough floaters, or juveniles disperse to neighbouring popula-tions. The status of Greenland skua populations therefore strongly depends upon floaternumbers and juvenile movements, which are not known. This reveals a need to intensifycolour-ringing efforts on the long-tailed skua at a circumpolar scale.Key-words: demographic buffering, environmental variance, floaters, population cycles,territoriality