1. The huge changes in population sizes of Arctic-nesting geese offer a great opportunity to study population limitation in migratory animals. In geese, population limitation seems to have shifted from wintering to summering grounds. There, in the Arctic, climate is rapidly changing, and this may impact reproductive performance, and perhaps population size of geese, both directly (e.g. by changes in snow melt) or indirectly (e.g. by changes in trophic interactions).2. Dark-bellied brent geese (Branta bernicla bernicla L.) increased 20-fold since the 1950s. Its reproduction fluctuates strongly in concert with the 3-year lemming cycle. An earlier analysis, covering the growth period until 1988, did not find evidence for density dependence, but thereafter the population levelled off and even decreased. The question is whether this is caused by changes in lemming cycles, population density or other factors like carry-over effects.3. Breeding success was derived from proportions of juveniles. We used an information-theoretical approach to investigate which environmental factors best explained the variation in breeding success over nearly 50 years (1960–2008). We subsequently combined GLM predictions of breeding success with published survival estimates to project the population trajectory since 1991 (year of maximum population size). In this way, we separated the effects of lemming abundance and population density on population development.4. Breeding success was mainly dependent on lemming abundance, the onset of spring at the breeding grounds, and the population size of brent goose. No evidence was found for carry-over effects (i.e. effects of conditions at main spring staging site). Negative density dependence was operating at a population size above c. 200 000 individuals, but the levelling off of the population could be explained by faltering lemming cycles alone.5. Lemmings have long been known to affect population productivity of Arctic-nesting migratory birds and, more recently, possibly population dynamics of resident bird species, but this is the first evidence for effects of lemming abundance on population size of a migratory bird species. Why lemming cycles are faltering in the last two decades is unclear, but this may be associated with changes in winter climate at Taimyr Peninsula (Siberia).

ABSTRACT. Least weasels are highly specialized small-rodent predators. Despite the fact that they are hypothesized to play an important role in generating the lemming cycles, a key process for the functioning of the terrestrial tundra ecosystem, very little is known about the biology of these miniature carnivores in the Arctic. At Mys Vostochny in western Taimyr, Russia, least weasels were observed for the first time during a lemming peak in 2005, but not in two subsequent years with low lemming densities. Here we report observations about weasel signs in lemming winter nests, body condition, habitat use, and diet in summer 2008, a year when lemmings had been numerous under the snow but populations crashed before the summer, and least weasels were abundant. Stable isotope analyses revealed that weasel diet was dominated by Siberian lemmings during spring. As expected, given lower resource availability when the lemming population crashed, weight (taking into account body length) was somewhat lower in 2008 than in 2005. Tracking tunnels and trapping showed that in summer least weasels mostly used sheltered habitats such as rocky outcrops and driftwood. Together with surveys of lemming winter nests, tracking tunnels appeared to be a promising method for monitoring least weasels in the Arctic tundra.Key words: least weasel, Mustela nivalis, predator, tundra, lemming cycles, Lemmus sibiricus, lemming winter nests, tracking tunnels, monitoring, stable isotope analysis, Taimyr RÉSUMÉ. La belette pygmée est un prédateur de petits rongeurs hautement spécialisé. Malgré les hypothèses lui attribuant un rôle important pour la génération des cycles de lemmings, un processus clé pour le fonctionnement de l'écosystème terrestre de la toundra, la biologie de ce carnivore miniature est très peu connue dans l'Arctique. À Mys Vostochny, dans l'ouest du Taimyr (Russie), des belettes pygmées ont été observées pour la première fois pendant une année d'abondance de lemmings en 2005, mais pas au cours des deux années suivantes caractérisées par de faibles densités de lemmings. Nous reportons ici des observations sur les signes de belettes dans les nids d'hiver de lemmings ainsi que sur la condition corporelle, les préférences d'habitat et le régime alimentaire de belettes à l'été 2008, une année où les lemmings étaient nombreux sous la neige, mais où leur effectif a chuté avant l'été, et où les belettes pygmées étaient de nouveau nombreuses. Des analyses d'isotopes stables ont montré que le régime alimentaire des belettes était dominé par le lemming sibérien au printemps. Comme attendu, étant donné la diminution de la quantité de ressources disponibles en présence de la chute de population de lemmings, la masse (corrigée par la taille) était un peu inférieure en 2008 par rapport à celle reportée en 2005. Les résultats de pièges à empreintes et de captures ont montré que les belettes pygmées utilisaient surtout des habitats protégés comme des rochers ou des amoncellements de bois échoué. Les pièges à empreintes semblent constituer une m...

Measuring the food left in experimental trays when study organisms cease feeding on them [so-called giving-up densities (GUDs)] is an accepted technique for assessing predation risk and disturbance. However, in natural settings, accessibility and energetic harvest costs may vary spatially, and GUDs may be confounded. In this study, we assessed whether at a heterogeneous site, non-experimental GUDs could reveal the effect of disturbance. We measured initial and GUDs of tubers of Fennel Pondweed Potamogeton pectinatus, which form here the exclusive food source of Bewick's Swans Cygnus columbianus bewickii during their migratory stopover. We calculated giving-up net energy intake rates (GUNs) by correcting for biomass accessibility and foraging costs. The study area was a shallow lake consisting of nine creeks, three of which were open to the public (i.e. disturbed). GUDs in creeks open or closed to the public were not significantly different. In contrast, GUNs were generally higher in creeks open to the public, after correcting for initial net energy intake rate. The results suggest that natural GUDs may not reflect the effects of disturbance in heterogeneous habitats. When environmental differences are large within a site, GUNs may be a useful alternative as a behavioural indicator.