A. 1995. Limitation of collared lemming population growth at low densities by predation mortality.-Oikos 73: 387-398. Populations of the collared lemming (Dicrostonyx kilangmiutak) and the tundra vole (Microtus oeconomus) have been at consistently low densities, and non-cyclic, at Pearce Point, Northwest Territories, Canada, for six yr. In most summers population densities decline, or only increase marginally, despite ongoing reproduction. We investigated the hypothesis that predation mortality is sufficient and necessary to curtail lemming population growth in summer. To test predictions of the hypothesis, we compared lemming demography, using mark-recapture and radiotelemetry, on a population from which predators were excluded (PE), using a perimeter fence and aerial mesh of nylon (11.4 ha), with demographies of three control populations (18-25 ha). Predation was the proximate cause of the majority of adult and neo-natal mortality, and was not replaced in a compensatory fashion by any other mortality factor in the PE. Significantly fewer adult lemmings died in the PE, and consequently survival inside the PE was significantly enhanced. Recruitment of juvenile lemmings to the adult population was significantly higher in the PE on a per unit area basis. The lemming population in PE followed a significantly different trajectory than the control populations in 1990 and 1991, remaining fairly stable while controls declined. However, the protected population did not grow, apparently because of juvenile dispersal: telemetered juveniles dispersed at an average rate of 53 m/d within the first ten d after weaning. We believe that the protected area was too small to encompass such dispersal, and that emigrants were not replaced by immigrating juveniles since the latter faced heavy mortality outside the exclosure. In 1992, numbers on PE and all controls grew, in conjunction with a regional absence of rough-legged hawks (Buteo lagopus) and a scarcity of red foxes (Vulpes vulpes), the two principal microtine predators. Tundra vole population growth was also limited by predation mortality, but to a lesser extent. We conclude that predation mortality is sufficient and necessary to limit summer population growth in these microtine species.

Canopy-forming shrubs are reported to be increasing at sites around the circumpolar Arctic. Our results indicate expansion in canopy cover and height of willows on Herschel Island located at 70 degrees north on the western Arctic coast of the Yukon Territory. We examined historic photographs, repeated vegetation surveys, and conducted monitoring of long-term plots and found evidence of increases of each of the dominant canopy-forming willow species (Salix richardsonii, Salix glauca and Salix pulchra), during the twentieth century. A simple model of patch initiation indicates that the majority of willow patches for each of these species became established between 1910 and 1960, with stem ages and maximum growth rates indicating that some patches could have established as late as the 1980s. Collectively, these results suggest that willow species are increasing in canopy cover and height on Herschel Island. We did not find evidence that expansion of willow patches is currently limited by herbivory, disease, or growing conditions.

Population fluctuations may occur in synchrony among several rodent species at a given site, and they may occur in synchrony over large geographical areas. We summarize information on synchrony in lemmings and voles from the Canadian Arctic for the past 20 years. The most detailed available information is from the central Canadian Arctic, where snap-trap samples have been taken annually at several sites for periods of up to 15 years. Geographical synchrony in the same species among different sites was strong, especially for the central and eastern Canadian Arctic. Synchrony among different species at a given site was also generally high. When one species is at high density, densities of all species at that site tend to be high. These results do not easily fit the mobile-predator hypothesis proposed to explain regional synchrony, and are more consistent with the weather hypothesis, which we suggest both entrains synchrony among sites and enforces synchrony among species within a site. We tentatively support the weather hypothesis for geographical synchrony in lemmings, and recommend the establishment of a circumpolar program to monitor lemming cycles and predator movements that would advance our understanding of these large-scale patterns of cyclic synchrony.Résumé : Les densités de plusieurs espèces de rongeurs peuvent fluctuer en même temps à un endroit donné et ces fluctuations peuvent aussi être synchrones sur une échelle géographique très grande. Nous résumons ici des informations sur le synchronisme des lemmings et des campagnols dans l'arctique canadien depuis 20 ans. Les informations les plus detaillées proviennent de la région centrale de l'arctique, où des recensements annuels par trappage destructif ont été effectués à plusieurs sites pour une période allant jusqu'à 15 ans. Pour chaque espèce, le synchronisme géogra-phique entre les différents sites est très fort entre les différents sites, et plus spécialement aux sites du centre et de l'est de l'arctique canadien. De même, le synchronisme entre les différentes espèces d'un même site est habituellement fort. Quand la densité d'une espèce devient haute, les densités de toutes les espèces à cet endroit ont aussi tendance à augmenter. Ces résultats sur le synchronisme géographique sont difficilement attribuables à l'hypothèse des prédateurs mobiles, mais semblent plutôt reliés à l'hypothèse météorologique selon laquelle les conditions météorologiques entraînent le synchronisme entre les sites et, de plus, forcent les espèces à être synchrones à un même site. Nous croyons pouvoir supporter l'hypothèse météorologique dans le cas du synchronisme géographique des lemmings et recommandons un suivi à l'échelle circumpolaire du cycle des lemmings et des déplacements des prédateurs ce qui contribuerait à parfaire nos connaissances sur les cycles synchrones à grande échelle. Krebs et al. 1333

Abstract:The Swedish Tundra Northwest Expedition of 1999 visited 17 sites throughout the Canadian Arctic. At 12 sites that were intensively sampled we estimated the standing crop of plants and the densities of herbivores and predators with an array of trapping, visual surveys, and faecal-pellet transects. We developed a trophic-balance model using ECOPATH to integrate these observations and determine the fate of primary and secondary production in these tundra ecosystems, which spanned an 8-fold range of standing crop of plants. We estimated that about 13% of net primary production was consumed by herbivores, while over 70% of small-herbivore production was estimated to flow to predators. Only 9% of large-herbivore production was consumed by predators. Organization of Canadian Arctic ecosystems appears to be more top-down than bottom-up. Net primary production does not seem to be herbivore-limited at any site. This is the first attempt to integrate trophic dynamics over the entire Canadian Arctic.Résumé : En 1999, l'Expédition suédoise de la toundra du nord-ouest à échantillonné 17 sites à travers l'arctique canadien. Nous avons utilisé différentes méthodes de trappage, des recensements visuels et des transects de décomptes de fèces pour estimer la biomasse végétale, ainsi que les densités des herbivores et des prédateurs aux 12 sites inventoriés plus en détail. Nous avons développé un modèle d'équilibre trophique à l'aide d'ECOPATH pour intégrer ces observations et déterminer le sort des productions primaire et secondaire de ces écosystèmes de toundra, entre lequels la biomasse végétale varie par un facteur de 8. Nous estimons qu'environ 13 % de la production primaire nette est consommée par les herbivores, tandis que plus de 70 % de la production des petits mammifères est consommée par les prédateurs. Seulement 9 % de la production des grands herbivores est consommée par les prédateurs. Ces écosystèmes semblent façonnés plus par les effets trophiques descendants (top-down) que par les effects trophiques ascendants (bottom-up). La production primaire nette de ces sites ne semble pas limitée par les herbivores. Notre étude constitue la première tentative pour intégrer les la dynamique trophique sur l'ensemble de l'arctique canadien. Krebs et al. 843

White spruce ( Picea glauca (Moench) Voss) cone crops were measured from 1986 to 2011 in the Kluane region of southwestern Yukon to test the hypothesis that the size of cone crops could be predicted from spring and summer temperature and rainfall of years t, t – 1, and t – 2. We counted cones in the top 3 m of an average of 700 white spruce trees each year spread over 3–14 sites along 210 km of the Alaska Highway and the Haines Highway. We tested the conventional explanation for white spruce cone crops that implicates summer temperatures and rainfall in years t and t – 1 and rejected it, since it explained very little of the variation in our 26 years of data. We used exploratory data analysis with robust multiple regressions coupled with Akaike’s information criterion corrected (AICc) analysis to determine the best statistical model to predict the size of cone crops. We could statistically explain 54% of the variation in cone crops from July and August temperatures of years t – 1 and t – 2 and May precipitation of year t – 2. There was no indication of a periodicity in cone crops, and years of large cone crops were synchronous over the Kluane region with few exceptions. This is the first quantitative model developed for the prediction of white spruce cone crops in the Canadian boreal forest and has the surprising result that weather conditions 2 years prior to the cone crop are the most significant predictors.