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Availability of food may play a number of different dynamical roles in rodent-vegetation systems. Consideration of a suite of rodent-vegetation models, ranging from very simple ones to a model of medium complexity tailored to a specific system (brown lemmings at Point Barrow, Alaska, USA), suggested several general principles. If vegetation grows logistically following an herbivory event (a standard assumption of previously advanced models for herbivore-plant interactions), then almost any biologically reasonable combinations of parameters characterizing rodent-vegetation systems would result in population cycles. We argue, however, that the assumption of logistic growth of the food supply may be appropriate for only a few species, such as moss-eating lemmings. The dynamics of food supply for many arvicoline (microtine) rodents may be better described by a \"linear initial regrowth\" model, which exhibits globally stable dynamics. If this is so, quantitative interactions with food supply are unlikely to explain multiannual population cycles for most boreal or temperate voles. The role of food in population dynamics, however, is not limited to its potential to generate cycles. A tritrophic model including vegetation, rodents, and their specialist predators suggests that food limitation may provide direct density dependence needed for sustained oscillations in this system (which is usually modeled by a phenomenological logistic term in the prey equation). We relate the general theory that we developed to one specific system where we have enough data to arrive at reasonable estimates for most of the parameters-brown lemmings at Point Barrow. The Barrow model exhibits oscillations of the approximately correct period and amplitude, thus giving some theoretical support to the food hypothesis. Nevertheless, we suggest that this result should be treated cautiously because key events explaining the population cycle in the model occur during winter, but winter biology of lemmings is still poorly understood.

Spatial synchrony in population dynamics can be caused by dispersal or spatially correlated variation in environmental factors like weather (Moran effect). Distinguishing between these mechanisms is challenging for natural populations, and the study of dispersal‐induced synchrony in particular has been dominated by theoretical modelling and laboratory experiments. The goal of the present study was to evaluate the evidence for dispersal as a cause of meso‐scale (distances of tens of kilometres) spatial synchrony in natural populations of the two cyclic geometrid moths Epirrita autumnata and Operophtera brumata in sub‐arctic mountain birch forest in northern Norway. To infer the role of dispersal in geometrid synchrony, we applied three complementary approaches, namely estimating the effect of design‐based dispersal barriers (open sea) on synchrony, comparing the strength of synchrony between E. autumnata (winged adults) and the less dispersive O. brumata (wingless adult females), and relating the directionality (anisotropy) of synchrony to the predominant wind directions during spring, when geometrid larvae engage in windborne dispersal (ballooning). The estimated effect of dispersal barriers on synchrony was almost three times stronger for the less dispersive O. brumata than E. autumnata. Inter‐site synchrony was also weakest for O. brumata at all spatial lags. Both observations argue for adult dispersal as an important synchronizing mechanism at the spatial scales considered. Further, synchrony in both moth species showed distinct anisotropy and was most spatially extensive parallel to the east–west axis, coinciding closely to the overall dominant wind direction. This argues for a synchronizing effect of windborne larval dispersal. Congruent with most extensive dispersal along the east–west axis, E. autumnata also showed evidence for a travelling wave moving southwards at a speed of 50–80 km/year. Our results suggest that dispersal processes can leave clear signatures in both the strength and directionality of synchrony in field populations, and highlight wind‐driven dispersal as promising avenue for further research on spatial synchrony in natural insect populations.

It is generally recognized that delayed density-dependence is responsible for cyclic population dynamics. However, it is still uncertain whether a single factor can explain why some rodent populations fluctuate according to a 3–4 yr periodicity. There is increasing evidence that predation may play a role in lemming population cycles, although this effect may vary seasonally. To address this issue, we conducted an experiment where we built a large exclosure (9 ha) to protect brown lemmings (Lemmus trimucronatus) from avian and terrestrial predators. We tested the hypothesis that predation is a limiting factor for lemmings by measuring the demographic consequences of a predator reduction during the growth and peak phases of the cycle. We assessed summer (capture-mark-recapture methods) and winter (winter nest sampling) lemming demography on two grids located on Bylot Island, Nunavut, Canada from 2008 to 2015. The predator exclosure became fully effective in July 2013, allowing us to compare demography between the control and experimental grids before and during the treatment. Lemming abundance, survival and proportion of juveniles were similar between the two grids before the treatment. During the predator-reduction period, summer densities were on average 1.9× higher inside the experimental grid than the control and this effect was greatest for adult females and juveniles (densities 2.4× and 3.4× higher, respectively). Summer survival was 1.6× higher on the experimental grid than the control whereas body mass and proportion of juveniles were also slightly higher. Winter nest densities remained high inside the predator reduction grid following high summer abundance, but declined on the control grid. These results confirm that predation limits lemming population growth during the summer due to its negative impact on survival. However, it is possible that in winter, predation may interact with other factors affecting reproduction and ultimately population cycles.

We examined logistic range dynamics of three cyclic subpopulations of migratory barren-ground caribou in northern Canada (Qamanirjuaq, Bathurst, and George River). We used time series census data from each subpopulation cycle to project numbers (Nt), calculate subpopulation annual finite rates of population increase (λt), and estimate the corresponding time series of range condition or carrying capacity (Kt) using an algebraically rearranged version of the discrete logistic growth equation. Range condition varied regularly over each cycle, presumably due to seasonal overgrazing and range recovery dynamics. Maximum and minimum annual rates of increase and decline for Qamanirjuaq caribou were 1.196 and 0.836. In contrast, maximum annual subpopulation growth rates for the Bathurst and George River herds were greater than intrinsically possible, indicating that immigration was a component of the irruption period of their recoveries. Subpopulation numbers for Qamanirjuaq, Bathurst, and George River barren-ground caribou subpopulations closely tracked carrying capacity throughout their cycles, with mean lag times of 3.95 (SE = 0.15), 3.65 (SE = 0.18), and 3.39 (SE = 0.19) years, respectively. Other factors appear to be of relatively minor or transitory importance to population growth for barren-ground caribou if barren-ground caribou are truly a logistic growth species. Range recovery and population increase did not occur until caribou numbers declined to a recovery threshold number (Qamanirjuaq = 41,971; Bathurst = 18,265; George River = 3141). Predator management and restrictive harvest practices during the low portion of the caribou cycle may unintentionally extend the time required for caribou to decline below the grazing threshold, and thus prolong the period of scarcity. Immigration from adjacent subpopulations played a role in the acceleration of the irruption period in the Bathurst and George River subpopulations, but not the Qamanirjuaq subpopulation. Once the subpopulation range begins to recover, the rapid recovery of subpopulation numbers suggests that other density-dependent and density-independent factors are of relatively minor importance compared to range condition. Continuation of barren-ground caribou cycles at historical levels is likely if habitat conservation measures are adopted so that annual migration patterns are not disrupted, summer and winter range remain undisturbed, and the natural decline of caribou to the threshold for range recovery is not artificially extended. Nous avons examiné la dynamique logistique de l’aire de répartition de trois sous-populations cycliques de caribous de la toundra migrateurs dans le Nord canadien (Qamanirjuaq, Bathurst et rivière George). Nous avons utilisé les données de recensement de séries chronologiques de chacun des cycles de sous-population pour projeter les effectifs (Nt), calculer les taux finis annuels de l’accroissement démographique des sous-populations (λt) et estimer les séries chronologiques correspondantes de l’état des aires de répartition ou de la capacité limite (Kt) à l’aide d’une version algébriquement réarrangée de l’équation de croissance logistique discrète. L’état des aires de répartition variait régulièrement pour chaque cycle, vraisemblablement en raison du surpâturage saisonnier et de la dynamique du rétablissement des aires de répartition. Les taux annuels maximaux et minimaux de croissance et de décroissance des caribous de Qamanirjuaq s’établissaient à 1,196 et à 0,836. Par contraste, les taux de croissance maximaux annuels des sous-populations des hardes de Bathurst et de la rivière George étaient plus élevés qu’intrinsèquement possible, ce qui indique que l’immigration constituait une composante de la période de pullulation de leurs rétablissements. Les effectifs des sous-populations de caribous de la toundra de Qamanirjuaq, de Bathurst et de la rivière George suivaient de près la capacité limite tout au long de leurs cycles, avec des temps de réponse moyens de 3,95 (écart-type = 0,15), de 3,65 (écart-type = 0,18) et de 3,39 (écart-type = 0,19) années, respectivement. D’autres facteurs semblent avoir une importance relativement mineure ou transitoire en matière de croissance de la population de caribous de la toundra si ces derniers sont vraiment une espèce à croissance logistique. Le rétablissement des aires de répartition et l’accroissement de la population ne se sont pas produits tant que les effectifs de caribous n’ont pas diminué pour atteindre un seuil de rétablissement (Qamanirjuaq = 41 971; Bathurst = 18 265; rivière George = 3 141). La gestion des prédateurs et les pratiques de récolte restrictive durant la partie basse du cycle des caribous peuvent avoir pour effet involontaire de prolonger le temps nécessaire au décroissement des populations de caribous sous le seuil de pâturage, et, par conséquent, de prolonger la période de rareté des caribous. L’immigration à partir des sous-populations adjacentes a joué un rôle dans l’accélération de la période de pullulation des sous-populations de Bathurst et de la rivière George, mais pas dans celle de la sous-population de Qamanirjuaq. Lorsque l’aire de répartition d’une sous-population commence à se rétablir, le rétablissement rapide des effectifs des sous-populations suggère que d’autres facteurs dépendants et indépendants de la densité revêtent une importance relativement mineure comparativement à l’état de l’aire de répartition. La continuation des cycles de caribous de la toundra à des niveaux historiques est probable si des mesures de conservation de l’habitat sont adoptées, de sorte que les régimes migratoires annuels ne soient pas perturbés, que les aires de répartition estivales et hivernales demeurent non perturbées et que le décroissement naturel des effectifs de caribou jusqu’au seuil de rétablissement pour les aires de répartition visées ne soit pas artificiellement prolongé.

The causes of cyclical fluctuations in animal populations remain a controversial topic in ecology. Food limitation and predation are two leading hypotheses to explain small mammal population dynamics in northern environments. We documented the seasonal timing of the decline phases and demographic parameters (survival and reproduction) associated with population changes in lemmings, allowing us to evaluate some predictions from these two hypotheses. We studied the demography of brown lemmings (Lemmus trimucronatus), a species showing 3‐ to 4‐year population cycles in the Canadian Arctic, by combining capture–mark–recapture analysis of summer live‐trapping with monitoring of winter nests over a 10‐year period. We also examined the effects of some weather variables on survival. We found that population declines after a peak occurred between the summer and winter period and not during the winter. During the summer, population growth was driven by change in survival, but not in fecundity or proportion of juveniles, whereas in winter population growth was driven by changes in late summer and winter reproduction. We did not find evidence for direct density dependence on summer demographic parameters, though our analysis was constrained by the paucity of data during the low phase. Body mass, however, was highest in peak years. Weather effects were detected only in early summer when lemming survival was positively related to snow depth at the onset of melt but negatively related to rainfall. Our results show that high mortality causes population declines of lemmings during summer and fall, which suggests that predation is sufficient to cause population crashes, whereas high winter fecundity is the primary factor leading to population irruptions. The positive association between snow depth and early summer survival may be due to the protective cover offered by snow against predators. It is still unclear why reproduction remains low during the low phase.

The sudden interruption of recurring larch budmoth (LBM; Zeiraphera diniana or griseana Gn.) outbreaks across the European Alps after 1982 was surprising, because populations had regularly oscillated every 8–9 years for the past 1200 years or more. Although ecophysiological evidence was limited and underlying processes remained uncertain, climate change has been indicated as a possible driver of this disruption. An unexpected, recent return of LBM population peaks in 2017 and 2018 provides insight into this insect’s climate sensitivity. Here, we combine meteorological and dendrochronological data to explore the influence of temperature variation and atmospheric circulation on cyclic LBM outbreaks since the early 1950s. Anomalous cold European winters, associated with a persistent negative phase of the North Atlantic Oscillation, coincide with four consecutive epidemics between 1953 and 1982, and any of three warming-induced mechanisms could explain the system’s failure thereafter: (1) high egg mortality, (2) asynchrony between egg hatch and foliage growth, and (3) upward shifts of outbreak epicentres. In demonstrating that LBM populations continued to oscillate every 8–9 years at sub-outbreak levels, this study emphasizes the relevance of winter temperatures on trophic interactions between insects and their host trees, as well as the importance of separating natural from anthropogenic climate forcing on population behaviour.

Wild mammal populations exhibit a variety of dynamics, ranging from fairly stable with little change in population size over time to high-amplitude cyclic or erratic fluctuations. A persistent question in population ecology is why populations fluctuate as they do. Answering this seemingly simple question has proven to be challenging. Broadly, density-dependent feedback mechanisms should allow populations to grow at low density and slow or halt growth at high density. However, experimental tests of what demographic processes result in density-dependent feedback and on what timescale have proven elusive. Here, we used replicated density perturbation experiments and capture-mark-recapture analyses to test density-dependent population growth in populations of meadow voles (Microtus pennsylvanicus) during the summer breeding season by manipulating founding population density and observing the pattern of survival, reproduction, and population growth. High population density had no consistent effect on survival rates but generally negatively influenced recruitment and population growth rates. However, these density-dependent effects varied within the breeding season and across years. Our study provides evidence that density-dependent feedback mechanisms operate at finer time scales than previously believed and that process, additively with delayed year effects, is key to understanding multiyear population demography.

Rodent population cycles have fascinated scientists for a long time. Among various hypotheses, an interaction of an extrinsic factor (predation) with intrinsic factors (e.g., sociality and dispersal) was suggested to lead to the generation of population cycles. Here, we tested this hypothesis with an individual‐based model fully parameterized with an exceptionally rich empirical database on vole life histories. We employed a full factorial design that included models with the following factors: predation only, predation and sociality, predation and dispersal, and predation and both sociality and dispersal. A comprehensive set of metrics was used to compare results of these four models with the long‐term population dynamics of natural vole populations. Only the full model, which included both intrinsic factors and predation, yielded cycle periods, amplitudes, and autumn population sizes closest to those observed in nature. Our approach allows to model, as emergent properties of individual life histories, the sort of nonlinear density‐ and phase‐dependence that is expected to destabilize population dynamics. We suggest that the individual‐based approach is useful for addressing the effects of other mechanisms on rodent populations that operate at finer temporal and spatial scales than have been explored with models so far.

Sustainable management of wildlife populations can be aided by building models that both identify current drivers of natural dynamics and provide near-term predictions of future states. We employed a Strategic Foresight Protocol (SFP) involving stakeholders to decide the purpose and structure of a dynamic state-space model for the population dynamics of the Willow Ptarmigan, a popular game species in Norway. Based on local knowledge of stakeholders, it was decided that the model should include food web interactions and climatic drivers to provide explanatory predictions. Modeling confirmed observations from stakeholders that climate change impacts Ptarmigan populations negatively through intensified outbreaks of insect defoliators and later onset of winter. Stakeholders also decided that the model should provide anticipatory predictions. The ability to forecast population density ahead of the harvest season was valued by the stakeholders as it provides the management extra time to consider appropriate harvest regulations and communicate with hunters prior to the hunting season. Overall, exploring potential drivers and predicting short-term future states, facilitate collaborative learning and refined data collection, monitoring designs, and management priorities. Our experience from adapting a SFP to a management target with inherently complex dynamics and drivers of environmental change, is that an open, flexible, and iterative process, rather than a rigid step-wise protocol, facilitates rapid learning, trust, and legitimacy. climate change; decision-making; food web; harvesting; near-term forecasting; population cycles; stakeholders; strategic foresight.

Determining the factors driving cyclic dynamics in species has been a primary focus of ecology. For snowshoe hares (Lepus americanus), explanations of their 10-year population cycles most commonly feature direct predation during the peak and decline, in combination with their curtailment in reproduction. Hares are thought to stop producing third and fourth litters during the cyclic decline and do not recover reproductive output for several years. The demographic effects of these reproductive changes depend on the consistency of this pattern across cycles, and the relative contribution to population change of late-litter versus early litter juveniles. We used monitoring data on snowshoe hares in Yukon, Canada, to examine the contribution of late-litter juveniles to the demography of their cycles, by assigning litter group for individuals caught in autumn based on body size and capture date. We found that fourth-litter juveniles occur consistently during the increase phase of each cycle, but are rare and have low over-winter survival (0.05) suggesting that population increase is unlikely to be caused by their occurrence. The proportion of third-litter juveniles captured in the autumn remains relatively constant across cycle phases, while over-winter survival rates varies particularly for earlier-litter juveniles (0.14–0.39). Juvenile survival from all litters is higher during the population increase and peak, relative to the low and decline. Overall, these results suggest that the transition from low phase to population growth may stem in large part from changes in juvenile survival as opposed to increased reproductive output through the presence of a 4th litter.