Winter Population Growth Research Articles

Snow physical properties may be a significant determinant of lemming population dynamics in the high Arctic

Cyclic population fluctuations are common in boreal and Arctic species but the causes of these cycles are still debated today. Among these species, lemmings are Arctic rodents that live and reproduce under the snow and whose large cyclical population fluctuations in the high Arctic impact the whole tundra food web. We explore, using lemming population data and snow modeling, whether the hardness of the basal layer of the snowpack, determined by rain-on-snow events (ROS) and wind storms in autumn, can affect brown lemming population dynamics in the Canadian high Arctic. Using a 7-year dataset collected on Bylot Island, Nunavut, Canada over the period 2003–2014, we demonstrate that liquid water input to snow is strongly inversely related with winter population growth (R2 ≥ 0.62) and to a lesser extent to lemming summer densities and winter nest densities (R2 = 0.29–0.39). ROS in autumn can therefore influence the amplitude of brown lemming population fluctuations. Increase in ROS events with climate warming should strongly impact the populations of lemmings and consequently those of the many predators that depend upon them. Snow conditions may be a key factor influencing the cyclic dynamics of Arctic animal populations.

Strong "bottom-up" influences on small mammal populations: State-space model analyses from long-term studies.

“Bottom‐up” influences, that is, masting, plus population density and climate, commonly influence woodland rodent demography. However, “top‐down” influences (predation) also intervene. Here, we assess the impacts of masting, climate, and density on rodent populations placed in the context of what is known about “top‐down” influences. To explain between‐year variations in bank vole Myodes glareolus and wood mouse Apodemus sylvaticus population demography, we applied a state‐space model to 33 years of catch‐mark‐release live‐trapping, winter temperature, and precise mast‐collection data. Experimental mast additions aided interpretation. Rodent numbers in European ash Fraxinus excelsior woodland were estimated (May/June, November/December). December–March mean minimum daily temperature represented winter severity. Total marked adult mice/voles (and juveniles in May/June) provided density indices validated against a model‐generated population estimate; this allowed estimation of the structure of a time‐series model and the demographic impacts of the climatic/biological variables. During two winters of insignificant fruit‐fall, 6.79 g/m2 sterilized ash seed (as fruit) was distributed over an equivalent woodland similarly live‐trapped. September–March fruit‐fall strongly increased bank vole spring reproductive rate and winter and summer population growth rates; colder winters weakly reduced winter population growth. September–March fruit‐fall and warmer winters marginally increased wood mouse spring reproductive rate and September–December fruit‐fall weakly elevated summer population growth. Density dependence significantly reduced both species' population growth. Fruit‐fall impacts on demography still appeared after a year. Experimental ash fruit addition confirmed its positive influence on bank vole winter population growth with probable moderation by colder temperatures. The models show the strong impact of masting as a “bottom‐up” influence on rodent demography, emphasizing independent masting and weather influences; delayed effects of masting; and the importance of density dependence and its interaction with masting. We conclude that these rodents show strong “bottom‐up” and density‐dependent influences on demography moderated by winter temperature. “Top‐down” influences appear weak and need further investigation.

Tidal marsh restoration stimulates the growth of winter shorebird populations in a temperate estuary

AbstractThe regional responses of winter shorebird populations in the nearly 3,000 ha estuary of Tomales Bay, California, to the restoration of 223 ha of historic tidal wetlands were evaluated for 27 years: 19 years prior to tidal reintroduction and 8 years after tidal reintroduction. We used interrupted time series analyses to measure the spatial extent of the restoration effect and to model the magnitude and length of time associated with the gradual, restoration‐induced growth of winter shorebird populations in the bay. Expanded, regional benefits of the restoration were revealed by consistent patterns of winter shorebird population growth. Eight years after tidal reintroduction, overall shorebird abundances in southern Tomales Bay nearly tripled in response to the restoration. Substantial winter population growth by most species in southern Tomales Bay was evident within 3 years after tidal reintroduction, and maximum responses to the restoration were estimated to be predominantly achieved within 8 years. In contrast to strong effects of tidal marsh restoration on winter shorebird populations in southern Tomales Bay, no significant overall responses were exhibited by shorebirds in the northern portion of the bay, although marginal evidence of expanded effects on a few species in northern Tomales Bay were suggested. The results illustrate the importance of accounting for restoration effects beyond the spatial and temporal boundaries of the restored habitat, to consider both the potentially expanded benefits and the spatial limits of those benefits to regional wildlife populations.

Long‐term pattern of population dynamics in the field vole from central Europe: cyclic pattern with amplitude dampening

AbstractThe subject of population cycles is regarded as controversial due to a number of unsettled questions such as whether or not cyclic patterns are governed by the same processes at high and low latitudes in Europe. Recent evidence suggests that the dynamics at high and low latitudes share the common temporal pattern of vole dynamics referred to as collapsing population cycles. Despite concurrent interest, the key contention around the causal mechanisms that drive population cycles remains a hot topic in ecology. The aims of this study are to supplement information on the seasonal population dynamics of the field vole Microtus agrestis in the Czech Republic by analysing 25 years of time series data. By applying robust estimation procedures, we estimated several parameters to describe population dynamics, such as population variability, amplitude dampening, cycle period, order of the dynamics and the structure of density dependence. The parameters indicate that field vole dynamics in central Europe are highly variable, cyclic dynamics of order two, with peaks in abundance occurring regularly at intervals of 4–5 years. In addition to exhibiting population cycles, the field vole populations show a pattern of dampened amplitude as observed elsewhere in Europe, including northern latitudes. By analysing temporal trends in seasonal abundances, population growth rates and environmental temperatures, we did not obtain evidence to support the hypothesis that amplitude dampening results from the negative effect of increasingly mild winters on winter population growth rates.

Seasonal demography of a cyclic lemming population in the Canadian Arctic.

1. 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. 2. 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. 3. 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. 4. 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. 5. 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. 6. 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.

Beating the odds: effects of weather on a short-season population of deer mice

We examined 11 years of data on reproductive success, survival, and population dynamics of two populations (Fortress and Grizzly) of deer mice (Peromyscus maniculatus) in the Kananaskis Valley, Alberta, to investigate the extent to which the dynamics of these populations is dictated by weather conditions. Summer population growth was not related to the population growth in the winter preceding the breeding season or to spring population density. Over the summer on the Fortress grid, population growth was positively related to adult survival, whereas on the Grizzly grid, population growth was positively related to nestling survival. Neither summer population growth nor demographic correlates of summer population growth was consistently related to weather patterns. On Fortress, adult survival during the breeding season was negatively correlated with precipitation. On Grizzly, nestling survival during the breeding season was negatively correlated with precipitation. Winter population growth was inversely proportional to the fall population density prior to the winter but neither was related to weather conditions. Climate limits seasonal breeding in these populations, but compensatory responses appear sufficient to accommodate extreme weather conditions during both the breeding and nonbreeding seasons.

Seasonal regulation in fluctuating small mammal populations: feedback structure and climate

We studied fluctuating populations of six small mammal species in the Appalachian Plateau of Pennsylvania, USA for 20 yr. We analyzed the feedback structure of these species using statistical time series models for spring and autumn abundances. All species showed a seasonal density‐dependent structure, and in five of them first‐order feedbacks were dominant in winter and summer. Instead, southern red‐backed voles (Clethrionomys gapperi) showed a different feedback structure during winter and summer. In three species (C. gapperi, Napaeozapus insignis and Peromyscus maniculatus), environmental factors were more important during summer, while the opposite pattern was found in Blarina brevicauda and Tamias striatus. Snowfall influenced positively the winter population growth rates of southern red‐backed voles, white‐footed mice, woodland jumping mice and eastern chipmunks. We found seasonal differences in the effects of the small mammals assemblage on population growth rates of the two Peromyscus species. The common feedback structure between seasons observed in most of the species, particularly among voles and mice, points to a different feedback structure from northern cyclic small mammals. We conclude that a seasonal feedback structure dominated by intra‐ and inter‐specific competitive interactions may be at the basis of the population dynamics of these species.

Limitations to collared lemming population growth in winter

We investigated the roles of ermine predation and temperature and snowfall regimes in limiting population growth of collared lemmings (Dicrostonyx kilangmiutak) at low densities (< 3/ha) in winters between 1987 and 1992 at Pearce Point, Northwest Territories, Canada. We estimated winter population growth, for four study populations, as the instantaneous weekly rate of population change from late August to early in the subsequent June. Population growth was not significantly related to the intensity of ermine predation, based on data from two winters. Variance in rates of growth among five winters was significantly explained by a combination of (i) cold intensity in autumn, i.e., the mean daily temperature in September and October, when lemmings change from summer to winter morphology, and (ii) the insulative potential of the snow, i.e., centimetre-days of snow cover per degree-day of frost from November through March, when lemmings potentially breed. Arvicoline winter nests were strongly associated with remnant snow in spring, indicating selection for areas with deepest snow. Low autumn temperatures and shallow winter snow appear to be strong limiting factors in winter, and may prolong the period of low density of Dicrostonyx populations.

Population dynamics of the collared lemming and the tundra vole at Pearce Point, Northwest Territories, Canada

From 1987 to 1989 we monitored population changes during summer of the collared lemming (Dicrostonyx groenlandicus) and the tundra vole (Microtus oeconomus) at Pearce Point, Northwest Territories, Canada (69° 48' N, 122° 40' W). Populations on four study areas did not cycle but remained at low density (<3/ha) each year and continued at low numbers for the following 3 years (Reid et al. 1995). Lemming numbers often declined throghout the summer in spite of continous reproduction, and population recovery occurred overwinter. Heavy predation losses of radio-collared lemmings occurred each summer and this lemming population may be trapped in a predator-pit. Collared lemmings breed in winter and only because of winter population growth do these populations persist. Tundra vole numbers increased rapidly in most summers but usually declined overwinter. Tundra voles do not seem able to sustain winter reproduction in this extreme environment and this prevents them from reaching high density because of the short summer. Population growth in both these rodents could be prevented by poor food or by predation losses, and landscape patchiness may also help to prevent population growth. For lemmings we do not think that a shortage of shelter or intrinsic limitations could be restricting population increase at Pearce Point. This is the first detailed study of a non-cyclic collared lemming population.