Population dynamics of Norwegian red deer: density-dependence and climatic variation

We present a model on plant-deer climate interactions developed for improving our understanding of the temporal dynamics of deer abundance and, in particular, how intrinsic (density-dependent) and extrinsic (plants, climate) factors influence these dynamics. The model was tested statistically by analysing the dynamics of five Norwegian red deer populations between 1964 and 1993. Direct and delayed density-dependence significantly influenced the development of the populations: delayed density-dependence primarily operated through female density, whereas direct density-dependence acted through both female and male densities. Furthermore, population dynamics of Norwegian red deer were significantly affected by climate (as measured by the global weather phenomenon, the North Atlantic Oscillation: NAO). Warm, snowy winters (high NAO) were associated with decreased deer abundance, whereas the delayed (two-year) effect of warm, snowy winters had a positive effect on deer abundance. Our analyses are argued to have profound implications for the general understanding of climate change and terrestrial ecosystem functioning.

Stochastic seasonality and nonlinear density-dependent factors regulate population size in an African rodent

Ecology has long been troubled by the controversy over how populations are regulated. Some ecologists focus on the role of environmental effects, whereas others argue that density-dependent feedback mechanisms are central. The relative importance of both processes is still hotly debated, but clear examples of both processes acting in the same population are rare. Key-factor analysis (regression of population changes on possible causal factors) and time-series analysis are often used to investigate the presence of density dependence, but such approaches may be biased and provide no information on actual demographic rates. Here we report on both density-dependent and density-independent effects in a murid rodent pest species, the multimammate rat Mastomys natalensis (Smith, 1834), using statistical capture-recapture models. Both effects occur simultaneously, but we also demonstrate that they do not affect all demographic rates in the same way. We have incorporated the obtained estimates of demographic rates in a population dynamics model and show that the observed dynamics are affected by stabilizing nonlinear density-dependent components coupled with strong deterministic and stochastic seasonal components.

Nonlinear effects of large-scale climatic variability on wild and domestic herbivores

Large-scale climatic fluctuations, such as the North Atlantic Oscillation (NAO), have been shown to affect many ecological processes. Such effects have been typically assumed to be linear. Only one study has reported a nonlinear relation; however, that nonlinear relation was monotonic (that is, no reversal). Here we show that there is a strong nonlinear and non-monotonic (that is, reversed) effect of the NAO on body weight during the subsequent autumn for 23,838 individual wild red deer (Cervus elaphus) and 139,485 individual domestic sheep (Ovis aries) sampled over several decades on the west coast of Norway. These relationships are, at least in part, explained by comparable nonlinear and non-monotonic relations between the NAO and local climatic variables (temperature, precipitation and snow depth). The similar patterns observed for red deer and sheep, the latter of which live indoors during winter and so experience a stable energy supply in winter, suggest that the (winter) climatic variability (for which the index is a proxy) must influence the summer foraging conditions directly or indirectly.

A geographic gradient in small rodent density fluctuations: a statistical modelling approach

The patterns of density dependence in Fennoscandian rodents are investigated statistically using a linear autoregressive scheme. Nineteen time series of microtine abundances along a latitudinal gradient in Fennoscandia from 60 degrees N to 69 degrees N are analysed. We provide statistical evidence that there exists a latitudinal gradient in density dependence in Fennoscandian microtines. Southern populations experience significantly stronger direct density dependence than northern populations. Delayed density dependence was significantly negative throughout the region and appeared constant across the latitudinal gradient. The populations consistently exhibit dynamics of second order throughout the region. Together, the clinal direct density dependence and constant delayed density dependence give rise to a cline in cycle period from 3 to 4.5 years. The statistical results are compared to assumptions and predictions made in previous studies on the geographic gradient in the population dynamics of these rodents. The results are in agreement with the predictions of the 'generalist predator hypothesis'.

Are low but statistically significant levels of genetic differentiation in marine fishes 'biologically meaningful'? A case study of coastal Atlantic cod

A key question in many genetic studies on marine organisms is how to interpret a low but statistically significant level of genetic differentiation. Do such observations reflect a real phenomenon, or are they caused by confounding factors such as unrepresentative sampling or selective forces acting on the marker loci? Further, are low levels of differentiation biologically trivial, or can they represent a meaningful and perhaps important finding? We explored these issues in an empirical study on coastal Atlantic cod, combining temporally replicated genetic samples over a 10-year period with an extensive capture-mark-recapture study of individual mobility and population size. The genetic analyses revealed a pattern of differentiation between the inner part of the fjord and the open skerries area at the fjord entrance. Overall, genetic differentiation was weak (average F(ST)  = 0.0037), but nevertheless highly statistical significant and did not depend on particular loci that could be subject to selection. This spatial component dominated over temporal change, and temporal replicates clustered together throughout the 10-year period. Consistent with genetic results, the majority of the recaptured fish were found close to the point of release, with <1% of recaptured individuals dispersing between the inner fjord and outer skerries. We conclude that low levels of genetic differentiation in this marine fish can indeed be biologically meaningful, corresponding to separate, temporally persistent, local populations. We estimated the genetically effective sizes (N(e) ) of the two coastal cod populations to 198 and 542 and found a N(e) /N (spawner) ratio of 0.14.

Common dynamic structure of canada lynx populations within three climatic regions

Across the boreal forest of Canada, lynx populations undergo regular density cycles. Analysis of 21 time series from 1821 onward demonstrated structural similarity in these cycles within large regions of Canada. The observed population dynamics are consistent with a regional structure caused by climatic features, resulting in a grouping of lynx population dynamics into three types (corresponding to three climatic-based geographic regions): Pacific-maritime, Continental, and Atlantic-maritime. A possible link with the North Atlantic Oscillation is suggested.

Global climate change and phenotypic variation among red deer cohorts

The variability of two fitness-related phenotypic traits (body weight and a mandibular skeletal ratio) was analysed among cohorts and age-classes of red deer in Norway. Phenotypic variation among cohorts was pronounced for calves, yearlings and reproductively mature adults. Fluctuations in cohort-specific mean body weights and skeletal ratios of adults correlated with global climatic variation in winter conditions influenced by the North Atlantic Oscillation while cohorts were in utero. Red deer born following warm winters were smaller than those born after cold winters, and this inter-cohort variability persisted into adulthood. Phenotypic variation among cohorts of red deer influenced by climate change may pose consequences for fitness of cohorts since body size and condition contribute to reproductive success and survival in male and female red deer. In particular, the recent trend of increasingly warm winters in northern Europe and Scandinavia may lead to reduced body size and fecundity of red deer, and perhaps other ungulates, in those areas.

Population regulation in snowshoe hare and Canadian lynx: asymmetric food web configurations between hare and lynx

The snowshoe hare and the Canadian lynx in the boreal forests of North America show 9- to 11-year density cycles. These are generally assumed to be linked to each other because lynx are specialist predators on hares. Based on time series data for hare and lynx, we show that the dominant dimensional structure of the hare series appears to be three whereas that of the lynx is two. The three-dimensional structure of the hare time series is hypothesized to be due to a three-trophic level model in which the hare may be seen as simultaneously regulated from below and above. The plant species in the hare diet appear compensatory to one another, and the predator species may, likewise, be seen as an internally compensatory guild. The lynx time series are, in contrast, consistent with a model of donor control in which their populations are regulated from below by prey availability. Thus our analysis suggests that the classic view of a symmetric hare-lynx interaction is too simplistic. Specifically, we argue that the classic food chain structure is inappropriate: the hare is influenced by many predators other than the lynx, and the lynx is primarily influenced by the snowshoe hare.

From patterns to processes: phase and density dependencies in the Canadian lynx cycle

Across the boreal forest of North America, lynx populations undergo 10-year cycles. Analysis of 21 time series from 1821 to the present demonstrates that these fluctuations are generated by nonlinear processes with regulatory delays. Trophic interactions between lynx and hares cause delayed density-dependent regulation of lynx population growth. The nonlinearity, in contrast, appears to arise from phase dependencies in hunting success by lynx through the cycle. Using a combined approach of empirical, statistical, and mathematical modeling, we highlight how shifts in trophic interactions between the lynx and the hare generate the nonlinear process primarily by shifting functional response curves during the increase and the decrease phases.

Effects of age, sex and density on body weight of Norwegian red deer: evidence of density-dependent senescence

There are only a few recent studies that have demonstrated senescence in ungulates and nothing is known regarding how patterns of senescence may vary as a function of density Senescence in general is linked to the cost of reproduction, which probably increases with density in ungulates and may differ between the sexes. Further, senescence in ungulates is also regarded to be a function of tooth wear rates. As density dependence and sexual differences in food choice have been well documented, this may lead to different tooth wear rates and, thus, possibly density-dependent and sex-specific patterns of senescence. We therefore investigated the effects of age, sex, density and their possible interactions on the variability of body weight in 29,047 red deer harvested during 1965-1998 from Norway, out of which 380 males and 1452 females were eight years or older. There was clear evidence that spatio-temporal variation in density correlated negatively with body weights. In addition, there was evidence of senescence in both male and female red deer. Age at onset of senescence in females was after 20 years of age and independent of population density. In males, the onset and rate of senescence increased with increasing population density. The onset of senescence for males was at ca. 12 years of age at low density, but decreased to approximately ten years of age at high density. The pattern of density-dependent senescence in males, but not that in females, can be explained if (i) the cost of reproduction increases with density more strongly in male than in female red deer, and/or (ii) tooth wear rates are density dependent in males, but not in females. We discuss the ability of these two different, not mutually exclusive hypotheses in explaining the observed pattern of senescence.

The impact of specialized enemies on the dimensionality of host dynamics

Although individual species persist within a web of interactions with other species, data are usually gathered only from the focal species itself. We ask whether evidence of a species' interactions be detected and understood from patterns in the dynamics of that species alone. Theory predicts that strong coupling between a prey and a specialist predator/parasite should lead to an increase in the dimensionality of the prey's dynamics, whereas weak coupling should not. Here we describe a rare test of this prediction. Two natural enemies were added separately to replicate populations of a moth. For biological reasons that we identify here, the prediction of increased dimensionality was confirmed when a parasitoid wasp was added (although this increase had subtleties not previously appreciated), but the prediction failed for an added virus. Thus, an imprint of the interactions may be discerned within time-series data from component species of a system.

Environmental variation shapes sexual dimorphism in red deer

Sexual dimorphism results from dichotomous selection on male and female strategies of growth in relation to reproduction. In polygynous mammals, these strategies reflect sexual selection on males for access to females and competitive selection on females for access to food. Consequently, in such species, males display rapid early growth to large adult size, whereas females invest in condition and early sexual maturity at the expense of size. Hence, the magnitude of adult size dimorphism should be susceptible to divergence of the sexes in response to environmental factors differentially influencing their growth to reproduction. We show that divergent growth of male and female red deer after 32 years of winter warming and 15 years of contemporaneously earlier plant phenology support this prediction. In response to warmer climate during their early development, males grew more rapidly and increased in size, while female size declined. Conversely, females, but not males, responded to earlier plant phenology with increased investment in condition and earlier reproduction. Accordingly, adult size dimorphism increased in relation to warmer climate, whereas it declined in relation to forage quality. Thus, the evolutionary trajectories of growth related to reproduction in the sexes (i) originate from sexual and competitive selection, (ii) produce sexual size dimorphism, and (iii) are molded by environmental variation.

Environmental change and rates of evolution: the phylogeographic pattern within the hartebeest complex as related to climatic variation

Global climate fluctuated considerably throughout the Pliocene-Pleistocene period, influencing the evolutionary history of a wide array of species. Using the phylogeographic patterns within the hartebeest (Alcelaphus buselaphus (Pallas, 1766)) complex, we evaluated the evolutionary consequences of such environmental change for a typical large mammal ranging on the African savannah. Our results, as generated from two mitochondrial DNA markers (the D-loop and cytochrome b), suggest an origin of the hartebeest in eastern Africa from where the species has colonized other parts of the continent. Phylogenetic analyses revealed an early diversification into southern and northern hartebeest lineages, an event that may be related to the formation of the Rift Valley lakes. The northern lineage has further diverged into eastern and western lineages, most probably as a result of the expanding central African rainforest belt and subsequent contraction of savannah habitats during a period of global warming. The diversification events appear to have coincided with major climatic changes and are highly correlated in time. These observations strongly suggest that large-scale climatic fluctuations have been a major determinant for the species' evolutionary history and that hartebeest evolution has mainly taken place in isolated yet environmentally favourable refugia during periods of global warming. Indications of sudden population expansion for two putative ancestral hartebeest populations provide further support for a refugia-based explanation of the diversification events. Reciprocal monophyly between southern and northern lineages may suggest that reproductive barriers exist and that the hartebeest complex comprises two different species.

Life-history traits of voles in a fluctuating population respond to the immediate environment

Life-history traits relating to growth and reproduction vary greatly among species and populations and among individuals within populations. In vole populations, body size and age at maturation may vary considerably among locations and among years within the same location. Individuals in increasing populations are typically larger and start reproduction earlier in the spring than those in declining populations. The cause of such life-history variation within populations has been subject of much discussion. Much of the controversy concerns whether the memory of past conditions, leading to delayed effects on life-history traits, resides in the environment (for example, predators, pathogens or food) or intrinsically within populations or individuals (age distribution, physiological state, genetic or maternal effects). Here we report from an extensive field transplant experiment in which voles were moved before the breeding season between sites that differed in average overwintering body mass. Transplanted voles did not retain the characteristics of their source population, and we demonstrate an over-riding role of the immediate environment in shaping life-history traits of small rodents.

Is spacing behaviour coupled with predation causing the microtine density cycle? A synthesis of current process-oriented and pattern-oriented studies

Current ecological information on periodically fluctuating microtine populations are demonstrated to support a hypothesis involving both predation and intrinsic self-regulation as necessary and sufficient factors for explaining the "microtine density cycle'. The structure of the cyclic time series is largely two dimensional with strong delayed density dependence. Together with recent field studies on rodent demography, our modelling suggests that trophic interaction is a likely candidate to generate the dimensionality observed for northern microtine rodent dynamics. It is shown that the trophic interaction must be fairly strong. This suggests that specialist predation is the most likely one among the classes of trophic interactions. We also argue that some - but not too strong - self-regulation must occur to generate the structure of the available time series on northern European microtines.

A gradient from stable to cyclic populations of Clethrionomys rufocanus in Hokkaido, Japan

A total of 31 years of abundance data from 90 populations of the grey-sided vole (Clethrionomys rufocanus) in northern Hokkaido (Japan) were analysed with respect to population dynamic characteristics. Both non-periodic and multi-annually periodic fluctuations occur among the studied populations. The length of the period varies from 2 to 5 years. The dynamics appear approximately linear on a logarithmic scale, and a log-linear stochastic difference model with one time-lag is found to recreate the periodograms of the time series. The deterministic (log-linear) component of these models determines the periodicity while stochasticity sustains the cycles. The dynamics of the Hokkaidian vole populations are found to vary clinally from the western coast eastwards and towards the interior of the study area. This gradient corresponds superficially to the latitudinal gradient seen in microtine populations in Fennoscandia. However, under close scrutiny, these gradients differ greatly: the Hokkaidian gradient is caused by a cline in delayed density dependence. Statistical delayed density dependence is more negative towards the east and the interior.

A new geographical gradient in vole population dynamics

A new geographical gradient in the dynamics of small rodents is demonstrated by analysing 29 time series of density indices of the common vole (Microtus arvalis) from Poland, the Czech Republic and the Slovak Republic. This gradient extends from more stable northerly populations in coastal Poland to more variable and cyclic populations in the southernmost parts of the Slovak Republic, and is hence a reversal of the Fennoscandian gradient. All studied variables (such as mean density, cycle amplitude, density variability and the coefficients in a second-order autoregressive model) exhibit consistent latitudinal variation. Possible underlying factors are discussed. In particular, we suggest that seasonality may be a key element in explaining the observed new gradient.

Interspecific and intraspecific competition as causes of direct and delayed density dependence in a fluctuating vole population

A 3- to 5-year cycle of vole abundances is a characteristic phenomenon in the ecology of northern regions, and their explanation stands as a central theoretical challenge in population ecology. Although many species of voles usually coexist and are in severe competition for food and breeding space, the role of interspecific competition in vole cycles has never been evaluated statistically. After studying community effects on the population dynamics of the gray-sided vole (Clethrionomys rufocanus) in the subarctic birch forest at Kilpisjärvi, Finland, we report statistical results showing that both interspecific and intraspecific effects are important in the direct year-to-year density dependence. However, interspecific effects are not detectable in the 2-year delayed density dependence that is crucial for generating the characteristic cycles. Furthermore, we show that most of the competition takes place during the winter. The results are evaluated against two models of community dynamics. One assumes that the delayed effects are caused by an interaction with a specialist predator, and the other assumes that they are caused by overgrazing food plants. These statistical results show that vole cycles may be generated by a species-specific trophic interaction. The results also suggest that the gray-sided vole may be the focal species in the birch-forest community, as field voles may be in the taiga and as lemmings may be on the tundra.

Glacial survival of the Norwegian lemming (Lemmus lemmus) in Scandinavia: inference from mitochondrial DNA variation

In order to evaluate the biogeographical hypothesis that the Norwegian lemming (Lemmus lemmus) survived the last glacial period in some Scandinavian refugia, we examined variation in the nucleotide sequence of the mitochondrial control region (402 base pairs (bp)) and the cytochrome b (cyt b) region (633 bp) in Norwegian and Siberian (Lemmus sibiricus) lemmings. The phylogenetic distinction and cyt b divergence estimate of 1.8% between the Norwegian and Siberian lemmings suggest that their separation pre-dated the last glaciation and imply that the Norwegian lemming is probably a relic of the Pleistocene populations from Western Europe. The star-like control region phylogeny and low mitochondrial DNA diversity in the Norwegian lemming indicate a reduction in its historical effective size followed by population expansion. The average estimate of post-bottleneck time (19-21 kyr) is close to the last glacial maximum (18-22 kyr BP). Taking these findings and the fossil records into consideration, it seems likely that, after colonization of Scandinavia in the Late Pleistocene, the Norwegian lemming suffered a reduction in its population effective size and survived the last glacial maximum in some local Scandinavian refugia, as suggested by early biogeographical work.

The timing of life-history events in a changing climate

Although empirical and theoretical studies suggest that climate influences the timing of life-history events in animals and plants, correlations between climate and the timing of events such as egg-laying, migration or flowering do not reveal the mechanisms by which natural selection operates on life-history events. We present a general autoregressive model of the timing of life-history events in relation to variation in global climate that, like autoregressive models of population dynamics, allows for a more mechanistic understanding of the roles of climate, resources and competition. We applied the model to data on 50 years of annual dates of first flowering by three species of plants in 26 populations covering 4 degrees of latitude in Norway. In agreement with earlier studies, plants in most populations and all three species bloomed earlier following warmer winters. Moreover, our model revealed that earlier blooming reflected increasing influences of resources and density-dependent population limitation under climatic warming. The insights available from the application of this model to phenological data in other taxa will contribute to our understanding of the roles of endogenous versus exogenous processes in the evolution of the timing of life-history events in a changing climate.

Ecosystem oceanography for global change in fisheries

Overexploitation and climate change are increasingly causing unanticipated changes in marine ecosystems, such as higher variability in fish recruitment and shifts in species dominance. An ecosystem-based approach to fisheries attempts to address these effects by integrating populations, food webs and fish habitats at different scales. Ecosystem models represent indispensable tools to achieve this objective. However, a balanced research strategy is needed to avoid overly complex models. Ecosystem oceanography represents such a balanced strategy that relates ecosystem components and their interactions to climate change and exploitation. It aims at developing realistic and robust models at different levels of organisation and addressing specific questions in a global change context while systematically exploring the ever-increasing amount of biological and environmental data.

Cryptic population structure in a large, mobile mammalian predator: the Scandinavian lynx

The Eurasian lynx (Lynx lynx) is an example of a species that has gone through a severe bottleneck, leading to near extinction in Scandinavia around 1930-- a pattern shared with several other large carnivorous mammals. Here we extend previous genetic analyses of northern European lynx, confirming that lynx from the Scandinavian Peninsula represent a distinct clade differing clearly from European conspecifics. Furthermore, and despite a recent bottleneck and subsequent range expansion, we detect marked genetic differentiation within Scandinavia. This differentiation is largely manifested as a north-south gradient, with a linear increase in the quantity FST/(1 - FST). Aided by computer simulations we find that this pattern is unlikely to have arisen by random genetic drift in the short time since lynx started to expand in the 1950s, suggesting that the spatial structure may predate the bottleneck. Individual-based analyses indicate that, instead of a continuous gradient, Scandinavian lynx may be structured into three more or less distinct groups, possibly corresponding to northern, central and southern subpopulations. The presence of such structuring was unknown previously and was unexpected from general considerations on the mobility of the species, historical data and the absence of geographical barriers. Our study demonstrates how molecular markers may be used to detect cryptic population structure, invisible using traditional methods.

Phase- and density-dependent population dynamics in Norwegian lemmings: interaction between deterministic and stochastic processes

We analysed two 26-year long (1970-1995) time-series on annual population growth rates of Norwegian lemmings (Lemmus lemmus) from Finse, south Norway, using a threshold autoregressive (TAR) approach. We demonstrate that the population dynamics is both phase- and density-dependent. The phase-dependence accounts for the observed nonlinearity. We used the deduced stochastic model structure as a basis for evaluating the dynamic properties of this system. The dynamics is characterized either by limit cycles or chaos (the latter with a strong semi-periodic component). Stochasticity is seen to play an important role in the determination of the periodicity. The ecological implications of these statistical and mathematical results are discussed.

Cycles and trends in cod populations

Year-to-year fluctuations in fish stocks are usually attributed to variability in recruitment, competition, predation, and changes in catchability. Trends in abundance, in contrast, are usually ascribed to human exploitation and large-scale environmental changes. In this study, we demonstrate, through statistical modeling of survey data (1921-1994) of cod from the Norwegian Skagerrak coast, that both short- and long-term variability may arise from the same set of age-structured interactions. Asymmetric competition and cannibalism between cohorts generate alternating years of high and low abundance. Intercohort interactions also resonate the recruitment variability so that long-term trends are induced. The coupling of age-structure and variable recruitment should, therefore, be considered when explaining both the short- and long-term fluctuations displayed by the coastal cod populations. Resonant effects may occur in many marine populations that exhibit this combination of traits.