Perspectives: evolutionary biology. The evolutionary synthesis.
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Academies and Institutes↗
Biomedical subjects
N C Stenseth
Publications and source records attributed to N C Stenseth.
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.
Age Factors↗
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.
Animal Feed↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Africa↗
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.
Animal Feed↗
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.
Animals↗
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.
Animals↗
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'.
Animals↗
Bootstrap estimated uncertainty of the dominant Lyapunov exponent for Holarctic microtine rodents.
The dominant Lyapunov exponent, as estimated from time series using the Jacobian-based method, is often used for indicating whether the underlying dynamic system is chaotic or not. The Jacobian-based method together with Response Surface Methodology has been suggested as a method for detecting chaotic dynamics in ecological time series. Besides pointing out that this may not be an appropriate method, we report on estimates of the uncertainty in the estimates of the dominant Lyapunov exponent. For this purpose, we have used time series data on Holarctic microtines. On the basis of our analyses, we are unable to find general evidence for chaotic dynamics in northern microtine populations (north of 60 degrees N) as recently suggested in the ecological literature. It seems, however, that the dynamic properties of the northern and southern populations are different. These patterns are supported by testing for nonlinearity.
Animals↗
Fingerprinting of diverse species with DNA probes generated from immobilized single-stranded DNA templates.
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Animals↗
Conservation biology: divided the fruitflies fall.
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Animals↗
Age-specific optimal diets and optimal foraging tactics: a life-historic approach.
By linking optimal foraging theory and optimal life history theory, we demonstrate that optimal diets, in general, may depend on the individual's age even when everything else remains the same. Older individuals (i.e., individuals with lower reproductive values) are predicted to have diets composed of highly nutritious food types that are possibly dangerous to pursue.
Aging↗
Coevolution of competing species: ecological character displacement.
Character displacement of competing species is studied. A model, originally developed by MacArthur and Levins (Proc. Natl. Acad. Sci. USA 51 (1964), 1207-1210) and further analyzed by Lawlor and Maynard Smith (Amer. Nat. 110 (1976), 70-99), has been reanalyzed. In the present paper, a more formally correct analysis of the MacArthur-Levins model is provided. A standard population genetics approach to sexually reproducing populations is adopted. The same conclusion as proposed by Lawlor and Maynard Smith emerges; competition can lead only to character divergence. In our analysis we either require that allopatrically evolved consumer populations must be able to coexist at an ecologically stable equilibrium (hence, we require mutual invasibility), or consider the feasibility of allopatric equilibria.
Analysis of Variance↗