Torbjørn Håkan Ergon

• Bommerlund, Julie; Ergon, Torbjørn Håkan; Filhol, Simon & Mysterud, Atle (2026). Estimating red deer Cervus elaphus population density using drones in a steep and rugged terrain. Wildlife Biology. ISSN 0909-6396. doi: 10.1002/wlb3.01533. Full text in Research Archive Show summary

  Precise and accurate information about population density, crucial for wildlife management, is difficult to obtain for elusive species living in dense forests or steep and inaccessible terrain. Using unmanned aerial vehicles (UAVs), we developed a method for obtaining absolute population estimates of ungulates living in steep, rugged, and partly forested terrain. The UAVs were used to collect data from four different case study areas in two consecutive months in winter/early spring (before leaf growth) in Lærdal, Norway. The detections of red deer Cervus elaphus from UAV images were done using a double observer protocol, and hierarchal state–space models were fitted to the data using a Bayesian framework to obtain posterior distributions of absolute deer density. Results indicate that availability bias was negligible when deer can be searched for in multiple UAV images of the sites (sample plots) taken from different angles by a UAV at 60 m above ground level but detection probability in the images was significantly below one and must hence be accounted for. Although the credible intervals were rather wide, our results demonstrate that absolute red deer density estimates with quantifiable uncertainties can be produced using this method. Lower credible limits for the estimates indicate that densities in two of the study areas were consistently well above management target densities during the two consecutive months. The high spatial variation in deer densities (at various scales) in our study pose a major challenge, as large areas with a high number of sampling sites must be covered to model the spatial variation and obtain reliable and precise density estimates at a scale relevant for management. We nevertheless conclude that the method has potential for reliable spatiotemporal deer density estimation, given sufficient resources and field efforts.

• Lalechère, Etienne; Lenoir, Jonathan; Marrec, Ronan; Essl, Franz; Kühn, Ingolf & Ergon, Torbjørn (2025). Assessing biodiversity trends in a quasi-permanent non-equilibrium state. Trends in Ecology & Evolution. ISSN 0169-5347. 40(10), p. 949–959. doi: 10.1016/j.tree.2025.07.003. Full text in Research Archive Show summary

  The ‘equilibrium assumption’ underlying biodiversity trends assessments in response to environmental changes is rarely challenged, the traditional assumption being that biodiversity is in an equilibrium state with its contemporary drivers. Existing non-equilibrium biodiversity frameworks still rely on the assumption that biodiversity is, at a given moment in time, in an equilibrium state with its contemporary drivers. In this opinion article we consider multiple trajectories of changes due to long-term disturbances that push biodiversity into a quasi-permanent non-equilibrium state. We propose a conceptual framework based on the use of temporal influence functions that can be integrated into a wide range of ecological models. The complete paradigm shift we provide can hindcast past, and project future, non-equilibrium biodiversity trends.

• Vindenes, Yngvild; Broch, Catharina; Andersen, Tom; Hessen, Dag Olav & Ergon, Torbjørn (2025). Understanding the role of ageing in the thermal responses of life history and fitness in Daphnia magna. Proceedings of the Royal Society of London. Biological Sciences. ISSN 0962-8452. 292(2051). doi: 10.1098/rspb.2025.0430. Full text in Research Archive Show summary

  The current climate warming has raised concerns that increased rates of ageing will lead to reduced fitness and population viability in ectotherms. However, it is currently not known whether temperature effects on the rate of ageing mainly reflect a change in pace of life or changes in the strength of ageing in the life history. Evaluating this question requires that ageing in different temperatures is compared on the same intrinsic pace-standardized time scale. We present results from a laboratory experiment with Daphnia magna , recording complete life histories of 240 individuals from four clonal lines at six temperatures (5°C to 30°C). For each clone and temperature, we calculated measures of pace and shape of ageing in survival and reproduction. Fitness (long-term population growth rate) and mean lifetime reproduction were calculated using matrix population models. Our results highlight three main points: (i) lifespan is not a good predictor of temperature effects on ageing; (ii) there is no strong consistent effect of temperature on shape of ageing; and (iii) within temperature, the shape of ageing was positively correlated with fitness. Hence, lower fitness and population viability of ectotherms in warmer environments may be driven by factors other than increased ageing rates.

• Kufa, Chala Adugna; Bekele, Afework; Atickem, Anagaw Meshesha; Chala, Desalegn; Alemu, Diress Tsegaye & Ergon, Torbjørn [Show all 8 contributors for this article] (2025). Mapping suitable habitat and Anthropocene refugia for Ethiopian Guerezas: Insights for their conservation. Global Ecology and Conservation. 59. doi: 10.1016/j.gecco.2025.e03547. Full text in Research Archive Show summary

  Ethiopia is home to two subspecies of Colobus guereza, C. g. guereza and C. g. gallarum. Whereas C. g. guereza is listed as Least Concern by IUCN, the conservation status of C. g. gallarum is unclear, but according to a recent assessment, it will most likely be listed as Vulnerable, because of habitat loss due to agricultural expansion. We used climate data to model the habitat suitability for both taxa in a comparative study to identify suitable habitats within and outside of protected areas that may serve as Anthropocene refugia. Our ensemble models estimated 168,731 km2 as climatically suitable habitat for C. g. guereza and 69,542 km2 for C. g. gallarum with an overlap between the two taxa of 17.2 %. Areas that qualified as refugia, i.e., areas covered by forest, were 47,101 km2 (only 27.9 % of the total suitable habitat) and 8430 km2 (12.1 % of the suitable habitat) for C. g. guereza and C. g. gallarum, respectively. Of these, 39.8 % (C. g. guereza) and 53.7 % (C. g. gallarum) are within Ethiopia’s current protected area network. Given that potential Anthropocene refugia are found only partly within protected areas, conservation management should include this information when developing conservation strategies for both taxa. As the majority of suitable habitats for the two colobus taxa exist in non-forested regions, afforestation in these areas would be highly beneficial and is strongly recommended.

• Reitan, Trond; Ergon, Torbjørn & Liow, Lee Hsiang (2022). Relative species abundance and population densities of the past: Developing multispecies occupancy models for fossil data. Paleobiology. ISSN 0094-8373. 49(1), p. 23–38. doi: 10.1017/pab.2022.17. Full text in Research Archive Show summary

  Abstract The number of individuals of species varies, but estimating abundance, given incomplete and biased sampling in both contemporary and fossilized communities, is challenging. Here, we describe a new occupancy model in a hierarchical Bayesian framework with random effects, in which multispecies occupancy and detection are modeled as a means to estimate relative species abundance and relative population densities. The modeling framework is suited for temporal samples of fossil communities with repeated sampling including multiple species with similar preservation potential. We demonstrate our modeling framework using a fossil community of benthic organisms to estimate relative species abundance dynamics and changing relative population densities of focal species in nine (geological) time intervals over 2.3 Myr. We also explore potential explanatory factors (paleoenvironmental proxies) and temporal autocorrelation that could provide extra information on unsampled time intervals. The modeling framework is applicable across a wide range of questions on species-level dynamics in paleoecological community settings.

• Nater, Chloe Rebecca; Stubberud, Marlene Wæge; Langangen, Øystein Ole Gahr; Rustadbakken, Atle; Moe, S. Jannicke & Ergon, Torbjørn [Show all 8 contributors for this article] (2022). Towards a future without stocking: harvest and river regulation determine long-term population viability of migratory salmonids. Climate Research (CR). ISSN 0936-577X. 86, p. 37–52. doi: 10.3354/cr01644. Full text in Research Archive Show summary

  Freshwater species are particularly vulnerable to emerging threats linked to climate change because they are often already heavily impacted by habitat destruction, pollution, and exploitation. For many harvested populations of freshwater fish, these combined impacts have been mitigated for decades through stocking with captive-bred individuals. However, stocking may lead to loss of genetic variation, which may be crucial for adaptation under climate change. Exploration of sustainable alternatives is therefore paramount. We used a female-based integral projection model (IPM) to assess the consequences of terminating a long-term stocking programme for a population of landlocked, migratory brown trout Salmo trutta, and to evaluate relative effectiveness of alternative management strategies involving harvest regulations and river habitat improvement. The IPM classified individuals by body size, life history stage, and location relative to a hydropower dam, and was parameterised with 50 yr of individual-based data, supplemented with literature values. Model simulations indicated a strong population decline of 22-29% per year without stocking, much of which was attributed to high harvest mortality. Consequently, drastic reductions in fishing pressure were predicted to be necessary to ensure population viability without stocking. Mitigation measures reducing mortality associated with the hydropower dam or restoring spawning areas could further contribute to population viability when combined with changes in harvest regulations. Our results thus emphasise that large changes in management strategies, such as termination of long-term stocking programmes, require a thorough assessment of potential consequences and alternative mitigation strategies using data and models, or, at the very least, a precautionary approach under consideration of on-going climate change. Migratory salmonid · Salmo trutta · Integral projection model · Harvest · Fishing · Stocking · Dam · Hydropower · Trout

• Turek, Daniel; Milleret, Cyril Pierre; Ergon, Torbjørn; Brøseth, Henrik; Dupont, Pierre & Bischof, Richard [Show all 7 contributors for this article] (2021). Efficient estimation of large-scale spatial capture–recapture models. Ecosphere. 12(2). doi: 10.1002/ecs2.3385. Full text in Research Archive Show summary

  Capture–recapture methods are a common tool in ecological statistics, which have beenextended to spatial capture–recapture models for data accompanied by location information. However,standard formulations of these models can be unwieldy and computationally intractable for large spatialscales, many individuals, and/or activity center movement. We provide a cumulative series of methodsthat yield dramatic improvements in Markov chain Monte Carlo (MCMC) estimation for two examples.These include removing unnecessary computations, integrating out latent states, vectorizing declarations,and restricting calculations to the locality of individuals. Our approaches leverage the exibility providedby the nimble R package. In our rst example, we demonstrate an improvement in MCMC efciency (therate of generating effectively independent posterior samples) by a factor of 100. In our second example, wereduce the computing time required to generate 10,000 posterior samples from 4.5 h down to ve minutes,and realize an increase in MCMC efciency by a factor of 25. These approaches can also be applied gener-ally to other spatially indexed hierarchical models. We provide R code for all examples, an executable web-appendix, and generalized versions of these techniques are made available in the nimbleSCR R package. Markov chain Monte Carlo; Mark–recapture; nimble; sampling efficiency; spatial capture–recapture

• Nater, Chloe R; Vindenes, Yngvild; Aass, Per; Cole, Diana; Langangen, Øystein & Moe, S. Jannicke [Show all 10 contributors for this article] (2020). Size‐ and stage‐dependence in cause‐specific mortality of migratory brown trout. Journal of Animal Ecology. ISSN 0021-8790. 89(9), p. 2122–2133. doi: 10.1111/1365-2656.13269. Full text in Research Archive Show summary

  Evidence‐based management of natural populations under strong human influence frequently requires not only estimates of survival but also knowledge about how much mortality is due to anthropogenic vs. natural causes. This is the case particularly when individuals vary in their vulnerability to different causes of mortality due to traits, life history stages, or locations. Here, we estimated harvest and background (other cause) mortality of landlocked migratory salmonids over half a century. In doing so, we quantified among‐individual variation in vulnerability to cause‐specific mortality resulting from differences in body size and spawning location relative to a hydropower dam. We constructed a multistate mark–recapture model to estimate harvest and background mortality hazard rates as functions of a discrete state (spawning location) and an individual time‐varying covariate (body size). We further accounted for among‐year variation in mortality and migratory behaviour and fit the model to a unique 50‐year time series of mark–recapture–recovery data on brown trout (Salmo trutta) in Norway. Harvest mortality was highest for intermediate‐sized trout, and outweighed background mortality for most of the observed size range. Background mortality decreased with body size for trout spawning above the dam and increased for those spawning below. All vital rates varied substantially over time, but a trend was evident only in estimates of fishers' reporting rate, which decreased from over 50% to less than 10% throughout the study period. We highlight the importance of body size for cause‐specific mortality and demonstrate how this can be estimated using a novel hazard rate parameterization for mark–recapture models. Our approach allows estimating effects of individual traits and environment on cause‐specific mortality without confounding, and provides an intuitive way to estimate temporal patterns within and correlation among different mortality sources.

• Ergon, Torbjørn; Borgan, Ørnulf; Nater, Chloé Rebecca & Vindenes, Yngvild (2018). The utility of mortality hazard rates in population analyses. Methods in Ecology and Evolution. 9, p. 2046–2056. doi: 10.1111/2041-210X.13059. Full text in Research Archive
• Nater, Chloé Rebecca; Rustadbakken, Atle; Ergon, Torbjørn; Langangen, Øystein; Moe, S. Jannicke & Vindenes, Yngvild [Show all 8 contributors for this article] (2018). Individual heterogeneity and early life conditions shape growth in a freshwater top predator. Ecology. ISSN 0012-9658. 99(5), p. 1011–1017. doi: 10.1002/ecy.2178. Full text in Research Archive Show summary

  Body size can have profound impacts on survival, movement, and reproductive schedules shaping individual fitness, making growth a central process in ecological and evolutionary dynamics. Realized growth is the result of a complex interplay between life history schedules, individual variation, and environmental influences. Integrating all of these aspects into growth models is methodologically difficult, depends on the availability of repeated measurements of identifiable individuals, and consequently represents a major challenge in particular for natural populations. Using a unique 30‐yr time series of individual length measurements inferred from scale year rings of wild brown trout, we develop a Bayesian hierarchical model to estimate individual growth trajectories in temporally and spatially varying environments. We reveal a gradual decrease in average juvenile growth, which has carried over to adult life and contributed to decreasing sizes observed at the population level. Commonly studied environmental drivers like temperature and water flow did not explain much of this trend and overall persistent and among‐year individual variation dwarfed temporal variation in growth patterns. Our model and results are relevant to a wide range of questions in ecology and evolution requiring a detailed understanding of growth patterns, including conservation and management of many size‐structured populations.

• Ergon, Torbjørn & Ergon, Rolf (2017). When three traits make a line: evolution of phenotypic plasticity and genetic assimilation through linear reaction norms in stochastic environments. Journal of Evolutionary Biology. ISSN 1010-061X. 30(3), p. 486–500. doi: 10.1111/jeb.13003. Full text in Research Archive Show summary

  Genetic assimilation emerges from selection on phenotypic plasticity. Yet, commonly used quantitative genetics models of linear reaction norms considering intercept and slope as traits do not mimic the full process of genetic assimilation. We argue that intercept–slope reaction norm models are insufficient representations of genetic effects on linear reaction norms and that considering reaction norm intercept as a trait is unfortunate because the definition of this trait relates to a specific environmental value (zero) and confounds genetic effects on reaction norm elevation with genetic effects on environmental perception. Instead, we suggest a model with three traits representing genetic effects that, respectively, (i) are independent of the environment, (ii) alter the sensitivity of the phenotype to the environment and (iii) determine how the organism perceives the environment. The model predicts that, given sufficient additive genetic variation in environmental perception, the environmental value at which reaction norms tend to cross will respond rapidly to selection after an abrupt environmental change, and eventually becomes equal to the new mean environment. This readjustment of the zone of canalization becomes completed without changes in genetic correlations, genetic drift or imposing any fitness costs of maintaining plasticity. The asymptotic evolutionary outcome of this three-trait linear reaction norm generally entails a lower degree of phenotypic plasticity than the two-trait model, and maximum expected fitness does not occur at the mean trait values in the population.

• Sengupta, Sagnik; Ergon, Torbjørn & Leinaas, Hans Petter (2017). Thermal plasticity in postembryonic life history traits of a widely distributed Collembola: Effects of macroclimate and microhabitat on genotypic differences. Ecology and Evolution. 7(19), p. 8100–8112. doi: 10.1002/ece3.3333. Full text in Research Archive Show summary

  Life history traits in many ectotherms show complex patterns of variation among conspecific populations sampled along wide latitudinal or climatic gradients. However, few studies have assessed whether these patterns can be explained better by thermal reaction norms of multiple life history traits, covering major aspects of the life cycle. In this study, we compared five populations of a Holarctic, numerically dominant soil microarthropod species, Folsomia quadrioculata, sampled from a wide latitudinal gradient (56–81°N), for growth, development, fecundity, and survival across four temperatures (10, 15, 20, and 25°C) in common garden experiments. We evaluated the extent to which macroclimate could explain differences in thermal adaptation and life history strategies among populations. The common garden experiments revealed large genotypic differences among populations in all the traits, which were little explained by latitude and macroclimate. In addition, the life history strategies (traits combined) hardly revealed any systematic difference related to latitude and macroclimate. The overall performance of the northernmost population from the most stochastic microclimate and the southernmost population, which remains active throughout the year, was least sensitive to the temperature treatments. In contrast, performance of the population from the most predictable microclimate peaked within a narrow temperature range (around 15°C). Our findings revealed limited support for macroclimate‐based predictions, and indicated that local soil habitat conditions related to predictability and seasonality might have considerable influence on the evolution of life history strategies of F. quadrioculata. This study highlights the need to combine knowledge on microhabitat characteristics, and demography, with findings from common garden experiments, for identifying the key drivers of life history evolution across large spatial scales, and wide climate gradients. We believe that similar approaches may substantially improve the understanding of adaptation in many terrestrial ectotherms with low dispersal ability.

• Liow, Lee Hsiang & Ergon, Torbjørn (2016). Fitting ancestral age-dependent speciation models to fossil data. In Allmon, Warren D. & Yacobucci, Margaret M. (Ed.), Species and Speciation in the Fossil Record. University of Chicago Press. ISSN 9780226377445. p. 198–216. doi: 10.7208/chicago/9780226377582.001.0001. Full text in Research Archive
• Sengupta, Sagnik; Ergon, Torbjørn & Leinaas, Hans Petter (2016). Genotypic differences in embryonic life history traits of Folsomia quadrioculata (Collembola: Isotomidae) across a wide geographical range. Ecological Entomology. ISSN 0307-6946. 41(1), p. 72–84. doi: 10.1111/een.12270. Full text in Research Archive
• Schnug, Lisbeth; Ergon, Torbjørn; Jakob, Lena; Scott-Fordsmand, Janeck J; Joner, Erik & Leinaas, Hans Petter (2015). Responses of earthworms to repeated exposure to three biocides Applied singly and as a mixture in an agricultural field. Science of the Total Environment. ISSN 0048-9697. 505, p. 223–235. doi: 10.1016/j.scitotenv.2014.09.089. Full text in Research Archive
• Ergon, Torbjørn & Gardner, Beth (2014). Separating mortality and emigration: modelling space use, dispersal and survival with robust-design spatial capture–recapture data. Methods in Ecology and Evolution. 5(12), p. 1327–1336. doi: 10.1111/2041-210X.12133. Full text in Research Archive
• Knudsen, Endre; Linden, Andreas; Both, Christiaan; Jonzén, Niclas; Pulido, Francisco & Saino, Nicola [Show all 27 contributors for this article] (2011). Challenging claims in the study of migratory birds and climate change. Biological Reviews. ISSN 1464-7931. 86, p. 928–946. doi: 10.1111/j.1469-185X.2011.00179.x. Full text in Research Archive
• Reimers, Eigil; Lund, Steinar & Ergon, Torbjørn (2011). Vigilance and fright behaviour in the insular Svalbard reindeer (Rangifer tarandus platyrhynchus). Canadian Journal of Zoology. ISSN 0008-4301. 89(8), p. 753–764. doi: 10.1139/Z11-040. Full text in Research Archive
• Ergon, Torbjørn; Ergon, Rolf; Begon, Mike; Telfer, Sandra & Lambin, Xavier (2011). Delayed density-dependent onset of spring reproduction in a fluctuating population of field voles. Oikos. ISSN 0030-1299. 120(6), p. 934–940. doi: 10.1111/j.1600-0706.2010.18983.x. Full text in Research Archive Show summary

  Delayed density-dependent demographic processes are thought to be the basis for multi-annual cyclic fluctuations in small rodent populations, but evidence for delayed density dependence of a particular demographic trait is rare. Here, using capture-recapture data from 22 sites collected over nine years, we demonstrate a strong effect of population density with a one-year lag on the timing of the onset of spring reproduction in a cyclically fluctuating population of field voles (Microtus agrestis, L.) in northern England. The mean date for the onset of spring reproduction was delayed by about 24 days for every additional 100 voles/ha in the previous spring. This delayed density dependence is sufficient to generate the type of cyclic population dynamics described in the study system.

• Hegel, Troy Michel; Mysterud, Atle; Ergon, Torbjørn; Loe, Leif Egil; Huettmann, Falk & Stenseth, Nils Christian (2010). Seasonal effects of Pacific-based climate on recruitment in a predator-limited large herbivore. Journal of Animal Ecology. ISSN 0021-8790. 79(2), p. 471–482. doi: 10.1111/j.1365-2656.2009.01647.x. Full text in Research Archive
• Liow, Lee Hsiang; Skaug, Hans Julius; Ergon, Torbjørn H. & Schweder, Tore (2010). Global occurrence trajectories of microfossils: environmental volatility and the rise and fall of individual species. Paleobiology. ISSN 0094-8373. 36(2), p. 224–252. doi: 10.1666/08080.1. Full text in Research Archive
• Ergon, Torbjørn; Yoccoz, Nigel G. & Nichols, James D. (2009). Estimating Latent Time of Maturation and Survival Costs of Reproduction in Continuous Time from Capture–Recapture Data. In Thomson, David L.; Cooch, Evan G. & Conroy, Michael J. (Ed.), Modeling Demographic Processes In Marked Populations. Springer. ISSN 9780387781501. p. 173–197. Full text in Research Archive
• Jonzén, Niclas; Lindén, Andreas; Ergon, Torbjørn; Knudsen, Endre; Vik, Jon Olav & Rubolini, Diego [Show all 17 contributors for this article] (2008). Rapid advance of spring arrival dates in long-distance migratory birds. In Kennedy, Donald (Eds.), Science Magazine's State of the Planet 2008-2009: with a Special Section on Energy and Sustainability. Island Press. ISSN 9781597264051. p. 1959–1961. Full text in Research Archive Show summary

  Taken from the pages of Science and supplemented by contributions from the magazine’s editors, State of the Planet 2008-2009 offers contemporary science writing that is sometimes provocative, frequently enlightening, and always authoritative. Published by the American Association for the Advancement of Science (AAAS), Science is one of the most respected scientific magazines in the world. With a readership of more than one million people, it offers “hard science” from top scientists to both educated lay readers and scientists alike. The articles collected here are arranged thematically and each section is introduced by a prominent scientist or science writer. Donald Kennedy, who was Editor-in-Chief of Science when these articles appeared in the magazine, contributes a preface and several short essays. Focusing on issues of energy and sustainability, sections of the volume are devoted to the prospects of energy-sparing technologies and alternatives to fossil fuel use, including ethanol and cellulosic digestion. Other sections center on climate change, led by a comprehensive essay on the state of scientific knowledge today and followed by contributions about the Antarctic and Greenland ice sheets, as well as the effects of climate change that have been measured to date, including changes in migration and breeding cycles of birds and flowering in plants, changing patterns of hurricanes and extreme weather events, and alterations in forest fire frequency. Interspersed throughout the book are Science news pieces that highlight particular issues and cases relevant to the main scientific findings. A glossary of key terms and concepts helps students and nonspecialists better understand the terminology and the issues.

• Ergon, Torbjørn (2007). Optimal onset of seasonal reproduction in stochastic environments: When should overwintering small rodents start breeding? Écoscience. ISSN 1195-6860. 14, p. 330–346. Full text in Research Archive Show summary

  Theories for optimal life history strategies in variable environments have until now focused on cases where the individuals have either no information about the environment (models maximizing geometric mean fitness) or full information about the environment (models predicting optimal reaction norms). In this paper I investigate the optimal time for multivoltine organisms to commence seasonal reproduction in a more general and realistic case where animals perceive the state of their environment through cues that are measured with varying degrees of precision. If there were only a trade-off between early reproduction and high reproductive success, and if animals had perfect information about their environment, it would be optimal to commence reproduction when the rate of change in reproductive success relative to its current value equals the difference between population growth during the reproductive and non-reproductive seasons. This implies that reproductive success at the optimum is independent of when (but not how) the environment improves over the season. However, because it is optimal to respond conservatively to uncertain cues, we should expect higher reproductive success during years when breeding conditions improve early than when they improve late. Nevertheless, a phenotypic correlation between reproductive success and timing of reproduction will probably not be detectable in a stochastic environment. Data from a cyclic population of field voles (Microtus agrestis L.) in northern England show a negative correlation between reproductive success and timing of reproduction among out-of-phase locations. Such a pattern may occur when there is a convex trade-off between pre-breeding survival and timing of reproduction, or if animals precipitate reproduction to avoid senescence when the environment improves late.

• Jonzén, Niclas; Lindén, Andreas; Ergon, Torbjørn; Knudsen, Endre; Vik, Jon Olav & Rubolini, Diego [Show all 17 contributors for this article] (2007). Response to comment on "Rapid advance of spring arrival dates in long-distance migratory birds". Science. ISSN 0036-8075. 315(5812), p. 598–598. doi: 10.1126/science.1136920. Full text in Research Archive
• Jonzén, Niclas; Ergon, Torbjørn; Lindén, Andreas & Stenseth, Nils Christian (2007). Bird migration and climate change: the general picture and beyond. Climate Research (CR). ISSN 0936-577X. 35(1-2), p. 177–180. doi: 10.3354/cr00724. Full text in Research Archive
• Saino, Nicola; Rubolini, Diego; Jonzén, Niclas; Ergon, Torbjørn; Montemaggiori, A & Stenseth, Nils Christian [Show all 7 contributors for this article] (2007). Temperature and rainfall anomalies in Africa predict timing of spring migration in trans-Saharan migritory birds. Climate Research (CR). ISSN 0936-577X. 35(1-2), p. 123–134. Full text in Research Archive
• Knudsen, Endre; Lindén, Andreas; Ergon, Torbjørn; Jonzén, Niclas; Vik, Jon Olav & Knape, Jonas [Show all 8 contributors for this article] (2007). Characterizing bird migration phenology using data from standardized monitoring at bird observatories. Climate Research (CR). ISSN 0936-577X. 35(1-2), p. 59–77. doi: 10.3354/cr00714. Full text in Research Archive
• Krafft, Bjørn A.; Kovacs, K.M.; Ergon, Torbjørn; Andersen, Magnus; Aars, Jon & Haug, T. [Show all 7 contributors for this article] (2006). Abundance of ringed seals (Pusa hispida) in the fjords of Spitsbergen, Svalbard, during the peak molting period. Marine mammal science. ISSN 0824-0469. 22(2), p. 394–412. Full text in Research Archive Show summary

  Ringed seal (Pusa hispida) abundance in Spitsbergen, Svalbard, was estimated during the peak molting period via aerial, digital photographic surveys. A total of 9,145 images, covering 41.7% - 100% of the total fast-ice cover (1,496 km(2)) of 18 different fjords and bays, were inspected for the presence of ringed seals. A total of 1,708 seals were counted, and when accounting for ice areas that were not covered by images, a total of 3,254 (95% Cl: 3,071-3,449) ringed seals were estimated to be hauled out during the surveys. Extensive behavioral data from radio-tagged ringed seals (collected in a companion study) from one of the highest density fjords during the molting period were used to create a model that predicts the proportion of seals hauled out on any given date, time of day, and under various meteorological conditions. Applying this model to the Count data from each fjord, we estimated that a total of 7,585 (95% CI: 6,332-9,085) ringed seals were present in the surveyed area during the peak molting period. Data on interannual variability in ringed seal abundance suggested higher numbers of seals in Van Keulenfjorden in 2002 compared to 2003, while other fjords with very stable ice cover showed no statistical differences. Poor ice conditions in general in 2002 probably resulted ill seals from a wide area coming to Van Keulenfjorden (a large fjord with stable ice in 2002). The total estimated number of ringed seats present in the study area at the time of the survey Must be regarded as a population index, or at least a minimum estimate for the area, because it does not account for individuals leaving and arriving, which might account for a considerable number of animals. The same situation is likely the case for many other studies reporting aerial census data for ringed seals. To achieve accurate estimates of populations sizes from aerial Surveys, more extensive knowledge of ringed seat behavior will be required.

• Jonzén, Niclas; Lindén, Andreas; Ergon, Torbjørn; Knudsen, Endre; Vik, Jon Olav & Rubolini, Diego [Show all 17 contributors for this article] (2006). Rapid Advance of Spring Arrival Dates in Long-Distance Migratory Birds. Science. ISSN 0036-8075. 312(5782), p. 1959–1961. Full text in Research Archive
• Krafft, Bjørn A.; Kovacs, Kit M.; Andersen, Magnus; Aars, Jon; Lydersen, Christian & Ergon, Torbjørn [Show all 7 contributors for this article] (2006). Abundance of ringed seals (Pusa hispida) in the fjords of Spitsbergen, Svalbard, during the peak moulting period. Marine mammal science. ISSN 0824-0469. 22(2), p. 394–412. doi: 10.1111/j.1748-7692.2006.00035.x. Full text in Research Archive
• Loe, Leif Egil; Bonenfant, Christophe; Mysterud, Atle; Gaillard, JM; Langvatn, R & Klein, F [Show all 10 contributors for this article] (2005). Climate predictability and breeding phenology in red deer: timing and synchrony of rutting and calving in Norway and France. Journal of Animal Ecology. ISSN 0021-8790. 74, p. 579–588. doi: 10.1111/j.1365-2656.2005.00987.x. Full text in Research Archive
• Cavanagh, Rachel; Lambin, Xavier; Ergon, Torbjørn; Bennett, Malcolm; Graham, Isla M. & Soolingen, Dick Van [Show all 7 contributors for this article] (2004). Disease dynamics in cyclic populations of field voles (Microtus agrestis): cowpox virus and vole tuberculosis (Mycobacterium microti). Proceedings of the Royal Society of London. Biological Sciences. ISSN 0962-8452. 217(1541), p. 859–867. Full text in Research Archive Show summary

  The possible role of pathogens in rodent population cycles has been largely neglected since Elton�s �epidemic hypothesis� of 1931. To re-visit this question, twelve adjacent, cyclic, but out-of-phase populations of field voles (Microtus agrestis) in North East England were studied and the initial results are presented here. Cowpox virus antibody prevalence and the prevalence of clinical signs of Mycobacterium microti infection (vole TB) showed delayed (not direct) density dependence (with a lag of 3-6 months). This did not result from changes in population structure, even though there were such changes associated with the different phases of the cycle. The prevalences rose as vole numbers rose, and peaked as numbers declined. The apparent lag in the numerical response of infection prevalence to changes in host abundance is consistent with the hypothesis that diseases, singly or in combination, play a hitherto under-estimated role in the dynamics of cyclic populations.

• Ergon, Torbjørn; Speakman, John R.; Scantlebury, Michael; Cavanagh, Rachel & Lambin, Xavier (2004). Optimal body size and energy expenditure during winter: Why are voles smaller in declining populations? The American Naturalist. ISSN 0003-0147. 163, p. E442–E457. doi: 10.1086/381940. Full text in Research Archive Show summary

  Winter is energetically challenging for small herbivores because of greater energy requirements for thermogenesis at a time when little energy is available. We formulated a model predicting optimal wintering body size, accounting for the scaling of both energy expenditure and assimilation to body size, and the trade-off between survival benefits of a large size and avoiding survival costs of foraging. The model predicts that if the energy cost of maintaining a given body mass differs between environments, animals should be smaller in the more demanding environments and there should be a negative correlation between body mass and daily energy expenditure (DEE) across environments. In contrast, if animals adjust their energy intake according to variation in survival costs of foraging, there should be a positive correlation between body mass and DEE. Decreasing temperature always increases equilibrium DEE, but optimal body mass may either increase or decrease in colder climates depending on the exact effects of temperature on mass-specific survival and energy demands. Measuring DEE with doubly-labelled water on wintering Microtus agrestis at four field sites, we found that DEE was highest at the sites where voles were smallest despite a positive correlation between DEE and body mass within sites. This suggests that variation in wintering body mass between sites was due to variation in food quality/availability, and not adjustments in foraging activity to varying risks of predation.

• Speakman, John R.; Ergon, Torbjørn; Cavanagh, Rachel; Reid, K.; Scantlebury, Michael & Lambin, Xavier (2003). Resting and daily energy expenditures of free-living field voles are positively correlated but reflect extrinsic rather than intrinsic effects. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 100(24), p. 14057–14062. Full text in Research Archive
• Cavanagh, Rachel; Begon, Michael; Bennett, Malcolm; Ergon, Torbjørn; Graham, Isla M. & Haas, Petra E. W. de [Show all 12 contributors for this article] (2002). Mycobacterium microti Infection (Vole Tuberculosis) in Wild Rodent Populations. Journal of Clinical Microbiology. ISSN 0095-1137. p. 3281–3285. Full text in Research Archive Show summary

  Mycobacterium microti (vole tuberculosis) infections in small wild mammals were first described more than 60 years ago in several populations in Great Britain. Few studies of vole tuberculosis have been undertaken since then, and little is known about the relationship between M. microti isolates originating from different populations or at different times or of the prevalence of this infection in wild rodent populations, despite human cases of M. microti infections being increasingly reported. In this study, field votes (Microtus agrestis), bank voles (Clethrionomys glareolus), and wood mice (Apodemus sylvaticus) were found to be infected, with up to 8% having external tuberculous signs, in wild populations in Northumberland and Cheshire, England. Spoligotyping applied directly to the clinical material simultaneously detected and typed M.. microti bacteria in skin lesions, lymph glands, and internal abcesses. IS6110 restriction fragment length polymorphism typing of cultured bacteria was used to compare these isolates with previously isolated strains from both animals and humans. This demonstrated that although the current rodent isolates were distinct from those isolated from voles in the 1930s in Great Britain, they had a high degree of similarity to these strains and were distinct from the M. microti isolates from humans, a pig, and a ferret from The Netherlands. Thus, M. microti infection seems to be widespread in wild rodent populations, but more studies are needed to understand how M. microti might be transmitted from animals to humans and to determine better the zoonotic risk posed.

• Ergon, Torbjørn; Lambin, Xavier & Stenseth, Nils Christian (2001). Life-history traits of voles in a fluctuating population respond to the immediate environment. Nature. ISSN 0028-0836. 411, p. 1043–1045. Full text in Research Archive Show summary

  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.

• Ergon, Torbjørn; Lambin, Xavier; MacKinnon, James L.; Boonstra, Rudy & Stenseth, Nils Christian (2001). Mechanisms for delayed density-dependent reproductive traits in field voles, Microtus agrestis: the importance of inherited environmental effects. Oikos. ISSN 0030-1299. 95, p. 185–197. Full text in Research Archive Show summary

  Reproductive traits of voles vary with the phases of the population density fluctuations. We sought to determine whether the source of this variation resides in the individuals or in their environment. Overwintering field voles (Microtus agrestis) from two cyclic out-of-phase populations (increase and peak phases) were sampled in early spring and bred in the laboratory for two generations under standardised conditions with ambient light and temperature. Monitoring of the source populations by capture-mark-recapture showed large differences in reproductive performance. In the increase area, reproduction started six weeks earlier, the probability of maturation of young-of-the-year was more than ten times higher during mid-summer, and reproduction continued nearly two months later in the autumn than in the peak area. These differences were not found to be associated with a difference in age structure between the two areas (assessed by the distribution of eye lens masses from autopsy samples). Although the population differences in reproductive traits were to some degree also present among the overwintered animals in the laboratory, we found no difference in reproductive traits in the laboratory-born generations. There was a strongly declining seasonal trend in probability of sexual maturation both in the field and in the laboratory under ambient light conditions. However, in the field there were large population differences in the steepness of the seasonal decline that were not seen under the standardised laboratory conditions. We conclude that seasonal decline in maturation rates is governed by change in photoperiod, but that the population level variation in the shape of the decline is caused by a direct response to the environment and not due to variation in any intrinsic state of the individuals reflecting the environment experienced by the previous generation(s).

• Johansen, Beate Strøm; Kooij, Jeroen van der; Breistøl, Arild; Ergon, Torbjørn; Lislevand, Terje & Rognes, Knut (1997). Fjellrotte funnet i Agderfylkene. Fauna. ISSN 0014-8881. 50(4), p. 176–177. Full text in Research Archive

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• Ulvund, Kristine R.; Jackson, Craig Ryan; Rød-Eriksen, Lars; Hovstad, Knut Anders; Areskoug, Veronika & Thierry, Anne-Mathilde [Show all 10 contributors for this article] (2022). From captive to free – survival and reproduction in Arctic foxes. Full text in Research Archive
• Valpine, Perry de; Turek, Daniel; Ponisio, Lauren C.; Milleret, Cyril Pierre; Ergon, Torbjørn & Brøseth, Henrik (2019). Hierarchical statistical modeling with NIMBLE for occupancy, abundance, and spatial capture-recapture models. Full text in Research Archive
• Moghadam, Maunya D. & Ergon, Torbjørn (2015). Estimating the animal movements from lightbased data by using state-space modeling. Full text in Research Archive
• Ergon, Torbjørn (2014). Ghosts, demons and elusive species; Adaptive multi-method occupancy sampling for monitoring of multiple species. Full text in Research Archive
• Hagelin, Jon Valbjørn; Ergon, Torbjørn & Slagsvold, Tore (2014). Change in distribution of Norwegian alpine breeding birds in response to global warming. Full text in Research Archive
• Ergon, Torbjørn (2013). Separating mortality and emigration with robust-design spatial capture-recapture data. Full text in Research Archive
• Areskoug, Veronika; Landa, Arild Magne; Eide, Nina Elisabeth; Flagstad, Øystein & Ergon, Torbjørn (2013). Reproductive biology of arctic fox (Alopex lagopus) under unlimited food resources and competitor exclusion : An experimental enclosure approach. Full text in Research Archive
• Ergon, Torbjørn (2012). Separating mortality and emigration: modelling space use, dispersal and survival with robust-design spatial capture-recapture data. Full text in Research Archive
• Ergon, Torbjørn (2012). Variation in life-history, space-use and survival in a cyclic population of voles. Full text in Research Archive
• Ergon, Torbjørn & Gardner, Beth (2012). Separating mortality and emigration: Modeling space use, dispersal and survival with robust-design spatial capture-recapture data. Full text in Research Archive
• Ergon, Torbjørn (2012). Small rodent cycles and phenotypic plasticity. Full text in Research Archive
• Ergon, Torbjørn (2011). Førkveld. [TV]. NRK 1. Full text in Research Archive
• Ergon, Torbjørn & Skaug, Hans Julius (2008). Estimating distributions of correlated individual life-history traits from mark-recapture data. Full text in Research Archive
• Jonzén, Niclas; Ergon, Torbjørn; Lindén, Andreas & Stenseth, Nils Christian (2007). Bird Migration and Climate Change - Introduction. Climate Research (CR). ISSN 0936-577X. 35(1-2), p. 1–3. doi: 10.3354/cr00710. Full text in Research Archive
• Ergon, Torbjørn (2007). Klimaendringer og Norsk natur; Pattedyr og fugler. Full text in Research Archive
• Ergon, Torbjørn; Yoccoz, Nigel G. & Nichols, James D. (2007). Estimating latent time of maturation and survival costs of reproduction in continuous time from capture-recapture data. Full text in Research Archive
• Knudsen, Endre; Ergon, Torbjørn & Stenseth, Nils Christian (2007). Trekkfugler kvitrer om klima. p. "Kultur" 4–"Kultur" 4. Full text in Research Archive
• Ergon, Torbjørn & Stenseth, Nils Christian (2006). Unikt samarbeid mellom forskere og fuglekikkere. [Journal]. Uniforum. Full text in Research Archive
• Ergon, Torbjørn (2006). Bird Migration and Climate Change. Full text in Research Archive Show summary

  One of the great ecological concerns about climate change is that the phenology of different trophic levels respond differently, causing a mismatch between the timing of peak food requirements and peak food availability. Such mismatch may have severe consequences for individual fitness, population growth and ecosystem functioning. Species of long-distance migratory birds that breed at northern latitudes and overwinter in the tropics have no or little information about the climate at their breeding grounds when they depart from the winter quarters several weeks before breeding can take place. They cannot therefore respond plastically to year-to-year fluctuations in the spring phenology, but have instead evolved endogenous “clocks” that initiate migration northwards. In this talk I present evidence for an evolutionary change in the tuning of these clocks that cause the long-distance migratory birds to arrive earlier to their breeding areas. Nevertheless, it is uncertain whether the birds can ever catch up with the mismatch between food requirements and availability that is caused by climate change

• Krafft, Bjørn Arne; Kovacs, Kit M.; Ergon, Torbjørn; Andersen, Magnus; Aars, Jon & Haug, Tore [Show all 7 contributors for this article] (2005). Abundance of ringed seals (Pusa hipida) in the fjords of Spitsbergen, Svalbard, during the peak moulting period. Full text in Research Archive
• Ergon, Torbjørn; Lambin, Xavier; Ergon, Rolf; Fairbairn, Jonathan P.; Graham, Isla M. & MacKinnon, James L. [Show all 7 contributors for this article] (2005). DELAYED DENSITY-DEPENDENT ONSET OF SPRING REPRODUCTION IN FIELD VOLES. Full text in Research Archive Show summary

  Capture-recapture data on field voles (Microtus agrestis) from 9 years and 22 forest clear-cuts in Northern England show that onset of reproduction after the non-breeding winter season varies by more than 8 weeks between years and locations. About 57% of this variation can be explained by estimates of population density in the previous spring. On average, onset of reproduction is delayed by 24 days (95% CI: 13 to 31 days) for every additional 100 voles/ha in the previous spring. Reproduction was also initiated late after steep winter declines, leading to a pattern in which low spring densities were associated with late onset of reproduction.

• Ergon, Torbjørn; Lambin, Xavier; Speakman, John R. & Stenseth, Nils Christian (2001). Life-history traits and energy expenditure of voles in a fluctuating population respond to the immediate environment. Full text in Research Archive Show summary

  Body mass and reproductive traits are known to vary greatly among locations and from year to year in fluctuating populations of small rodents. By transplanting voles between sites during mid-winter we showed that large differences in body growth and timing of reproduction in the following spring were brought about by the immediate environment. Transplanted animals did not retain any characteristics of their source population. Energetic measurements during mid-winter show that the environmental conditions in the sites with small voles and late reproduction were energetically more demanding. Variation in the quality/availability of food plants is a likely explanation for the observed pattern.

• Ergon, Torbjørn; Lambin, Xavier; MacKinnon, James L.; Boonstra, Rudy & Stenseth, Nils Christian (1999). Phase-dependent reproductive traits in spring cohorts of field voles (Microtus agrestis): the importance of environmentally inherited effects in a cyclic population. Full text in Research Archive Show summary

  We investigated whether field voles in fluctuating out-of-phase populations show different reproductive performance under standardised lab-conditions. Although overwintering animals showed differences in reproductive traits, this effect was not carried over to later generations. The most prominent pattern observed related to the seasonal changes in the ambient lab-conditions.

• Molværsmyr, Sindre; Ergon, Torbjørn; Nilsson, Anna & Helberg, Morten (2019). Colony formation and breeding success in the rapidly declining Black-headed Gull. Universitetet i Oslo. Full text in Research Archive Show summary

  Black-headed Gulls are in rapid decline in Norway and is categorized as VU (Vulnerable) on the national Red List. The Norwegian population has decreased from 30 000 pairs around 1990 to just 7000 pairs in 2011. In the study area, the inner Oslo Fjord, the total breeding population counted about 3000 pairs in 2018. This study aims to explain variations in nest and chick survival both within and between colonies, with respect to date of individual nest settlement or colonies establishment, as well as stress elements and colony defence. Multiple colonies were monitored with a combination of nest cameras and drone footage throughout the breeding cycle. From this, data regarding formation, as well as nest and chick survival, were analysed. Of 3050 Black-headed Gull nests monitored in this thesis, one in five produced chicks. Pairs settling early in the breeding season had a much higher nest survival probability than those that settled later. This was true both within and between colonies. Larger colonies had higher nest survival than smaller colonies. The most observable reason for nest and colony failures were predation from Crows, large Gulls, Red Foxes and Badgers. Colonies that failed completely were likely to have a weaker colony defence due to behavioural changes, which made the colony more accessible for predators. Low food availability and a combination of other stress factors are presumed to have made many pairs in the colonies invest less energy in current reproduction and rather focus on own survival and future breeding attempts. From a population perspective, this strategy is sustainable for a few years, but if breeding seasons with low reproduction success are too frequent, the species decline will continue, possible until extinction in Norway.

• Gonzalez, Fernando Arce & Ergon, Torbjørn (2014). Spatiotemporal distribution of a small mammal assemblage at Finse: A methodological perspective. Universitetet i Oslo. Full text in Research Archive
• Edvardsen, Erik; Røer, Jan Erik; Solvang, Rune; Ergon, Torbjørn; Rafoss, Trond & Klaveness, Geir (2004). Bestandsovervåking ved Standardisert Fangst og Ringmerking ved Fuglestasjonene. Direktoratet for Naturforvaltning. Full text in Research Archive
• Ergon, Torbjørn H. (1996). On the effects of wound induced plant resistance on Norwegian lemmings (<EM>Lemmus lemmus</EM>) : responses at the individual level and their ecological consequences. [Mangler utgivernavn]. Full text in Research Archive

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