Eivind Østbye

Linking climate change to lemming cycles

The population cycles of rodents at northern latitudes have puzzled people for centuries, and their impact is manifest throughout the alpine ecosystem. Climate change is known to be able to drive animal population dynamics between stable and cyclic phases, and has been suggested to cause the recent changes in cyclic dynamics of rodents and their predators. But although predator–rodent interactions are commonly argued to be the cause of the Fennoscandian rodent cycles, the role of the environment in the modulation of such dynamics is often poorly understood in natural systems. Hence, quantitative links between climate-driven processes and rodent dynamics have so far been lacking. Here we show that winter weather and snow conditions, together with density dependence in the net population growth rate, account for the observed population dynamics of the rodent community dominated by lemmings (Lemmus lemmus) in an alpine Norwegian core habitat between 1970 and 1997, and predict the observed absence of rodent peak years after 1994. These local rodent dynamics are coherent with alpine bird dynamics both locally and over all of southern Norway, consistent with the influence of large-scale fluctuations in winter conditions. The relationship between commonly available meteorological data and snow conditions indicates that changes in temperature and humidity, and thus conditions in the subnivean space, seem to markedly affect the dynamics of alpine rodents and their linked groups. The pattern of less regular rodent peaks, and corresponding changes in the overall dynamics of the alpine ecosystem, thus seems likely to prevail over a growing area under projected climate change.

Effects of Snow Depth on Food and Habitat Selection by Roe Deer Capreolus capreolus along an Altitudinal Gradient in south-central Norway

Roe deer Capreolus capreolus food and feeding habitat selection was studied by snow tracking on transects along an altitudinal gradient in Flatdal, the county of Telemark, south-central Norway, during winter 1979/80. The main food was bilberry Vaccinium myrtillus (36.8%), rowan Sorbus aucuparia (24.4%) and arboreal lichens Alectoria sarmentosa, Bryonia spp., Hypogymnia physodes, Usnea spp. (17.4%), which were also the only preferred food plants. The amount of food removed from the field layer was stable through the winter. Roe deer used the lower part of the study area most heavily. At lower altitudes, older mature stands and edges between medium-aged stands and clear-cuts/plantations were preferred, pole-sized stands and edges between older mature stands and clear-cuts/plantations were used as expected from their availabilities, whereas medium-aged stands, clear-cuts and young plantations were avoided. That snow depth was an important factor determining habitat use by roe deer was evident in three ways: 1) use of areas at higher elevations decreased as winter progressed, 2) use of mature forest stands increased from early to late winter relative to open habitats, and 3) use of mature forest stands was more extensive at higher than at lower elevations compared to open habitats.

Effect of climate and density on individual and population growth of roe deer Capreolus capreolus at northern latitudes: the Lier valley, Norway

Abstract The growth of individuals and populations can be affected by both density-dependent and density-independent factors. Severe environmental conditions typically affect young and very old individuals more than prime-aged individuals, so that limiting factors such as climate and density frequently interact with the sex- and age-structure of the population. For roe deer Capreolus capreolus explicit analyses of growth rates of individuals and populations at northern latitudes are rare. In this article, we present the first analysis of a 17-year record of body weight data (N = 286) and harvest statistics (a proxy for population size) from the Lier valley, Norway. We tested whether climate (winter and spring) and population density affected individual body weight in autumn and the growth rate of the population as indicated by harvest statistics. We found that population growth rate in the Lier valley was negatively affected by increasing snow depth during winter. There was also a tendency for body weight to be lower after snowy than after less snowy winters. We found no significant effect of spring temperature or population density, though parameter estimates of both were negative. Our findings provide quantitative data supporting the ‘common knowledge’ that winter is the critical period for roe deer at northern latitudes, and that population density is unlikely to be a regulating factor in most inland areas of Norway with todays low population densities.

Paleoclimate deduced from a multidisciplinary study of a half-million-year-old stalagmite from rana, northern Norway

A 7-cm-thick flowstone sequence has been dated by extended Uranium series techniques to less than 1.25 myr, with a probable growth interval of ≥350,000–≤730,000 yr B.P. The time span is in accordance with paleomagnetic results revealing normal polarity for the whole sequence, i.e., a depositional age of <730,000 yr B.P. Oxygen isotope variations suggest deposition during three warm periods, interrupted by two isotopically cold hiatuses characterized by bulk resolution and detrital laminae. Calculations suggest that climatic transitions may have involved a shift of 1.1–2.4°C in mean annual temperatures. Contemporary flooding of the cave due to adjacent glacier expansion provides evidence for the widely held view that speleothem deposition is halted by glacier proximity. The flowstone matrix contained appreciable amounts of pollen (pine, birch) as well as larger amounts of charcoal dust. The pollen assemblage suggests a climate comparable with the present, implying that Norway has experienced at least three glacial/interglacial transitions during the growth interval of the speleothem. The warmer periods were characterized by a forest environment, with ample evidence of fire.

Comparing simple methods for ageing roe deer Capreolus capreolus: are any of them useful for management?

Abstract The dynamics of ungulate populations depend not only on the size, but also on the sex- and age-structure of the population. Successful management therefore depends on obtaining estimates of the age composition. Variation in performance due to age can be fairly well described by stages, and simple, rough methods for ageing cervids can therefore be useful to management. We assessed the performance of three relatively simple and objective methods based on tooth wear (height of molar), weight of eye lenses and diameter of pedicles (males only) on a sample of 77 female and 81 male European roe deer Capreolus capreolus from Lier, Norway. The relationship between tooth wear and age was linear, whereas the relationship between weight of eye lenses and diameter of pedicles was curvilinear with age, likely making them unreliable for old age classes. However, as only three males and six females ≥ 6 years old were included, we were unable to assess the uncertainty in age estimation for older age classes precisely. No simple method could precisely age roe deer, even up to five years of age. Our results do suggest that tooth wear, i.e. height of molar, can serve as a very simple and objective measure of age in roe deer, given that moderate precision (an error rate of ± 1 year and a success rate of 70% up to four years of age) is sufficient to reach management aims. As residuals between age estimates based on tooth wear and diameter of pedicle were not correlated, combining these methods improved the fit slightly. Since tooth wear may differ between areas, the scales presented here may perform less well in other areas, and a calibration for each area is clearly recommended.

Roe Deer Capreolus capreolus Feeding on Yew Taxus baccata in Relation to Bilberry Vaccinium myrtillus Density and Snow Depth

The extent of roe deer Capreolus capreolus feeding on the highly toxic yew Taxus baccata during winter was quantified from 1989–1994 in areas with varying density of bilberry Vaccinium myrtillus. Yew was most heavily harvested by roe deer in areas with low densities of bilberry. In areas with high densities of bilberry shrub, roe deer started browsing on yew when snow depth increased and reduced access to the field layer. However, in areas with low densities of bilberry, roe deer browsed heavily on yew also when snow was absent. Our observations suggest that roe deer may be the main factor affecting yew survival in areas where other preferred feeding plants are not available.

Roe deer Capreolus capreolus use of agricultural crops during winter in the Lier valley, Norway

The quality of common agricultural crops relative to natural plants was measured and the spatial feeding pattern of roe deer Capreolus capreolus on strawberry Fragaria ananassa fields in the Lier valley, southeastern Norway, were investigated during a winter (1992/93) with shallow snow depth. Strawberry plants were easily digestible, had a high mineral content and were readily harvested by roe deer. Distance to the nearest houses and forest edge affected the spatial pattern of feeding intensity differently when considered at between-field or within-field selection level. Distance to the forest edge or houses did not seem to affect choice of fields. Distance to the forest edge had no effect on feeding intensity within fields when fields were situated far from houses, but had a significant effect when fields were close to houses. When fields were far from the forest edge but close to houses, the distance to the houses but not to the forest edge affected within-field use by roe deer. Hence, roe deer seem to assess risk factors (distance to houses) and vary their response to the forest edge accordingly.

Application of a Model for Territorial Behaviour and Density Fluctuations in Alpine Passerines

A model for territorial behaviour developed by Maynard Smith is applied for interpreting data collected in a Norwegian alpine habitat on the density changes of passerine birds, especially the meadow pipit Anthus pratensis. Our data represent 12 breeding seasons. A basic assumption in application of the model is that the rate of snow-melt in spring may affect the synchrony of territorial establishment. The model predicts that territorial birds may exhibit density fluctuations as a result of varying degrees of synchronous territorial establishment. The analysis demonstrates no reason to reject this hypothesis so far. A brief discussion of the possible theoretical relation of such density fluctuations in passerines to small rodent cycles is presented.

Morphology and life history divergence in cave and surface populations of Gammarus lacustris (L.)

Cave animals provide a unique opportunity to study contrasts in phenotype and life history in strikingly different environments when compared to surface populations, potentially related to natural selection. As such, we compared a permanent cave-living Gammarus lacustris (L.) population with two lake-resident surface populations analyzing morphology (eye- and antennal characters) and life-history (size at maturity, fecundity and egg-size). A part of the cytochrome c oxidase subunit I gene in the mitochondrion (COI) was analyzed to contrast genetic relationship of populations and was compared to sequences in GenBank to assess phylogeography and colonization scenarios. In the cave, a longer life cycle was implied, while surface populations seemed to have a shorter life cycle. Egg size, and size at maturity for both sexes, were larger in the cave than in surface populations, while fecundity was lower in the cave than in surface populations. The cave population had longer first- and second antennae with more articles, longer first- and second peduncles, and fewer ommatidia than surface populations. The cold low-productive cave environment may facilitate different phenotypic and life-history traits than in the warmer and more productive surface lake environments. The trait divergences among cave and surface populations resembles other cave-surface organism comparisons and may support a hypothesis of selection on sensory traits. The cave and Lake Ulvenvann populations grouped together with a sequence from Slovenia (comprising one genetic cluster), while Lake Lille Lauarvann grouped with a sequence from Ukraine (comprising another cluster), which are already recognized phylogenetic clusters. One evolutionary scenario is that the cave and surface populations were colonized postglacially around 9 000–10 000 years ago. We evaluate that an alternative scenario is that the cave was colonized during an interstadial during the last glaciation or earlier during the warm period before onset of the last glaciation.