Harry P. Andreassen

Spatial synchronization of vole population dynamics by predatory birds

Northern vole populations exhibit large-scale, spatially synchronous population dynamics. Such cases of population synchrony provide excellent opportunities for distinguishing between local intrinsic and regional extrinsic mechanisms of population regulation. Analyses of large-scale survey data and theoretical modelling have indicated several plausible synchronizing mechanisms. It is difficult, however, to determine the most important one without detailed data on local demographic processes. Here we combine results from two field studies in southeastern Norway—one identifies local demographic mechanisms and landscape-level annual synchrony among 28 enclosed experimental populations and the other examines region-level multi-annual synchrony in open natural populations. Despite fences eliminating predatory mammals and vole dispersal, the growth rates of the experimental populations were synchronized and moreover, perfectly linked with vole abundance in the region. The fates of 481 radio-marked voles showed that bird predation was the synchronizing mechanism. A higher frequency of risky dispersal movements in slowly growing populations appeared to accelerate predation rate. Thus, dispersal may induce a feedback-loop between predation and population growth that enhances synchrony.

The return of the giants: ecological effects of an increasing elephant population.

Abstract Northern Botswana and adjacent areas, have the worlds largest population of African elephant (Loxodonta africana). However, a 100 years ago elephants were rare following excessive hunting. Simultaneously, ungulate populations were severely reduced by decease. The ecological effects of the reduction in large herbivores must have been substantial, but are little known. Today, however, ecosystem changes following the increase in elephant numbers cause considerable concern in Botswana. This was the background for the “BONIC” project, investigating the interactions between the increasing elephant population and other ecosystem components and processes. Results confirm that the ecosystem is changing following the increase in elephant and ungulate populations, and, presumably, developing towards a situation resembling that before the reduction of large herbivores. We see no ecological reasons to artificially change elephant numbers. There are, however, economic and social reasons to control elephants, and their range in northern Botswana may have to be artificially restricted.

Mice, rats, and people: the bio‐economics of agricultural rodent pests

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SPACE‐USE RESPONSES TO HABITAT FRAGMENTATION AND CONNECTIVITY IN THE ROOT VOLE MICROTUS OECONOMUS

We radiotracked 184 reproductive active root vole Microtus oeconomus in- dividuals in 12 experimentally fragmented populations to test predictions regarding re- sponses in spatiosocial organization to habitat fragmentation and connectivity. We used two genetically distinct strains with different intrinsic characteristics to test explicitly for interactions between spacing behavior and the degree of habitat fragmentation and con- nectivity. The experimental habitat configurations were created by mowing enclosed mead- ow plots so that habitat (meadow) fragments were imbedded in a hostile, barren matrix of nonhabitat. We used three types of habitat patch configurations to contrast two levels of fragmentation (large vs. small habitat fragments) and two levels of connectivity (small isolated vs. small corridor connected habitat fragments), while keeping the total area of habitat and distances between fragments constant. The least fragmented systems contained two habitat fragments large enough to encompass several female home ranges (large frag- ment plots). Both of the two other habitat configurations contained six small habitat frag- ments, each of the size corresponding to the home range of a single female. The two small fragment configurations differed with respect to connectivity in that one configuration had 0.5 m wide corridors connecting two triplets of patches (corridor plots), while the other small patch configuration had no corridors (small fragment plots). Four population replicates (two of each root vole strain) of each configuration were employed over two years. Home range area estimates showed rather complex responses to habitat fragmentation depending on strain and sex. This was expected as the two strains and males and females had been previously shown to exhibit different space requirements. Core areas were, how- ever, unaffected by fragmentation pattern at the spatial scale employed. The most pro- nounced and consistent responses to habitat fragmentation concerned movement rates for both sexes and the degree of space sharing between matrilineally related and unrelated females. The frequency of home range overlaps (irrespective of matrilineal relatedness and strain) was highest in the large-fragment plots and lowest in the small-fragment plots. The degree of overlap between matrilineally related females showed a reverse trend, however, indicating that space sharing was more directed towards kin in the most fragmented systems. The rate of interfragment movements increased with habitat fragmentation. Corridors in- duced more individuals to move between the smallest fragments. Males generally moved more frequently between fragments than did females. Our results match the hypothesis predicting fusion and fission responses based on intrinsic social mechanisms in patchy populations.

DISPERSAL IN PATCHY VOLE POPULATIONS: ROLE OF PATCH CONFIGURATION, DENSITY DEPENDENCE, AND DEMOGRAPHY

We studied dispersal movements in 12 enclosed, patchy populations of root voles (Microtus oeconomus) during the breeding season. The habitat was manipulated experimentally so that there were four replicates of three types of habitat patch configuration: two large patches, six small isolated patches, and six small patches of which two patch triplets were connected by corridors. The total habitat area (1350 m2) and the distance to nearest neighboring patch (15 m) were the same in the three different configurations. The matrix area between the patches was kept open and uninhabitable for voles by weekly mowing. Dispersal was defined as shifts between patches. Movements across the open matrix habitat to traps located along the edge of the enclosures were also recorded. The frequency of shifts between patches was higher among small than among large patches. Corridors channeled dispersal between corridor-connected patches but did not enhance the frequency of shifts between patches at the population level. Disper...

Effects of spatiotemporal scale on autocorrelation and home range estimators

We compared 3 home range estimators (kernel estimator [Kernel], multiple polygons by clustering [Cluster], and minimum convex polygon [MCP]) and evaluated a measure of autocorrelation (Schoeners ratio), with respect to the effects of sampling frequency, spatial resolution of the sampling reference grid, and sample size. We also used Schoeners ratio as a descriptor of within home range movements. An extensive dataset from radiotracking of root voles (Microtus oeconomus) formed the basis for these comparisons. The degree of autocorrelation was sex specific. In particular, locations of reproductive females were significantly autocorrelated for a sampling interval equal to the period of the populations ultradian activity rhythm, indicating territory patrolling behavior in this sex. We assessed the effect of spatial resolution of animal location data on home range descriptors by manipulating the cell size of the sampling reference grids. The Kernel estimator was robust to changes in spatial resolution of the data. In contrast, the polygon estimates were severely biased upwards with decreasing spatial resolution (increasing grid cell size). Therefore, comparative studies based on polygon methods must use the same spatial resolution. The sampling frequency affected all estimators, but qualitative differences were found among the specific estimators. Numerical resampling methods indicated that home range sizes were underestimated, and that the precision of the estimators was generally low.

Density-dependent dispersal and spatial population dynamics.

The synchronization of the dynamics of spatially subdivided populations is of both fundamental and applied interest in population biology. Based on theoretical studies, dispersal movements have been inferred to be one of the most general causes of population synchrony, yet no empirical study has mapped distance-dependent estimates of movement rates on the actual pattern of synchrony in species that are known to exhibit population synchrony. Northern vole and lemming species are particularly well-known for their spatially synchronized population dynamics. Here, we use results from an experimental study to demonstrate that tundra vole dispersal movements did not act to synchronize population dynamics in fragmented habitats. In contrast to the constant dispersal rate assumed in earlier theoretical studies, the tundra vole, and many other species, exhibit negative density-dependent dispersal. Simulations of a simple mathematical model, parametrized on the basis of our experimental data, verify the empirical results, namely that the observed negative density-dependent dispersal did not have a significant synchronizing effect.

Supplemental feeding of migratory moose Alces alces: forest damage at two spatial scales

Landowners in southeast Norway have supplied moose Alces alces with ensilaged bales of hay for up to eight winters. The incentive has been to limit migratory movements into heavily trafficated winter areas and away from young forest plantations. In this article, we report on landscape characteristics that increase the use of feeding stations, and on how browsing activity on Scots pine Pinus sylvestris twigs as well as leader stems of both pine and Norway spruce Picea abies was associated with distance to feeding stations at two different spatial scales: 1) at a local scale we sampled data from 50 m2 observational plots up to 200 m from the feeding stations, and 2) at a regional scale we sampled data at all available young forest plantations up to 7 km from a feeding station. The probability that a feeding station was used increased towards the bottom of side valleys. The frequency of use of feeding stations increased annually, with increasing distance to other feeding stations, and with increasing distance to the more populated main valley. Moose-induced browse damage was extensive at proximate distances (< 200 m) to feeding stations, decreased to a minimum at 1–2 km, and increased slightly again at 3–7 km distance from less intensely used feeding stations. Indices of moose activity distribution (i.e. piles of faecal pellets) resembled browsing activity.

THE EFFECT OF SCENT‐MARKING, FOREST CLEARING, AND SUPPLEMENTAL FEEDING ON MOOSE‐TRAIN COLLISIONS

Abstract We analyzed how the application of scent-marking, forest clearing, and supplemental feeding correlated with the number of moose (Alces alces)–train collisions along the most vulnerable railroad stretch in Norway. Data on 1,045 collisions has been compiled for 18 years since 1985, and remedial actions have occurred during various periods since 1990. We used sections of the rail line where remedies had never been applied as control sections to estimate the expected number of collisions per year and per km. In this way, we took into account the yearly variation in the number of accidents by using the difference between the actual number of accidents and the expected number of accidents as our response variable. We compared the difference between periods when remedies were applied to periods without any remedy. We found a general 46% decrease in the number of accidents during years with a remedy compared to what would have been expected the same years without any remedy. Forest clearing and supplemental feeding seem to be reliable ways of reducing the number of collisions. Scent was only applied for short distances in a few years, and the beneficial effects we observed were questionable. We conclude that mitigative efforts may substantially reduce accidental mortality in moose populations if applied for long distances. We discuss the economics of game-vehicle collisions by performing a simple calculation to visualize the need for a bio-economic approach to the problem.

Discontinuous habitat corridors : effects on male root vole movements

We tested how movement rates of male root voles were affected by discontinuities (gaps) in a 1-m wide habitat corridor. Gaps of increasing size (0.25-4m) were created experimentally by mowing the vegetation. Movement rates (recorded by radiotelemetry) were compared between the manipulated corridor and a contemporal, unmanipulated (continuous) control corridor. Male root voles did not respond to gaps until the gap size became 4m which is equivalent to 10-20% of the normal home range diameter. Gap sizes of 4 m in the corridor decreased the movement rates significantly and to the same degree for two behaviourally distinct strains of root voles. The similar responses of the two vole strains indicated that the results may have validity beyond the particular experimental setting employed.