Edda Johannesen

Demographic Consequences of Movements in Subdivided Root Vole Populations

We studied three types of movements: (1) movements leading to permanent transfer of individuals between habitat patches, (2) movements (excursions) into habitat corridors and (3) into a barren matrix area, and the demographic consequences in 12 enclosed populations of the root vole, Micrototus oeconomus. Each population was subdivided into two demes inhabiting one habitat patch each. The two patches were approximately two male home range diameters apart in a vole-hostile, devegetated matrix. While the patches were connected by a narrow (0.5 m) habitat corridor for six of the populations, the other six populations inhabited isolated patches. The experiment was initiated by introducing laboratory raised founder demes onto each patch in the beginning of July. The populations were thereafter monitored by live trapping for the next 4-5 months during the snow-free season. After the snow melted the following spring the experiment was terminated by removal trapping. The experiment was run over two years (1994 and 1995) with six population replicates each year. Movements into the corridors and permanent transfers of animals between patches were registered during ordinary live trapping at 15-d intervals. Movements into the barren matrix habitat were registered continuously during the snow-free season by activating edge traps along the fences of the enclosures every night. Except for males early in the summer, movements leading to transfer of individuals between demes were rare relative to mortality and recruitment, and transfer did not act to synchronize the dynamics between demes. Moreover, transferred animals possessed the same survival probability as those staying in their natal deme. Corridors slightly enhanced the rate of transfer in females, but not in males. Excursions into corridors and the matrix area took place much more frequently than transfer and most frequently in the first cohorts early in the season and more in males than in females. Movements in the hostile matrix had a considerable negative effect on survival and, thus on the demography of the populations. Predation by birds is the most likely cause of this movement related mortality which may play an important role in the dynamics of patchy vole populations during the snow-free season. Movements into the corridors did not have any independent effect on survival probability, and habitat corridors may thus act to transfer animals more safely from one patch to another.

Modelling Survival Rates: Habitat Fragmentation and Destruction in Root Vole Experimental Populations

The effects of habitat destruction and fragmentation on survival rate and population size were tested in an experimental model system consisting of seven enclosed populations of root voles, Microtus oeconomus. The study comprised two consecutive parts. First, the effect of habitat fragmentation was investigated by comparing survival in three populations inhabiting continuous habitat with that in four populations inhabiting frag- mented habitat. Second, the continuous habitats were experimentally fragmented by de- stroying 50% of the area inhabited by the populations, producing the same configuration as in the fragmented habitats. These extensive habitat manipulations were expected to affect survival rates due to changes in the spatiosocial structuring of the populations. Throughout the study, populations were monitored by an intensive live-trapping program. Cormack- Jolly-Seber methodology was used for statistical modelling and estimation of survival rates. The seven experimental populations differed considerably in population dynamics: some grew, while others remained remarkably stable throughout the experiment. In contrast to expectations, neither population size nor survival rates seemed to be affected by the ex- perimental treatments. Survival rate seemed to play a minor role in determining population size. Emigration rates, which also seemed to have little direct demographic importance, were considerably higher in fragmented than in continuous habitats during the first (pre- destruction) part of the study. Surprisingly, this difference persisted after habitat destruction, when all populations experienced the same habitat configuration. We conclude that survival rates in these root vole populations were not dependent on major alterations of spatiosocial structure, possibly owing to adaptation to natural distur- bance regimes.

Effect of Radiocollars on Survival of Root Voles

Radiocollars, which commonly are used to study mammals, might increase the mortality of the animals studied. We used Cormack-Jolly-Seber modeling on data of root voles ( Microtus oeconomus ), to test whether an increase in mortality due to radiocollars occurred. Higher mortality was not detected in animals wearing collars compared with animals that did not wear collars.

A demographic analysis of vole population responses to fragmentation and destruction of habitat

Abstract Fragmentation and destruction of natural habitats is currently considered to be the major threat to wildlife populations. We here perform a comprehensive analysis of the demographic effects of habitat fragmentation and destruction on 14 populations of the root vole. The experiment was divided into two consecutive periods. During the first period, we contrasted populations with the same initial size and structure in continuous and fragmented habitat. During the second period, we fragmented the continuous habitat into the same configuration as the permanently fragmented habitat so that the effect of habitat destruction could be evaluated. We estimated survival and fecundity parameters and combined them into population projection matrices to evaluate their relative impact on population growth. In the first period of the experiment there was no difference in population growth rate between fragmented and continuous populations, although litter size was significantly higher in the continuous populations. In the second period, we found higher population growth rates in populations that had experienced habitat destruction. By applying the transition matrix model to empirical estimates of demographic parameters, we demonstrate that the difference in population growth rate in the second period of the experiment was the result of a nonsignificant difference in adult survival. Movements out of the habitat patches were significantly lower in populations that had experienced habitat destruction. We conclude that predator-caused mortality of animals moving out of the habitat patches was the main determinant of demographic variation in this system.

The effect of breeding success on nest and colony fidelity in the Little Penguin (Eudyptula minor) in Otago, New Zealand

Abstract Both individual breeding success and breeding success of conspecifics can be used to indicate breeding-site quality and thus to determine site fidelity. Using data taken from two adjacent colonies differing in quality, we studied site fidelity of Little Penguins (Eudyptula minor) at two spatial scales: the nest and the colony. We tested for the effect of individual breeding success, nest type (natural and nest boxes) and mate change on nest fidelity. Nest fidelity was highest for penguins with high breeding success and for those that bred with the same partner as in the previous year. Within the same colony, penguins were more faithful to boxes than to natural nests. We tested for the effect of both individual and conspecific breeding success on colony fidelity. Over the five years of study, there were no recorded movements of breeding birds between the two study colonies. There was no evidence for any effect of conspecific breeding success on colony fidelity. Individual breeding success was found to have an effect on colony fidelity: successful breeders were more likely than unsuccessful breeders to return to breed in the colony the subsequent year.

Density-dependent variation in body mass of voles

We studied inter-annual, spatial and sexual variation in the body mass of bank volesMyodes glareolus Schreber, 1780 and grey-sided volesMyodes rufocanus Sundevall, 1846 using live trappings from two grids on the southand north-facing slopes of a mountain valley in Southern Norway. Variation in spring density of the four populations was consistent with cyclic dynamics (n=7,s-values >0.5). Individuals caught on the south-facing slope were larger than those caught on the north-facing slope. Reproductively mature bank vole males were smaller than females, whereas reproductively mature grey-sided vole males were larger than females. Body mass was related to density in both species. In bank voles, we found a direct positive density dependence caused by a higher rate of survival at higher densities resulting from individual allocation of resources from reproduction to survival and growth. In grey-sided voles, we found a negative delayed density dependence resulting from grazing on preferred plants that determined the resource available for individual vole growth the following year.