How predation and landscape fragmentation affect vole population dynamics

Background: Microtine species in Fennoscandia display a distinct north-south gradient from regular cycles to stable populations. The gradient has often been attributed to changes in the interactions between microtines and their predators. Although the spatial structure of the environment is known to influence predator-prey dynamics of a wide range of species, it has scarcely been considered in relation to the Fennoscandian gradient. Furthermore, the length of microtine breeding season also displays a north-south gradient. However, little consideration has been given to its role in shaping or generating population cycles. Because these factors covary along the gradient it is difficult to distinguish their effects experimentally in the field. The distinction is here attempted using realistic agent-based modelling. Methodology/Principal Findings: By using a spatially explicit computer simulation model based on behavioural and ecological data from the field vole (Microtus agrestis), we generated a number of repeated time series of vole densities whose mean population size and amplitude were measured. Subsequently, these time series were subjected to statistical autoregressive modelling, to investigate the effects on vole population dynamics of making predators more specialised, of altering the breeding season, and increasing the level of habitat fragmentation. We found that fragmentation as well as the presence of specialist predators are necessary for the occurrence of population cycles. Habitat fragmentation and predator assembly jointly determined cycle length and amplitude. Length of vole breeding season had little impact on the oscillations. Significance: There is good agreement between our results and the experimental work from Fennoscandia, but our results allow distinction of causation that is hard to unravel in field experiments. We hope our results will help understand the reasons for cycle gradients observed in other areas. Our results clearly demonstrate the importance of landscape fragmentation for population cycling and we recommend that the degree of fragmentation be more fully considered in future analyses of vole dynamics

Experimental demonstration of the antiherbivore effects of silica in grasses: impacts on foliage digestibility and vole growth rates

The impact of plant-based factors on the population dynamics of mammalian herbivores has been the subject of much debate in ecology, but the role of antiherbivore defences in grasses has received relatively little attention. Silica has been proposed as the primary defence in grasses and is thought to lead to increased abrasiveness of foliage so deterring feeding, as well as reducing foliage digestibility and herbivore performance. However, at present there is little direct experimental evidence to support these ideas. In this study, we tested the effects of manipulating silica levels on the abrasiveness of grasses and on the feeding preference and growth performance of field voles, specialist grass-feeding herbivores. Elevated silica levels did increase the abrasiveness of grasses and deterred feeding by voles. We also demonstrated, for the first time, that silica reduced the growth rates of both juvenile and mature female voles by reducing the nitrogen they could absorb from the foliage. Furthermore, we found that vole feeding leads to increased levels of silica in leaves, suggesting a dynamic feedback between grasses and their herbivores. We propose that silica induction due to vole grazing reduces vole performance and hence could contribute to cyclic dynamics in vole populations

Temporal and spatial dynamics of willow grouse Lagopus lagopus

This study is a first attempt to analyze large-scale willow grouse (Lagopus lagopus) population dynamics. In this thesis I studied the population dynamics of willow grouse in the Swedish mountain range. I estimated the scale of synchrony in both breeding success, and the adult segment between populations throughout the mountain range by using time-series of harvest data and population densities estimated from line transect counts. Little evidence of regular fluctuations/cycles in juveniles in autumn was found, and chick production in willow grouse appears to fluctuate more irregularly than was previously believed. Breeding success estimated as number of juveniles per two adults in autumn, showed a spatial correlation between sites up to 200 km apart, but there was no spatial correlation in the adult segment of the populations. When analyzing the presence and strength of time-lags in density dependent processes, a first order process was detected in the southernmost region, and a combination of weak first and second order processes in the northern part of the mountain range. There was a positive relationship between breeding success the previous year and the intrinsic rate of increase in the northernmost region where first and second order density dependence was weak and of equal strength. No such relationship was found in the southern region where density dependence was stronger and dominated by a first order process. Natal dispersal resulted in 80% of the juvenile females dispersing more than 5 km from their summer area, with a mean dispersal distance of 10.2 km, three times further than for juvenile males (3.4 km). Adult females were migratory between wintering areas used as a juvenile and their first breeding site. At landscape scales, only juvenile females and migratory adult females are important in exchange of individuals between populations. Hunters’ effort provided a useful relationship with harvest rate. Setting limits to the totally allowable effort within an area has a stronger potential for controlling harvest than daily bag limits or adjusting the length of the open season. Another simple harvest strategy would be to prohibit harvest in parts of the total area hunted. By setting aside a part of the total area as a buffer, and placing limits on the harvest effort in the open parts of the hunting area, it would be possible to achieve a cost-efficient system with a small risk of over-harvesting

Interactions between landscape changes and host communities can regulate echinococcus multilocularis transmission

An area close to the Qinghai-Tibet plateau region and subject to intensive deforestation contains a large focus of human alveolar echinococcosis while sporadic human cases occur in the Doubs region of eastern France. The current review analyses and compares epidemiological and ecological results obtained in both regions. Analysis of rodent species assemblages within quantified rural landscapes in central China and eastern France shows a significant association between host species for the pathogenic helminth Echinococcus multilocularis, with prevalences of human alveolar echinococcosis and with land area under shrubland or grassland. This suggests that at the regional scale landscape can affect human disease distribution through interaction with small mammal communities and their population dynamics. Lidicker's ROMPA hypothesis helps to explain this association and provides a novel explanation of how landscape changes may result in increased risk of a rodent-borne zoonotic disease

Additional Comments on Reproductive Strategies and Population Fluctuations in Microtine Rodents

Recently, Schaffer and Tamarin (1973) proposed a model relating changes in reproductive effort (RE) to fluctuating densities in microtine rodents (lemmings and voles). They assumed (and presented data supporting this assumption) that the major effect of increased crowding would be a reduction in survival among prereproductives, thereby lowering the effective fecundity (Schaffer and Rosenzweig 1977) of their parents. As a consequence, Schaffer and Tamarin argued that the optimal reproductive expenditure, E(N), should decline with increasing population size, N. They also deduced the shape of the zero-growth isocline, N*(E), for differing levels of RE and plotted both E(N) and N*(E) on a graph whose axes are reproductive expenditure and population density (Fig1 a)

Space use and habitat affinities of the singing vole on the northern foothills of the Brooks Range, Alaska

Arctic tundra is being affected by a rapidly warming climate, which is accompanied by shifts in plant community composition and structure. Shrub expansion, a predominant consequence of this warming, is linked with changes in nutrient cycling and has direct implications to global change biology. Habitats are being altered across the landscape, with subsequent changes to arctic faunal communities. While herbivory has been noted as important in contributing to plant community composition in the arctic, with the potential to both exacerbate and mitigate shifts toward shrub-dominated tundra landscapes, little research has been conducted on herbivore dynamics. Microtine rodents (i.e., voles and lemmings) are the dominant vertebrate herbivores in the Alaskan Arctic. Through mark-recapture surveys and analysis of individual and population-level space use, I studied the population ecology of the microtine community to better establish the role of these small mammals in this rapidly changing region. Co-dominant species, the singing vole and the tundra vole, exhibit limited ecological overlap, preferring different habitat types and food sources. Results from surveys confirmed previously documented spatial segregation of the two co-dominant species by habitat along a moisture gradient. Interpretation of results suggest that extrinsic factors, possibly relating to stochastic winter climatic events impact these co-dominant species differently. Over the duration of the study, the singing vole was locally more abundant despite preferring regionally less available habitat, which suggests that its habitat may buffer singing vole populations from the affects of stochastic events. Analysis of space use by the singing vole indicated that both intraspecific interaction and microhabitat affinities played a role in local scale space use, which, through selective herbivory and concentrated deposition of nutrients, has implications on its role in structuring tundra plant communities. Further research on these species over a longer duration will classify the impact of extrinsic factors on population dynamics and the impact of resource use on local and landscape level changes to the tundra ecosystem

Population Dynamics of \u3ci\u3eMicrotus Ochrogaster\u3c/i\u3e in Eastern Kansas

Four eastern Kansas populations of the prairie vole, Microtus ochrogaster were live trapped from 1970—73 to gain insight into the population regulation of this species. All four populations exhibited a 2—yr cycle in numbers with peak densities generally occurring in June 1972. Peak densities were followed by a decline in numbers, a recovery, and a population crash in spring 1973. Reproductive parameters changed dramatically as density rose and fell. The summer breeding season in the crash year of 1973 was shortened by at least 3 mo. A reduction in breeding activity occurred during the summer of every year of the study. The highest amount of reproduction occurred during the spring and fall. More voles were breeding during the winter before the peak year (1971—72) than during either the preceding or succeeding winter. There was no deviation from a sex ratio of 1 in the populations. Mortality rates had a strong impact on changes in numbers. Survival rates of juveniles and subadults in the population were significantly lower than adults in the summer breeding season. Adults survived better during winter than during summer. Survival of ♂ ♂ and ♀ was correlated and was relatively low during episodes of decreasing density. The survival of voles between weaning and trappable size was high during periods of increasing density and low during periods of declining density. A multiple regression analysis was performed to determine the relative usefulness of four demographic variables in predicting mean rate of population increase. The analysis indicated that early juvenile and ♀ survival are the best predictors of population growth. Growth of voles in the populations was assessed from body weight distributions and instantaneous growth rate per body weight. There was no shift in body weight toward heavier animals in peak populations and instantaneous growth rates were erratic. Finally, inconsistencies in these results compared with other microtine studies, such as the short peak phase, lack of a well—defined breeding season, a summer breeding depression, and the absence of a shift towards heavier animals in the peak phase, are discussed in relation to a single or multifactor hypothesis for explaining population cycles

Ecological Distribution of Small Mammals in the De Long Mountains of Northwestern Alaska

The ecological distribution of small mammals (<200 g) was studied in the foothill tundra of the De Long Mountains in northwestern Alaska. Three species of shrews and five species of microtine rodents were trapped on 15 live-trapping grids during 1978 and 1979. Emphasis was placed on the three most abundant microtine species (Clethrionomys rutilus, Microtus oeconomus and M. gregalis). During late summer up to six species of small mammals were captured per habitat type which ranged from wet meadow through mesic shrubland to dry ridges. Following snowmelt most habitats contained only a single species and some contained none. Only four habitat types were continuously occupied by small mammals during both summers. Species diversity was variable among habitats. Most species of small mammals were captured on eight or fewer of the 15 trapping grids. Only Clethrionomys rutilus was captured on all grids. The number of habitats occupied by Clethrionomys rutilus increased from 4 to 14 as population densities increased. The number of habitats occupied by the other species seemed to be independent of population density. Average population densities for the microtines were low (<15/ha) and, for each species, varied according to habitat type. Only Clethrionomys rutilus populations demonstrated marked intra-annual fluctuations (3/ha to 37/ha).Key words: rodents, habitat, Alaska, populations, Clethrionomys rutilus, Micro oeconomus, Microtu gregalisMots clés: rongeurs, habitat, Alaska, populations, Clethrionomys rutilus, Microtus oeconomus, M. gregali