Predator–vole interactions in northern Europe: the role of small mustelids revised

The cyclic population dynamics of vole and predator communities is a key phenomenon in northern ecosystems, and it appears to be influenced by climate change. Reports of collapsing rodent cycles have attributed the changes to warmer winters, which weaken the interaction between voles and their specialist subnivean predators. Using population data collected throughout Finland during 1986–2011, we analyse the spatio-temporal variation in the interactions between populations of voles and specialist, generalist and avian predators, and investigate by simulations the roles of the different predators in the vole cycle. We test the hypothesis that vole population cyclicity is dependent on predator–prey interactions during winter. Our results support the importance of the small mustelids for the vole cycle. However, weakening specialist predation during winters, or an increase in generalist predation, was not associated with the loss of cyclicity. Strengthening of delayed density dependence coincided with strengthening small mustelid influence on the summer population growth rates of voles. In conclusion, a strong impact of small mustelids during summers appears highly influential to vole population dynamics, and deteriorating winter conditions are not a viable explanation for collapsing small mammal population cycles.

Effect of cyclic and declining food supply on great grey owls in boreal Sweden

In this study of 35 years of data, we examine the short-term (cyclic) and long-term relationship between breeding success of great grey owls ( Strix nebulosa Forster, 1772) and their food supply (bank voles ( Clethrionomys glareolus (Schreber, 1780)), grey-sided voles ( Clethrionomys rufocanus (Sundevall, 1846)), and field voles ( Microtus agrestis (L., 1761))) in northern Sweden. Annual number of owl nests showed a 3 year cyclicity, which as predicted, corresponded to the length of the vole cycle in the region. Mean annual brood size also fluctuated and was positively dependent on the vole supply during the same spring. In this region, there has also been a decline in vole numbers in recent decades, from high-amplitude cycles in the 1970s to subsequent low-amplitude cycles. Correspondingly, and as predicted, mean annual brood size of the owls also declined, although only during the third years of the vole cycle when vole supply in spring and brood size of the owls is at its highest level in high-amplitude cycles. We predict that in the long run the vole decline, associated with increasingly milder winters, and the reduction of the brood size of the owls, especially in years of high owl breeding success, will have serious implications for the population of great grey owls in Scandinavia.

Reproduction and Survival in a Variable Environment: Ural Owls (Strix Uralensis) and the Three-Year Vole Cycle

We analyzed data on 535 Ural Owl (Strix uralensis) breeding attempts and consecutive survival of both adults and offspring from 1987–1998 in relation to the regional abundance of the Ural Owl's main prey, voles, which show a cycle of low, increase, and peak phases in their population numbers. Vole abundance varied up to 49×, crashing during spring–summer every three years. The breeding population tracked abundance of voles in the previous autumn with respect to percentage of pairs breeding and their reproductive output (laying date, clutch size), largely irrespective of phase. Survival depended on vole density in the preceding autumn, but was generally highest in the increase phase. There was thus a paradoxical situation in the peak phases, when vole populations crashed; the owls produced large clutches, but those survived poorly. Some adaptive and nonadaptive scenarios of the Ural Owl's life history are discussed.

Seasonal dynamics of Pneumocystis carinii in the field vole, Microtus agrestis, and in the common shrew, Sorex araneus, in Finland

Seasonal dynamics of Pneumocystis carinii in the field vole, Microtus agrestis, and in the common shrew, Sorex araneus, were investigated in southern and central Finland by microscopical examination of methenamine silver-stained tissue sections. In both host species at both localities the number of P. carinii cyst forms was highest in late autumn (November). In S. araneus, prevalence was higher than in M. agrestis during all seasons. None of the animals was heavily infected or apparently ill, and neither species showed any extrapulmonary dissemination. In this study covering an increase phase and 4 peak host-density phases of the vole cycle, the occurrence of P. carinii seemed to be related to the population density of M. agrestis.

Diet composition, prey choice, and breeding success of Long-eared Owls: effects of multiannual fluctuations in food abundance

Common voles (Microtus epiroticus) were the main prey of Long-eared Owls (Asio otus) breeding in western Finland during 1977 – 1989. They constituted, on average, 58% (range < 15 to > 75%) of the number of prey. Field voles (Microtus agrestis), common shrews (Sorex araneus), bank voles (Clethrionomys glareolus), and water voles (Arvicola terrestris) were the most frequent alternative prey. The owls showed strong preference for common voles over the alternative prey species and this preference increased with vole density. If the common vole is the most profitable prey of Long-eared Owls, the between-years variation in the diet was consistent with the three predictions of the conventional model of the optimal diet theory. (1) Predators should feed on the most valuable prey type when prey are abundant. There was a positive correlation between the abundance of common voles and their proportions in the food. (2) No relationships for alternative prey types agreed with the prediction that the inclusion of a prey type in the diet depends only on the abundance of the preferred prey. (3) As predicted by the optimal diet theory, the diet width expanded when the density of common voles decreased and shrank when vole density increased. Annual breeding density (range 0.0–0.4 nests/km2), mean clutch size (3.0 – 6.3), and mean brood size (0.0 – 3.5) were positively related to the spring abundance of common and field voles. Accordingly, it is adaptive for Long-eared Owls to breed in a good area for voles. Because vole abundances in Fennoscandia fluctuate markedly between years and asynchronously between areas, Long-eared Owls should stay in the same area in the increase phase of the vole cycle, but move away when voles decline.