Minna Koivula

WINTER FOOD SUPPLY LIMITS GROWTH OF NORTHERN VOLE POPULATIONS IN THE ABSENCE OF PREDATION

Mathematical models have suggested that population cycles of northern voles are generated by a combined effect of delayed and direct density-dependent mechanisms. Predation is considered to be the most likely mechanism affecting vole populations in a delayed density-dependent manner. We conducted a replicated two-factor experiment with the field vole (Microtus agrestis) during 1999-2001 in western Finland, manipulating both predation rate and winter food supply to evaluate whether a shortage of winter food has the potential to limit the growth of vole populations in a direct density-dependent manner. Vole populations in fenced predator exclosures rapidly attained higher densities than in unfenced areas, with the difference persisting until the end of the experiment. In the first winter, food supplementation increased vole population growth in fenced areas, but not in unfenced areas. The growth of vole populations in both supplemented and nonsupplemented fenced areas became limited in a direct density-dependent manner during the first winter. During the second winter, food supplementation prevented the crash of vole populations within fences, whereas again no obvious effect was found in the areas exposed to predation. Furthermore, supplemental winter food increased the overwinter survival of voles in fenced areas in both winters. Our results indicate that Microtus vole populations that have succeeded in escaping regulation by predators are limited in growth by a lack of winter food. This factor is thus a strong candidate for the direct density dependence inherently necessary for the occurrence of population cycles.

Small Mustelid Predation Slows Population Growth of Microtus Voles: A Predator Reduction Experiment

1. The effects of predator reduction on the breeding performance and sex ratio of Microtus voles were studied in a cyclically fluctuating vole assemblage in western Finland, where the field vole (Microtus agrestis) and the sibling vole (M. rossiaemeridionalis) are the main prey of small mustelids. 2. The densities of small mustelids [the least weasel (Mustela nivalis nivalis) and the stoat (M. erminea)] were reduced experimentally in three large (2-3 km 2 ) unfenced areas in 1992, during a crash phase of the vole cycle, and in three different areas during the following crash phase in 1995. The reproductive performance of Microtus voles was compared between control and manipulation areas before and after reducing predators. 3. The reduction of predators increased the productivity of female voles, mainly due to an increased proportion of pregnancies, whereas the body condition of voles was not affected by the manipulation. This suggests that high predation risk in control areas suppressed the breeding of free-living voles. 4. The sex ratio of trapped voles before the manipulation did not differ between reduction and control areas, but after predators were reduced the sex ratio was more male-biased in the control areas. This indicates that small mustelids selectively killed female voles. 5. Our results suggest two possible mechanisms which may have operated concurrently. An adaptive explanation is that voles traded their current reproductive investment against future breeding under high predation risk. A simpler explanation is that small mustelids selectively preyed on pregnant female voles. Both suppressed breeding and selective killing would lead to a lower proportion of reproducing individuals in the vole population. Therefore, our results show that the presence of small mustelid predators slowed the population growth of Microtus voles.

Rough-legged Buzzards use vole scent marks to assess hunting areas

Voles scent mark their territories with urine and faeces that are visible in ultraviolet (UV) light. Rough-legged Buzzards Buteo lagopus feed mainly on voles. We performed a field experiment in northern Finland to test whether buzzards are attracted to UV-visible vole scent marks. The buzzards hunted more often on plots with vole scent marks than on control plots. Our results indicate that Rough-legged Buzzards may be able to use vole scent marks as a cue when searching for profitable hunting and breeding areas.

Do Tengmalm's owls see vole scent marks visible in ultraviolet light?

Scent markings (urine and faeces) of small mammals are visible in ultraviolet (UV) light. Diurnal kestrels, Falco tinnunculususe them as a cue to find areas of food abundance. We studied whether vole-eating, nocturnal Tengmalms owls, Aegolius funereuscan see vole scent marks using UV-vision. In a laboratory experiment, 14 young (less than 6 months old) and 14 adult (more than 6 months old) owls were individually given a choice between four adjacent arenas: (1) an arena with vole urine and faeces in UV light; (2) an arena with vole urine and faeces in visible light; (3) a clean arena in UV light; and (4) a clean arena in visible light. Owls did not prefer any of the four arenas. Our results suggest that Tengmalms owls probably do not use UV light as a cue to detect vole scent marks.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Size of Internal Organs and Forage Quality of Herbivores: Are There Differences between Cycle Phases in Microtus Voles?

We studied the possible role of food quality in cyclic vole populations by using the size of internal organs as an indicator of forage quality. We trapped field (Microtus agrestis) and sibling (M. rossiaemeridionalis) voles during two increase and two decline phases of the 3-yr population cycle in western Finland, recorded total body mass and weighed their pancreas and liver. There were no obvious differences in the relative size of organs related to phase of the cycle, which suggests voles from declining populations do not exhibit pancreatic hypertrophy. These results do not support the hypothesis that proteinase inhibitors in food plants that have adverse effects on metabolic processes of herbivores, and which are induced by grazing, cause regular cycles in small rodent populations.

Kestrels prefer scent marks according to species and reproductive status of voles

AbstractWe have previously documented that European kestrels (Falco tinnunculus L.) are attracted to the scent marks of small rodents in the presence of ultraviolet light (UV) but not visible light...