Evidence for different bottom-up mechanisms in wood mouse (Apodemus sylvaticus) and bank vole (Myodes glareolus) population fluctuations in Southern Norway

Abstract

Animals that feed on forest tree seeds, such as Apodemus mice, increase in number after a mast year. At high latitudes, there is a similar delayed response by Myodes voles to high seed crops of bilberry (Vaccinium myrtillus), but here the mechanism is hypothesised to be increased forage quality, caused by a trade-off between reproduction and defence in the plants. Both Apodemus mice and Myodes voles eat berries, but only the latter feed on bilberry plants. Hence, only Myodes voles are predicted to respond to bilberry peak years. A second prediction is that the effect should last longer than any possible direct impacts of bilberries, because the plants would not be able to rebuild their defence until the succeeding summer. During a 21-year snaptrapping study of small rodents in Southern Norway, the spring population of bank vole (Myodes glareolus) was positively related to a bilberry seed index of the previous year, indicating increased winter survival, whereas the wood mouse (Apodemus sylvaticus) was not affected. Also the succeeding autumn population index of the bank vole was positively related to the bilberry index of the previous year, even when controlling for spring population levels. The wood mouse population responded to mast years of sessile oak (Quercus petraea), whereas seeds of Norway spruce (Picea abies) seemed to have some impact on both species. It is concluded that these rodents are mainly limited from below, but by different mechanisms for the granivorous and the herbivorous species.

Bilberry seed production explains spatiotemporal synchronicity in bank vole population fluctuations in Norway

“…In both Agder and Varaldskogen, there was a rather high berry index in 1997 and 2001, but in 1998, voles were abundant only in Agder, and in 2002 only in Varaldskogen. The suggested negative factor for small rodents in Agder in 2002 was low temperatures prior to the first snowfall (Selås, 2020), whereas at Varaldskogen the ground was already well covered with snow prior to the cold spell.…”

“…There is no doubt that predators affect prey abundance, but in our view, their contribution is most likely to enhance or dampen population cycles, depending on type of predator (specialist or generalist) and the availability of alternative prey. Temporal asynchrony in the fluctuation pattern of sympatric rodent species is not unusual (Framstad, 2020; Hörnfeldt, 1994; Krebs, Boonstra, Gilbert, Kenney, & Boutin, 2019; Selås, 2020), a pattern not in accordance with the specialist predation hypothesis. Also the fact that rodent cycles exist in the absence of small mustelids (Graham & Lambin, 2002; Krebs et al, 2002; Menyushina, Ehrich, Henden, Ims, & Ovsyanikov, 2012) refutes predation as a universal explanation (Oli, 2019).…”

“…The annual berry index is the number of berries per m 2 . In Agder (Vegårshei), all berries in 15 fixed plots of 2 × 2 m 2 were counted in late July or early August, that is, at 60 m 2 each year, from 1999 onward (Selås, 2020). In Varaldskogen, berries were counted in mid‐August in three circles of 0.5 m 2 in the same 6–8 forest stands each year, corresponding to 9–12 m 2 each year, from 2003 onward.…”

“…The spatiotemporal dynamic may, however, change over time (Bierman et al, 2006). Populations of bank vole ( Myodes glareolus ) in eastern Norway fluctuated in synchrony within a spatial scale of up to 30–40 km during 1990–1994 (Steen, Ims, & Sonerud, 1996), whereas population peaks observed in 2007 and 2014 occurred synchronously in areas situated several hundreds kilometers apart (Andreassen, Johnsen, Joncour, Neby, & Odden, 2020; Selås, 2020; Selås et al, 2011; Wegge & Rolstad, 2018).…”

“…Bilberry is a staple food plant for Myodes voles in winter (Hansson, 1985; Hansson & Larsson, 1978), when food appears to be a limiting factor (Haapakoski & Ylönen, 2013; Johnsen et al, 2017; Ylönen & Eccard, 2004). In Fennoscandia, the autumn population level of Myodes voles usually peaks 1 year after peaks in the berry production of bilberry ( Vaccinium myrtillus ) (Laine & Henttonen, 1983; Selås, 2020). A similar berry‐vole relationship has been reported from Canada (Krebs, Cowcill, Boonstra, & Kenney, 2010).…”