Moose recruitment in relation to bilberry production and bank vole numbers along a summer temperature gradient in Norway

Abstract

The plant stress hypothesis states that plant stress factors other than herbivory improve herbivore performance due to changes in the content of nutritive or defensive compounds in the plants. In Norway, the bilberry (Vaccinium myrtillus) is important forage for the bank vole (Myodes glareolus) in winter and for the moose (Alces alces) in summer and autumn. The observed peaks in bank vole numbers after years with high production of bilberries are suggested to be caused by increased winter survival of bank voles due to improved forage quality. High production of bilberries should also lead to higher recruitment rates in moose in the following year. We predict, however, that there is an increasing tendency for a 1-year delay of moose indices relative to vole indices with decreasing summer temperatures, because low temperatures prolong the period needed by plants to recover in the vole peak year, and thus positively affect moose reproduction also in the succeeding year. In eight out of nine counties in south-eastern Norway, there was a positive relationship between the number of calves observed per female moose during hunting and a bilberry seed production index or an autumn bank vole population index. When dividing the study area into regions, there was a negative relationship between a moose-vole time-lag index and the mean summer temperature of the region. These patterns Communicated by C. Gortázar suggest that annual fluctuations in the production of bilberries affect forage quality, but that the effect on moose reproduction also depends on summer temperatures.

Population fluctuations of moths and small rodents in relation to plant reproduction indices in southern Norway

“…In Fennoscandia, there is a one-year lagged correlation between seed production of bilberry and population peaks of Myodes-voles (Laine and Henttonen 1983, Selå s 1997, Selå s et al 2011a. A similar relationship between berry crops and Myodes-voles has been reported from Canada (Krebs et al 2010).…”

“…Because flowering and seed production of bilberry are synchronized by physical factors (Selå s 2000), such as temperatures and precipitation, which usually operates at regional or national scales, we assume that this index will reflect much of the variation in bilberry seed crops in large parts of southern Norway, including the moth and rodent trapping areas. A significant positive correlation between the bilberry index and the bank vole index from Vang has already been demonstrated, for the period 2000(Selå s et al 2011a). …”

“…The bank vole series also correlated with a time series on bank vole from the area where our bilberry index (see below) was obtained (Selå s et al 2011a, b). In the latter area, the wood lemming is absent.…”

“…The reason is probably that the timing of sexual reproduction in plants is trigged by weather events (Selå s 2000, Hohe et al 2002, Sundberg 2002, Howe et al 2012), in particular temperatures, which are usually common for large regions. One deviation is that in 2008, there appeared to be a high seed crop of bilberry only in parts of Aust-Agder and Akershus County, and not in Hedmark County (Selå s et al 2011a). In 2009, there was a high seed crop in all three counties.…”

“…In Norway, the deciduous tree rowan (Sorbus aucuparia L.) and the deciduous dwarf shrub bilberry (Vaccinium myrtillus L.) produce conspicuous fleshy fruits with direct or indirect economic value, and information on seed crops of these species have revealed a more or less regular fluctuation patterns. In rowan, a positive correlation between fruit production and the number of aphids has been found (Schaefer and Rolshausen 2007), whereas several herbivore species seem to be positively affected by high seed crops of bilberry (Nordhagen 1928, Laine and Henttonen 1983, Selå s et al 2011a.…”