Diet Quality Limits Summer Growth of Field Vole Populations

Kristian M Forbes,Tapio Mappes,Peter Stuart,Heikki Henttonen,Otso Huitu,Katrine S Hoset,Gabriele Sorci

doi:10.1371/journal.pone.0091113

Kristian M Forbes, Tapio Mappes + Show 5 more

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https://doi.org/10.1371/journal.pone.0091113

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Abstract

Marked variation occurs in both seasonal and multiannual population density peaks of northern European small mammal species, including voles. The availability of dietary proteins is a key factor limiting the population growth of herbivore species. The objective of this study is to investigate the degree to which protein availability influences the growth of increasing vole populations. We hypothesise that the summer growth of folivorous vole populations is positively associated with dietary protein availability. A field experiment was conducted over a summer reproductive period in 18 vegetated enclosures. Populations of field voles (Microtus agrestis) were randomised amongst three treatment groups: 1) food supplementation with ad libitum high protein (30% dry weight) pellets, 2) food supplementation with ad libitum low protein (1% dry weight; both supplemented foods had equivalent energy content) pellets, and 3) control (no food supplementation), n = 6 per treatment. Vole density, survival, demographic attributes and condition indicators were monitored with live-trapping and blood sampling. Highest final vole densities were attained in populations that received high protein supplementation and lowest in low protein populations. Control populations displayed intermediate densities. The survival rate of voles was similar in all treatment groups. The proportion of females, and of those that were pregnant or lactating, was highest in the high protein supplemented populations. This suggests that variation in reproductive, rather than survival rates of voles, accounted for density differences between the treatment groups. We found no clear association between population demography and individual physiological condition. Our results demonstrate that dietary protein availability limits vole population growth during the summer growing season. This suggests that the nutritional quality of forage may be an underestimated source of interannual variation in the density and growth rates of widely fluctuating populations of herbivorous small mammals.

Highlights

Populations of northern small mammals are renowned for their high-amplitude density cycles, with peaks every 3–5 years [1,2,3,4,5]
By September densities were greater in high protein than low protein treatment populations, while control groups displayed an intermediate level of density, which did not differ from either supplementation group
Consistent with our hypothesis, the summer growth of vole populations was limited by the availability of dietary proteins

Summary

Introduction

Populations of northern small mammals are renowned for their high-amplitude density cycles, with peaks every 3–5 years [1,2,3,4,5]. Delayed density-dependent predation is often considered the principle driver of cyclic dynamics [6,7,8,9,10], regulatory processes are likely to be multifactorial and geographically variable [1,11]. Boreal vole cycles typically involve two successive years of variable but positive population growth in summer and negative or zero population growth in winter [2,15,16]. The peak of a multiannual cycle is usually attained in late summer to autumn of the latter increase year, after which winter food depletion initiates a population decline [17,18]. The year following peak density is characterized by a summer decline, when populations typically decrease in size from spring to autumn [1,2,15]

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