Seasonal Changes in Nycthemeral Availability of Sympatric Temperate Mixed Forest Rodents: The Predators' Perspective

Abstract

(1) Background: Bank voles (Clethrionomys glareolus) and Apodemus mice are of exceptional importance as prey for predators in temperate mixed forests. We hypothesized that overall prey availability would increase linearly with prey frequency, and that the daylight hours, which are considered particularly dangerous, would be used only during seasonal rodent population peaks and only in the twilight hours. (2) Methods: We conducted a two-year camera-trapping study in an inner alpine mixed forest and collected 19,138 1 min videos in 215 camera-trap nights. Prey availability was defined as the pseudo-replication-limited maximum number of the respective rodent taxon per 30 min period, summed per season. (3) Results: Overall prey availability increased with frequency, i.e., the maximum number of rodent individuals per camera-trap night. Seasonally, Apodemus mice were particularly available to predators in the summer and bank voles in the autumn after a tree mast year. In both cases, this was accompanied by a significant increase in diurnal availability. During the population peak of Apodemus mice, the nocturnal availability of bank voles decreased without a concurrent increase in absolute diurnal availability, even though the significant relative shift to diurnal activity superficially suggested this. Bank voles were active throughout the day, while Apodemus mice were nocturnal and (rarely) crepuscular. (4) Conclusions: Availability of rodents to predators, especially during daylight hours, was mainly dependent on their tree mast-induced increased frequencies. Bank voles likewise responded strongly to interspecific competition with the larger and aggressive Apodemus mice, which negatively affected availability to predators. At our seasonal level of evaluation, we conclude that nycthemeral availability of forest-dwelling rodents to generalist predators of temperate mixed forests is predominantly driven by bottom-up mechanisms.

Keywords: Apodemus mice; Clethrionomys glareolus; bank vole; diurnality; predator-prey activity patterns; seasonal prey availability; temperate mixed forest.

Figure 1

Seasonal overall availability of forest-dwelling rodents to their predators in an inner alpine study site. Significantly increased availability correlated with high frequencies of Apodemus mice and bank voles about one year after tree seed masting. The peak of availability occurred in the summer of 2021 for Apodemus mice, while it was less pronounced in autumn 2021 for bank voles. Each point represents one camera trap.

Figure 2

Absolute and relative diurnal availability of Apodemus mice in an inner alpine study site. Both availabilities increased in parallel and significantly in summer 2021. Each point represents one camera trap.

Figure 3

Absolute and relative diurnal availability of bank voles in an inner alpine study site. In contrast to Apodemus mice, absolute diurnal availability increased significantly only in autumn 2021. Bank voles were relatively, but not absolutely, more available to their predators during daylight hours from winter 2020/2021 into summer 2021. Each point represents one camera trap.

Figure 4

Seasonal diurnal availability of Apodemus mice on an inner alpine study site. Diurnal availability was an extension of nocturnal availability, primarily occurring at the edges of the light day and coinciding almost exclusively with the time of population maximum.

Figure 5

Seasonal diurnal availability of bank voles in an inner alpine study site. Availability is given for the entire light day.