1. Introduction
In the arms race against predators, prey have evolved powerful defense mechanisms. Antipredator protection includes avoidance, warning signals, such as aposematism and calls, physical deterrence, and group defense [
1,
2,
3]. In a restricted, functional sense, avoidance means staying away spatially or temporally from predators and thereby eluding detection [
3]. To avoid encounters with predators, prey may alter their activity patterns according to the risk allocation hypothesis, i.e., adaptively changing temporal exposure to predation across high- and low-risk situations [
4,
5,
6,
7,
8]. The majority of small, ground-dwelling mammals in the world are nocturnal, but diurnal, crepuscular, and cathemeral strategies also exist [
9]. Nocturnal activity is presumably an evolutionary adaptation to an increased predation risk in the day due to the superior sensory abilities of diurnal predators [
10,
11,
12,
13].
Most of the time, however, there is a trade-off between activities like foraging, mating, intraguild interaction, and antipredator behavior [
14,
15], and multiple predator communities may increase the temporal overlap with prey [
16,
17]. Therefore, it is not possible for prey species, such as temperate mixed forest-dwelling rodents like
Apodemus spp. mice (hereafter referred to as
Apodemus mice) and bank voles (
Clethrionomys glareolus), formerly known as
Myodes glareolus [
18], to completely avoid exposure to predators or to be active only at night [
19,
20,
21,
22]. In fact, mice of the genus
Apodemus are predominantly nocturnal, whereas bank voles exhibit a more flexible activity pattern [
23,
24]. Factors influencing (increased) diurnal activity in these small mammals can be diverse and include abiotic factors such as temperature and precipitation [
25,
26,
27,
28], as well as biotic factors such as nutrition, intraspecific organization, and intraguild competition [
29,
30,
31,
32,
33].
From the perspective of a wide range of predators,
Apodemus mice and bank voles are important prey, and a seasonally increased prey availability is of particular interest with regard to survival and reproduction [
34,
35]. In inner alpine mixed forests such as our study area, this includes diurnal predators such as common buzzard (
Buteo buteo, [
36]), nocturnal predators such as tawny owl (
Strix aluco, [
37]), stone marten (
Martes foina, [
38]) and European polecat (
Mustela putorius, [
39]), as well as nycthemeral more flexible predators such as red fox (
Vulpes vulpes, [
40]), pine marten (
Martes martes, [
41]) and domestic cat (
Felis catus, [
42]).
Our dataset covers two years of camera-trapping. We define availability as the probability that prey will be accessible to above ground hunting predators [
43]. As an approximation of the overall availability of prey, the pseudo-replication-limited maximum numbers of
Apodemus mice and bank voles of the 48 30-min periods per day were added up, separately in each of the eight seasons investigated. The highest prey frequency per camera trap-night, i.e., the maximum number of prey individuals in any of the 48 given 30-min periods per day, served as an approximation of prey abundance. We address questions related to seasonal availability of
Apodemus mice and bank voles to their predators: 1. At first, we predicted that a higher frequency of forest rodents would lead to a proportionally increased availability, i.e., daily overall and seasonally enhanced access to prey (Prediction 1). 2. In the next step, we separated overall prey availability into different components, namely diurnal and nocturnal availability, and the relative ratio between these two. Because diurnal activity is considered particularly dangerous, we predicted that increased diurnal and in parallel nocturnal availability would only occur at high prey frequencies. Increased diurnal availability would result from an expansion of nocturnal activity, but not from a mere shift of availability into the day (Prediction 2). 3. Finally, we predicted that increased diurnal availability would result in greater use of the crepuscular margins of the day rather than mid-day. If ecologically necessary but dangerous, the activity of small forest-dwelling rodents should extend just into the light day, but not into the brightest hours (Prediction 3).
4. Discussion
Diel activity pattern of prey, such as bank voles and
Apodemus mice, is thought to be influenced by a trade-off between physiological needs and the reduction of predation risk through spatiotemporal avoidance [
4,
5]. This paper examines the seasonal availability of these sympatric, forest-dwelling rodents to generalist predators in an inner alpine mixed forest, with particular emphasis on the risks associated with diurnal activity [
56]. We discuss the nycthemeral availability pattern in terms of intraspecific and intraguild bottom-up mechanisms (changes in prey frequency and concurrence among prey).
Only
Apodemus mice conformed to our predictions. When they were particularly frequent during a population peak induced by a tree mast, they were more available to predators both at night and during the day. Their heightened diurnal activity during this season was an extension of their nocturnal activity rather than a shift to daytime. This markedly increased availability of
Apodemus mice during the summer of 2021 was undoubtedly advantageous in terms of nutrition for both territorial predators (e.g., energy-demanding, post-breeding molt [
57]) and dispersing predators [
39,
58]. However, after the population collapsed in the autumn of 2021, availability significantly decreased also at night from spring to summer 2022, thus possibly negatively affecting the predators’ reproductive phase in the following year [
59].
Therefore, the availability to predators in this genus was strongly influenced by intraspecific and density-dependent mechanisms.
Apodemus mice possess several traits that enable them to evade diurnal activity. They dominate over competitors such as bank voles [
60], consume and store high-energy food [
31], and can slow down their metabolism under certain circumstances [
61,
62]. In addition, sharpened sensory capabilities make them well-equipped to survive nocturnal predation attempts [
23]. The increased diurnal availability cannot be attributed solely to the shorter nights in summer because it was not observed at low population densities in summer 2022 and availability was primarily concentrated to the twilight hours of the day. In conclusion, for predators, this translates to particularly high availability of
Apodemus mice during population peaks, both at night [
37,
63,
64] and even during the day [
65].
Bank voles displayed considerably more varied responses in their chronoecology and barely met our predictions. Particularly in the spring and summer of 2021, both overall and absolute nocturnal activity was reduced, and availability relatively shifted into the daytime. This resulted in a decrease in availability to nocturnal predators without a corresponding increase in absolute diurnal availability, despite the calculation of the relative value superficially suggesting otherwise [
66]. This inverse activity pattern closely coincided with the population maximum of the dominant
Apodemus mice and can be interpreted as a strategy to avoid intraguild competition [
33]. It was not until bank voles themselves reached a small population peak in the autumn of 2021 that absolute diurnal availability increased. By this time, the population of
Apodemus mice had already collapsed, suggesting an intraspecific and density-driven effect as well. It can be surmised that the competition-induced reduction in overall availability in spring was facilitated by the availability of high-energy, less foraging time-consuming food obtainable during this season. However, this reduction possibly had a negative impact on population growth and may have shifted the frequency peak further into the autumn.
Bank voles were thus much more likely than
Apodemus mice to exhibit diurnal activity, and this was not restricted to the twilight hours of the day. The increase in absolute diurnal availability observed in the autumn of 2021 was not accompanied by a parallel increase in absolute nocturnal availability either. Therefore, bank voles must have evolved effective antipredator strategies to keep diurnal mortality low or to derive other advantages from diurnal activity. Some potential benefits include reduced daily energy expenditure, as predicted by the circadian thermo-energetics hypothesis [
61], as well as advantages in foraging and digestion [
31,
67]. Individual personality differences in bank voles may also account for some of the diurnal variation in risky behaviors such as foraging [
68,
69]. From the predators’ perspective, bank voles are known for their use of cover-rich microhabitats [
70,
71], making them relatively difficult to detect and capture, even during the day. Extensive pellet analyses in Central Europe involving a variety of diurnal and nocturnal predators, both avian and mammalian, have shown that the bank vole is notably underrepresented relative to its abundance [
72]. The same is true for the boreonemoral region, where mice of the genus
Microtus are preferred by various owl species [
73].
At our seasonal day-night evaluation level, we hypothesize a bottom-up controlled predator-prey system. Seasonal and diurnal variations in prey availability were strongly influenced by intraspecific, density-dependent organization in
Apodemus mice and bank voles as well. Furthermore, multiple regression analysis indicated a linear relationship between frequency and availability. Non-linear or exponential correlations were ruled out through examination of scatter plots. Bank voles were further influenced by competitive mechanisms within the guild of forest-dwelling rodents. As expected, when both
Apodemus mice and bank voles were more frequent, their availability increased overall [
74], but especially during the day. Surprisingly, at the peak of
Apodemus mice frequency, interspecific competition significantly reduced bank vole availability both overall and during the night, without causing a shift in absolute availability to the daytime.
While we were able to detect the avoidance of moonlight as an indirect cue of predation risk in our study area [
33], we found no evidence indicating that the generalist predators of this temperate forest ecosystem were responsible for the seasonal changes in the overall nycthemeral activity patterns of their rodent prey. We mainly recorded nocturnal generalist predators in the non-mast year (
n = 13; red fox, 53.8%; stone marten, 23.1%; pine marten, 7.7%; European polecat, 7.7%; tawny owl, 7.7%), i.e., at the time when nocturnal activity was prevalent in
Apodemus mice. Moreover, there was no difference in absolute diurnal availability with respect to the detection or non-detection of nocturnal predators in bank voles as well (W = 306.50,
p = 0.811). Therefore, it is highly unlikely that predators induced increased diurnal activity, even via indirect cues such as feces, urine, and anal gland secretions [
75,
76]. In the predator-prey-system we investigated, the system-stabilizing generalist predators [
77] are apparently unable to induce a temporal niche switching in rodents, from primarily nocturnal activity to predominantly diurnal activity, or vice versa. We conclude that predators in our study area need to adapt their hunting patterns to match the temporal availability of prey [
13]. Resident specialized predators
sensu [
78] may have a greater influence on the nycthemeral activity of their rodent prey in their ongoing “David and Goliath” arms race [
21,
35,
79,
80,
81,
82]. However, we never detected highly specialized vole-hunting species such as stoat (
Mustela erminea) and least weasel (
Mustela nivalis) during our two years of study, neither by the camera traps nor by observations during field work.
We were only able to identify bank voles up to species and
Apodemus mice up to genus level. Regarding behavioral choices made in response to predation risk, especially the important decisions about when, where, and what to feed [
83], we primarily highlighted the first aspect. We had no data on the specific diets of our prey taxa, and our camera trap locations shared a high degree of habitat similarity [
33]. Nevertheless, we did observe a “thigmotaxis parameter” (% of cover with lying deadwood, snags, and rocks in a 10 m radius of the camera trap) targeting the bank vole’s need for cover, which showed a negative correlation with overall availability (Spearman’s ρ: r = −0.217,
p = 0.011). This suggested that with increasing frequency, bank voles possibly had to leave the sheltered cover more frequently. Conversely, there was a positive correlation for
Apodemus mice, which are generally more socially tolerant (Spearman’s ρ: r = 0.865,
p < 0.001). For a more comprehensive understanding of species-, sex-, age-, nutrition-, and habitat-specific characteristics, as well as effects of diel vulnerability to predation in the future, it would be promising to combine the camera trap survey with live-trapping [
84,
85,
86], diet tracing [
31,
87,
88,
89,
90], and further related aspects [
69,
91,
92,
93,
94,
95,
96]. This approach would also enable a more precise measurement of the crucial parameter of availability [
97].
We are, nevertheless, convinced to have made a methodological and subject-related contribution to the understanding of seasonal changes in nycthemeral availability of a temperate mixed forest-dwelling rodent community from the predators’ perspective:
We used the video function of camera traps, which is better suited than the photo function for determining the frequency (and behavior) of small mammals.
Camera traps reduce the need for handling and thus minimize disturbance of the target organisms. We were able to accurately determine the nycthemeral activity of Apodemus mice and bank voles because we completely avoided manipulations during the twilight hours.
We conducted a two-year, year-round study, allowing us to cover all seasons with a large sample of videos.
The tree mast/nil crop-induced outbreak-crash pattern in Apodemus mice in our study provided us with a quasi-experimental situation to measure the influence of the dominant competitor.
Overall above ground availability to predators (summed maxima of prey individuals of the 48 30-min periods/camera trap-night) increased linearly with frequency (maximum number of prey individuals/camera trap-night) in Apodemus mice as well as in bank voles.
Seasonally, Apodemus mice were only available to diurnal predators at times of high population densities; in bank voles, diurnal activity increased at a (small) population peak.
We were able to show that the commonly used relative measure of nycthemeral activity in prey animals can lead to misconceptions about availability to predators. During the population peak of Apodemus mice, bank voles were diurnally active for up to three quarters of their activity time, without changing the absolute duration and, thus, their availability in daylight hours to predators.
Our study suggests that in a temperate mixed forest, prey availability is bottom-up controlled. This mainly depends on intraspecific, density-dependent population phenomena and is also influenced by intra-guild competition with Apodemus mice in the case of the bank vole. We found no evidence for control of this forest predator-prey system by the generalist, predominantly non-migratory predators.