Introduction
Lack of resources, including availability of nest substrates and food,
can limit abundances of small mammals and birds (Aitken & Martin, 2012;
Berthier, Leippert, Fumagalli, & Arlettaz, 2012; Hanski, Hansson, &
Henttonen, 1991). In turn, predators are frequently limited by prey
availability and increases in abundance of small mammals, at least
locally, can support increased predator numbers albeit temporally lagged
behind abundances of their prey (Hanski et al., 1991). Artificially
increasing resources, including structural habitat (e.g. nest boxes),
can provide a unique opportunity to examine intrinsic increases in
abundances of the targeted population but also potential responses of
the broader vertebrate community, including predators (Aitken & Martin,
2012; Cockle & Martin, 2015; Dunn, 1977). Yet, studies of wild small
mammal (<1 kg) population response to increases in structural
habitat and the potential responses of predators and competitors remains
relatively scarce (Newton, 1994).
Predators can reduce abundances of their prey directly through predation
(lethal) and indirectly by inducing non-lethal behavioral constraints to
foraging, resting, and reproduction (Preisser, Bolnick, & Benard,
2005). Prey can exhibit differential responses to predator foraging
mode, including active avoidance of locations where active seeking
predators (e.g. weasels, Mustela spp. ; (King & Powell, 2007)
have visited or relying on indirect cues, such as overhead cover and
light intensity to select areas with high overhead cover to reduce risk
while foraging from sedentary ambush predators such as most forest owls
(Jaksić & Carothers, 1985; Jędrzejewski & Jędrzejewska, 1990;
Jędrzejewski, Rychlik, & Jędrzejewska, 1993; Kotler, Brown, & Hasson,
1991). Prey vulnerability to predators exhibiting different foraging
modes may thus vary depending on where encounters occur, whether at prey
resting or foraging sites.
The timing and extent of biological phenomena and the statistical models
used to interpret can be sensitive to the period in which observations
are made (Steenweg, Hebblewhite, Whittington, Lukacs, & McKelvey,
2018). Continuous monitoring, such as remote camera or video devices,
enables varying the temporal grain of observation (e.g., time-bin width
of 1-hour vs. 1-day), providing insights into statistical sensitivity
but also potentially to observed biological phenomenon. The decision to
bin continuous data to a coarser temporal grain is often arbitrary or
based on properties of statistical models rather than biological
phenomena (Sollmann, 2018).
In 2015, Linnell et al. (2018) initiated a study examining the response
of arboreal rodents (red tree vole, Arborimus longicaudus ;
Humboldt flying squirrel, Glaucomys oregonensis ; Douglas’
squirrel, Tamiasciurius douglasii ) to an increase in nest
substrates in young forests (<80 years old), a resource
hypothesized to be limiting there as compared to old forests (≥80 years
old). They observed a 5.8-fold increase (95% confidence interval(CI) : 2.9, 9.2) in plot-level occupancy of the main target
population (red tree voles, henceforth: tree voles), a small arboreal
rodent that builds nests and forages exclusively in the live-tree
canopy, and that was not likely to be limited by food as their diet
primarily consists of conifer needles, which are readily available in
conifer forests. Tree voles are important prey for predators that
exhibit different foraging modes, including forest owls (Strix
occidentalis caurina, Strix varia, Aegolius acadicus ) and weasels, and
have low annual survival of 0.15 (95% CI : 0.06, 0.31), due
primarily to high predation rates (Forsman, Anthony, & Zabel, 2004;
Forsman & Maser, 1970; Swingle, Forsman, & Anthony, 2010; Wiens,
Anthony, & Forsman, 2014) .
Herein we describe predation and non-lethal avoidance of the suite of
nest predators and competitors of arboreal rodents, in particular tree
voles, during monitoring of artificial nest substrates (henceforth, nest
platforms) for three years. We describe the indirect and direct short
(i.e. ~1 week) and long term effects (12-weeks) of four
taxa that are documented predators of tree voles (weasels, owls) or that
may exhibit competition but represent low predation risk (flying
squirrels, probing or digging birds) on patterns of nest occupancy by
tree voles as observed by remote cameras placed directly above nest
platforms (Graham & Mires, 2005; Swingle et al., 2010). We predicted
that weasels and forest owls would have an immediate lethal effects
(mortality) and cause longer term non-lethal avoidance of nest
platforms. Interactions between Humboldt flying squirrels (henceforth,
flying squirrels) and tree voles, as estimated from nest platform
occupancy, are likely to be more subtle involving weak correlations
although have the potential to influence nest occupancy if those
interactions occur frequently.