INTRODUCTION
Although the distinction between
active search and incidental encounter of prey is key to understanding
predator-prey dynamics, whether, and under what conditions predators
actively search for particular prey species is poorly understood. Diet
breadth theory suggests that predators seeking to maximize their rate of
energy acquisition should have a more generalist prey profile until the
encounter rate with the most profitable prey item crosses a critical
threshold that makes it suboptimal to include lower-profitability prey
in their diet (MacArthur & Pianka 1966; Figure 1; Supporting
Information Text S1). Diet
specialization on abundant prey is necessarily associated with active
search for that prey, but as the density of the focal prey species
declines, additional species enter the diet which reduces the rate of
acquisition of the focal prey. A switch from active search for a prey
species at high density to incidental encounter of the same species at
low density can have stabilizing effects on prey population dynamics
(Murdoch 1969) and results in the canonical sigmoid Type III functional
response that is associated with many of the most interesting dynamics
in predator-prey systems including alternative stable states,
bifurcations, and predator pits (Holling 1959; Messier 1994; Leviet al. 2015).
The density-dependent response of predators that is evidence for active
search can be found by observing disproportionate predation in years in
which a given prey species is at high density. However, in many systems
prey densities vary predictably within a year due to a birth pulse that
provides a uniquely vulnerable life stage for predators. A classic case
of this phenomenon is the seasonal pulse of songbird nests (Schmidt,
Goheen & Naumann 2001; Vigallon & Marzluff 2005), and both active
search (Pelech, Smith & Boutin 2010) and incidental encounter (Schmidt
2004) strategies have been documented as a function of nest density,
vulnerability, and maternal vigilance or defense (Schmidt 1999). In
addition to songbirds, large herbivore systems worldwide feature a
predictable birth pulse of neonates that are vulnerable to predators
across a wide body size gradient. Although mortality of ungulate
neonates has been studied extensively (Linnell, Aanes & Andersen 1995),
it is still largely unknown whether, and under what conditions predators
engage in active search behavior during the birth pulse, which has
important implications for prey population dynamics.
The propensity of carnivores to target neonates likely depends on
factors intrinsic to both the predator and prey. For predators, this may
vary by body size, hunting proficiency or mode, and ability to cope with
maternal defense by the large herbivore. For example, ungulate neonates
are highly vulnerable to predation immediately following parturition
from a suite of carnivores, but quickly become sufficiently vagile to
elude those that are less predaceous or of smaller body size. Thus,
species such as bears and many mesopredators experience a particularly
short resource pulse of neonates (Linnell, Aanes & Andersen 1995; Zager
& Beecham 2006; Griffin et al. 2011) (Figure 1a, b). In
contrast, the birth pulse may be less consequential to the largest
felids and canids that can efficiently capture large-bodied ungulates
throughout the first year of life and beyond (Figure 1c). Even within a
species, the response of predators to the ungulate birth pulse may vary
by individual or sex as has been shown in bears (Jacoby et al.1999; Zager & Beecham 2006; Rayl et al. 2015).
Because predatory behavior during the ungulate birth pulse is influenced
by idiosyncratic combinations of factors intrinsic to both predators and
prey, studies involving multiple carnivore and multiple ungulate species
will be needed to elucidate the generality of search behavior. Previous
research has largely focused on identifying spatial shifts in habitat
use by bears (but see Bastille-Rousseau et al . 2016; Svobodaet al . 2019) toward birthing grounds or areas on the landscape
more likely to contain neonates, which has resulted in different
conclusions wherein both active search (Rayl et al . 2018) and
incidental encounter (BastilleāRousseau et al. 2011; Bowersocket al. 2021) were inferred. Much stronger inference is possible
using analytical methods that utilize spatiotemporal encounters between
individual predators and individual prey from contemporaneous GPS
telemetry data. Further, identifying general conditions associated with
incidental encounter and active search requires relocation data on
multiple species of predators and prey across gradients in body size,
abundance, and life histories.
Here
we use contemporaneous GPS tracking data at the level of encounters
between individual predators and individual prey to assess whether
carnivores actively searched for or incidentally encountered ungulate
neonates in a multi-predator, multi-prey system. Each carnivore species
(cougar [Puma concolor ], coyote [Canis latrans ],
black bear [Ursus americanus ], bobcat [Lynx
rufus ]) varied in size, life history, and predatory ability, ranging
from large-bodied obligate predators to smaller-bodied omnivores, while
prey species (mule deer [Odocoileus hemionus ] and elk
[Cervus canadensis ]) varied dramatically in abundance. Our
primary objective was to determine whether predators encountered
parturient female ungulates more often than expected by chance,
indicating active search, while controlling for their habitat
preferences. If predators did indeed exhibit targeted search behavior, a
secondary objective was to determine whether a shift in space use toward
parturition habitat tracked the phenology of the birth pulse consistent
with an effort to maximize detections of neonates. We hypothesized that
the magnitude of response by predators to the birth pulse would be
greater toward elk than mule deer for all carnivore species because elk
were approximately 5 times more abundant than mule deer and could thus
cross the profitability threshold associated with specialization under
diet-breadth theory. We expected the carnivore species with a more
generalist diet profile (bears and coyotes) would alter their foraging
behaviors more strongly than cougars or bobcats because generalist
consumers are more fluid in their response to changing resources
(Ostfeld & Keesing 2000; Yang et al. 2008). Bears are the least
carnivorous of these taxa, and previous research suggests that male
bears are disproportionately predaceous (Rode, Robbins & Shipley 2001,
Boertje et al. 1988; Jacoby et al. 1999), so we
additionally hypothesized that male bears would be more likely to
exhibit active search behavior. Further, cougars kill mule deer and elk
of all age classes year-round (Clark et al. 2014), so we expected
that the ungulate birth pulse may be less consequential to cougars given
their adeptness at killing larger prey such that they may not exhibit a
change in behavior during the earliest neonatal period. Finally, we
hypothesized that bobcats would show the weakest response, since they
rarely consume elk and mule deer in our study area (Ruprecht et
al. 2021b).