1 | INTRODUCTION
Some terms in ecology hold specific connotations, that when used, can
limit our perceptions about ecological processes, bias our results, and
hinder our understanding of the natural world. For instance, the term
“carnivore” can be confusing in ecology, either referring specifically
to members of the mammalian order Carnivora, or more generally referring
to animals (typically mammals) that kill and consume other vertebrates
(i.e., meat-eaters; Gittleman, 2013; Yoshimura et al., 2021). However,
outside a few families of obligate carnivores (felids and hyenids;
Clauss et al., 2010; Yoshimura et al., 2021), members of Carnivora eat a
wide variety of foods, including invertebrates, plants and fungi
(Edwards et al., 2011). Additionally, many species in groups outside
Carnivora, such as primates, rodents, raptors, reptiles, amphibians,
fish, invertebrates, and plants (Román-Palacios et al., 2019), kill and
consume vertebrates. Thus, classic foraging categories of carnivore,
omnivore, and herbivore may be overly simplistic (Harrer & Levi, 2018;
Leigh et al., 2018; Vazquez et al., 2023) and lead to a narrow focus on
an organism’s diet. Using a more precise term, such as polyphagous
(Loxdale & Harvey, 2023), may combat this issue.
Food webs have traditionally modelled polyphagous species as static
consumers exerting constant pressure on multiple resources, but diet
specificity changes with environmental conditions and is essential to
ecosystem stability (Gutgesell et al., 2022). Primary productivity
varies annually related to climate and insect or pathogen abundance
(Bjerke et al., 2014) and organisms can respond to this variation by
changing their diet. Thus, polyphagous species can rapidly respond to
changes in resource availability (Deacy et al., 2018), relieving
pressure from an exhausted resource while increasing pressure on
another, more abundant resource, even if the second resource is a
less-preferred food (Zhang et al., 2021). The historical focus on
predators through simple interactions between large, charismatic,
obligate carnivores and their prey over-simplifies many polyphagous
species’ roles within ecological communities (Miller et al., 2001). This
likely overlooks trophic interactions which may be important to
community stability (Kratina et al., 2012) and essential to predict
community shifts in a changing climate (Gutgesell et al., 2022).
Despite the need for a deeper understanding of polyphagous species
beyond their roles as predators, measuring diet in wild species is
difficult (Davis & Pineda‐Munoz, 2016) and modeling environmental
features associated with diet changes is challenging due to the
complexity of ecological communities. One way to estimate diet in
wildlife is through stable isotope analysis (Tieszen & Boutton, 1989).
This method is based on the principal that the ratio of naturally
occurring stable isotopes varies across the earth and among different
food types, such as C-4 and C-3 plants or marine and terrestrial
animals, and all organisms must build their tissues from molecules they
consume (Tieszen & Boutton, 1989). Thus, the stable isotope value of an
organisms’ tissue will most closely resemble that of its dominant foods
(Semmens et al., 2009).
In addition to diet estimation, accurate measures of food availability
are difficult to obtain, especially over time periods long enough to
detect change (Davis & Pineda‐Munoz, 2016). Even within the same
population of a single species, there will be differences in diet among
individuals (Edwards et al., 2011), demographic classes (Beck et al.,
2007), across space (Stern et al., 2024), and time (Davis &
Pineda-Munoz, 2016), which can obscure general patterns. For example,
within a species different populations may have different responses to
variation in primary productivity, such as mast crops (Hertel et al.,
2019; Schwartz et al., 2010). Thus, determining the effect of variation
in primary productivity on diet of polyphagous species is challenging.
We used a dataset on a polyphagous mammal, the brown bear (Ursus
arctos ), to estimate annual diet proportions of common foods as well as
identify diet patterns and drivers over 25 years. We used carbon
(δ 13C) and nitrogen
(δ 15N) stable isotopes measured in hair of
known individual bears in south-central Sweden to estimate annual diet
among different demographic classes. We focused on the five primary diet
components of bears in this system: ants (Formnica spp. and Camponotus spp. ), bilberry (Vaccinium
myrtillus ), crowberry (Empetrum nigrum), lingonberry(Vaccinium vitus-vitae), and moose (Alces alces ; Stenset
et al., 2016). Berry production in Scandinavia is variable both
temporally and spatially (Hertel et al., 2016), while moose populations
showed less variation (Jensen et al., 2020). Based on previous stable
isotope analyses (Mikkelsen et al., 2023), we expected bilberry to make
up the greatest proportion of the brown bear diet, however meat is
considered higher quality food than berries (Pritchard & Robbins,
1990). Thus, while it accounts for a smaller diet proportion, we
expected brown bear diet to respond to moose availability (Hypothesis
1); years with greater numbers of moose will be associated with greater
proportions of moose in the diet. Moose populations have been increasing
across Scandinavia (Jensen et al., 2020), and proportions of moose in
the diet should increase through time as bears respond to increasing
moose availability (Hypothesis 2). Annual variation in food productivity
will also result in annual variation in stable isotope values.
Specifically, we expected δ 15N values to be
related to indices of moose population size andδ 13C values to be related to annual bilberry
production (Hypothesis 3).