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Tools and Technology
Estimating Biomass of Berries Consumed by
THOMAS D. GABLE,
Northern Michigan University, Department of Biology, 1401 Presque Isle Avenue, Marquette, MI 49855, USA
STEVE K. WINDELS, Voyageurs National Park, 360 Highway 11 E, International Falls, MN 56649, USA
JOHN G. BRUGGINK, Northern Michigan University, Department of Biology, 1401 Presque Isle Avenue, Marquette, MI 49855, USA
ABSTRACT Gray wolves (Canis lupus) consume berries and other wild fruits seasonally when available or
abundant. However, a method to convert percent frequency of occurrence or percent volume of berries in wolf
scats to percent biomass has not yet been developed. We used estimates of the average number of blueberry
(Vaccinium spp.) seeds in 10 individual wolf scats collected in and adjacent to Voyageurs National Park,
Minnesota, USA, along with published values of the number of seeds per blueberry and blueberry masses to
estimate that a wolf scat containing only berries equated to an average of 0.468 kg of berries consumed.
We recommend using this berry conversion factor (0.468 kg/scat) to convert the percent frequency of
occurrence or percent volume of berries and other wild fruits to percent biomass when estimating wolf diets
from scats. Ó2017 The Wildlife Society.
KEY WORDS Canis lupus, correction factor, fruits, scat analysis, wolf diet.
Wolves (Canis lupus) are carnivorous mammals that feed
primarily on ungulates and other prey species such as beavers
(Castor spp.) or hares (Lepus spp.; Newsome et al. 2016).
Wolves are opportunists, however, and will take advantage of
other food sources such as human garbage, ﬂightless molting
birds, and spawning salmon (Oncorhynchus spp.) when
available (Szepanski et al. 1999, Peterson and Ciucci
2003, Wiebe et al. 2009). Wolves also consume fruits
such as wild blueberries (Vaccinium spp.) and raspberries
(Rubus spp.) when these fruits are abundant. In areas where
berry consumption occurs, berries typically constitute a
minor (<10% frequency) portion of the summer diet
(Messier and Cr^ete 1985). However, in some areas, berries
can be a signiﬁcant summer food item for wolves. Berries
(primarily blueberries) constituted 10–30% (frequency) of
the diet of wolves from 1 June to 15 September in southern
Quebec, Canada (Tremblay et al. 2001). Similarly, vegeta-
tion (primarily berries) occurred in 52% of scats collected at
home sites in July and 20% of scats collected on trails in
August and September in north-central Minnesota, USA
(Fuller 1989). In Voyageurs National Park, Minnesota,
berries constituted 30–50% (volume) of wolf diets in July and
August 2015 (T. D. Gable, personal observation). Though
berries can be an important summer food for wolves in boreal
systems, the percent biomass of wolf diets composed of
berries is largely unknown.
Percent frequency of occurrence or percent volume of a
particular species in wolf scats does not always equate to
percent biomass consumed of that species because smaller
prey have a larger proportion of indigestible material than
larger prey. The following equation (Weaver 1993) has been
used to correct for this bias and convert percent frequency of
occurrence or percent volume of mammalian prey to percent
Y¼0:439 þ0:008 Xð1Þ
where Xis the average live mass of a prey species and ^
prey mass per scat. The biomass of each prey species in the
wolf diet is determined by multiplying the prey mass per scat
by the proportion (based on volume or frequency) of that
species in the diet. The percent biomass of each prey species
is determined by dividing the biomass value of each prey
species by the summation of all biomass consumed and
multiplying by 100.
However, Weaver’s equation is not applicable to nonmam-
malian food items. Diet correction factors are generally
determined by feeding captive animals a known mass (or
volume) of food and measuring the mass (or volume) of fecal
material produced (e.g., Hewitt and Robbins 1996) or
counting the number of collectible scats produced (e.g., Floyd
et al. 1978). Such work has not been done for wolves and
berries, and digestibility of soft fruits by wolves is unknown.
Therefore, a method to convert percent frequency of
occurrence or percent volume of berries in wolf diets to
percent biomass of berries consumed needs to be developed to
better understand the contribution of berries and other fruits to
the diet of wolves. Wild fruits contain seeds that cannot be
digested by wolves; therefore, the biomass of ingestedfruits can
be estimated based on the number of seeds that pass in fecal
material. Thus, our objective was to develop a conversion factor
to convert the percent frequency of occurrence or percent
volume of berries in wolf diet to percent biomass by estimating
Received: 25 July 2016; Accepted: 27 November 2016
Wildlife Society Bulletin; DOI: 10.1002/wsb.730
Gable et al. Estimating Berry Consumption by Wolves 1
the mass of blueberries consumed to produce one scat. Use of
this conversion factor should reduce overestimation of the
percent biomass of mammalianprey species in wolf diets where
berry consumption is high.
As part of a larger study of wolf diets in and around
Voyageurs National Park, Minnesota (488300N, 938000W),
we collected 557 wolf scats during July–August 2015 from 3
wolf packs with 1 wolf/pack ﬁtted with a Global
Positioning System (GPS) collar. Scats were collected on
trails and logging roads, at home sites, and at clusters of GPS
locations. We transferred individual scats to nylon stockings
and sterilized them by boiling in water for >45 min. We then
washed the scats in a washing machine and allowed them to
air dry for >12 hr (Gable 2016).
To estimate the biomass of berries consumed, we randomly
selected 10 out of 46 scats that contained only blueberries.
We spread the contents of each cleaned scat over an 8 8-cm
grid so that the seeds were uniformly distributed across each
grid cell. We then counted the number of seeds in one
randomly selected grid cell. We estimated the total number
of seeds in each scat by multiplying the number of seeds
counted in one grid cell by the total number of grid cells (64),
and then calculating an average number of seeds per scat. We
estimated the number of blueberries consumed by dividing
the average number of seeds per scat by the average number
of seeds per blueberry (14.6 seeds/berry; based on 12 seeds/
berry [Vander Kloet and Hill 1994] and 17.2 seeds/berry
[Usui et al. 2005]). This value was multiplied by the average
mass of a wild blueberry (0.335 g/berry; based on 0.300
[Welch et al. 1997] and 0.369 g [Usui et al. 1994]) to produce
an estimate of the mean biomass of blueberries (the berry
conversion factor) consumed per scat. Although we think
that the estimates derived from the literature represent a
broader sample of wild blueberries from across a larger
geographical region, we wanted to evaluate how the mean
mass of wild blueberries derived from Welch et al. (1997) and
Usui et al. (1994) compared with those in our study area. To
do so, we collected 2,000 individual blueberries from one
location in our study site and divided by the total mass of the
berries to derive an estimate of mass/berry.
We counted an average of 31993 (SD, range ¼209–477)
seeds/grid cell in the 10 scats examined, which was an average of
20,415 5,938 (SD, range ¼13,372–30,508) seeds per scat.
Our minimum estimateof the biomass of blueberries consumed
by wolves was 0.356kg/scat based on 17.2 seeds/berry (Usui
et al. 2005) and 0.300g/berry (Welch et al. 1997). Our
maximum estimate was 0.628 kg/scat based on 12 seeds/berry
(Vander Kloet and Hill 1994) and 0.369 g/berry (Usui et al.
1994). Using combined averages of 14.6 seeds/berry and
0.335 g/berry, we estimated the berry conversion factor, which
is the mean biomass of blueberries consumed per scat, to be
0.468 0.136 kg/scat (SD, range ¼0.306–0.699 kg/scat). Our
estimate of mass of wild blueberries in our study area was
We recommend using our conversion factor of 0.468 kg/
scat (prey mass per scat) to estimate the percent biomass
of berries in wolf diet from percent frequency of
occurrence or percent volume in wolf scats. For scats
that are only partially composed of berries, the results can
be scaled to the volumetric proportion of the scat that is
berries (e.g., 50% of scat volume consisting of blueberries
¼0.234 kg blueberries consumed). The estimated mass of
a blueberry (0.329 g) in our study area was similar to the
combined mean mass (0.335 g) that we used from
literature values, though we acknowledge the limitations
of this simple evaluation. Future studies that use our
method could be improved by deriving local estimates of
The contribution of berries relative to mammalian prey in
summer wolf-diet biomass can now be examined because
the prey mass per scat of both berries (using the berry
conversion factor) and mammalian prey (using Weaver’s
equation) can be determined. However, percent
biomass in the diet should not be confused with the energy
derived from a prey source. For example, wild blueberries
contain 0.51 kcal/g of energy, whereas ungulate prey
contains 1.87 kcal/g (Usui et al. 1994, Peterson and Ciucci
2003). Wolves almost certainly cannot digest berries as
efﬁciently as they can digest ungulate prey (Litvaitis and
Mautz 1976). However, even if digestibility of berries is
low, great abundance of berries on the landscape might
make berries an important food source because berries can
be acquired with little energy expenditure in the summer
months when availability of mammalian prey is low
(Tremblay et al. 2001).
In systems where berry consumption is high, use of the
berry conversion factor could help reduce overestimation of
the consumption of mammalian prey. Our conversion factor
is most appropriately applied to gray wolves that consume
blueberries. However, our conversion factor can also be
applied to other fruits of similar size and digestibility (e.g.,
raspberries) that wolves may eat, because each scat should
represent the same amount of food consumed. We caution
against extrapolating our results to other canids because our
results are based on individual wolf scats, which are larger
than those of other canids. Thus, using the berry conversion
factor for other canids would overestimate the biomass of
berries consumed. Further, differences in the digestibility of
berries by wolves and other canids are largely unknown and
as a result, the mass of scat produced per berry consumed by
other canids will likely be different from wolves. Additional
research may improve the accuracy and precision of our
berry conversion factor, especially for use in other regions
where the types of soft fruits consumed by wolves may be
We thank W. Severud and S. Barber-Meyer for reviewing an
earlier version of this manuscript and providing helpful
suggestions. We thank Voyageurs National Park and
2 Wildlife Society Bulletin 9999()
Northern Michigan University for funding and logistical
Floyd, T. J., L.D. Mech, and P. A. Jordan. 1978. Relating wolfscat content to
prey consumed. Journal of Wildlife Management 42:528–532.
Fuller, T. K. 1989.Population dynamics of wolvesin north-central Minnesota.
Wildlife Monographs 105.
Gable, T. D. 2016. Wolf predation: where and how wolves kill beavers, and
confronting the biases in scat-based diet studies. Thesis, Northern
Michigan University, Marquette, USA.
Hewitt, D. G., and C. T. Robbins. 1996. Estimating grizzly bear food habits
from fecal analysis. Wildlife Society Bulletin 24:547–550.
Litvaitis, J. A., and W. W. Mautz. 1976. Energy utilization of three diets fed
to a captive red fox. Journal of Wildlife Management 40:365–368.
Messier, F., and M. Cr^ete. 1985. Moose–wolf dynamics and the natural
regulation of moose populations. Oecologia 65:503–512.
Newsome, T. M., L. Boitani, G. Chapron, P. Ciucci, C. R. Dickman, J. A.
Dellinger, J. V. Lopez-Bao, R. O. Peterson, C. R. Shores, A. J. Wirsing,
and W. J. Ripple. 2016. Food habits of the world’s grey wolves. Mammal
Peterson, R. O., and P. Ciucci. 2003. The wolf as a carnivore. Pages 104–130
in L. D. Mech and L. Boitani, editors. Wolves: behavior, ecology, and
conservation. University of Chicago Press, Illinois, USA.
Szepanski, M. M., M. Ben-David, and V. Van Ballenberghe. 1999. Assessment
of anadromous salmon resources in the diet of the Alexander Archipelago wolf
using stable isotope analysis. Oecologia 120:327–335.
Tremblay, J. P., H. Jolicoeur, and R. Lemieux. 2001. Summer food habits of
gray wolves in the boreal forest of the Lac Jacques-Cartier highlands,
Quebec. Alces 37:1–12.
Usui, M., P. G. Kevan, and Y. Kakuda. 1994. Composition and energy
values of wild fruits from the boreal forest of northern Ontario. Canadian
Journal of Plant Science 74:281–287.
Usui, M., P. G.Kevan, and M. Obbard. 2005.Pollination and breedingsystem
of lowbush blueberries, Vaccinium angustifolium Ait. and V. myrtilloides
Michx. (Ericacaeae), in the boreal forest. Canadian Field Naturalist
Vander Kloet, S. P., and N. M. Hill. 1994. The paradox of berry production
in temperate species of Vaccinium. Canadian Journal of Botany 72:52–58.
Weaver, J. L. 1993. Reﬁning the equation for interpreting prey occurrence in
gray wolf scats. Journal of Wildlife Management 57:534–538.
Welch, C. A., J. Keay, K. C. Kendall, and C. T. Robbins. 1997. Constraints
on frugivory by bears. Ecology 78:1105–1119.
Wiebe, N., G. Samelius, R. T. Alisauskas, J. L. Bantle, C. Bergman, R. De
Carle, C. J. Hendrickson, A. Lusignan, K. Phipps, and J. Pitt. 2009.
Foraging behaviours and diets of wolves in the Queen Maud Gulf Bird
Sanctuary, Nunavut, Canada. Arctic 62:399–403.
Associate Editor: Glenn.
Gable et al. Estimating Berry Consumption by Wolves 3