Editor
Edward J. Heske
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Abstract
Human-altered
landscapes are thought to reduce habitat quality for many
forest-dependent species, but some omnivorous, opportunist species take
advantage of human-related food sources, such as garbage and crops. It
is unknown, however, whether anthropogenic foods can sustain populations
in areas with relatively little natural habitat. We studied (2007–2012)
a population of American black bears (Ursus americanus) at the
western periphery of their range in Minnesota, in a landscape that was
heavily impacted by agriculture (~50%). We estimated the dietary
contributions of corn and sunflowers (2–4% of the landscape) versus
seasonally available natural foods (spring vegetation, ants, deer,
fruit, and nuts) with stable isotope analyses (δ13C and δ15N)
of 110 hair samples from 51 bears. We identified associations between
diet and sex, age, body size and condition, reproductive status, space
use, habitat connectivity, and natural food abundance. At the population
level, adult males and adult females without cubs consumed considerable
crops in fall (95% credible intervals: males = 19–46% of diet, females =
10–40%), but females with cubs and juvenile bears rarely consumed
crops. Individual estimates of crop consumption were positively
correlated with the proportion of GPS-collar locations in crop fields.
Females, but not males, decreased crop consumption in years with high
availability of natural fall foods. Further, the degree of crop
consumption was more closely tied to local crop availability and
landscape composition in females than in males. Weight and fat were
positively correlated with crop use for both sexes, and males’ use of
crops also increased with their physical stature, suggesting that crops
provided substantial caloric benefits to bears and that social dominance
may have influenced foraging decisions. However, a large segment of
this bear population (44% of sampled bear–years) made little use of
crops, and crops accounted for more than half the fall diet for only 14%
of the population. Whereas some bears clearly benefited from
consumption of crops, we conclude that a population of bears could be
sustained in this largely agricultural region even without crops as a
food source.
- agricultural landscape
- corn
- crops
- foraging strategies
- Minnesota
- stable isotope
- sunflowers
- Ursus americanus
- © 2015 American Society of Mammalogists, www.mammalogy.org
