Mark Ditmer holding a black bear cub
If I asked you to name three favorite foods of bears, you would probably list off honey, berries, and salmon. But did you know that some bears also like corn and even sunflower seeds? Mark Ditmer, the Consbio program’s most recent grad, has spent the last several years studying what black bears eat in the far extreme of the Eastern black bear population’s range. His work attempts to explain how bears are managing to succeed in Northwestern Minnesota, an area previously thought to represent marginal habitat, but that is now the only part of Minnesota with an increasing bear population.
In order to survive a winter’s hibernation, bears need to pack on the pounds during the fall. Ditmer found that captive bears, given a choice, were basing their food intake on which items had the highest caloric content: fatty oil sunflower seeds were preferred over corn, acorns, and lower-calorie convection sunflower seeds. His stable isotope analysis of wild bears similarly showed that while corn and sunflowers made up a relatively small proportion of the available landscape, they were accounting for a disproportionately large amount of the diet.
As it turns out, crop fields provide easy foraging. By utilizing heart rate biologgers developed by Medtronic, Inc. and GPS-collars, Ditmer found that whereas when bears move across areas of open agricultural lands (without crops that bears prefer to eat) they exhibit faster movement and heart rates, their movement will actually decrease, and so may their heart rates, when they are foraging in crop fields. What could be easier than sitting in the middle of a corn field, and munching on every ear within arm’s reach? Of course, this kind of behavior gets bears into trouble with farmers. The havoc caused by a bear eating corn is much more visible than when a deer wanders through a field taking a bite here and a bite there.
So to answer Ditmer’s question, (how are bears managing to thrive in the marginal habitat of Northwestern Minnesota?) it turns out, bears in Northwestern Minnesota have much larger home ranges than bears in other places. Typically large home ranges are associated with poor habitats but Ditmer’s findings suggest that the males’ roaming is used to seek out and then hone in on the best available forage. Bear home ranges expand and contract in the short-term as different food resources become available. For instance, bears tend to stick to a smaller area if they have ripe crops in their home range, but before the crops ripen they are using a much larger area. As a general rule of thumb, the higher the caloric density (the more food there is) the smaller the bear’s home range will be. Crop fields are particularly dense in calories, so bears, especially the bolder males, are likely to use crops to forage to prepare for a long winter without any food. However, female bears seem willing to reduce or avoid raiding crop fields if other options in natural habitat, such as an oak forest full of acorns, are available in a given year.