Effect of habitat quality on killings by black bears

February 15, 2010 - 9:12 PM CST

In yesterday’s update we began addressing a viewer’s question about whether familiarity with people makes black bears more dangerous.  The viewer wanted to know if the 63 killings by black bears since 1900 were in places where black bears had grown used to humans and perhaps been hand-fed.  Although there are exceptions to just about everything a person can say, the data show that most black bear killings are in areas where bears and people have the least contact.  Killings generally have not been where bears have grown used to people or been hand-fed.  We ended yesterday’s update by comparing areas where 40 percent of the killing by wild bears had occurred to areas where no killings had occurred despite a high amount of bear-human interaction.

Before we go further, though, we want to say we are not writing about captive bears.  Unlike wild bears, which we find to be fairly predictable, we have no idea what to expect from caged bears.  If captive bears don’t like someone, they can’t run away.  Some are excessively defensive.  Individual bears develop quirks.  They can even develop what veterinarians call cage psychosis.  Caged bears give bears a reputation for being unpredictable.

We ended yesterday saying the areas we compared had another difference beyond how much contact there was between humans and bears.  That difference is habitat quality.  The two areas with the highest number of killings, northern Ontario and British Columbia, have generally low natural food abundance for black bears.  Northern Ontario sits on the Canadian Shield, a bedrock formation that’s close to the surface.  The shallow soil has notoriously low fertility and easily dries out in years of drought.  In years of drought or late frost, berries are scarce and there’s little else to turn to except ant pupae and vegetation that’s become mature and hard to digest.  Oak trees are scarce in most areas, and good hazelnut crops are infrequent.  Other nut-producing trees like hickory, beech, and the many others, are nonexistent.  The rugged mountain areas of interior British Columbia have similar problems of low fertility and scarce hard mast.

Growing seasons are short, and black bears have less than five months to fatten for 6-7 months of hibernation.  Coastal British Columbia is richer in food and has had no killings despite a higher human population.

Compare Ontario and BC with Pennsylvania and New Jersey, arguably the best black bear habitat anywhere.  Bears there grow faster and reproduce sooner than black bears anywhere else.  Black bears in those states are blessed with lush vegetation in spring and many kinds of berries, nuts, and acorns into late fall.  They have 8-9 months to fatten for 3-4 months of hibernation.  In addition, the high human population inadvertently provides food with garbage and bird feeders or on purpose with bear feeding troughs.

Pennsylvania is home to one of the top examples of bear-human coexistence.  Dr. Gary Alt, a bear researcher, studied a gated community called Hemlock Farms where a thousand people per square mile coexisted with three bears per square mile.  That’s a higher density of black bears than is found in any national park or national forest and might be the highest human density to coexist with that many bears.  People fed bears in backyard feeding troughs there for decades.  People enjoyed seeing the bears.  The bears got used to seeing people.  Many were hand-fed.  No one was attacked.

Does the difference in food supply between the area with highest number of killings and the area with no killings explain the difference?  Not necessarily, which is why we said there is no good explanation for the 63 killings since 1900.  In areas of moderate to poor bear habitat, there are years when bear food is very scarce due to drought, frost, and other factors.  Whole populations become so malnourished that cubs are starving to death, mature females cannot get fat enough to maintain pregnancies, and bears enter their dens with marginal fat reserves.  Despite all that, there usually are no killings and no attacks.

So is there something else to account for killings by black bears?  What about wounded bears, cornered bears, and sudden encounters?  What about mothers with cubs, or the odd rogue bear?  What about bears that lose their fear of people at bear-viewing areas or garbage dumps?  Do some black bears develop a taste for human food, seek it out, and eventually seek the humans themselves?  What about roadside bears in national parks where people sometimes put their kids on the bears’ backs.  Are menstruation and sex factors?  Do bears hold grudges if they were shot and wounded by hunters or landowners?  What if a bear is malnourished due to intestinal parasites?  Do some human behaviors trigger attacks?  Do these factors account for enough of the 63 killings to explain them?  Or is there a bell-shaped curve for bear behaviors and the occasional bear is out in the tail as either a rogue or a super bear?  These are topics for upcoming updates.

Thank you again for your continued support to make this and more possible.  Thank you also for supporting the work through your NABC Gift Shop purchases.

—Lynn Rogers and Sue Mansfield, Biologists, North American Bear Center