The “tug of war” facing urban wildlife: When to forage on human foods?

An urban squirrel feasting on its bird feeder treasures.

An urban squirrel feasting on its bird feeder treasures.

By Jerod A. Merkle

Human dominated landscapes can provide a food haven for certain species. Think squirrels and pigeons, animals many of us see almost every day. These species have adapted so well to human infrastructure and its seemingly endless bounty, that they can live their entire lives within cities with millions of people.

Of course, there are other species that also capitalize on foods available in urban areas.  But they do so with much less visibility.  In fact, they spend much of their time in more natural areas, only venturing into urban areas to feed.  In this case, think of animals such as raccoons, coyotes, and bears. Although the act of moving from wildlands into urban areas is for the most part driven by food, our understanding of how animals make these decisions is still not complete.  Understanding how and why these behaviors develop can help direct efforts to minimize human-wildlife conflicts.

: Panorama of Rattlesnake Valley, Missoula Montana, showing the shift from wildlands (right) to the urban center (left).

: Panorama of Rattlesnake Valley, Missoula Montana, showing the shift from wildlands (right) to the urban center (left).

Theory provides us some basic ground work, where animals must trade-off the benefits and costs of feeding on a particular food item or within a particular food patch.  Benefits can include caloric intake rate of a certain food item, and costs can include probability of death while eating that food item.  This seemingly simple balancing act becomes a little less clear in urban areas, where cues that animals normally use to predict costs and benefits can be distorted.  Think birds flying into windows or getting tangled in power lines, or animals mistakenly eating small plastic items for food.

White-tailed deer foraging within a fenced yard.

White-tailed deer foraging within a fenced yard.

When trying to understand foraging ecology and how to manage associated conflicts along urban-wildland interfaces, there is an important question to consider.  Is the animal in question “pushed” or “pulled” out of the nearby wildlands?  More specifically, do the benefits of foraging in wildlands fall below some threshold and then animals are pushed into urban areas in search of food?  Or, can peaks in urban food availability entice wildlife, pulling animals into urban areas, regardless of the quality of wildland foods?

Along with several collaborators, I tested just this question in a population of black bears living in and around Missoula, Montana.  Between 2008 and 2010, we marked 16 bears with GPS collars, followed their movements in and out of town, and concurrently monitoring wildland foods (berry availability and wildland green-up) and urban foods (fruit and garbage availability and urban green-up).

Young black bear walks on a Missoula resident’s front porch.  Photo credit: R. Wolfe

Young black bear walks on a Missoula resident’s front porch. Photo credit: R. Wolfe

We found that the probability of a bear entering the urban area was influenced by urban food availability, not wildland food availability.  In other words, when urban foods were available, bears moved (or were pulled) into the urban area, regardless of the level of availability of wildland foods.  Furthermore, their movements were particularly driven by apple availability and to a lesser extent urban green-up and garbage availability.  These results were supported by a concurrent diet study which showed that 50% of feeding sites were related to fruit trees, and < 10% were not human-related foods.

These results are somewhat surprising, as other authors have speculated that human-bear interactions occur more often when wildland foods are unavailable.  Our findings clearly illustrate the profound effect human-dominated landscapes can have on the behavior of wild animals.  Future researchers must be cognizant of the potential for drastic behavioral change.  Likewise, management and conservation plans must incorporate how urban food availability can be more important than wildland food availability.

Black bear foraging on apples in Missoula, Montana.  Photo credit: D. Leparko

Black bear foraging on apples in Missoula, Montana. Photo credit: D. Leparko

The bottom line is that we humans coexist with wildlife, for better or worse.  As researchers, we realize there are a lot of unanswered questions.  As neighbors, asking a few questions can go a long way toward reducing human-wildlife conflicts.  As we said in a popular article about the study, think of it this way:

“Do you live within a wildland-urban interface? If so, take a look at your landscaping next time you walk outside. Think about the food that it produces for wildlife, and ask yourself a couple of key questions. Am I indirectly contributing to human-wildlife conficts? Is it possible that animals are finding food on my property and then moving on to other areas where conflicts may develop?”

For more information, read the full article in the Journal of Mammalogy, and read an in depth popular article about the study in Fair Chase magazine.  All photos by Jerod A. Merkle unless otherwise noted.

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Suggested readings:

Beckmann, Jon P., and Joel Berger. (2003) “Rapid ecological and behavioural changes in carnivores: the responses of black bears (Ursus americanus) to altered food.” Journal of Zoology 261.2: 207-212. (article)

Niemelä, Jari. (1999) “Is there a need for a theory of urban ecology?.” Urban Ecosystems 3.1: 57-65. (article)

Tuomainen, Ulla, and Ulrika Candolin. (2011) “Behavioural responses to human‐induced environmental change.” Biological Reviews 86.3: 640-657. (article)

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