Home Range Size and Habitat Use of Urban Black Bears in the San Gabriel Mountains

It may come as a surprise to some, but bears living in the San Gabriel Mountains do not know that they are "supposed" to stay on their side of the Angeles National Forest boundary. As a result of more people living close to or actually within that boundary, and a possible increase in the bear population after the heavy rain years from 1994 to 1998, bear sightings in people's yards have become more common. For example, 17 bear sightings were reported to Monrovia police from 4/11/99 to 5/27/99, and most people remember the videotape of a bear enjoying a swimming pool in a resident's back yard.

I studied how bears are reacting to this urban expansion by capturing and tracking four urban bears from June 1997 to June 1998. Three of these bears relied heavily on urban areas for their food, which was mostly human garbage and domestic plants.

The bears in my study were captured in urban areas, using a culvert trap. Two were captured in Bradbury, two in Monrovia, two in Glendora and one in Sierra Madre. The bears were measured for size and health (see Table 1) and fitted with ear tags and radio collars.

Table 1

No.SexDate CapturedAge (years)Girth (cm)Length (cm)Weight (kg)
F-465F6/24/97> 293143.559(100)^
M-225*M6/25/97> 6113160128--
M-425M7/15/97> 6135184148184
M-290/226M10/15/97< 6127183125183
M-545*M10/19/97< 210214791147
M-023*M1/13/98< 6142181159181

* Bear subsequently lost radio collar. Two bears lost their collars within one month; another after three months.
^ The measurements were probably inaccurate for this first bear that we captured; I estimate that 100 kg is closer to her true weight.

Bear M-290/226 was recaptured on 11/15/97, and had gained ~50 kg in weight.

To convert to inches, divide cm by 2.54; to convert to pounds, multiply kg by 2.2. Thus the lengths ranged from 58-72" (4.8-6') and the calculated weights from 315-405 pounds.

The bears were tracked for 6-12 months about three times weekly, with observation times distributed throughout 24 hours. Triangulation produced 241 positions, with 16 additional positions from aerial data and from sightings of collared bears by researchers and volunteers.

Their home ranges, as defined by a convex polygon containing 95% of the positions for each bear, are displayed in Figure 1, with sizes give in Table 2.

No.Map colorNo. positionsHome range size (km2)% Urban Area% Sightings in Urban Area

Map color is the color used to plot the 95% confidence home range in Figure 1. The home range size is from the minimum convex polygon, both in Table 2and in the Figure 1. The column "% Urban Area" is the percent cover of the home range by Urban Area. The column "% Sightings in Urban Area" is the percentage of position determinations that were within the Urban Area.

The combined home range is bounded roughly by SR2 on the west and north, Forest Service Road 2N24 on the north, Glendora Mountain Road on the east, and Foothill and Sierra Madre Boulevards on the south.

Even though all four bears were captured in urban areas, there is dramatic variability in how much bears like to "hang out" in urban areas. Bear M-425 spent most of his time outside of urban areas, whereas the other three bears spent much more time in the urban areas than expected just on the basis of the urban area percentage of his total home range. Bear M-425 was probably not a true urban bear, since his behavior at capture indicated a lack of familiarity with the area. When pursued after release, he ventured farther into the residential area rather than retreating to the adjacent mountainous areas. After his capture, he was found in urban habitat only once, instead spending most of his time in Van Tassell and Fish Canyons away from urban habitat.

The lone female in this sample had not only the smallest range in this sample, 5.4 km2, but the smallest range reported for any single female bear in previous Southern California studies. Previous studies reported home range sizes of 8.6, 19.5 and 25 km2. This may be due to her over-reliance on concentrated urban food sources. This bear was seen numerous times foraging in trash, especially in late summer and early fall. She only left urban habitat at that time for two three-day trips to the Monrovia Peak / White Saddle area when an abundant crop of hollyleaf cherry (Prunus ilicifolia) was ripening. During the winter, she denned in that area. Excluding those few trips, her core home range was only ~3 km2, almost all of it urban habitat. In fact, on several occasions during summer and fall of 1997 and summer 1998, she was observed soaking in, swimming in, or drinking from swimming pools and spas!

Similarly, bear M-290 has a home range among the smallest reported for male North American black bears, 7.4 km2. Other Southern California black bears have reported home ranges of 24.2, 32.3 and 36 km2. Again, this is most likely due to trash foraging. In fact, this bear loved foraging so much that he was recaptured in a trash-can trap, despite his first capture in such a trap!

Both bears F-465 and M-290 had day beds only a few hundred meters from human dwellings. M-290 used many day beds in thickets of Arundo donax a few meters from residential streets, and had one day bed under the floor of a house that was under construction!

None of the male bears migrated to a denning area during the very mild winter of 1997-1998, nor did they "winter sleep".

I analyzed the data to determine whether there was any evidence of bears spending more time at higher elevation in a particular season. Only Bear F-465 had data for all four seasons: Her mean elevation was 1056 m in winter, and 460 m in summer.

I collected 20 bear scats from urban areas and 23 from wild or rural areas, and analyzed them with a 10x hand lens or a 25x Tasco dissecting microscope to identify at least the genus of all vegetable remnants. Table 3 summarizes the results:

Table 3

ContentUrban ScatRural Scat
Non-native plants160
Human garbage92
Native plants3633
Animal Matter (total)625
Animal Matter (- bear hair)218

If a particular scat contained two species of plants, it was counted twice in the above table.

Urban scat contains a much higher level of non-native, cultivated plant remnants; 80% of urban scats vs. none of the wild scats. The urban scats contained such plants as figs, peaches, apples, apricots, avocados and domestic cherries. Native plants found in both urban and rural scats were most frequently hollyleaf cherry and manzanita, followed by redberry, grasses and coast live oak. Human garbage fragments included paper, plastic and metal, including wrappers from fast food vendors.

Animal remains were found in only two urban scats, one had fly pupae and the other bird bones. Animal remains were found in 74% of the wild scats: bee remains, lepidoptera parts, reptile bones, beetles, ants, wasps and bird feathers.

For complete details on this study, including references, see my master's thesis Home Range Size and Habitat Use of Urban Black Bears in Southern California, California State University, Northridge, December 1998.

I thank Tom Chester for extracting the information from my thesis for this webpage.

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Copyright © 1998-2001 by Greg E. Van Stralen.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: care of Tom Chester
Updated 17 January 2001