Population Dynamics and Denning Ecology of Black Bears in Shenandoah National Park, Virginia.  (Daniel W. Carney, Master's thesis, 1985)

ABSTRACT

In his study from 1982-1985, taking place in the Central District of the SNP, 115 bear were captured in foot snares and culvert traps a total of 149 times (75% caught by snares, 25% by culvert traps). 47 bear were fit with radio transmitter collars (mainly sows). Minimum breeding age of sows was two years, but the average was three, with a mean litter size of two, giving birth every other year (except when litters were lost during the summer, in which case they gave birth in consecutive years. Annual mortality rates were estimated at 30% for cubs, 54% for yearlings, 39% for two year olds, and 21.5% for older bears. Radio collared males had a mortality rate over five times that of females (41.5% versus 7.5%). Most bears were aged from one to three. Bears of every age up to 14 were captured. Bear density was estimated at one bear/0.96-1.49 sq km. The population appeared stable.

29 of 61 den types were rock cavities and 19 of 61 were above-ground tree cavities. Males did not use tree cavities. Differences in dates of den entry, den emergence, and parturition (birth) were unrelated to weather and hard mast production.

POPULATION DYNAMICS

In 1976, a bear management plan was implemented to (1) restore and maintain the natural integrity, distribution, and behavior of the bear population. (2) minimize nuisance bear conflicts, and (3) provide the opportunity for visitors to see bears. By 1981, the plan was judged successful and the objectives fulfilled. In 1980, 232 bears were harvested in VA. In 1982, 432 bears were harvested. The mean annual bear harvest from 1981 to 1984 was 399 bears, compared to the mean annual bear harvest of 225 from 1972 to 1979. About half the bears harvested in VA have been from the eight counties surrounding the SNP.

RESULTS

Sex and Age Structure . 108 captures were male compared to 41 female (115 bear captured a total of 149 times). 77 males were captured versus 38 females. The major difference was in sub-adults (1-2 years), with the more roaming males outnumbering females 54 to 11.

Reproduction. The sex ratio of cubs were 0.85M:1F. Mean litter size was 2.0. Six litters had one cub, 9 litters had two cubs, and 6 litters had three cubs. Statistically, there was no correlation of age with number of cubs. Sows up to 15 gave birth (sows age of 9,11,14, and 15 had litters of 1,2,1, and 2).

Cub Mortality. 14 families were radio monitored for more than one year with four losing entire litters and two losing 1of 3 (three cases, females left their cubs after the den was disturbed-in which case, the cubs were introduced to other females - two cases of entire litter loss and both cases of partial litter loss occurred to females that had accepted orphaned cubs introduced to them while the sow was still in the winter den and lost over the summer). Ignoring these cases of introduced cubs, 3 of 10 litters were lost in the first summer (two of the three litters were first litters for the sows).

Adult Mortality. Of the 47 radio-collared bear, an average mortality of 6.1% (18.4% over three years) was found. Annual survival rates for males were about 60% and females about 90%.

Population Density. Differing models gave figures for the Central District population ranging from 30 to 196 for the northern section of the study area and 145 for the southern section (of the Central District).

DISCUSSION

Population Structure. Although cub sex ratios usually do not differ from 1:1, most studies report capturing more males, due to their larger home ranges and more extensive traveling. Mean age was 3.5, lower than studies in Yosemite and the Smokies. The lower age composition is indicative of an exploited (hunted) population.

Reproduction. Mean litter size of 2.0, based on cub counts, is smaller than studies in PA and the Smokies, larger than reported in Montana, but similar to most. The two year old breeding age is common only in PA and the southern Appalachians, where 38% and 65%. respectively, of the two year olds had bred. Montana reported a minimum breeding age of 6 years old. In general, western bear studies showed a breeding every 3 or 4 years, compared to 2 years of the eastern studies.

Mortality. 21.5% adult mortality rate is similar to PA and Smokies studies, but higher than several western bear studies (13% for Montana and 15-18% for Washington). These figures support an exploited population. The difference between male (41.5%) and female (7.5%) mortality rates was related to the long and narrow shape of the Park, which made males more likely to travel outside the Park. Other studies show more male mortality, but nothing like this study population. Subadult and yearling mortality was difficult to summarize due to the small sample size. However, various studies have shown a correlation between mast crops and overwintering subadult and yearling success. Additionally, first litters show a increase in mortality.

Population Size. The estimate of 194 for the northern section of the Central District, with a density of 1 bear / 0.91 square km is reliable. The best model for the southern section was 145 bear, with a density of 1 bear/ 1.03 square km. Thus, the total density is estimated at 1 bear / 0.96 square km. This figure is higher than most studies, related to the higher mortality of adult male versus female (a function of Park shape), which enables more subadults to remain in the area (with less adult males to force the young to evacuate the area and increased survivability of cubs and subadults). Modeling supports that the bear population in the Park is stable. A search of the literature supports that, of 16 studies in North America, only two areas have a higher density of bears than the SNP (In Washington and Alaska, with 0.67-0.89 and 0.32 sq km/bear respectively). Smokies in 1983 were shown to have a density of 1.7-11.8 sq km / bear. (Using these units, this study indicates 0.96-1.49 sq km/bear.)

DENNING ECOLOGY OF BLACK BEARS IN SHENANADOAH NATIONAL PARK

RESULTS

Den Sites and Den Characteristics. 61 sites were observed; 56 with radio-collared bear, five used by unmarked bear. 10 male sites were found, 46 female sites found, and 5 of undetermined sex. No males denned in trees (17 females used tree cavities). 7 of 10 male dens were rock cavities (21 of 46 females used rock cavities). 47% of all dens (29) were in rock cavities, 31% in above ground cavities (19). 21 tree dens were in northern red oak (62%), chestnut oak (29%), and tulip poplar (9.5%). Two were in dead snags broken off 12-18 feet off the ground. The mean height of the tree den was 28 feet. The mean depth of the den inside was 9 feet below the entrance. Mean dbh was 37". Elevation and aspect were random. Overall, dens were found in appropriate ratio to forest types, with the exception of fewer dens found in tulip poplar forest types. Four denned in an open bed, one each in excavations, upturned root system, hollow log, and brush pile. Three were located in ground level tree cavities. Four cases of bears not denning were recorded. Two were subadult males, and two were females with yearlings. In general, den entry was earlier for females than males (sows with cubs had a later entrance date than pregnant sows- you know how kids are) with late November for sows and males a week later for 1982-82, mid December for females and early January for males in 1983-84 and late December for females and early January for males in 1984-85. Parturition (birthing) took place around Jan 20 in 1983, and around February 5 and 2, respectively, in 1984 and 85. Den emergence was latest for sows with cubs, averaging mid to late April, with solitary females and females with yearlings emerging a little earlier, and males being first out, averaging late March to early April. Dates of parturition were not related to age or den entry. Denning periods ranged widely, with males being the shortest, then females with yearlings (those crazy kids), followed by solitary females and lastly, females with cubs.

Only two cases of den reuse were identified, both by the same solitary female. This sow used three different dens in each winter; using one den two successive winters and another den also two successive winters.

DISCUSSION

Den Selection and Availability. Rock and tree cavities are most common, as reported in PA, Montana, Idaho and Tennessee. Most females used both types over a few years. Tree cavities may be preferred by females due to their smaller size than males. One study showed a decrease in tree cavity use after a good mast year, suggesting larger female size than the preceding year. Another study suggests females, by denning earlier than males, pick the better tree cavities, but in this SNP study, good tree cavities were not used throughout the winter. Behavioral differences shows females more often found foraging or loafing in trees than males, and hunters note females more likely to climb trees when pursued by dogs than males. In this study, it didn't appear that den site availability was a limiting factor, with disturbed bears heading directly to other sites to den in, both from tree to rock cavities and the reverse. Low den reuse is commonly found, but in areas of recent logging, den reuse is more common, due to limited available denning options. Ground dens were found on sloping grounds, perhaps to reduce likely disturbances. Six sites were within 90 feet of heavily used foot trails, with only one (less than 30 feet from a prominent overlook on the trail, left the den probably as a result of human disturbance.

Chronology. Other studies have shown females with yearlings may not den. Also, males have been known not to den, sometimes the result of poor mast years, prohibiting the production of an adequate brown fat layer. Contradicting study results have been suggested for den entry stimuli. Heavy snowfall, low temperatures and above average precipitation, physical conditions, food availability , low pressure systems, poor mast availability and mild weather stimulating earlier entries than good mast and bad weather. However, other studies have shown no correlation between food availability and den entry, den entry earlier in good mast years, and poor correlation between weather and entry dates.

 

SURVIVAL OF BLACK BEAR CUBS INTRODUCED TO FOSTER MOTHERS AND LITTER SUCCESS IN SHENANDOAH NATIONAL PARK, VIRGINIA

Six females and one male were introduced to 8 foster mothers. Only one introduced cub was known to be rejected. (In this case, the researcher approached a tree cavity den, only to chase the female from a new nearby ground bed, leaving her own cub behind. She returned four hours later, picking up her own cub, leaving the foster cub abandoned. The cub was successfully fostered by another female., who later lost her radio collar. Another female lost her radio collar, thus her (and her foster cub) success are unknown. The other five were successfully introduced to foster mothers. Three already had two cubs. By the following winter, only one cub of the three was still with the mother. Two other females lost all of their litter during the summer. Thus, of the seven introduced, only one is known to survive the first year.

10 natural family groups were monitored, of which three did not survive their first summer ( litter sizes were 1, 3, and an unknown number of cubs. Only the female with three had had previous litters.)

DISCUSSION

Other studies show similar high percentages in successful adoptions (this study was 85%), but better results in survival rates than this study's. First time mothers have been shown to have more cub mortality. Females with introduced cubs have higher cub mortality than natural families, perhaps due to the nutritional limits of the mother. Thus, it seems that introducing cubs to foster mothers should only be attempted in areas of high quality habitat. The low success rate does not justify the possibility of reducing survival of the mother's natural cubs. Other techniques of reintroducing cubs into the wild, such as feeding them through the summer and then releasing them or introducing them to females with cubs after den emergence, might be more successful.