Impacts of Acorns and Oaks on Bear Populations

In the southern Appalachians, hard mast, particularly red oak and white oak acorns, is the driving force in the population dynamics of black bears (Pelton 1989). This conclusion could be generalized to include all areas where bear habitat is dominated by oak forests or where oak is an important component of the forest. In the fall, when bears must prepare for the coming winter, for denning, and for producing young, they make physiological, behavioral, and ecological adjustments to their fall food source (Pelton 1989); and these adjustments affect every aspect of their existence, including movements, home range, growth and condition, vulnerability to harvest, survival, and reproduction. Thus, an acorn crop failure or a bumper crop in a given year can alter the distribution and abundance of bear populations on a local or regional scale.

Fall Movements

During fall, black bears depart from their normal crepuscular feeding pattern and begin almost continuous foraging (Garshelis and Pelton 1980). In order to acquire the food necessary to accumulate the fat reserves to survive the winter denning period, bears must locate reliable food sources, which may require extensive movements. The magnitude of these seasonal shifts is dependent upon fall acorn production. In good years, bears may simply move within their normal home range to elevations or aspects that produce more hard mast (Garshelis and Pelton 1981), but in poor years they may leave their home ranges entirely and travel great distances to find areas of high food abundance (Rogers 1987).

Pelton (1989) described four types of movements or adjustments bears in Tennessee made in response to spotty mast production: (1) general long-range movements, (2) intensive use of small areas resulting in significant home range overlap, (3) accommodation of a prime acorn site within the normal annual home range, and (4) departure from traditional spring and summer ranges to pockets of concentrated and abun dant sources of acorns in fall. in years of poor acorn production, bears moved two to four times farther than in good production years. This also was the case for bears studied in Virginia, where the movement from summer range to fall range was two to three times as great (1.7 km versus 6.7 km for solitary females and 1.2 km versus 3.4 km for females with cubs) during years of gypsy moth induced acorn crop failures as it was prior to the gypsy moth infestation (Kasbohm et al. 1998). Black bears in Minnesota traveled farther and became more attracted to human-related food sources when fall acorn crops failed (Noyce and Garshelis 1997).

While many others have reported long-range movements of bears in response to fall acorn crop failures (Jonkel and Cowan 1971, Reynolds and Beecham 1980, Garshelis and Pelton 1981, Novick and Stewart 1982, Carlock et al. 1983, Powell et al. 1997), some of the most extensive movements reported for bears in search of fall food (primarily acorns) came out of Minnesota (Rogers 1987). Rogers reported several movements of bears in excess of 90 km; 1 male moved 201 km to an area of high acorn production. it is unclear how bears find these distant food sources, but Rogers (1987) suggested they may travel to these areas as cubs, and return as adults, thus passing on this knowledge from generation to generation.

Fall movements by bears in search of food do not always result in success. Of 59 marked bears residing in gypsy-moth defoliated areas of Shenandoah National Park, 35 did not move, and only 14 of 24 that did move found acorns (Kasbohm et al. 1998). These kinds of long fall movements into marginal areas probably result in increased mortality and reduced reproduction (Carlock et al. 1983).

Home Range in general, animal home range size varies inversely with food production (Schoener 1983), and bears are no exception. For instance, bears in Virginia increased the size of their fall ranges twofold in areas of gypsy moth-induced acorn failure (Kasbohm et al. 1998). But, home range size also may depend on the complexity of the diet as was the finding in a study in Arkansas, where fall home ranges of bears feeding on acorns were smaller than summer ranges (27 vs. 97 km2) when bears fed on a variety of foods (Smith and Pelton 1994).

Fall home ranges of female bears in the Pisgah National Forest, North

Carolina, were found to be small when fall production of acorns was great and large when production was small (Powell et al. 1997). Powell et al. (1997, pp. 96-105) tested hypotheses concerning the relationship between three important food groups of bears (squaw root, berries, and acorns) and home range size. Acorn production explained 76% of the variation in fall home range size of females and 49% for males. Neither of the other food groups affected fall home ranges. Powell et al. also found a close relationship between fall acorn production and annual home range size, highlighting the importance of fall production of this fruit to the annual movement patterns of bears.

Habitat quality in the fall depends, in part, on the abundance of oaks at mast-producing age (Powell et al. 1997), thus a bear's fall home range tends to include a high proportion of mast-producing oak trees. Garshelis (1978), for instance, documented that fall home ranges of bears in Tennessee had significantly more oak trees than their spring and summer ranges. Garner (1986) noted the same for bears in Virginia. Bears are adapted to long periods of fasting, but their ability to locate these pockets of food abundance in fall is critical if they are to accumulate fat reserves so they can survive the winter.

Survival

Mortality rates of adults and of 1- and 2-year-old bears tend to be highest in fall, due to harvest, or in winter, due to starvation (Bunnell and Tait 1981). Cubs suffer highest mortality soon after birth (Bunnell and Tait 1981, Higgins 1997). The effect of fall mast production on cub survival is unclear. There is evidence to support a positive relationship between cub survival and fall hard mast production (Pelton 1989) and evidence to suggest that no relationship exists (Elowe and Dodge 1989). In Tennessee, cubs and yearlings suffered 80% mortality in years of poor acorn production (Pelton 1989), and females lost entire litters only in years following poor acorn production (Eiler et al. 1989). Only 2 of 16 cubs died following good years, but 7 of 8 died following poor crop years (Eiler et al. 1989), leaving Eiler to conclude that "yearly variation in cub mortality was probably related to differences in mast yields." In Minnesota, cub survival was 88% when food was abundant in the year of conception and the year of birth, but only 59% when food was poor in both years (Rogers 1987). Others studies in Tennessee (Wathen 1983) and Minnesota (Rogers 1976) found positive relationships between cub sur vival and fall mast production the previous year. In the latter study, cubs weighing less than 1.8 kg in late March (n = 15) had 4 times the mortality rate prior to family breakup (67% died) as heavier cubs (n = 47).

Conversely, cub survival in Massachusetts (Elowe and Dodge 1989) and Arizona (LeCount 1982) was not related to fall mast production; high and low cub survival occurred under similar forage conditions. Data collected prior to, during, and following a gypsy moth infestation in Shenandoah National Park, seemingly support these findings. Cub survival rates, and survival rates of all age classes, were unchanged following massive defoliation by gypsy moths and complete loss of the acorn crop. Prior to defoliation (Carney 1985) and following defoliation (Schräge and Vaughan 1998), acorn crops had been good. However, despite the loss of the acorn crop during years of defoliation, bears found alternative food sources and their condition remained good (Kasbohm et al. 1995).

Our work at Virginia Tech with captive bears indicates that fall nutrition for adult females does affect survival of their cubs (Vaughan, unpublished data). In food trials with some bears on ad lib diets and others on maintenance diets, all bears produced cubs, but those on maintenance diets did not have the energy to lactate, and their cubs died.

Yearling bears, on their own for the first time, likely are more vulnerable to starvation than other age classes, but not much information is available on survival of yearling bears. Yearling bears in Minnesota weighing less than 10 kg at the end of denning had higher mortality rates than those weighing more than 10 kg (Rogers 1983, 1987). Only 1 of 25 weighing less than 10 kg, but 53 of 73 weighing more than 10 kg survived. In this study, acorns were a component of the fall diet, but not the predominant food.

Most adult bears die from hunting or removal because of nuisance activity (Bunnell and Tait 1981), and there appears to be a strong relationship between fall hard mast production and both nuisance activity and harvest rates (Kasbohm et al. 1994). In Wisconsin (unpublished reports for 1954-1969 cited in Rogers 1976), failure of blueberry and red oak crops were correlated with increased damage by bears. The number of bears killed on depredation complaints was greater than 100 only when acorn and berry production fell below 20-25% of normal production. In Tennessee (Pelton 1989), game officers found 21 bears dead following the 1984 oak mast crop failure; an additional 20 had been shot after wandering onto farms and into subdivisions.

Fall food supply appears to affect the vulnerability of bears to hunting (Gilbert et al. 1978, Alt 1980, Pelton et al. 1986, Litvaitis and Kane 1994). In Pennsylvania (Alt 1980), hunting success was high when acorns were abundant, likely because the abundant food supply kept bears from entering dens until later in the fall. In New Hampshire (Kane 1989) and Massachusetts (McDonald et al. 1994), hunting success went up when fall mast (primarily acorns and beechnuts) was low. In both New Hampshire and Massachusetts, baiting was legal, thus hunters were successful in attracting bears to bait when natural food was scarce.

In Virginia, archery hunters were more successful in years when acorn production was low (Martin and Steffen 1999). During 19731998, the four worst years for hard mast production coincided with the four most successful years for archers. During those four years, archers accounted for 26-36% of the harvest; in all other years they never exceeded 17% of the harvest.

Noyce and Garshelis (1997) examined the relationship between natural food abundance and the bear harvest in Minnesota. They found that the percent of females in the harvest, the mean age of females killed, and hunting success were inversely related to fall production of hazelnuts (Corylus spp.) and acorns (Figure 15.2). Hunter success ranged from 26% to 43% and was highest when the food index (hazelnuts and acorns) was lowest (r2 = 0.66; P = 0.0013). The percentage of females in the harvest was independent of population size, the percentage of females in the population ratio, and the number of hunters, but was in-

Figure 15.2. Percentage of Minnesota hunters shooting female and male black bears (number killed/number hunters) versus annual hazel + oak index of relative productivity of the trees. Each square represents one year of data during 1984-95. (Reprinted from Noyce and Garshelis 1997 by permission of The Wildlife Society.)

Figure 15.2. Percentage of Minnesota hunters shooting female and male black bears (number killed/number hunters) versus annual hazel + oak index of relative productivity of the trees. Each square represents one year of data during 1984-95. (Reprinted from Noyce and Garshelis 1997 by permission of The Wildlife Society.)

versely related to food abundance (r2 = 0.62; P = 0.002). Both of these relationships seem to indicate that bears, particularly female bears, delayed den entry in years of poor food abundance (pregnant females are normally the first to enter dens; Kasbohm et al. 1996b). The relationship between harvest rates and food abundance was strongest for adults, less for juveniles, and not significant for yearlings, thus the average age of females in the harvest increased with decreasing food.

All of the cases cited above are clear evidence of the importance of fall production of hard mast on all age classes of bears. The relationship can be direct (starvation) or indirect (e.g., vulnerability to hunting), but the impact on the growth rate and overall health of the population can be tremendous.

Reproduction

Numerous studies indicate that every aspect of reproduction in bears, including proportion of females breeding, age of first reproduction, litter size, and interbirth interval, is affected by the nutritional condition of females in fall (Rogers 1976, Beeman and Pelton 1980, Willey 1978, Beecham 1980, Bunnell and Tait 1981, Young and Ruff 1982, Eagle and Pelton 1983, Clark et al. 1987, Eiler et al. 1989, Elowe and Dodge 1989, Clark et al. 1994). The range for each of these variables is narrow, but the impact on population growth caused by these variations can be great. Among eastern bear populations, reproductive performance differs for areas that have high versus low acorn production. in northern Minnesota, age of first reproduction was 6.3 years, average interbirth interval was 2.3 years, and average litter size was 2.4, whereas in Pennsylvania 88% of females produced young by age 4, interbirth interval was 2.0 years, and average litter size was 3.0 (Rogers 1987).

The black bear population in the Great Smoky Mountains of Tennessee and North Carolina is typical of bear populations in the mid and southern Appalachian mountains, where acorns of white and red oak are the primary fall food (Pelton 1989). There, during a 10-year period, the difference in reproductive success following years of poor acorn production versus years of good acorn production was striking. in years following poor acorn production, reproductive success was only 26% and cub mortality was 87.5% (Eiler et al. 1989). in years following good acorn production reproductive success was 89% and cub survival was high. Nine of 10 skips in the normal two-year reproductive cycle followed years of poor acorn production, and only 1 of 30 females that were trapped the summer following poor acorn production was lactating (Eiler et al. 1989). In fact, there was a linear relationship between white oak acorn abundance and percentage of females lactating the following summer (Pozzanghera 1990). These results led Eiler et al. (1989) to conclude that "the relationship between oak mast availability and black bear reproduction and cub survival emphasizes the importance of maximizing mast production from the oak component of the southern Appalachian forests."

In another region of the Appalachian range, loss of the acorn crop did not result in reproductive failure, because bears found an alternative food source of grapes and pokeweed (Kasbohm et al. 1995). During three years of a gypsy moth infestation in Shenandoah National Park, which resulted in complete loss of the acorn crop, litter size, interbirth interval, age at first reproduction, and cub survival remained unchanged between years prior to infestation (Carney 1985) and years following infestation (Schräge and Vaughan 1998). No female skipped an opportunity to breed. We concluded from this that bears have high behavioral plasticity and are extremely flexible in their diet and that acorn production alone cannot completely explain fluctuations in bear survival and reproduction; total food production and the relative nutritional value of all food items must be considered (Kasbohm et al. 1996b).

The same kind of diet flexibility was evident in Minnesota bears that fed on beaked hazel (C. rostrata) and red oak (Rogers 1976, 1983). When these bears had access to garbage, a higher percentage of females produced larger litters at shorter intervals. This led to the blastocyst resorption hypothesis, which hypothesized that bears in poor nutritional condition would not implant blastocysts in the uterus and would skip a year of reproduction. My students and I tested the blastocyst resorption hypothesis in captive bears and found that even bears in poor nutritional condition implanted blastocysts and in most cases brought fetuses to term. However, their inability to lactate led to starvation of the newborns, which sows devoured leaving no evidence of reproduction (Vaughan unpublished data).

Since bears breed in the summer, but implantation of the fertilized eggs and growth of fetuses is delayed until late fall or early winter (Wimsatt 1963), fall nutrition has a tremendous impact on reproductive success of female, but not male, bears. However, the idea that fall nutrition the previous year affects the breeding potential of male bears has not been examined and is a potential field for investigation.

Was this article helpful?

0 0

Post a comment