This study was conducted from 1985 through 1991 to assess the
effects of gypsy moth infestation and subsequent defoliation on the resident
population of black bears in the Shenandoah National Park.
Significant data exists documenting pre-infestation conditions (see
Garner and Carney studies) enabling a comparison based on radio telemetry,
population, and behavioral data.
Gypsy moth defoliation (>60% canopy loss) increased from 546 ha in 1986, to
2,304 ha in 1987, 6,227 ha in 1988, and 17,736 ha in 1989 (34% of the study
area). Chestnut oak and red oak
habitat types received the greatest defoliation, with 60% and 45% of these
types suffering greater than 60% loss in the Northern and Central Districts
resulted in a 99% reduction in acorn production in defoliated stands.
Long-Term Ecological Monitoring System (LTMS) sites experienced a 17.6%
oak mortality in areas of 2 - 3 years defoliation.
Areas with multiple years of defoliation were projected to have 50% oak
mortality over 20% of the oak habitats in SNP from the initial gypsy moth
Short-term gypsy moth-induced habitat alterations included dramatic increases
in soft mast production (blackberries, raspberries, cherry, grape, and
pokeweed). Thus, short-term
impacts have not yet jeopardized the SNP bear population.
The reoccurrence of extensive defoliation events will dictate oak
mortality and long-term habitat quality, and ultimately will determine bear
habitat suitability in the future.
Bears were able to switch from hard mast (acorns) to soft
mast (previously mentioned) with little detectable effect on bear nutrition,
physical condition, reproduction, or survival, despite acorn failure.
Litter size did not differ, nor did females skip opportunities (years)
to reproduce. Survival rates were
not different from pre-defoliation levels.
Bears used cove hardwood and black locust habitats more often than expected,
likely due to increased cherry abundance.
Bears did not avoid defoliated habitat, nor did they significantly
increase use of gypsy moth non-host stands (tuliptree forests).
Fall range areas were significantly greater during infestation than
before, implying that acorn loss stimulated fall bear movements.
In defoliated areas of Virginia, 220 bear were “harvested” compared
to 138 harvested prior to defoliation; a reflection of greater bear movement.
Den entry dates were equivalent, but den emergence and length of
denning were significantly later and longer during infestation.
Of the 70 dens used from 1896 to 1990, cavities in live (mostly oak)
trees were the predominant den types. Defoliation
resulted in 54% mortality of oak den trees during this period.
Since the accidental release of gypsy moths in 1869 in Medford, Massachusetts,
the gypsy moth has slowly spread south and west, and has defoliated up to 25
million ha of forest since 1924.
Gypsy moths defoliate forest cover from early June to late July and can result
in permanent forest species composition changes through large scale mortality
of host species. Preferred hosts
starts with chestnut oak, followed by northern red oak, white oak, scarlet
oak, and black oak. Heavy
defoliation aborts acorn production and can prevent full acorn production for
Effects of this defoliation on wildlife has been virtually never studied
(except a few bird studies). Black
bears in the southern Appalachians are adapted to habitats that are based on
acorn hard mast production. Thus,
the impacts of the gypsy moth defoliation has the potential to significantly
modify bear habitat through forest cover and yearly food supply changes that
can be expected to alter many aspects of bear behavior and population
The gypsy moth was first discovered in SNP in 1984 and began extensively
defoliating the Park by 1987. With
the intensive black bear radio telemetry study conducted by Carney (1985) and
Garner (1986) documenting the basic biology of the population, a unique
opportunity exists to investigate the effects of gypsy moth infestation, and
the resultant habitat defoliation, on the SNP black bear population.
The specific objectives for this study were:
1. To determine the short-term effects of gypsy moth infestation on aspects of
the SNP black bear habitat.
quantify short-term demographic changes in the SNP
bear population during years of extensive gypsy moth infestation.
3. To identify bear behavioral alterations attributable to
gypsy moth defoliation.
The 522km2 study area was the North and Central Districts of
CHAPTER 1: EFFECTS OF GYPSY MOTH INFESTATION ON BEAR HABITAT
Extensive defoliation of canopy trees results in an immediate
decrease in overstory foliage and can cause large scale tree mortality,
resulting in short-term and long-term changes in vegetation structure,
wildlife cover, and hard and soft mast food abundance. Potentially significant
habitat composition shifts away from oak toward non-gypsy moth host species
such as red maple, black birch, and tulip tree.
Prior to 1989, defoliation was restricted primarily to the
North District. Most areas were
defoliated only one year, but significant acreage of the North District was
defoliated 2 - 3 years form 1986 - 1989.
Defoliation occurred in SNP as a patchwork of heavily defoliated
(>60%) mixed with areas with lesser levels of defoliation (<60%), and
stands relatively untouched by the moth.
Non-defoliated areas tended to occur in drainages, primarily along
streambeds, where non-gypsy moth host trees (tulip trees) were in greater
Effects of Defoliation on Acorn Production
Overall, non-defoliated plots produced 119,500 acorns/ha compared to a yield
of only 1,600 acorns in areas of first time defoliated stands; a 99% reduction
in hard mast production. The one plot defoliated for two consecutive years produced no
acorns the second year.
Effects of Defoliation on Forest Temperature
Maximum daily temperatures in defoliated stands averaged 4.7 degrees C higher
than in non-defoliated stands during peak defoliation.
After refoliation was complete, maximum daily temperatures continued to
be 2.5 degrees C warmer in defoliated areas.
Effects of Defoliation on SNP Vegetation
Sites with 2 - 3 years defoliation experienced the largest oak and maple
density declines and the greatest mortality per year.
For oak species, yearly mortality was greater than 20% in three of the
five sites with 2 -3 years defoliation. The
most severely affected site experienced 85% oak mortality over a three year
period (from 304 trees/ha to 43 trees/ha).
At sites of 0 to 1 year defoliation, oak tree losses were less than 4%
/year with one exception, which experienced a 17% oak mortality/year.
Hickory mortality was related directly to the number of years of
defoliation, with some significant plot losses, although overall loss of
hickory and maple averaged only 3 hickories and 5 maples/ha died during gypsy
moth infestation. Depressed
growth rates were found among hickories and oaks with 2 to 3 years of
defoliation. Soft mast trees (dogwood, hackberry, blackgum, sweet and
black cherry, sassafras, and elderberry) exhibited significantly greater
growth in increased years of defoliation.
Similarly, soft mast shrubs and vines (spicebush, Virginia creeper,
gooseberry, black raspberries, greenbriar, poison ivy, viburnums and grape)
increased dramatically in three of the five 2 - 3 year defoliated sites.
Despite extensive gypsy moth defoliation and immediate habitat and
microclimate changes, bears in general did not avoid defoliated areas.
Elevated temperatures did not affect bear ranges.
Hard mast loss was compensated for by soft mast production.
Hard mast is known to be an important fall bear food for maintaining
bear nutrition and reproduction. Areas
of 60% defoliation were subjected to a complete acorn failure, and even
moderate defoliation (>20% and <60%) resulted in a 50% reduced acorn
production. Limited studies show
acorn reduction lasts for 1 to 5 years following foliage defoliation.
With oak mortality, acorn production will decrease, in all cases,
except where less than 25% mortality occurs, in which case, increased growth
of surviving trees may offer partial compensation and actually increase
production as much as 20%.
Multiple years of defoliation resulted in substantial, short-term oak
mortality (17% on average), up to 85% in 3 years at one site, and decreased
oak density. Another study found
one year defoliation resulting in 18% oak mortality, with 2 and 3 year
defoliation resulting in 89% and 98% mortality respectively.
Such statistics indicate that a conservative estimate suggests that a
greater than 50% oak mortality is likely over at least 20% of the oak habitats
and 15% of all habitat types in the North District.
(This is higher than PA studies that reported only 4% of stands
received greater than 50% mortality, but about 30% suffered 20 - 50%
mortality. Although not born out
by this study, other studies often reveal overall oak regeneration failure due
to competition to less preferred species (such as black birch, beech, red and
striped maple), loss of acorns as a seed reserve, the inability of oaks killed
following defoliation to produce vigorous stump sprouts, and the death of
existing seedlings. Studies have
also described reduced woody seedling regeneration due to increases in pioneer
species such as blackberries and understory trees and shrubs.
The immediate and short-term effects of gypsy moth infestation reveal
trade-offs with respect to long-term bear habitat quality.
While substantial and widespread oak decline will unquestionably reduce
hard mast production, especially with replacement by birch and maple forests,
moderate oak decline will result in increased acorn production from surviving
oak trees, diversify Park habitat, increase production of soft mast, and
thereby increase habitat quality. Gypsy
moth infestation in SNP has not yet resulted in sufficient habitat alterations
to jeopardize the Park bear population. However,
repeated defoliation events will dictate oak mortality and long-term habitat
changes and ultimately will determine SNP bear habitat suitability in the