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Predicting Bird Community Changes to Invasion of
Hemlock Woolly Adelgid in Kentucky
David R. Brown1,* and Todd Weinkam1
Abstract - Adelges tsugae (Hemlock Woolly Adelgid [HWA]) is rapidly spreading across
Kentucky, yet the potential effects on bird communities of the region are not known. We
surveyed birds in Tsuga canadensis (Eastern Hemlock) and other forest types across the
Appalachian mountain region of Kentucky during the early stages of infestation when
HWA was absent from most sites. Based on associations between birds and forest types, we
identified 4 bird species likely to be negatively affected by Eastern Hemlock decline. For
one of these species, Setophaga virens (Black-throated Green Warbler), we used landscape
metrics of forest composition to make predictions about the density of Eastern Hemlock
necessary for persistence. This type of information can help managers establish control
strategies for HWA.
Introduction
Non-native invasive species have the potential to dramatically disrupt ecosystem
dynamics at rapid rates. The effects of such disturbances can be extensive and
result in a cascade of biological responses (Crooks 2002, Vitousek et al. 1997).
The loss of a foundation species, such as Tsuga canadensis (L.) Carrière (Eastern
Hemlock, hereafter Hemlock), can have dramatic effects on the composition
of communities at different trophic levels (Ellison et al. 2005). In this study, we
examine the bird communities of intact Hemlock forests in Kentucky, and predict
changes to these communities in response to rapid Hemlock decline caused by an
invasive pest, Adelges tsugae Annand (Hemlock Woolly Adelgid, hereafter HWA).
Hemlock is a shade-tolerant conifer with a range that extends across much of
eastern North America, including eastern Kentucky. In the southern Appalachians,
most Hemlock occurs along stream drainages (Narayanaraj et al. 2010), but in Kentucky
stands also occasionally occur on the north- and east-facing slopes of ridges.
Where abundant, Hemlock plays a foundation role by stabilizing various ecosystem
dynamics and influencing composition of plant and animal communities (Ellison
et al. 2005). Hemlocks influence forests’ environmental and ecological conditions
including local air and hydrologic temperatures, soil pH, and the physical structure
of the forest (Ellison et al. 2005, Snyder et al. 2002), which influence the biologic
community, forest, and associated streams.
HWA is an exotic invasive insect responsible for widespread mortality of Hemlock
across eastern North America. HWA feeds on plant nutrients in leaf bases,
causing decline in tree health and eventually mortality. It appears to disperse during
1Department of Biological Sciences, Eastern Kentucky University, Richmond, KY 40475.
*Corresponding author - david.brown@eku.edu.
Manuscript Editor: Jason Davis
Forest Impacts and Ecosystem Effects of the Hemlock Woolly Adelgid in the Eastern US
2014 Southeastern Naturalist 13(Special Issue 6):104–116
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the first instar stage via wind, humans, birds, and other animals (McClure 1990). In
recent decades, HWA has spread over most of the native range of Hemlock resulting
in changes to forest structure and ecosystem functions (Ellison et al. 2005, Nuckolls
et al. 2009, Stadler et al. 2005). The range of HWA has recently expanded to
include parts of Kentucky, and the insect is predicted to spread throughout the range
of Hemlock in Kentucky (Clark 2010). In the southern Appalachians, Hemlock
mortality exceeded 85% seven years after invasion (Ford et al. 2012). Forest simulation
models suggest that Hemlock in Kentucky could be completely extirpated
within 20 years of HWA invasion, leading to altered forest structure with increased
canopy gaps, expansion of Rhododendron spp. (rhododendron), and succession to
forests dominated by deciduous hardwood trees (Spaulding and Rieske 2010). The
loss of Hemlock and its replacement by other species is also likely to have effects
on animal communities.
Several studies have highlighted strong positive associations of bird species,
including Empidonax virescens (Acadian Flycatcher), Vireo solitarius (Blueheaded
Vireo), Setophaga virens (Black-throated Green Warbler), Setophaga
fusca Statius Muller (Blackburnian Warbler), and Catharus guttatus Pallas (Hermit
Thrush) with Hemlock forests in other regions of the eastern United States
(Ross et al. 2004, Tingley et al. 2002). For at least one species, Acadian Flycatcher,
breeding density was negatively correlated with HWA-induced Hemlock
defoliation, indicating that this species may experience decline due to HWA
(Allen et al. 2009). A number of bird species have been associated with positive
responses to Hemlock decline due to HWA including Contopus virens (Eastern
Wood-Pewee), Molothrus ater Boddaert (Brown-headed Cowbird), Baeolophus
bicolor L. (Tufted Titmouse), Sitta carolinensis (White-breasted Nuthatch), Vireo
olivaceus (Red-eyed Vireo), Setophaga citrina Boddaert (Hooded Warbler), and
several woodpecker species (Tingley et al. 2002).
As HWA spreads through Kentucky, land managers are using a combination of
chemical and biological control methods to protect individual and forest stands.
Thus far, the biological methods, primarily predaceous beetle releases, have been
localized and their efficacy largely unkown. Chemical treatment for HWA has
been the predominant management approach, but land managers do not have the
resources to treat every tree. Unfortunately, there are few guidelines and almost no
research on the number, density, spatial extent, and age-classes of trees that need to
be preserved to maintain biodiversity and ecosystem functions. During this critical
period of early invasion in Kentucky, land managers need accurate information on
how bird communities will respond to HWA infestation and how to best manage
forests to protect at-risk species. Large-scale coordinated efforts to treat Hemlock
for HWA are underway, but these efforts lack specific guidance on the extent and
density of Hemlock required to preserve the biological communities that associate
with these forests.
Our objectives were to describe the bird communities in Kentucky’s Hemlock
forests and compare the overall bird community composition to other forest
types within the Appalachian mountain region of Kentucky. We also identified
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bird species likely to benefit or decline in response to changes in the forest
community composition and structure as HWA invades Kentucky. Finally, we
used landscape metrics of forest community composition to make predictions
about the density of Hemlock necessary for persistence of one at-risk species,
Black-throated Green Warbler.
Field Site Description and Methods
This study took place within Hemlock forests of the Appalachian Mountain region
of southeast Kentucky (Fig. 1). This area is in the Coal Fields physiographic region
of the Cumberland Plateau, and includes the Level III Ecoregions of the southwestern
Appalachians, the central Appalachians, and the western Allegheny Plateau (Woods
et al. 2002). We used bird-survey data collected in 2009 from 271 locations including
Hemlock forests (n = 123) and other forest types (n = 148). At each location, we assessed
the forest community and stand structure using standard forestry techniques
(James and Shugart 1970). Protocols differed slightly at sites on US National Forest
(n = 16). For all sites, we used a general classification based on the dominant overstory
tree species. We categorized sites as Hemlock if it was among the three most
dominant overstory tree species within a 50-m radius of the bird-survey location; otherwise,
we considered sites non-Hemlock forest. Non-Hemlock habitat was typically
deciduous forest with mixed-mesophytic or Quercus spp.(oak)-Carya spp. (hickory)
Figure 1. Study locations were clustered on public lands in the Appalachian Mountain region
of Kentucky. Solid circles are study locations with Hemlock present. Open circles are
forested locations with Hemlock absent within 50-m radius of the site location.
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communities (Braun 1950). Many survey locations were clustered on public lands
and were established as part of other land-bird monitoring programs. Birds were surveyed
by biologists with special training in bird identification, including personnel
from Eastern Kentucky University, Richmond, KY; Kentucky Department of Fish
and Wildlife Resources (KDFWR), Frankfort, KY; and the US Forest Service, Daniel
Boone National Forest, KY. Bird surveys were conducted as unlimited-distance point
counts for 10 minutes. At a subset of 75 sites, we surveyed for an additional 5 minutes
using playback of song and calls of Limnothlypis swainsonii Audubon (Swainson’s
Warbler) to increase detections of this secretive species.
We used non-metric multidimensional scaling (NMS) to illustrate the overall
difference in bird communities in Hemlock stands compared to other forest types
(Legendre and Legendre 2012). NMS is robust to matrices with many zeroes, which
are typical of animal community data, because it uses rank-distances between
sampling locations (Clarke 1993, McCune and Grace 2002). This analysis was
conducted in Program R using Package Vegan (R Development Core Team 2012).
We used Bray-Curtis dissimilarity to construct the distance matrix and limited the
analysis to a two-axis solution.
To describe the strength of Hemlock habitat associations for individual bird species
and to develop predictions for which species will be affected by HWA, we used
logistic regression to model presence/absence of bird species based on presence/
absence of Hemlock within 50 m of each survey location using the same surveys
(n = 271) described above. To control for variation attributable to the clustered
distribution of survey locations, we statistically blocked our data based on the local
watershed at the 12-digit hydrologic unit code (HUC-12) scale (n = 24 watersheds).
We used the HUC-12 scale because it better reflected the clustered pattern of our
sampling design than the next available landscape data at smaller (HUC-14) and
larger (HUC-10) scales. We applied this analysis to the 23 bird species that we detected
at 50 or more locations. We report the results using Wald’s statistic, P values,
and Odd’s ratios.
We developed a model of the Hemlock density at the sub-watershed scale that
best predicted the occurrence of one at-risk bird species, the Black-throated Green
Warbler. Of all bird species within the range of Hemlock, that is the one cited
most frequently as being positively associated with Hemlock (Becker et al. 2008,
Howe and Mossman 1995, Keller 2004, Tingley et al. 2002). Hemlock density
was estimated from a Hemlock distribution map developed specifically for eastern
Kentucky using a decision-tree classification based on topographic and climatic environmental
variables, and Landsat 7 ETM+ satellite imagery at a resolution of 30
x 30 m (Clark 2010). The Hemlock distribution map covered 25,790 km2 of eastern
Kentucky, but did not provide complete coverage of our study area, so we reduced
our sample for this analysis by 10 bird-sampling sites. For each bird-survey site,
we used ArcGIS v. 9.2 (Environmental Systems Research Institute 2005) to create
landscape-sampling polygons based on the sub-watershed described by the USGS
14-digit hydrologic unit code (HUC-14) (n = 70 sub-watersheds with mean size ±
SD = 15.0 ± 15.8 km2).
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Sub-watersheds included 1–12 bird-sampling locations (mean ± SD, n = 3.8 ±
3.2). We used ArcGIS Spatial Analyst extension to calculate the total number of
cells dominated by Hemlock forest within each sub-watershed and then calculated
the density of Hemlock habitat in each sub-watershed as the area of Hemlock
divided by the area of each HUC-14 sub-watershed. We used the HUC-14 subwatershed
scale because this is the smallest watershed-scale data available and we
assumed it better reflected the scale at which land managers would make forestry
decisions related to management for HWA and its impacts. We calculated the average
abundance of Black-throated Green Warblers among all point-count sampling
locations within each sub-watershed. The statistical model included the average
bird abundance per sub-watershed as the response and a single predictor variable,
the total density of Hemlock within the HUC-14 sub-watershed. We used a generalized
linear model with a Tweedie distribution and a log link. The Tweedie distribution
was appropriate because we used the average bird abundance which had
frequent zeros but was otherwise continuously and normally distributed. We used
the model-estimated coefficients and standard errors and the average sub-watershed
size of sampled sites (15.73 km2) to estimate the minimum density of Hemlock
within sub-watershed landscape patches necessary to support the presence of at
least one Black-throated Green Warbler per sample. We used SPSS v 19 to conduct
the logistic regressions and generalized linear model.
Results
Based on the spatial separation of bird communities in Hemlock compared to
deciduous forest sites, the NMS ordination analysis (stress = 0.18) showed differences
in the composition of bird communities between Hemlock and deciduous
forest habitats in the Appalachian mountain region of Kentucky (Fig. 2). Birds
are highly mobile, and many of the 75 species included in the analysis are habitat
generalists, so despite the moderate degree of overlap between communities, the
overall pattern suggests bird community structure differs between Hemlock and
deciduous forest. Logistic regression indicated 4 of 25 bird species with high detection
frequency were positively associated with Hemlock: Acadian Flycatcher,
Blue-headed Vireo, Black-throated Green Warbler, and Mniotilta varia (Black-andwhite
Warbler) (Table 1). Black-throated Green Warblers were more than 9 times
more likely to be detected in Hemlock than deciduous forest types, and Blue-headed
Vireos were almost 6 times more likely to be found in Hemlock forests than in
deciduous forests. We found ten species to be negatively associated with Hemlock
Table 1). The statistical blocking on clustered study areas was significant for some
species, suggesting that this approach was warranted and helped to strengthen our
results of the regional patterns of bird associations with Hemlock forest. Swainson’s
Warblers were detected at only 2 of 271 locations using standard point-count
methodology, and at 19 of 75 locations where supplemental playback of song and
call was used following standard point counts.
Region-wide, Hemlock forests composed 5% of the landscape. Among the
70 HUC-14 sub-watersheds sampled, the average proportion of area classified
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as Hemlock forest was 7% (± 6% SD). For the generalized linear modeling, the
average abundance of Black-throated Green Warblers showed a strong positive
exponential relationship with Hemlock density at the HUC-14 sub-watershed scale
(Wald’s Χ2 = 7.35, n = 70, P = 0.007, Log link coefficients: intercept = -0.484,
Hemlock = 5.63; Fig. 3). The minimum area of Hemlock based on the averagesized
sub-watershed necessary for the presence of an average of one Black-throated
Green Warbler per sample location is 1.35 km2 (95% CI: 0.06–9.35 km2).
Discussion
This study describes the bird communities of Hemlock forests as having distinct
structure and different species associations than bird assemblages in the dominant
deciduous forest of the Appalachian mountain region of Kentucky. Three of the 4
species we found to be positively associated with Hemlock—Acadian Flycatcher,
Figure 2. Graphical analysis illustrating difference in bird communities in sites with
Hemlock forest (solid circles) compared to deciduous forest sites (open circles) in the Appalachian
mountain region of Kentucky. Each dot represents a field site. Axis 1 and 2 represent
summary values of abundance for all bird species detected.
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Blue-headed Vireo, and Black-throated Green Warbler—have been similarly found
to associate with Hemlock in other states (Allen et al. 2009, Becker et al. 2008, Keller
2004, Ross et al. 2004, Tingley et al. 2002). Thus, our results are consistent with other
Table 1. Association of bird species with Hemlock forest in the Appalachian mountain region of Kentucky.
Detections indicate the number of point-count locations where a species was observed. Positive
slopes (b) indicate species that were more likely detected in Hemlock forest, whereas negative slopes
indicate species more frequently detected in forest without Hemlock. Odd’s ratios indicate the likelihood
that a species was detected in Hemlock forest compared to other forest types.
Odds
Species Detections Wald’s b P-value ratio
Positive associations
Empidonax virescens (Vieillot) (Acadian Flycatcher) 124 17.68 1.06 less than 0.001 2.90
Vireo solitarius (Wilson) (Blue-headed Vireo) 74 32.13 1.77 less than 0.001 5.88
Setophaga virens (Gmelin) (Black-throated Green 130 62.10 2.25 less than 0.001 9.52
Warbler)
Mniotilta varia (L.) (Black-and-white Warbler) 96 18.19 1.14 less than 0.001 3.12
Negative associations
Zenaida macroura (L.) (Mourning Dove) 51 6.29 -0.84 0.012 0.42
Coccyzus americanus (L.) (Yellow-billed Cuckoo) 58 5.67 -0.77 0.017 0.46
Melanerpes carolinus L. (Red-bellied Woodpecker) 67 19.00 -1.46 less than 0.001 0.23
Contopus virens (L.) (Eastern Wood-Peewee) 61 26.21 -2.07 less than 0.001 0.12
Vireo olivaceus (L.) (Red-eyed Vireo) 188 4.36 -0.55 0.037 0.57
Cyanocitta cristata (L.) (Blue Jay) 64 4.93 -0.66 0.026 0.51
Sitta carolinensis Latham (White-breasted Nuthatch) 65 13.44 -1.17 less than 0.001 0.30
Hylocichla mustelina (Gmelin) (Wood Thrush) 115 12.15 -0.91 less than 0.001 0.40
Piranga olivacea (Gmelin) (Scarlet Tanager) 132 14.47 -0.96 less than 0.001 0.38
Passerina cyanea (L.) (Indigo Bunting) 50 6.40 -0.92 0.011 0.39
Figure 3. Average
s u b - w a t e r s h e d -
scale abundance
per sampling location
of Blackthroated
Green
Warblers in relation
to the relative
density of Hemlock
within subwatersheds
as delimited
by 14-digit
Hydrologic Unit
Codes. The solid
line represents the
exponential best-fit
model and dashed
lines are 95% confidence
intervals.
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research from across the range of Hemlock and provide state-specific information on
bird–Hemlock associations. We also found Black-and-white Warbler to be positively
associated with Hemlock, but to a lesser degree than the Blue-headed Vireos and
Black-throated Green Warblers. No other studies have identified Black-and-white
Warblers as Hemlock associates, and one study found this species to be negatively
associated with Hemlock (Keller 2004). Most other bird species previously described
as positively associated with Hemlock but not found to be significantly associated
with Hemlock in this study, including Setophaga caerulescens Gmelin (Blackthroated
Blue Warbler) and Sitta canadensis L. (Red-breasted Nuthatch), occur in
only isolated locations within our study region, and their rarity in the study region
may mask a possible association.
Swainson’s Warbler, a species of special concern in Kentucky, may associate
with Hemlock, so it could also be at further risk from the spread of HWA. This
species preferentially nests in habitats with dense shrub layers (Bassett-Touchell
and Stouffer 2006, Graves 2002), including in young Hemlock in the Appalachian
mountains of South Carolina (Lanham and Miller 2006). Swainson’s Warblers are
notoriously difficult to detect using traditional survey methods, and our detection
rate was less than 1%, yet we detected Swainson’s Warblers at more than 25% of
Hemlock sites where we used playback vocalizations of the bird’s song and call.
Thus, this species may also be associated with Hemlock habitat in Kentucky, but
additional targeted research is needed.
It is possible that the positively associated species are actually selecting habitat
based on environmental conditions or landscape cues other than the presence of
Hemlock. For instance, most Hemlock in Kentucky occurs in stream bottoms. The
bird species we report to be positively associated with Hemlock might be selecting
habitat based on proximity to streams or other topographic features such as slope,
aspect, or elevation. If such characteristics are more important to habitat selection
than forest composition and structure, then bird communities may change little in
response to the decline of Hemlock. Similarly, the species we describe as being
associated with Hemlock also occur in other mesic habitats. If habitat selection
is based more on environmental conditions than stand structure, then we may see
negligible change in the bird communities as Hemlock declines; however, because
Hemlock shows keystone ecosystem functions, the loss of Hemlock may shift local
conditions to be more xeric and thus lead to indirect effects on bird communities.
Our study was not designed to test environmental factors other than the presence
or density of Hemlock, and such research might provide managers with additional
valuable knowledge. However, regardless of the proximal causes for the habitat
associations, these birds may benefit from the presence of Hemlock in subtle ways
that influence survival and reproductive success. For instance, Swainson’s Warblers
in the Appalachian mountains of South Carolina preferentially nest in young
Hemlock, possibly because leaf litter accumulations in the branches serve as nest
decoys that reduce nest predation (Lahham and Miller 2006). Additional research
focused on the demographic response, home-range use, and foraging behavior of
individual species, such as the Black-throated Green Warbler, would allow more
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specific predictions about how the bird community and populations may change
due to HWA and other future invasions.
Our analysis of the relationship between landscape density of Hemlock and the
abundance of Black-throated Green Warblers suggests that relatively high densities
of Hemlock within watersheds need to be protected to maintain current densities of
this bird. Black-throated Green Warblers often use transitional areas between coniferous
and deciduous forests, but they associate with some type of coniferous habitat
in most of their range (Collins 1983). In eastern Kentucky, Picea spp. (spruce) and
Abies spp. (fir) are absent, and Pinus spp. (pine) forms a minor component of the forest
community. Thus, Hemlock represents the primary coniferous species available
to Black-throated Green Warblers in Kentucky. As Hemlock distribution shrinks,
Black-throated Green Warbler populations may be buffered by resources available
in nearby deciduous forests, but the gradual decline of this bird species in Kentucky
seems likely without HWA management success. Further research will be necessary
to determine the spatial arrangements and age-class structure of Hemlock within
these areas that will best support Black-throated Green Warbler and other species.
In contrast to our findings regarding likely declines in certain bird populations,
our results also enable us to predict that several species will benefit from the decline
of Hemlock and its replacement by deciduous forest communities. Of the species
we describe as negatively associated with living Hemlock, Eastern Wood-peewee
and Red-eyed Vireo were similarly found to associate with non-Hemlock forest
types elsewhere in the range of Hemlock (Becker et al. 2008, Howe and Mossman
1995, Keller 2004, Tingley et al. 2002). In Connecticut, these species, among others
such as woodpeckers, appeared to benefit from the presence of dead Hemlocks
(Tingley et al. 2002). Widespread Hemlock mortality has yet to occur in Kentucky,
so we may see increases in these species in future years.
Based on our findings, we infer that bird communities of Hemlock forests in
Kentucky will likely change in predictable patterns as Hemlocks die and are replaced
by other tree species. Several bird species appear to be at risk of losing
preferred habitat as Hemlock disappears from eastern Kentucky. Species that are
the most closely associated with Hemlock (i.e., Acadian Flycatcher, Blue-headed
Vireo, and Black-throated Green Warbler) will likely experience the most dramatic
declines in abundance due to HWA, and it is possible that these species will disappear
entirely from some areas. As Hemlocks are replaced and forests transition to
other communities such as oak- or Acer spp. (maple)-dominated forests, some bird
species will likely benefit by the addition of these breeding and foraging areas. This
study focused on breeding birds and does not address the impact of Hemlock loss
on birds during migration or the winter season. Although winter birds in Kentucky
forests tend to be generalists in their habitat use, the loss of cover as Hemlocks decline,
at least in areas lacking rhododendron or other year-round cover, could lead
to local changes in the winter bird community.
The KDFWR, the state agency responsible for monitoring biodiversity in
Kentucky, has closely followed the spread of HWA. In anticipation of the effects
of HWA, KDFWR recently listed the Black-throated Green Warbler as a species of
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greatest conservation concern under the state Wildlife Action Plan (KY DFWR
2013). This proactive approach to management suggests that managers appreciate
the risk of HWA. We recommend that resource managers continue to monitor bird
populations and forest ecology, and maintain flexibility to adjust management approaches
as research advances. Vose et al. (2013) present a framework for HWA
management decisions in the southern Appalachians that could be modified and
adopted for use in Kentucky.
There are at least two potential biases that could have influenced our results.
First, almost all of our 70 study sites were located on protected public lands, 2 of
which included old-growth forest, which may differ in composition and age-class
structure compared to stands on private lands in the region. However, our results
should be broadly applicable because of the large spatial scale of the sampling design
and the fact that the relative density of Hemlock in our study sites was similar
to the regional average, and to the average within public lands in the region (5%).
Another potential bias is that HWA had already invaded the most southern sites at
the start of this study. Although there was some evidence of infestation at these
locations, it was at an early stage with little or no apparent effects on health of trees
and no mortality (T. Weinkam, pers. observ.) Thus, the effect of HWA on birds was
likely minimal.
Because of Hemlock’s foundational role and the patterns of association described
here and in other locations, we predict that a shift in bird communities
will occur because of HWA. However, in areas of active HWA control, it may be
possible to mitigate changes to existing bird communities. Management for HWA
typically involves the release of beetles as HWA predators or insecticide treatments
of individual trees. For biological control efforts, rearing of predatory beetles is
now occurring at several institutions and millions of beetles have been released
across the Hemlock’s range (Vose et al. 2013). In some cases, predator populations
have become established and appear to be associated with increased health of Hemlock
(Mausel et al. 2010, Onken and Reardon 2010).
For chemical HWA control, soil soaking and soil injections of insecticide appear
to be the most effective and efficient short-term methods and can provide protection
for several years (Cowles 2009, Cowles et al. 2006). Recent research on the
primary pesticide used for such treatments, imidacloprid, suggests that it has minimal
negative effects on nearby stream invertebrates (Churchel et al. 2011) and soil
microarthropods (Knoepp et al. 2012).
Other research suggests that 1–2 years following insecticide treatment, HWA
abundance does not differ from untreated sites (Falcone and DeWald 2010). Research
also suggests non-target effects of insecticide treatment on the abundance of
Lepidoptera, an important food for insectivorous birds during critical brood-rearing
periods (Dilling et al. 2009, Falcone and DeWald 2010). Furthermore, Falcone and
Dewald also found that the combined density of 3 insectivorous migrant bird species,
Blue-headed Vireo, Black-throated Green Warbler, and Black-throated Blue
Warbler, did not differ between imidacloprid-treated and untreated sites (Falcone
and DeWald 2010). However, they did find a higher density of these bird species
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in areas with relatively healthy Hemlock cover as measured by foliage mass. This
finding suggests a tradeoff between food availability and other factors such that
treated Hemlock has reduced food availability for birds, but the increased foliar
mass provides benefits such as cover from predators or for nest concealment. Biological
control efforts may be successful, but forest management strategies will
need to consider tradeoffs including the economic cost of treating Hemlock, alternative
strategies of treatment, and the potential for complex indirect ecological
effects of control (Vose et al. 2013).
In Kentucky, approximately 100,000 trees have been treated with insecticide
to date, with some areas receiving follow-up applications (Alice Mandt, Kentucky
Division of Forestry, Frankfort, KY, pers. comm.). The baseline data we collected
for our study provide opportunities to study the impacts of HWA on birds and the
efficacy of control efforts in Kentucky.
Our findings provide a starting point for long-term monitoring of bird communities
in the Hemlock forests of Kentucky and testable predictions of how species
abundance will change with altered forest conditions. Our results also provide
land managers with specific guidance on the density of Hemlock necessary to prevent
local decline and extirpation of one at-risk bird species, the Black-throated
Green Warbler. In combination with other recent and ongoing studies of HWA
management approaches, this work helps form a basis for informed species-specific
decisions.
Acknowledgments
We thank all of the biologists that conducted the bird surveys. We thank the Kentucky
State Nature Preserves Commission, Kentucky State Parks, Cumberland Gap National
Historic Park, Kentucky Department of Fish and Wildlife Resources, US Forest Service,
Eastern Kentucky University Division of Natural Areas, and private landowners for permission
to conduct this research. Songlin Fei provided the Hemlock map for eastern Kentucky.
Support was provided by KY EPSCoR. We also appreciate the hard physical work of all
the people, including many volunteers, conducting HWA-control work in Kentucky. This
is contribution No. 43 of Lilley Cornett Woods Appalachian Ecological Research Station,
Eastern Kentucky University, Richmond, KY.
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