Summer-Habitat Suitability Modeling of Myotis sodalis (Indiana Bat) in the Eastern Mountains of West Virginia
Jesse L. De La Cruz and Ryan L. Ward
Northeastern Naturalist, Volume 23, Issue 1 (2016): 100–117
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J.L. De La Cruz and R.L. Ward
22001166 NORTHEASTERN NATURALIST 2V3(o1l). :2130,0 N–1o1. 71
Summer-Habitat Suitability Modeling of Myotis sodalis
(Indiana Bat) in the Eastern Mountains of West Virginia
Jesse L. De La Cruz1,* and Ryan L. Ward1
Abstract - Little information exists with regard to suitable summer habitat of Myotis sodalis
(Indiana Bat) in West Virginia. Our research objectives were to use ultrasonic acoustic
equipment and automated identification software to collect presence data for Indiana Bats
and to examine habitat characteristics and availability across the local landscape. We used
a maximum entropy (MAXENT) approach to determine if the distribution of various ecological
factors such as landuse/landcover, forest fragmentation, aspect, area solar radiation,
slope, proximity to permanent water, and elevation influenced foraging-habitat suitability
of Indiana Bats. We sampled across the 1160-ha Camp Dawson Collective Training Area in
Preston County, WV, to determine Indiana Bat presence. We employed the collected presence
data to examine habitat suitability within a 16,151-ha study area encompassing the
training facility. Based on MAXENT results, we characterized highly suitable Indiana Bat
habitat as including large tracts of contiguous forest cover (>200 ha) associated with low to
modest slopes (<20°), road corridors, and areas of high solar radiation (>5.5 x 105 WH/m2).
High (81–100%) and medium-high (61–80%) suitability classes were uncommon across the
landscape (0.6% and 2.7%, respectively), with the broad medium-to-high suitability classes
(41–100%) collectively comprising only 11.4% of the study area. Elevation (m) and aspect
contributed little to the model and displayed low permutation importance that did not vary
notably from the corresponding percent contribution. These variables, along with close
proximity to permanent water (≤200 m away), are likely not limiting ecological factors.
The results of this study supplement current knowledge of summer habitat of the Indiana
Bat and provide land and wildlife managers localized guidance on conservation priorities
within the region.
Introduction
It is critical to understand habitat relationships in order to enact effective species
management and conservation plans, particularly for endangered, obligate
migratory species (Morrison 2001). Knowledge regarding bat habitat in relation
to land management is lacking (Keeley et al. 2003). Habitat requirements of
Myotis sodalis Miller and G.M. Allen (Indiana Bat) are not fully understood, and
information is often anecdotal, with only a few studies concentrating their efforts
in West Virginia (Ford et al. 2005, Johnson et al. 2010, Weber and Sparks 2013).
Science-based conservation of species such as the Indiana Bat is hindered because
of the lack of research concerning local resource relationships within the central-
Appalachians (Ford et al. 2005). Anthropogenic land use and development-based
disturbances have caused extinctions or population declines in numerous wildlife
species (Fahrig 1997, Fischer and Lindenmayer 2007, Haila 2002). The Indiana Bat
1AllStar Ecology LLC, 1582 Meadowdale Road, Fairmont, WV 26554. *Corresponding
author - jesse@allstarecology.com.
Manuscript Editor: Joseph Johnson
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has been on the US Endangered Species List since 1967 (USFWS 2013a), and is a
focal species for many management efforts. Understanding how landscape-level alterations
and forest management potentially affect habitat suitability of the Indiana
Bat is important for making informed management decisions. Variation in the scale
and intensity of forest-management activity may have wide-ranging effects on bat
populations (Britzke et al. 2003, Callahan et al. 1997, Gardner et al. 1991a). Management
techniques should attempt to conserve or create suitable maternity habitat
(i.e., roost trees; Timpone et al. 2010) and feeding and flyway habitat (e.g., canopy
perforations, forested corridors; Hein et al. 2009) across the landscape (Womack et
al. 2013). Because the removal of contiguous, mature forest potentially eliminates
optimal habitat for Indiana Bats (Britzke et al. 2003, Callahan et al. 1997, Gardner
et al. 1991a, Kurta 2005), silvicultural techniques should mimic small-scale natural
disturbances, such as those caused by fire, wind, and disease or pests (Womack et
al. 2013).
Few studies have described landscape-level summer-habitat suitability and
resource selection concerning Indiana Bats in West Virginia. Characterization of
roost-tree species and location, particularly in the Midwest, is the primary subject
matter of summer research (Britzke et al. 2006, Callahan et al. 1997, Gardner et al.
1991a, Kurta 2005). Furthermore, studies that examine foraging habitat are often
inconclusive and qualitative (Murray and Kurta 2004) and relate foraging to a wide
variety of habitat types (Brack et al. 2002, Butchkoski and Mehring 2004, Carter
2006, Clark et al. 1987, Gardner et al. 1991b, Humphrey et al. 1977, Jachowski et al.
2014, Kurta and Whitaker 1998, Menzel et al. 2005, Murray and Kurta 2002, Owen
et al. 2004, Tuttle et al. 2006). Although Indiana Bat roosts are often described as
large snags, a variety of other roosts have also been documented (Battle and Stone
2003, Gardner et al. 1991a, Kurta et al. 2002). Despite discrepancies among studies
of foraging- and roosting-habitat use in Indiana Bats, the species shows no selection
for early successional forests, old fields, or shrublands in relation to either roosting
or foraging habitat (Fuller and DeStefano 2003, Jachowski et al. 2014). Agricultural
land is a dominant cover-type throughout much of the Midwestern range of the Indiana
Bat; however, these areas are often not selected by the species (Humphrey et
al. 1977, Menzel et al. 2005, Murray and Kurta 2004). Furthermore, Indiana Bats
do not select large open-water sites or deforested creeks as flyways or feeding areas
(Menzel et al. 2005).
In West Virginia, Johnson et al. (2010) found that male Indiana Bats selected
roosts in forests with low basal area associated with large canopy perforations,
independent of roost-decay class. Weber and Sparks (2013) were unable to draw
conclusions on habitat suitability for the species due to an overall lack of occurrence
data in the state. Ford et al. (2005) found that foraging activity in West Virginia
was associated with small canopy perforations or closed forest-canopy in close
proximity to 2nd-order streams. These results contradict those reporting behavior of
the species in its Midwestern range, where large, flooded bottomland and forested
wetlands are often selected as foraging habitat. Due to this apparent overall plasticity
of the species in relation to resource selection, specifically foraging habitat
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(Jachowski et al. 2014), research that describes local resource relationships is still
needed to enhance the conservation and management of the species, particularly in
West Virginia, where research is often localized or inconclusive.
Maximum entropy models use a machine-learning process to assess the probable
distribution of a species by evaluating presence data in combination with available
ecological resources (Jepsen et al. 2011, Weber and Sparks 2013). Species distribution
modeling methods are being used more frequently in ecology (Elith et al. 2006,
Peterson 2006) to assess potential relationships between species-presence data and
ecological conditions within a study area (Kumar and Stohlgren 2009). Our goal
was to identify areas of ecological importance in West Virginia, determine the
geographic distribution of suitable Indiana Bat foraging habitat, and describe these
resource distributions so that management decisions are specific for the species.
Because Indiana Bats have been shown to select foraging areas associated with
high canopy-cover, forested streams (Ford et al. 2005), warmer ridges, and bottomlands
(Brack et al. 2002, Butchkoski and Mehring 2004), we predicted that suitable
habitat would be distributed throughout large tracts of contiguous forest cover that
receive high levels of solar radiation and are situated near persistent aquatic resources.
We used a geographical distributional approach to evaluate our hypothesis
that landuse/landcover, slope, aspect, forest fragmentation, elevation, proximity to
permanent water, and solar radiation affect the geographical distribution of suitable
Indiana Bat foraging habitat. We fit models based upon population-level observations
for the species. In order to better understand and conserve potential summer
habitat, we assessed habitat distributions at the local-landscape level in a 16,151-ha
study area within a 3.2-km buffer of the surveyed tracts.
Field-site Description
We conducted presence/absence surveys during July and August 2013 at the
Camp Dawson Collective Training Area (Camp Dawson) located southeast of
Kingwood in central Preston County, WV. We examined habitat suitability across
a 16,151-ha study area that encompassed Camp Dawson (Fig. 1). The study area
is in the Allegheny Highlands physiographic province of northern West Virginia.
Elevation at the site ranges from 360 m to 890 m above sea level (asl) and precipitation
averages 89–102 cm annually (Brack et al. 2005). Camp Dawson encompasses
1160 ha of primarily forested land across its 3 main tracts—Pringle, Briery, and
Volkstone (Fig. 1). Road construction and creation of roads for training activities
disturb vegetation on Camp Dawson and cause the formation of numerous habitat
types including: disturbed riparian systems, open meadows, contiguous core-forest,
and fragmented forest patches. There are significant edge effects throughout the 3
surveyed tracts (Brack et al. 2005, De La Cruz et al. 2013). Forest types range from
mature stands to open, regenerating areas of dense shrub and understory species.
Forest types present are Allegheny hardwoods—without Castanea dentata (Michx.)
Raf. (American Chestnut)—and mixed mesophytic at low elevations and in
coves, and Appalachian Quercus spp. (oak) on western and southern aspects (Dyer
2006). Forests on undisturbed ridges and upper slopes were dominated by Acer
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rubrum L. (Red Maple) and various oak species including Q. alba L. (White Oak),
Q. prinus Willd. (Chestnut Oak), and Q. velutina Lam. (Black Oak). Undisturbed
mesic coves were dominated by Liriodendron tulipifera L. (Yellow Poplar), Q. rubra
L. (Northern Red Oak), A. sacchrum Marsh. (Sugar Maple), and Betula lenta
L. (Black Birch). Disturbed slopes, particularly mined areas on the Pringle Tract,
were dominated by Robinia pseudoacacia L. (Black Locust), Pinus strobus L.
(White Pine), and the invasive shrub Eleagnus umbellata Thunb. (Autumn Olive).
Undisturbed lower slopes and riparian forest near the Cheat River were dominated
by Platanus occidentalis L. (American Sycamore) and Northern Red Oak, while
the forest along Pringle Run contained a Tsuga canadensis (L.) Carrière (Eastern
Hemlock) and Rhododendron maximum L. (Great Rhododendron) community.
Methods
Indiana Bat occurrence data
We derived Indiana Bat occurrence data used for this project from acoustic
recordings collected during July–August 2013, at Camp Dawson. Based upon previous
acoustic recordings, bat population declines have been observed in the region
since the 2010 arrival of white-nose syndrome (Johnson et al. 2013). Our study site
was within 15 km of the 668,589-ha Monongahela National Forest and ~10 km
from a suitable hibernaculum—potential refugia for the species. We placed Binary
Acoustic iFR-IV field recorders (Binary Acoustics Technology, Phoenix, AZ) along
suitable flyways, within open feeding areas and canopy perforations, and in close
proximity to open aquatic resources in accordance with federal sampling protocols
(USFWS 2013b). We programmed recorders to sample from before dusk (1930 h)
Figure 1. Study area (16,151 ha) and 3 primary tracts of Camp Dawson: Pringle Tract, Briery
Tract, and Volkstone, Preston County, WV.
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to just before dawn (0530 h). We placed ultrasonic microphones oriented ≥45° from
parallel on tripods elevated ~1.5 m above the ground (USFWS 2013b). We recorded
data in full-spectrum format at a 10-factor time-expansion rate. We programmed
recorders to only activate and record in response to high-frequency sounds ranging
from 16 kHz to120 kHz. We used 2 candidate automated identification-software
programs to identify any recorded bat-call sequences (USFWS 2013b): Kaleidoscope
Pro Version 2.0.4 (Wildlife Acoustics, Inc., Maynard, MA) and Bat Call
Identification Version 2.6a (BCID; Bat Call Identification, Kansas City, MO) set
on manufacturer defaults to identify bat-call sequences. We used Kaleidoscope to
filter all noise and convert the collected full-spectrum data into zero-cross data;
we set a division ratio of 8 for the conversion of all full-spectrum data to zerocross
data (Agranat 2012). We made initial Indiana Bat presence determinations
via significant (P < 0.05) maximum likelihood estimates (MLE; USFWS 2013b).
We visually vetted all files identified as Indiana Bats by automated classifiers to
verify call quality (e.g., sequences with n ≥ 5 call pulses) and call characteristics of
the species (e.g., high frequency, call duration) in AnalookW 3.9c to help mitigate
false-positive/false-negative identification bias (De La Cruz et al. 2013). We recorded
all acoustic sampling-site locations with a Trimble Geoexplorer 6000 Series
GPS unit capable of sub-meter accuracy.
Habitat data
To characterize Indiana Bat habitat, we assessed landscape-level landuse/landcover,
forest fragmentation, slope, area solar radiation, proximity to permanent
water, elevation, and aspect. We derived cover classes from the raster dataset Landuse/
Landcover of West Virginia 2011 (Strager 2012a). Landuse/landcover data
were produced using an object-based image-analysis methodology. Basic landcover
classes were extracted from 1-m-cell-size, 4-band, and uncompressed 2011
National Agricultural Imagery Program (NAIP) orthophotography. The analysis
produced the following classes: forested, grasslands, barren/developed, open water,
mine grass, mine barren, pre-Surface Mining Control and Reclamation Act (Pre-
SMCRA) permit forested, Pre-SMCRA grass, Pre-SMCRA barren, herbaceous
wetlands, woody wetlands, and census roads 2011 (Strager 2012a). We reviewed
the Forest Fragmentation of West Virginia 2011 dataset (Strager 2012b) with Arc-
GIS (version 10.1; Environmental Systems Research Institute, Inc. Redlands, CA)
to examine the differentiated classifications of patch, edge, perforated, core (<100
ha), core (100–200 ha), and core (>200 ha); all non-forest areas were reclassified
as a 7th variable within the raster dataset. We derived measures of elevation directly
from 3-m–resolution digital elevation model (DEM) raster data (USGS and
WVSAMB 2003), made available through the West Virginia GIS Technical Center.
We obtained solar radiation, slope, and aspect from DEM raster data using the spatial
analyst extension of ArcGIS. Solar radiation represents watt-hours per square
meter (WH/m2); both aspect and slope are reported in degrees. We classified land as
either near (≤200 m) or far (>200 m) from permanent water (Humphrey et al. 1977).
We determined distance to permanent water by applying a 200-m buffer to perennial
streams, open-water resources, and herbaceous and woody wetlands as mapped
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in the National Hydrography Dataset (100 k) (NHD; USGS 2002) and extracted
from the landuse/landcover raster dataset. We converted NHD stream data to raster
data using the polyline to raster tool in ArcGIS. We isolated both NHD streams and
landuse/landcover aquatic resources using the reclassification and combined them
with the raster calculator spatial analyst tools in ArcGIS. We examined raster data
in ArcGIS using the extract by mask and zonal geometry spatial ana lyst tools.
MAXENT and GIS analysis
We used the machine-learning program MAXENT (version 3.3.3k) that estimates
probable species occurrence or habitat suitability of a species based on
ecological variables (Phillips et al. 2006). MAXENT operates under the theory
of maximum entropy in which the distribution of a population and its habitat will
become uniform as ecological variables are taken into consideration (Phillips et al.
2004). MAXENT has been found to perform well in comparison to other modeling
methods, particularly when utilizing occurrence data (Dudik et al. 2007, Phillips
and Dudik 2008, Phillips et al. 2006), and shows promise even in cases of low
(n < 20) sample sizes (Hernandez et al. 2006, Kumar and Stohlgren 2009, Papeş and
Gaubert 2007, Pearson et al. 2007). Habitat-modeling techniques provide useful
tools to relate occurrence data with ecological variables for the creation of detailed
maps (Elith et al. 2006). MAXENT measures the statistical relationship between
ecological variables at presence locations versus background locations within study
areas (Muscarella et al. 2014). MAXENT controls for model overfitting by excluding
ecological variables that contribute little or nothing to the model (Merow et al.
2013). Default settings of MAXENT are based on a substantial evaluation study of
the program (Phillips and Dudík 2008).
We followed Phillips and Dudík (2008) and analyzed our data by running
the program primarily on default settings. We used a random test-percentage of
20% so that each replicate MAXENT model used n = 9 observations as training
data and n = 2 observations to test the model (Jepsen et al. 2011). A single model
can show overfitting and bias toward areas around initial training-data points;
thus, we followed Jepsen et al. (2011) and used the average area under the curve
(AUC) across model replicates to represent our analysis. By measure of standard
deviation (SD), mean AUC values have been shown to converge and stabilize
with 50 model replicates, with no change in AUC with further replication (Jepsen
et al. 2011). We replicated models using a bootstrap run-type, in which
replicate sample sets are chosen by sampling with replacement (Phillips and
Dudík 2008). The bootstrap run-type requires the use of a random seed, such that
the testing and training data used is randomly sampled for each model replicate
(Phillips and Dudík 2008). Validation is required to assess the performance of
the model (Kumar and Stohlgren 2009). The most common approach is to partition
occurrence data into training and test data, thereby creating data for model
testing (Fielding and Bell 1997, Guisan and Thuiller 2005). We used a jackknife
procedure to assess model performance based on its ability to predict the locality
of the training dataset (Pearson et al. 2007). Model AUC values over 0.5 indicate
probable occurrence or likely suitable habitat conditions, with increased model
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efficiency as AUC approaches 1.0 (Swets 1988).
We used the ENMeval package of the statistical software program R (R Core
Team 2015) to construct habitat-suitability models evaluated by Akaike information
criterion value corrected for small sample size (AICc; Muscarella et al.
2014). The model with the lowest AICc value (i.e., ΔAICc = 0) reflects both model
goodness-of-fit and complexity (Burnham et al. 2010, Warren and Seifert 2010).
Models are based on presence locations and 500 randomly selected background locations
(Muscarella et al. 2014). Because numerous models evaluated by AICc may
have substantial support (ΔAICc < 2; Burnham and Anderson 2004) and because
these models often possess a lower ability to discriminate between testing and
background localities (Muscarella et al. 2014), we interpreted this analysis as an
additional measure of model validity. We assessed model similarity by measure of
spatial correlation between our habitat-suitability model created via 50 model replicates
and the model assessed by AICc using the band collection statistics spatial
analyst tool of ArcGIS. Correlation ranges from +1 to -1, with positive correlation
indicating a direct spatial relationship between the 2 models.
We used ArcGIS to analyze the data across the local landscape within specified
habitat-suitability classes (Jepsen et al. 2011). Using ArcGIS, we exported the habitat-
suitability distribution to raster-data format to create the following classes: low
(0–20%), low-medium (21–40%), medium (41–60%), high-medium (61–80%), and
high (81–100%). We used these classes to assess the probable availability of suitable
habitat within the study area.
Results
Indiana Bat observations
During our acoustic survey at Camp Dawson, we recorded 9200 potential
bat-call sequences. From our total dataset, 152 (1.7%) potential Indiana Bat
echolocation-call sequences were identified using automated classifiers, with 106
sequences (1.2%) displaying high quality and species-specific call characteristics
during qualitative analysis. Initial results indicated recordings of Indiana Bats at 15
(35.7%) acoustic sampling sites; however, after qualitative analysis, we eliminated
4 and used 11 (26.2%) locations in our analysis. The elevation range of observations
was from 451 m asl near the Cheat River canyon to 890 m asl near Briery
Mountain (Fig. 1).
MAXENT models
The MAXENT model depicting 20% habitat-suitability classes for Indiana Bats
within the 16,151-ha study area surrounding Camp Dawson is shown in Figure 2.
The average training AUC for 50 model replicates was 0.946 for training data
(Fig. 3) and 0.886 with a SD = 0.018 for test points, indicating the stable and robust
predictive capabilities of the model. Furthermore, our model constructed via
50 model-replicates displayed a positive spatial correlation (0.71) with the lowest
scoring AICc model (ΔAICc = 0, AUC = 0.74; Fig. 4).
Slope was the variable with the highest percent contribution (30.3%) to the
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Figure 2. Twenty percent habitat-suitability classes for summering Indiana Bats in Preston
County, WV, 2013. Low = 0–20%, medium-low = 21–40%, medium = 41–60%, mediumhigh
= 61–80% and high = 81–100% habitat-suitability classes.
Figure 3. Receiver operating curve (ROC) for 50 MAXENT model replicate runs concerning
habitat suitability of Indiana Bat in Preston County, WV, 2013.
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model (Table 1). Based upon the marginal response curves, low to modest slopes
of 1º–20º were most suitable, and there was a strong negative relationship between
bat occurrence and steeper areas. Several other variables contributed positively to
the model. Specifically, landuse/landcover contributed 23.2% to the model, with a
strong positive correlation between bat occurrence and the presence of roads and
barren ground. Forest fragmentation provided a 19.2% contribution to the model.
Large tracts (>200 ha) of contiguous forest cover provided the maximum probability
of predicting habitat suitability concerning any fragmentation variable, followed by
open non-forest areas and isolated forest patches. Area solar radiation contributed
9.9% to the model. Based upon the marginal response curves, areas of ≥5.5 x 105
Figure 4. Model comparison of (a) MAXENT averaging based on 50 model replicates
(b) MAXENT model having the lowest AICc value (i.e., ΔAICc = 0) concerning habitat
suitability of Indiana Bats in Preston County, WV, 2013.
Table 1. Selected ecological variables and their percent contribution to the MAXENT model for Indiana
Bat habitat suitability in Preston County, WV, 2013. DEM = digital elevation-model.
Permutation
Variable % Contribution importance Source
Slope 30.3 32.1 Generated in GIS from DEM
Landuse/Landcover of WV 23.2 25.6 Strager 2012a
Forest Fragmentation of WV 19.8 21.5 Strager 2012b
Area solar radiation 9.9 14.1 Generated in GIS from DEM
Proximity to permanent water 9.2 0.7 Generated in GIS
Elevation 6.5 5.1 Generated in GIS from DEM
Aspect 1.1 0.9 Generated in GIS from DEM
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WH/m2 were most suitable for Indiana Bats, with a strong decrease in suitability
across areas of lower solar radiation. Proximity to permanent water provided a
9.2% contribution to the model; the absence of permanent water (>200 m away)
was a better predictor of habitat suitability. The jackknife procedure indicated that
neither elevation (6.5%) nor aspect (1.1%) contributed greatly to the model. Individual-
permutation importance of these variables did not vary appreciably from the
corresponding percent contribution (Table 1, Fig. 5), further suggesting that these
variables are not predictive of suitable Indiana Bat habitat. Solar radiation was the
environmental variable with the highest gain when used in isolation, whereas forest
fragmentation decreased the gain the most when omitted (Fig. 5).
Distribution of habitat-suitability classes
The geographic analysis of habitat distribution showed that our analysis classified
only 0.6% of the study area in the high-suitability class (Table 2, Fig. 2).
Furthermore, only 2.7% and 8.1% of the landscape was classified as medium-high
and medium, respectively. The low-suitability and low-medium suitability classes
accounted for 88.6% of the landscape. Habitat variables within the high-suitability
Table 2. Total contribution of 20% habitat-suitability classes for summering Indiana Bats within a
study area located in Preston County, WV, 2013.
Habitat suitability class Total hectares Present composition
Low (0–20%) 10,021.9 62.0
Low-medium (21–40%) 4292.9 26.6
Medium (41–60%) 1310.7 8.1
High-medium (61–80%) 433.6 2.7
High (81–100%) 92.2 0.6
Total 16,151.3 100.0
Figure 5. Jackknife evaluation of relative importance of predictor variables for MAXENT
habitat-suitability models of Indiana Bats in Preston County, WV, 2013. Aspect = cardinal
direction of slope, elevation = height above sea level, frag = Forest Fragmentation of WV
value (Strager 2012b), lulc = Landuse/Landcover of WV value (Strager 2012a), slope = degree
of land gradient, solar = incoming solar radiation, and water = presence of permanent
water within 200 m.
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class indicate positive influences of contributing variables in the model. Areas
within the high-suitability class reflect the presence of large tracts of contiguous
forest cover (>200 ha) associated with low to modest slopes (less than 20°), road corridors,
and areas of high solar radiation (≥ 5.5 x 10 5 WH/m2).
Discussion
Our 11 occurrence observations of Indiana Bats were within the minimum values
necessary to effectively model habitat suitability using MAXENT (Kumar and
Stohlgren 2009). Our high and stable AUC values across 50 model replicates indicated
that the MAXENT model effectively predicted suitable areas of Indiana Bat
foraging habitat (Jepsen et al. 2011, Swets 1988). Our results were similar to those
of Muscarella et al. (2014) in that our AICc model showed an inability to discriminate
between testing and background localities when qualitatively compared to the
model constructed via 50 model replicates. However, our overall results suggest
that large-scale ecological variables are capable of predicting habitat suitability for
Indiana Bats at the local-landscape level. Based upon our results, high-suitability
areas concerning summer Indiana Bat habitat were rare across the local landscape
and accounted for only 0.6% of the study area. Because these highly suitable areas
for Indiana Bats are uncommon across the landscape, risks to the species due to
land use and development can be addressed within conservation and habitat mitigation
efforts on a finer scale. Furthermore, the broader and potentially suboptimal
classes of medium to medium-high suitability (41–80%) accounted for only 10.8%
of the landscape, which allows for further focus of conservation efforts.
Indiana Bats have been shown to select for highly variable amounts of canopy
closure, ranging from less than 20% to 88% (USFWS 2007). Regional differences in canopy
cover have been noted, and cover values are typically higher in areas were live
trees such as Carya ovata (Mill.) K. Koch (Shagbark Hickory) are used for roosts
(Palm 2003). Gardner et al. (1991a) found that a majority of located roost trees
were within forests with 80–100% canopy closure, and Carter et al. (2002) found
that roosts were surrounded by larger patches of closed-canopy forest than random
points. Likewise, Indiana Bats have been shown to forage in areas of high canopycover
in West Virginia (Ford et al. 2005), while individual roost trees are often
located in areas of low basal area and occur within large canopy gaps (Johnson et
al. 2010). The importance of large tracts of contiguous forest cover in our model
support these earlier findings. The use of larger tracts of forest displaying closed to
semi-open canopies likely provides a more diverse range of roost trees (Callahan
1993) and foraging habitat (Brack 1983, Humphrey et al. 1977, Laval and Laval
1980) available to colonies.
Although Indiana Bats have been found in variable canopy conditions, and
foraging has been attributed to warmer conditions potentially driven by elevation
and aspect (Brack et al. 2002, Butchkoski and Mehring 2004), solar radiation at the
larger landscape level has not been addressed. Our results suggest that large tracts
of closed canopy forest that receive high levels of solar radiation (with increased
ambient temperature) are important foraging habitats for Indiana Bats in West VirNortheastern
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ginia. Such areas are likely limited due to highly contrasting topographic features
(i.e., slope) that alter available solar radiation, causing variation in the microclimate.
Indiana Bats have been shown to select roosting habitats in close proximity
to foraging sites (Carter et al. 2002, Kurta et al. 2002, Murray and Kurta 2004,
Timpone et al. 2010), suggesting that our results may provide insight into roosting
habitat in West Virginia.
Maternity roosts are typically associated with increased solar radiation and are
found along forest edges and within canopy perforations (Miller et al. 2002, Whitaker
and Brack 2002). Our analysis found that comparable variables were important
determinants of foraging activity, particularly the significance of fringe-forest
fragments and core forest with high levels of solar radiation. Similar to our results,
Gardner and Cook (2002) found a high correlation between occurrence of maternity
colonies and areas of non-forested landscape. These areas are generally considered
valuable in the production of the insect prey required by actively reproductive
females. However, Indiana Bats typically avoid human urbanization and may tend
not to utilize such areas due to reduced foraging and roosting habitat (Carter et
al. 2002). Similar to our results, Gardner et al. (1991a) found that an Indiana Bat
maternity colony was closer to unpaved than paved roads and nearer to intermittent
than perennial streams. Such areas likely provide commuting corridors that link
various habitats (Carter 2003, Gardner et al. 1991a, Murray and Kurta 2004, Winhold
et al. 2005).
Gumbert et al. (2002) suggest that Indiana Bats may be more dependent upon
site characteristics than the continued suitability of individual roost trees. Roosts
are an ephemeral resource that may quickly become unsuitable due to the loss of
exfoliating bark, animal or parasite occupation, or collapse (Belwood 2002, Gardner
et al. 1991a, Kurta 1994). Abiotic factors such as slope and solar radiation
appear to be limiting ecological factors in the region. Steep slopes may accelerate
roost loss, prevent the formation of preferred foraging areas such as wetlands, and
alter microclimatic conditions. Our results suggest that solar radiation is the most
useful ecological variable for predicting suitable Indiana Bat habitat; however, our
results also suggest that solar radiation is potentially correlated with several other
variables, while forest fragmentation is likely the least correlated. Variation in solar
radiation across the landscape likely limits the suitable foraging range of Indiana
Bats in West Virginia. If these abiotic factors affect habitat suitability within the
region, Indiana Bats should seek them out annually; similar behavior has been observed
throughout the species’ range (Gardner et al. 1991a; Kurta et al. 1996, 2002;
Murray and Kurta 2002)
The conservation of summer maternity roosts is a cornerstone of Myotine bat
management in North America (Brooks and Ford 2005, Jung et al. 2004, Loeb
and O’Keefe 2006). Summer maternity roosts are assumed to be critical, limiting
resources for bats in both forested and formerly forested environments (Fenton
1997, Kunz and Fenton 2003), and management techniques should attempt to
conserve or create suitable maternity habitats (Timpone et al. 2010), with roosts
being maintained on the landscape for the long term (Lacki and Schwierjohann
Northeastern Naturalist
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J.L. De La Cruz and R.L. Ward
2016 Vol. 23, No. 1
2001, Owens et al. 2004, Perry and Thill 2007) or created through active management
(Carter 2006, Johnson et al. 2010). However, management decisions must
also consider landscape features such as trails and roads located within intact
forests (Palmeirim and Etheridge 1985, Zimmerman and Glanz 2000) and welldefined
forest edges and perforations (Grindal 1996, Hogberg et al. 2002). Such
landscape structures may be necessary to facilitate foraging success of bats in
forests (Lacki et al. 2007). Our results suggest that conservation efforts for the
species should be focused within large tracts of contiguous forest cover (>200 ha)
associated with low-to-modest slopes (less than 20°), road corridors, and, notably, areas
of high solar radiation (≥5.5 x 105 WH/m2).
Our results represent a landscape-level analysis conducted to quantify the distribution
of suitable Indiana Bat summer habitat. This work can be used to identify
candidate areas for conservation planning in West Virginia. Future research should
focus on microhabitat features associated with areas of high habitat-suitability for
further model validation. For example, small-scale aquatic resources such as wildlife
ponds and road ruts are likely valuable to Indiana Bats (USFWS 2007, Wilhide
et al. 1998) but were underrepresented in our analysis. However, our results suggest
that road corridors are predictive of suitable Indiana Bat habitat and they likely
link roosting sites to necessary aquatic resources (Murray and Kurta 2004). Mist
netting and radio-telemetry studies will be necessary to investigate microhabitat
requirements within the state. Conservation efforts should be based upon information
regarding the species at the local level, and our results provide additional data
to address conservation priorities of summer foraging habitat. Although we could
not determine the sex of bats recorded during this study, our results may still describe
resources necessary for the formation of maternity colonies such as access to
large tracts of core-forest that display stable thermal conditions and link essential
resources. Such areas are potentially critical as summer Indiana Bat habitat and are
likely limited across the topographically diverse landscape of West Virginia.
Acknowledgments
We thank R. Chaney and R. Snyder of Camp Dawson for allowing use of occurrence data
collected onsite, C.R. Allen of BCID for providing qualitative technical assistance on the
project, E.S. Schroder for providing technical field assistance, A.L. Runner for her editorial
contributions, and AllStar Ecology LLC, Fairmont, WV, for supporting this project.
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