159
Roosts of Rafinesque’s Big-Eared Bats and Southeastern
Myotis in East Texas
Leigh A. Stuemke1, Christopher E. Comer1,*, Michael L. Morrison2,
Warren C. Conway1, and Ricky W. Maxey3
Abstract - Because diurnal roosts can be important in determining bat occupancy and abundance
in forested habitats, we identified characteristics of cavity trees that influence roost
selection by Corynorhinus rafinesquii (Rafinesque’s Big-eared Bats) and Myotis austroriparius
(Southeastern Myotis) in east Texas. We identified used and non-used cavity trees
with a combination of transect searches, radiotelemetry, and historical records at 7 study
areas. Both bat species selected similar cavity trees for summer diurnal roosts, showing an
affinity for tupelo trees (Nyssa spp.), with 55% of diurnal roosts in Nyssa aquatica (Water
Tupelo) and 33% in N. sylvatica (Blackgum). Of 17 tree and habitat variables we measured
at used and unused cavity trees, those related to cavity size and availability (cavity height
and diameter, tree diameter, density of large trees in the area) were the most important predictors
of use. Characteristics of the surrounding stand at both local and landscape scales
were less important. Rafinesque’s Big-eared Bats and Southeastern Myotis appeared to use
the largest cavity trees present and we speculate that the availability of suitable trees with
large cavities may limit abundance in this region.
Introduction
Suitable diurnal roosts are vital for the success and persistence of all bat species.
Diurnal roosts provide protection from predators and ambient environmental
conditions, and aid in reducing energetic costs associated with parturition and
thermoregulation (Barclay and Kurta 2007). Within forested systems, bats utilize
a variety of structures for diurnal roosts, including tree foliage, exfoliating bark,
cracks or crevices in the tree, and internal cavities (Brigham 2007). Due to the
importance of diurnal roosts and the relative ease of studying bats in their roosts,
roost selection has been extensively studied for some species; however, much of
the research been focused on certain species (e.g., Myotis sodalis [Indiana Bat]) or
certain regions (e.g., the Pacific Northwest) (Brigham 2007). Because roost characteristics
are often very species-specific, local information on individual species’
needs is needed to formulate effective plans for conservation.
Corynorhinus rafinesquii Lesson (Rafinesque’s Big-eared Bat) and Myotis austroriparius
Rhoads (Southeastern Myotis) are closely associated with bottomland
hardwood forests and forested swamps throughout their ranges in the southeastern
1Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University,
Box 6109 SFA Station, Nacogdoches, TX 75962. 2Department of Wildlife and Fisheries
Sciences, Texas A&M University, 210 Nagle Hall, College Station, TX 77843-2258. 3Texas
Parks and Wildlife Department, PO Box 226, Karnack, TX 75661. *Corresponding author
- comerce@sfasu.edu.
Manuscript Editor: Jerry Cook
Proceedings of the 5th Big Thicket Science Conference: Changing Landscapes and Changing Climate
2014 Southeastern Naturalist 13(Special Issue 5):159–171
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United States (Clark 2003, Lance et al. 2001, Mirowsky 1998). Although habitat associations
are poorly understood, this association is believed to be related primarily
to their preference to roost in large, hollow trees of certain bottomland hardwood
species. In East Texas, natural roosts occur in mature to over-mature hardwood trees
that have formed cavities with basal and/or top openings in a variety of species,
including Nyssa sylvatica Marshall (Blackgum), N. aquatica L. (Water Tupelo),
Magnolia grandiflora L. (Southern Magnolia), Fagus grandifolia Ehrh. (American
Beech), Platanus occidentalis L. (Sycamore), and Taxodium distichum (L.) (Baldcypress)
(Clark 1990, Gooding and Langford 2004, Lance et al. 2001, Mirowsky et al.
2004, Trousdale and Beckett 2005). In most cases, Rafinesque’s Big-eared Bats used
multiple roosts (i.e., roost switching) in close proximity (e.g., <1 km) within a given
season (Stevenson 2008, Trousdale and Beckett 2005,). The use of multiple roosts
may be related to site, stand, and/or landscape characteristics that minimize energetic
costs of survival (Lewis 1995). In addition to tree roosts, these bats also utilize caves
and various anthropogenic structures (e.g., abandoned buildings, bridges, wells, culverts,
and artificial roosts) for roosting in the southeastern coastal plain ( Clark 1990,
Hoffmeister and Goodpaster 1962, Lance et al. 2001).
Many factors related to individual trees, forest stands, and forested landscapes
influence selection of diurnal roost sites for various species of forest bats (Barclay
and Kurta 2007). For example, tree height, tree diameter at breast height (DBH),
percent canopy cover, density of surrounding vegetation, stand age, and proximity
to water influence tree roost selection by forest bats (Crampton and Barclay 1998,
Jung et al. 2004, Ober and Hayes 2008, Willis and Brigham 2005). Due to the perceived
importance of diurnal roosts in the distribution of our target species, recent
research activity has defined natural and artificial roost requirements at small spatial
extents, including characteristics of the roost itself or the immediately surrounding
forest stand. Roosts that were spacious and partially lit (Lance et al. 2001) and
within close proximity to water (<1 km) were selected more often by both species
(Mirowsky et al. 2004). Rafinesque’s Big-eared Bats generally chose larger trees
(82–124 cm DBH) than Southeastern Myotis (76–108 cm DBH) as diurnal roosts
(Carver and Ashley 2008, Gooding and Langford 2004, Rice 2009). In Texas, areas
with historic records of occurrence consisted of relatively large (>100 ha) patches
of contiguous forest. These patches generally consisted of >30% bottomland hardwood
forest with at least a portion in cypress-tupelo forest (Mirowsky et al. 2004;
Meg Goodman, Texas Parks and Wildlife Department, Austin, TX, unpubl. data).
Roosts for both species occurred in landscapes with a minimum of 1/3 bottomland
hardwood forest and a prevalence of large Baldcypress or Water Tupelo. Habitat
surrounding bottomland forest stands used by Rafinesque’s Big-eared Bats and
Southeastern Myotis can be quite variable and may consist of mixed pine-hardwood
stands, upland pine forest, pine plantations, and urban interfaces (Carver and Ashley
2008, Gooding and Langford 2004, Trousdale et al. 2008).
In Texas, the Rafinesque’s Big-eared Bat is a threatened species with a high
conservation priority based on perceived population decline (Bender et al. 2005,
Mirowsky et al. 2004). The Southeastern Myotis is considered rare but is not
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afforded any state listing; however, the Texas Wildlife Action Plan lists this bat as
a high priority for conservation (Bender et al. 2005). Much of the concern related
to these species is due to the loss of up to 75% of original bottomland hardwood
forests in the region. Many of the remaining bottomland hardwood stands were
logged in the middle to late 20th century, and perceived low abundance may be
related to limited availability of trees suitable for roosting. Within East Texas, data
about roost requirements are limited to a single study performed in the mid-1990s
(Mirowsky 1998). Recent data in the region are limited to semiannual monitoring
of known roosts for both species. Our objectives in this study were to locate additional
roost trees and identify factors to differentiate between used and non-used
cavity trees for Rafinesque’s Big-eared Bats and Southeastern Myotis. The results
provide additional information on roosting ecology in the region that may assist
with prioritizing conservation actions for these species.
Study Area
Rafinesque’s Big-eared Bat and Southeastern Myotis reach the western extent
of their ranges in the Pineywoods eco-region of east Texas. This eco-region occurs
in the Gulf Coastal Plain physiographic region (Nixon 2000) and extends
from the Red River along the northern border of East Texas southward to the
northern suburbs of Houston. The Pineywoods covers 6.3 million ha in East Texas
and is an extension of the pine, mixed hardwood and bottomland hardwood forests
of the southeastern United States (Diggs et al. 2006). Topography of the area is
mostly flat with low rolling hills and elevation ranging from 15–230 m. Average
annual precipitation across the region varies from 89–127 cm and, combined with
hot summers (mean high temperature 24–25 °C) and mild winters (mean low temperature
11–12 °C), produces a long growing season of 220–270 days, depending
on location (Nixon 2000).
We selected 7 study areas within the Pineywoods eco-region based on historic
occurrence records for our target species, habitat conditions, and accessibility:
Caddo Lake National Wildlife Refuge (CLNWR: Harrison County, 3440 ha),
Caddo Lake State Wildlife Management Area (CLWMA: Marion County, 3240
ha), Little Sandy National Wildlife Refuge (LSNWR: Wood County, 1538 ha),
Big Thicket National Preserve (BTNP: Hardin County, 42,770 ha), Trinity River
National Wildlife Refuge (TRNWR: Liberty County, 10,117 ha), The Nature Conservancy’s
Roy E. Larsen Sandyland Sanctuary (TNC: Hardin County, 2250 ha),
and Village Creek State Park (VCSP: Hardin County, 441 ha) (Fig. 1). Among the
7 sites, ecological communities were variable, but the basic habitat structure that
supports Rafinesque’s Big-eared Bat and Southeastern Myotis populations was
present: cypress and tupelo swamps, bottomland hardwood forests, and mixed deciduous/
pine upland forests. Dominant overstory species for these areas included:
Liquidambar styraciflua L. (Sweetgum), Southern Magnolia, Water Tupelo,
Blackgum, Pinus taeda L. (Loblolly Pine), Quercus lyrata Walter (Overcup Oak),
Q. michauxii Nutt. (Swamp Chestnut Oak), Q. nigra L. (Water Oak), Q. phellos
L. (Willow Oak), Baldcypress, Ulmus americana L. (American Elm), and
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U. crassifolia Nutt. (Cedar Elm). Dominant midstory species included Cephalanthus
occidentalis L. (Buttonbush), Forestiera ligustrina (Michx.) Poir. (Swamp
Privet), Fraxinus pennsylvanica Marshall (Green Ash), and Planera aquatica J.F.
Gmel. (Water Elm).
Figure 1. Study area locations for Rafinesque’s Big-eared Bat and Southeastern Myotis
roost studies conducted in East Texas, 2008–2009.
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Methods
Bat surveys
We used standardized transect searches and radiotelemetry to locate cavity trees
used as diurnal roosts by the target species. In addition, we opportunistically identified
a small number of roosts during other research activities and recorded data
at used cavity roost-trees identified by historical records. A tree was classified as
used when one or more individual(s) of our target species were documented inside
the tree. Field surveys were conducted during the season of high bat activity: 12
May–15 August 2008, and 19 May–13 August 2009.
We located potential roost trees by visual observation along transects located
in 100-ha study blocks randomly selected from our 7 study areas. We
overlaid systematic block-sampling grids (100 ha) onto aerial photos of each
study area using ArcGIS 9.3 (Environmental Systems Research Institute, Redlands,
CA), then used a random number generator to select blocks for transect
searches. The block size of 100 ha was selected to match the estimated homerange
size of the Rafinesque’s Big-eared Bat (Hurst and Lacki 1999, Menzel
et al. 2001). There were 4 study blocks at LSNWR, 4 study blocks at CLNWR,
4 study blocks at CLWMA, 2 study blocks at TRNWR, 3 study blocks at BTNP,
2 study blocks at TNC, and 1 study block at VCSP for a total of 20 study blocks.
We randomly located 10 1-km-long by 40-m-wide transects in each study block,
and searched all trees within the transect boundary to identify potential cavity
openings. If we noted a potential cavity, we investigated for signs of bat presence.
Depending on the characteristics of the tree and cavity opening, we used
either direct observation with Surefire 9p flashlights (105 lumens) with red filters
(SureFire, LLC, Fountain Valley, CA), or hand-held mirrors and flashlights
to inspect tree cavities. If the tree cavity possessed a bend or some other hindrance
that prevented visual inspection, we used acoustic detectors (Pettersson
240X, Pettersson Elektronik AB, Uppsala, Sweden) to determine if bats were
using the tree. Identification of species was made through visual observation,
photographic evidence, or acoustic analysis of call characteristics. We used both
our own site-specific and freely available call libraries to identify calls to species
using Sonobat 2.5.9 (SonobatTM, Arcata, CA). If target bat species were not
present in a cavity tree at the time of survey and the tree was ≥30 cm DBH (Clark
2003), the tree was classified as a non-used cavity tree for our analyses. Due to
time and logistical constraints, we did not make repeat visits to potential roost
trees; therefore, we classified them as used or non-used based on their status the
day of the survey.
For our radiotelemetry activities, we captured bats by mist net or hand capture at
known roost trees. We opportunistically set mist nets in areas of appropriate habitat
and hand captured bats from known roosts in artificial structures. Upon capture, we
recorded species, sex, forearm length, hind foot length, tragus length, ear length,
body weight, reproductive condition, and age class (juvenile or adult, as determined
by the epipheyseal-diaphyseal fusion of the finger joints) (Anthony 1998). Captured
Rafinesque’s Big-eared Bats and Southeastern Myotis weighing a minimum
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of 7 g were fitted with 0.52-g transmitters (Model BN-2N, Holohil Systems, Ltd.,
Carp, ON) or 0.50-g transmitters (Philip Blackburn, SFASU, Nacogdoches, TX).
Transmitters were affixed with surgical adhesive (Torbot®, Torbot Group Inc.,
Cranston, RI) between the shoulder blades without clipping fur, and we did not find
that the fur hindered attachment duration. To allow for proper attachment of the
transmitters, a thin layer of glue was placed on both the transmitter and the bat and
allowed to stand for 5 min or until the glue bubbled as indicated by the adhesive
instructions. Tags were then affixed to the bats, and the bat was held for another
15–20 minutes to ensure that the glue had set completely prior to release.
We attempted to locate all radiomarked bats daily from the day after transmitter
attachment until transmitter failure or drop-off. We tracked bats using a handheld
R2000 receiver (Advanced Telemetry Systems, Isanti, MN) with a three-element
yagi antenna. Whenever radiotracking led to a tree roost, we marked that tree with
a handheld GPS unit and visually verified the presence of the bat in the tree. If additional
bats were present in the tree, we attempted to determine species and number
by visual or photographic examination.
Habitat measurements
We recorded 17 habitat-related variables at all used and non-used cavity trees
at three spatial extents (tree, stand, landscape). Tree characteristics included
species, DBH, and tree height. We also measured several characteristics of the
cavity, including interior diameter and height, width and height of the main cavity
opening, distance from the bottom of the main cavity opening to ground, and
total number of entrances to the cavity. We estimated the height of the cavity by
locating a point on the exterior of the tree that corresponded with the top of the
cavity. We then used a clinometer to measure the height to this point and used
this number to determine the height of the cavity. For trees that had multiple cavity
openings, the largest opening was used for measuring height and width of the
opening and distance to the ground.
We collected stand information in 0.01-ha circular plots centered at each focal
tree (used or non-used) to characterize the forest immediately surrounding the focal
tree. For each plot, we used a spherical densiometer to measure canopy closure on
the perimeter of the plot at each cardinal direction. We also recorded stem density
in three size classes according to DBH (0–26 cm, 27–52 cm, and ≥53 cm). For permanent
water and habitat edge data, we used georeferenced electronic data or 2010
National Agricultural Imagery Program (NAIP) aerial photographs, both available
from the Texas Natural Resources Information System (TNRIS). We used handheld
GPS units to obtain data points for anthropogenic structures not clearly visible on
aerial photographs. To derive distances, we used the Near tool in ArcGIS 9.3 (Environmental
Systems Research Institute, Redlands, CA). Because we were concerned
that non-used cavity trees were clustered at the landscape scale, we used random
points within the study blocks for comparisons using landscape-scale variables. For
these measurements, we generated 34 random points in each study block using the
Create Random Points tool in ArcGis 9.3.
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Data analysis
We used univariate analysis of variance (PROC ANOVA; SAS Institute, Inc. 2004)
to identify differences between roost trees for the two target species and to identify
factors differentiating between used and non-used cavity trees. We set alpha = 0.05.
Results
We documented 18 trees used as diurnal roosts by our target species, including
11 used by Rafinesque’s Big-eared Bats and 7 used by Southeastern Myotis. This
total included 7 trees discovered on 200 roost-search transects, 7 from radio-telemetry
methods, and 2 discovered opportunistically during other activities. Two roosts
from Texas Parks and Wildlife Department’s occurrence records for the target species
were also included in the analysis.
We radiomarked and tracked 7 Rafinesque’s Big-eared Bats (4 females, 3 males)
to 6 new tree roosts and 3 Southeastern Myotis (all females) to 1 new tree roost for a
total of 54 days. We observed very little roost switching during this study: 6 of the 7
Rafinesque’s Big-eared Bats and all of the Southeastern Myotis used only one tree
roost during the period they were radiomarked. The remaining Rafinesque’s Bigeared
Bat (a male) switched approximately every 2 days among 4 Water Tupelo trees.
Two of the male Rafinesque’s Big-eared Bats roosted in an artificial structure for one
night each: a concrete bridge and the attic above a garage in an occupied dwelling).
One post-lactating female frequently roosted under a concrete bridge. We did not observe
radiomarked Southeastern Myotis roosting in any artificial structures.
While conducting the 200 roost-search transects, we also documented and recorded
data for 244 non-used cavity trees. Based on these data, the study blocks
had approximately 12.3 cavity trees (≥30 cm DBH) per 100 ha of habitat.
Nyssa spp. were the most common trees used for roosts by both species, with
55% of diurnal roosts in Blackgum and 33% in Water Tupelo. These two species
comprised approximately 50 of 262 (15%) trees with cavities in the area but 15 of
the 18 (83%) used cavity trees. The remaining three used cavity trees were a Sweetgum,
a Baldcypress, and a Quercus laurifolia Michx. (Swamp Laurel Oak). All but
one of the used cavity-trees contained a hollow trunk cavity with a wide interior
diameter and smooth interior walls. Of the 18 used cavity trees measured, 11 (61%)
possessed a main cavity that was at ground level, four (23%) had cavities located
in the trunk of the tree above ground, and three (18%) were chimney trees with the
top broken off allowing entrance to the cavity.
Characteristics of cavity trees used by Rafinesque’s Big-eared Bats and Southeastern
Myotis were similar (Table 1). In general, Rafinesque’s Big-eared Bats used
trees with larger cavities than Southeastern Myotis, but the differences did not reach
statistical significance. Roosts used by Southeastern Myotis were in areas with
higher stem density at the smallest size class, 0–26 cm; however, other stand and
landscape characteristics were similar (Table 1). Used cavity trees for both species
combined, had cavities that were nearly twice as large (in both height and diameter)
as the cavities in non-used trees (Table 2). Perhaps reflecting the cavity size, used
trees were larger in diameter than unused trees, but height did not differ. Cavities
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of used trees also had more entrances and cavity entrances were higher above the
ground. Used trees were located in areas with a higher density of large (≥53 cm)
trees, but other stand and landscape characteristics were similar for used and nonused
trees (Table 2).
Table 1. Means, standard errors (SE), numbers of trees (n), and P-values (P) for tree habitat, and
landscape variables for tree roosts used by Rafinesque’s Big-eared Bats and Southeastern Myotis at 7
study areas in East Texas, 2008–2009.
Rafinesque’s
Big-eared Bat Southeastern Myotis
Variable Mean SE n Mean SE n P
Tree height (m) 17.6 1.37 11 20.9 2.00 7 0.1903
Tree diameter (cm) 101.3 8.65 11 81.9 4.77 7 0.1140
Cavity, interior diameter (cm) 174.9 74.01 8 80.4 9.43 7 0.1794
Cavity, interior height (cm) 813.6 135.71 10 766.7 358.10 6 0.8707
Cavity, distance from ground (cm) 261.4 117.39 11 53.7 33.71 7 0.1883
Cavity, opening width (cm) 25.6 4.04 8 42.6 10.66 7 0.0804
Cavity, opening height (cm) 96.2 31.11 8 70.0 12.04 7 0.3909
Cavity, number of entrances 1.8 0.30 11 1.1 0.14 7 0.1038
Plot, percent canopy closure 85.8 2.67 11 93.3 1.58 6 0.0508
Plot, stem density 0–26 (cm) 48.6 12.10 11 92.3 10.62 7 0.0236
Plot, stem density 27–52 (cm) 7.0 1.71 11 4.7 1.69 7 0.3804
Plot, stem density ≥53 (cm) 2.5 0.71 11 1.7 0.56 6 0.3132
Plot, canopy height (m) 21.1 2.12 11 21.3 1.58 7 0.9487
Distance to permanent water (m) 452.6 101.85 11 234.0 120.84 7 0.1909
Distance to habitat edge (m) 82.0 12.95 11 81.3 17.56 7 0.9722
Distance to human structures (m) 821.0 129.82 11 632.6 191.87 7 0.4103
Table 2. Means, standard errors (SE), number of trees measured (n) and P-value (P) for univariate
ANOVAs for tree, habitat, and landscape variables measured at used roosts and non-used cavity trees
for Rafinesque's Big-eared Bats and Southeastern Myotis at 7 stu dy areas in East Texas, 2008–2009.
Used Trees Non-usedTrees
Variable Mean SE n Mean SE n P
Tree height (m) 18.9 1.17 18 21.3 0.45 244 0.1532
Tree diameter (cm) 93.8 5.94 18 77.2 1.90 244 0.0216
Cavity, interior diameter (cm) 130.8 40.47 15 58.3 2.16 205 <0.0001
Cavity, interior height (cm) 796.0 151.56 16 431.7 23.56 164 <0.0001
Cavity, distance from ground (cm) 180.6 75.58 18 29.6 7.86 237 <0.0010
Cavity, opening width (cm) 33.6 5.68 15 25.4 1.21 237 0.1024
Cavity, opening height (cm) 84.0 17.30 15 78.3 3.50 237 0.6958
Cavity, number of entrances 1.6 0.20 18 1.2 0.04 242 0.0440
Plot, percent canopy closure 88.5 1.99 17 88.9 0.89 243 0.8882
Plot, stem density 0–26 (cm) 65.6 9.73 18 61.2 3.27 244 0.7208
Plot, stem density 27–52 (cm) 6.1 1.23 18 5.1 0.25 243 0.2824
Plot, stem density ≥53 (cm) 2.2 0.49 17 1.3 0.11 240 0.0432
Plot, canopy height (m) 21.2 1.40 18 22.6 0.44 244 0.3841
Distance to permanent water (m) 367.6 80.01 18 278.0 9.34 680 0.1280
Distance to habitat edge (m) 81.7 10.13 18 111.6 6.00 680 0.4194
Distance to human structures (m) 747.8 107.74 18 964.1 18.34 680 0.0584
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Discussion
Cavity size was the most important factor in determining use of a tree as a diurnal
roost by rare forest bats in East Texas, which was similar to the findings of
a study in Mississippi (Stevenson 2008). Tree DBH was also important, and may
be an appropriate proxy for cavity size. Because both target bat species are known
to switch roosts regularly under normal conditions (Lewis 1995), and we visited
each potential roost only once, we may have classified some trees as unused that
were used at other times. There is no reason to suspect bias based on the day of
survey (e.g., we were more likely to find them in larger trees on the survey days)
and we feel that our findings are representative of roost tree use by these bats in the
region. The importance of large trees for roosting by these species has been welldocumented,
and roost trees in this study (mean = 93.8 cm) were similar in diameter
to Rafinesque’s Big-eared Bat roost trees previously described in Texas (mean =
99.8 cm, Mirowsky 1998) and Louisiana (mean = 59–103 cm, Lance et al. 2001),
and slightly larger than roost trees in Mississippi (mean = 79.4 cm, Trousdale and
Beckett 2005). However, several studies have found that Rafinesque’s Big-eared
Bats selected roosts that were considerably larger, including in Arkansas (mean
= 155.3 cm, Cochran 1999), Louisiana (mean = 120.1 cm; Gooding and Langford
2004), and Tennessee (mean = 124.5 cm; Carver and Ashley 2008).
The reason that bats in Texas used smaller trees is not clear but it may reflect a
paucity of overmature, very large cavity-trees on the East Texas landscape. We found
approximately 12.3 cavity trees per 100 ha on our study sites; however, many of
these cavities were small. Using the smallest used cavity size (DBH ≥ 62.9 cm, cavity
diameter ≥ 40.6 cm, cavity height ≥ 63.5 cm) as a minimum suitable cavity size, we
found only 6 suitable roost trees per 100 ha (1 suitable tree per 16 ha) on these study
sites. In approximately 2000 ha surveyed for this study, we measured only 20 trees
(12 with large cavities) that were ≥124 cm. Although few studies of roost-selection
report cavity-tree density, Gooding and Langford (2004) estimated 65.5 cavity trees/
ha and both these authors and Carver and Ashley (2008) noted that large, hollow trees
were common on their sites. In contrast, Trousdale and Beckett (2005) commented
that suitable trees were uncommon on their study site in Mississippi.
Although the implications of using smaller cavities for roosting by Rafinesque’s
Big-eared Bats are unknown, our observations suggest that availability of suitable
roosts was low in East Texas. In our radiotracking, we observed very little of the
roost-switching behavior often observed in both species (Lewis 1995), perhaps due
to the large distances between suitable roost trees (Gooding and Langford 2004).
Only one male Rafinesque’s Big-eared Bat switched among multiple tree roosts,
using four large Water Tupelos that were all within approximately 200 m of each
other. Use of anthropogenic structures (abandoned homes, occupied buildings, concrete
bridges, concrete bunkers, abandoned wells) was common in our study, and
roost switching that did occur often included moving between a single tree-roost
and a structure. No information exists comparing relative selection for natural and
anthropogenic roosts by Rafinesque’s Big-eared Bats; however, it is illustrative to
note that studies with few large trees for roosting (Lance et al. 2001, Mirowsky
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1998, Trousdale and Beckett 2005) documented >40% of roosts in manmade structures
and studies with more suitable tree roosts found fewer (Carver and Ashley
2008) or none (Gooding and Langford 2004).
It is difficult to quantify the relationship between bat abundance and availability
of suitable roost sites, but most authors agree that roosts, particularly maternity
roosts, are critical components of habitat for forest bats (e.g., Barclay and Kurta
2007). In this study, only one of the 11 cavity trees used by Rafinesque’s Big-eared
Bats contained more than two bats, suggesting that use of natural tree roosts as
maternity colonies is uncommon. In fact, of 7 maternity roosts documented for
this species in East Texas, only one occurs in a natural tree roost (Mirowsky et al.
2004). This roost was located in a large (124 cm DBH, 21 m tall) Black Gum tree at
LSNWR in Wood County and has contained at least 30 bats during previous surveys
by Texas Parks and Wildlife (Meg Goodman, unpubl. data). The remainder were
in anthropogenic structures, primarily abandoned buildings, with several colonies
containing >40 individuals. If the availability of suitable natural roosts is an important
limiting factor in abundance of Rafinesque’s Big-eared Bats, the lack of trees
with cavities in the optimal size range may be limiting abundance in this region.
Abundance of forest bats is difficult to estimate (Weller 2007), and we do not have
reliable estimates of abundance for either target species. In bat surveys conducted
over the same study areas, we found that Rafinesque’s Big-eared Bats were widely
distributed but never abundant compared to other species (Stuem ke 2011).
In contrast to Rafinesque’s Big-eared Bats, we located four apparent maternity
roosts for Southeastern Myotis in natural tree roosts. One maternity roost located in
a structure (concrete bridge), was opportunistically located by communication with
personnel from the Big Thicket National Preserve (Mona Halverson, pers. comm.).
These colonies contained 50–150 bats, and the tree roosts occurred in trees ranging
from 71–86 cm DBH. Although direct comparisons are relatively rare, Southeastern
Myotis generally use smaller cavities than Rafinesque’s Big-eared Bats (Carver
and Ashley 2008, Rice 2009). We did not find differences between tree size used
by the two species, but this may reflect the lack of trees in the preferred size range
for Rafinesque’s Big-eared Bats or the presence of more large maternity colonies
in the Southeastern Myotis tree roosts. If Southeastern Myotis readily use smaller
natural tree-roosts, this may help explain why it was among the most common species
detected in bat surveys at these study sites (Stuemke 201 1).
Although we did not survey comprehensively across the landscape of East Texas,
our study areas were located throughout the region. Furthermore, the areas we
surveyed included several of the highest quality remaining bottomland hardwood
and forested wetland sites in the region. Therefore, it is likely that the relative
scarcity of large, hollow trees is common across the region. The reasons for this
are unclear and may be both natural and anthropogenic. The East Texas Pineywoods
are at the western extent of the range for both preferred roost tree species (Water
Tupelo and Black Gum) and both bat species. The extensive forest swamps of the
Mississippi River valley and large river systems in the eastern Gulf Coastal Plain
may never have been present in Texas (Conner and Buford 1998). Furthermore,
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2014 Vol. 13, Special Issue 5
logging of East Texas cypress and tupelo swamps may have occurred later than
locations farther east (Brandt and Ewel 1989, Diggs et al. 2006). More recently,
hurricanes Rita (in 2005) and Ike (in 2008) severely impacted the remaining stands
of mature bottomland forest in southeastern Texas. In 2006, we documented at
least three Southeastern Myotis maternity roosts and one Rafinesque’s Big-eared
Bat roost at Village Creek State Park near Beaumont, TX (see Fig. 1) that were
either knocked over completely or broken off in Hurricane Rita (C.E. Comer, pers.
observ.). None of these trees has been occupied by the target species since. Historic
range and abundance for rare forest bats are unknown, so it is difficult to determine
the impact of hurricanes on these species.
Long-term management goals for Rafinesque’s Big-eared Bat and Southeastern
Myotis in Texas should include preservation of known roosts and areas of matureovermature
bottomland cypress-tupelo swamp. Additionally, preserving younger
stands and allowing them to reach older age classes will potentially improve habitat
conditions for these bats. Apparently, anthropogenic roost structures (including artificial
towers constructed specifically for Rafinesque’s Big-eared Bats) are playing
an important role as maternity roost sites in the region (Mirowsky et al. 2004), but
the implications for long-term population health are unknown. Particularly in areas
with few large tupelo trees, structures known to support maternity colonies should
be preserved.
Acknowledgments
We thank the Texas Parks and Wildlife Department for providing the primary funding
for our study. Additional funding came from the US Department of Agriculture’s McIntire-
Stennis Program and from the Arthur Temple College of Forestry and Agriculture, Stephen
F. Austin State University. Thanks go to the staff and constituents of Little Sandy National
Wildlife Refuge/Little Sandy Fish and Hunt Club, Caddo Lake National Wildlife Refuge,
Caddo Lake Wildlife Management Area, Big Thicket National Preserve, Trinity River
National Wildlife Refuge, The Nature Conservancy’s Roy E. Larsen Sandylands Unit, and
Village Creek State Park for allowing us access to their properties and for providing field
lodging. Special thanks go to G. Winans, J. Deatherage, and K. Hammond for their assistance
in the field.
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