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2010 SOUTHEASTERN NATURALIST 9(1):35–46
Salamander Use of Karst Sinkholes in Montgomery
Karen E. Francl1,*, Clayton R. Faidley1, and Christine J. Small1
Abstract - To better understand salamander-habitat relationships in karst sinkholes,
we surveyed 25 sinkholes and three upland control sites at the Selu Conservancy
(Montgomery County, VA) in the spring and summer of 2008. At each site, we
measured 26 habitat parameters (e.g., number/decomposition stage of downed
logs and snags, soil moisture, soil temperature, canopy coverage, leaf-litter depth).
With these data, we questioned if sinkholes supported higher salamander densities
than non-sinkhole habitats, if salamander densities changed throughout the spring
and summer months in response to changes in soil temperature and soil moisture, and
which microhabitat measures were most strongly correlated with average salamander
densities. In seven rounds of surveys, we captured 292 Plethodon cinereus (Eastern
Red-backed Salamanders) and 223 Plethodon glutinosus (Northern Slimy Salamanders).
Although we found that capture success did vary among rounds, we found no
significant differences in salamander densities between sinkholes and upland sites.
We discovered a weak positive relationship between total captures per round and
percent soil moisture. Non-metric multidimensional scaling ordination suggested
that capture success for both species was markedly lower in sinkholes in and surrounded
by early successional habitats than those within a forest matrix. Indirect
indicators of soil fertility (e.g., percent organic matter, bryophyte cover, litter depth)
were correlated to salamander capture success. Our study serves as a springboard for
an ongoing project that examines patterns in salamander genetic diversity across a
wider range of sinkholes with varying historical land-use patterns.
Karst sinkholes in southwestern Virginia are small (typically <1 ha)
depressions in the landscape resulting from the gradual dissolution of
subsurface limestone or other water-soluble carbonate bedrock material
(Kastning 2003, Woodward and Hoffman 1991). The rate of such dissolution
depends greatly on natural (water table levels, rainfall) and anthropogenic
(various types of human disturbance) factors. Oftentimes, sinkholes are the
avenue by which surface water pools and then seeps into the groundwater
table (Hubbard 2007).
Kastning (2003) mapped and described the topography and geology of 41
karst sinkholes located on Radford University’s Selu Conservancy in Montgomery
County, VA (156 ha; Fig. 1). Her work emphasized the importance
of these habitats on a regional basis and the stresses they encounter due
to their placement on “weak zones,” which make them highly susceptible to
groundwater pollution and surface instability (Kastning 2003).
1Biology Department, Radford University, Radford, VA 24142. *Corresponding author
36 Southeastern Naturalist Vol. 9, No. 1
Although these karst sinkholes on the Selu Conservancy have been thoroughly
described from a topographic and geologic perspective (Kastning
2003), little is known about the biotic communities of these unique habitats.
Furthermore, as the forested sections of this tract (ca. 120 ha) are relatively
protected (restricted public access, hiking trails maintained to limit erosion;
Kastning 2003), biotic surveys of these habitats will provide important baseline
data for future research investigating salamander use of regional sinkholes
subject to differing anthropogenic land-use histories. Given their life-history
characteristics and suspected habitat preferences from preliminary upland forest
and forest sinkhole surveys in 2007 (K. Francl, unpubl. data), we deemed
salamanders as an appropriate choice for assessing the ecological value of
these isolated landscape features (karst sinkholes). Two species captured in
sinkholes in 2007— Plethodon cinereus Green (Eastern Red-backed Salamander)
and the Plethodon glutinosus Green (Northern Slimy Salamander)—were
the focus of this investigation. These two species are capable of traversing
forested landscapes and moving among sinkholes, yet their small home-range
size (both average <15 m2; Kleeberger and Werner 1982, Merchant 1972)
shows them capable of living exclusively within even the smallest of recognized
sinkhole habitats at the Conservancy (34 m2; Table 1).
We questioned whether biotic or abiotic factors (or a combination of
the two) were attracting Red-backed and Slimy Salamanders to these sinkhole
habitats. Previous studies in southwestern Virginia (Giles County)
in non-sinkhole upland forest habitats found that both Slimy and Redbacked
Salamander abundances were positively related to increased cover
(rocks, downed logs) and increased soil moisture (Grover 1998, Marsh and
Figure 1. (A) Aerial photograph (2002; Aerial Imagery, Commonwealth of Virginia)
showing field and forest habitats and locations of survey sites within the boundaries
of the Selu Conservancy, Montgomery County, VA. (B) Topographic map (1-m contours;
Kastning 2003) showing locations of sinkhole (circles) and upland (triangles)
survey sites within the boundaries of the Conservancy.
2010 K.E. Francl, C.R. Faidley, and C.J. Small 37
Beckman 2004). Higher soil temperature also was linked to greater captures
(ergo, inferred abundances) of Red-backed Salamanders (Grover 1996).
From these recognized salamander preferences, we sought to determine:
1) if sinkholes supported higher salamander densities than non-sinkhole
habitats 2) if salamander densities changed throughout the spring and summer
months in response to changes in soil temperature and soil moisture and
3) which microhabitat measures were most strongly correlated with average
salamander densities. We expected higher salamander densities (restricted
to surface activity as estimated through capture success) in sinkhole sites
as compared to non-sinkhole sites, given that sinkholes pool water and that
soil moisture would therefore be greater at these sites. We expected both
species to respond positively to increases in soil temperature and soil moisture.
Finally, we expected that habitat cover availability (e.g., percent cover
of rocks, snag, and log density) would be positively related to salamander
Table 1. Sinkhole (n = 25) and non-sinkhole upland (n = 3) sites surveyed for salamanders in
spring and summer 2008 at Radford University’s Selu Conservancy (Montgomery County,
VA). UTM (NAD83, Zone 16N) northing and easting values listed for each site. Also listed
is area of sinkhole, plus number of minutes each site was surveyed per round (based on area
UTM Northing UTM Easting Area (m2) Minutes surveyed
Baby 539742 4104904 34 1.6
Big Field 539707 4104752 1732 83.1
Brambles 539531 4104717 1658 79.6
East Ridgecrest 539748 4104897 74 3.5
Elbow 539717 4104928 326 15.7
Entrance 539888 4104570 1276 61.3
Fallen Logs 539575 4104785 652 31.3
Fenceline 539911 4104543 123 5.9
Frog Pond 539761 4104885 704 33.8
Hole-in-the-Wall 539642 4105034 534 25.6
Lower Valley 539560 4104919 1135 54.5
Marshy 539803 4104845 1073 51.5
North Loop 539717 4104884 972 46.7
North Ridgecrest 539569 4104990 1486 71.3
Oak Leaf 539607 4104985 1567 75.2
Quad-North 539633 4104888 1125 54.0
Quad-South 539618 4104855 1463 70.2
Rockpile Burn Field 539755 4104696 316 15.2
South Loop 539734 4104855 241 11.6
Subtle 539530 4104963 733 35.2
Tiny 539694 4104939 39 1.9
Upper Valley 539418 4105031 1276 61.2
Valley Train 539438 4104631 789 37.9
Valley Train 2 539475 4104587 261 12.5
West Field 539655 4104734 1047 50.3
Upland 1 539760 4104785 900 43.2
Upland 2 539500 4104850 900 43.2
Upland 3 539450 4105050 900 43.2
38 Southeastern Naturalist Vol. 9, No. 1
Radford University’s Selu Conservancy tract is a 156-ha natural area in
Montgomery County, VA (Fig. 1). Historically, much of this site was cleared
for agriculture through the 1960s (Leftwich 1992). Today, much of the eastern
half of the Conservancy (the focus of this study) lies in 50–60-year-old
second-growth forest, dominated by Quercus spp. (oak), Liriodendron tulipifera
L. (Tulip Poplar), Fraxinus americana L. (White Ash), and hickories
(esp. Carya ovata [Mill.] K. Koch [Shagbark Hickory]and C. alba [L.] Nutt.
ex Ell. [Mockernut Hickory]). About 4 ha are maintained through prescribed
burning or mowing as early successional native grassland; three sinkholes
occur within this area (Fig. 1).
We selected 25 sinkholes (size range = 34–1732 m2) mapped by Kastning
(2003) (Table 1). Three upland sites (area = 900 m2, the average size of the
surveyed sinkholes) also were selected—one in the fire-maintained field, and
two in upland mature, closed-canopy forests.
We surveyed each of the 25 sinkholes seven times from February to July,
2008, at approximately 3-week intervals. The three upland sinkholes were
added after the completion of the first round, and were therefore surveyed
six times (March to July, 2008). We conducted these hand-capture surveys
on days in which the temperature met or exceeded those seasonal minimums
set by the Virginia Frog and Toad Calling Survey (Garrett 2002) and lacked
heavy rain. Ambient temperature was measured with a Kestrel 3000 pocket
weather meter (Nielsen Kellerman, Boothwyn, PA).
We surveyed sites for time periods based on the estimated total area (maximum
length x maximum width; calculated area of an ellipse to account for
circular and oblong-shaped sinkholes) of the sinkhole (Table 1; Francl 2005),
so that effort-per-square-meter (4.8 minutes per 100-m2) did not differ across
sites. All searches focused on the overturning of logs, rocks, and boulders, as
well as examinations of snags and leaf litter in a non-destructive manner.
All captured salamanders were identified, weighed, and measured (total
length, snout–vent length). Additionally, ca. 3 mm of each salamander’s tail
was clipped and transferred to vials containing 70% ethanol. These tails will
be utilized in on-going salamander genetics research (R. Sheehy, Radford
University, Radford, VA, pers. comm.). The clips also served as a method
to note recaptures within the same season. After measuring and clipping,
individual salamanders were released near the point of capture within the
surveyed sinkhole. All methods were approved by the Radford University
Animal Care and Use Committee (Protocol #FY08-007).
We measured 26 habitat and environmental variables at each site. Variables
measured in January/February 2008 included: sinkhole or upland survey
area (described above); the number of upright snags (diameter at breast
2010 K.E. Francl, C.R. Faidley, and C.J. Small 39
height [1.37 m; DBH] > 5 cm) and downed logs (length > 2 m, diameter > 5
cm; Francl 2005) per plot; snag and log DBH and respective stage of decomposition
(utilized averages per site; Maser et al. 1979); number of exposed
boulders (>15 cm) per sinkhole; spring (pre-leaf-out) and summer canopy
cover (using a concave spherical densiometer; measured in four cardinal
directions from center of sinkhole); average leaf-litter depth (average of 20
measures randomly placed within each sinkhole); average slope (measured in
four cardinal directions from center of sinkhole with a clinometer).
Vegetation and associated habitat variables were assessed in June and
July 2008, in sample plots ranging in size from 36–400 m2. Plot sizes were
established to most closely approximate the size of each sinkhole, with
a maximum plot size of 400 m2. Within each plot, the percentage ground
area covered by each vascular plant species was estimated, as well as total
ground, shrub layer, and canopy cover. For all woody stems reaching or exceeding
1.37 m (breast height), plot basal area and density were determined.
In addition, bryophyte, decaying wood, and leaf-litter cover were estimated.
Range pole measures in each plot were used to calculate the Levins index
of vertical diversity (Levins 1968) and total vegetation volume (TVV, Mills
et al. 1989). Five replicate measures of photosynthetically active radiation
(PAR, LI-250A quantum sensor light meter, LI-COR, Lincoln, NE) and
percent soil moisture (Kel Instruments, Inc., Wyckoff, NJ) were taken from
each plot to characterize canopy light penetration and moisture availability.
Composite soil samples from the upper 10 cm were collected from each plot
and used in laboratory determinations of pH (glass-electrode method; Oakton
Double Junction pHTestr 20, Oakton Instruments, Vernon Hills, IL) and
percent organic matter (dry ash method; Shepard et al. 1993).
Furthermore, each time we surveyed for salamanders, we measured soil
temperature and percent soil moisture at five random points within the sinkhole.
These five measures were averaged and compared to round-by-round
Trends across rounds were examined with a repeated measures ANOVA
to determine if captures varied by round and between sinkhole and nonsinkhole
sites; however, because non-sinkhole sites were not surveyed
in round 1, only rounds 2 through 7 were statistically analyzed. We then
utilized a repeated measures mixed regression to examine additional trends
in all seven rounds (SAS 9.1, SAS Institute, Cary, NC). We utilized soil
moisture, soil temperature, and the interaction between the two to examine
site-by-site trends between these measures and the average number of salamanders
captured per square meter searched.
Non-metric multidimensional scaling (NMS) ordination was used to examine
salamander capture success relative to stand and site characteristics
across sample plots. NMS is an indirect ordination technique, assessing relationships
among sample plots based on species composition and abundance.
This ordination method differs from other commonly used techniques (e.g.,
40 Southeastern Naturalist Vol. 9, No. 1
DCA, PCA) in being non-parametric and iterative, using ranked distances
to arrange sample plots along a number of axes determined by a minimal
stress configuration. NMS has been shown to perform well with ecological
data that tend to be non-normal and contain numerous zero entries (Minchin
1987). Mean number of salamander captures by species and total salamander
captures at each of the 28 sample sites were analyzed using the global
NMS procedure performed by PC-ORD for Windows (version 5.14; MjM
Software, Gleneden Beach, OR). Following ordination analysis, stand and
environmental variables were correlated with NMS axis scores to identify
habitat variables most strongly infl uencing salamander capture success
(Pearson product-moment correlations, PC-ORD for Windows).
Across seven rounds of surveys, we captured 515 salamanders: 223 Slimy
Salamanders from 22 sites, and 292 Red-backed Salamanders from 23 sites.
In 26 of the 28 sites, we captured at least one salamander during the survey
period. The two remaining sites (East Ridgecrest [a forested sinkhole] and
Upland 1 [a non-sinkhole grassland control]) had no salamander captures.
Salamander capture success (hereafter, defined as salamanders captured
per m2) per round varied significantly among rounds 2–7 (F = 7.786, df =
5, P < 0.001), but the difference in capture success between sinkholes and
non-sinkholes was not significant (F = 2.238, df = 1, P = 0.147) (Fig. 2).
When examining the temporal infl uence of soil moisture and soil temperature
on the salamander capture success, we found mixed results. When
examined separately, neither salamander species showed any significant
Figure 2. Average number of salamander captures per hectare per round (± standard
error) for 25 sinkhole (dark grey) and 3 non-sinkhole (light grey) sites (non-sinkholes
not surveyed in round 1). Statistical analyses revealed significant differences among
rounds 2–7 (F = 7.786, df = 5, P < 0.001), but not between sinkholes and non-sinkholes
(F = 2.238, df = 1, P = 0.147).
2010 K.E. Francl, C.R. Faidley, and C.J. Small 41
relationship between capture success and soil temperature or moisture
(Table 2). However, when examining both salamanders collectively, we
found a weak (P = 0.051) relationship between soil moisture and salamander
captures (Table 2).
The NMS ordination was best fit by a two-axis solution, as determined by
a Monte Carlo randomization test (P < 0.05) and NMS scree plot. The first two
NMS axes accounted for 95.1% of the variability in the data (axis 1 = 87.5%,
axis 2 = 7.5%; final stress = 2.38; final instability less than 0.001; 82 iterations). To accommodate
statistical assumptions of ordination analysis, the two survey sites
lacking salamander captures were excluded from the NMS ordination.
Slimy and Red-backed Salamanders showed similar habitat preferences,
based on NMS ordination results. Capture success for both species showed
strong positive correlations with NMS axis 1 (Red-backed Salamanders, r =
0.75, P. glutinosus, r = 0.67) and negative correlations with axis 2 (Slimy
Salamanders, r = -0.44, Slimy Salamanders, r = -0.90). Thus, the greatest
numbers of captures per square meter occurred in survey sites positioned
high on NMS axis 1 and low on axis 2 (Table 3, Fig. 3). Both salamander
species showed markedly higher numbers of captures in forested sinkholes
than in early successional (field) sinkholes or upland sites (Fig. 3). Note that
abundance (mean number of captures per round) of Slimy Salamanders was
similar across nearly all forested sinkholes, whereas trends for Red-backed
Salamanders were infl uenced by greater abundance in a single forested sinkhole
site (Fig. 3).
Following ordination analysis, habitat variables were correlated with
NMS axis scores to identify the variables most strongly infl uencing
salamander capture success (Table 3). The variables correlated with the
greater capture successes found in forested sinkholes (i.e., positively correlated
with axis 1 and negatively correlated with axis 2) were leaf-litter
depth, bryophyte cover, tree canopy cover, soil organic matter, and number
of standing snags (Table 3, Fig. 4). Surveys in upland controls and field sites
Table 2. Results from repeated measures mixed regressions, examining salamander capture
success (for each species separately, plus all captures combined) as the dependent variable and
soil temperature and moisture (and their interaction) as predictive variables.
Num df Den df F P
Eastern Red-backed Salamander
Soil temperature 1 161 0.16 0.694
Soil moisture 1 161 1.92 0.168
Soil temperature * soil moisture 1 161 0.90 0.344
Northern Slimy Salamander
Soil temperature 1 161 0.57 0.453
Soil moisture 1 161 2.12 0.147
Soil temperature * soil moisture 1 161 1.27 0.261
Soil temperature 1 161 0.56 0.456
Soil moisture 1 161 3.87 0.051
Soil temperature * soil moisture 1 161 2.02 0.158
42 Southeastern Naturalist Vol. 9, No. 1
had poor capture success. These low numbers of salamander capture success
were associated with greater TVV, higher sinkhole area, and higher spring
canopy cover (i.e., fields and thickets with tall grasses and shrubs persisting
in late winter and early spring; Table 3, Fig. 4).
Our results indicated that salamander populations in sinkholes did not
markedly differ from our three upland control sites. We attribute this trend
to the immense variability in sinkhole microhabitat features, combined
with a lack of captures at one of our three upland control sites (that resulted
in our removing it from some analyses). The remaining upland sites were
both forested habitats with similar microhabitat features as other forested
However, the lack of salamanders in a field habitat lends credence to
our assumption that landscape-level habitat designations (whether the
Table 3. Pearson product moment correlations between NMS axis scores for each sample site
and salamander capture success or quantitative habitat variables.
Variable r Axis 1 r Axis 2
Salamander capture success
Total salamander 0.84 -0.66
Plethodon cinereus 0.75 -0.44
Plethodon glutinosus 0.67 -0.90
Total vegetation volume (TVV) -0.55 0.63
Sinkhole area (m2) -0.45 0.57
Spring canopy cover (%) -0.28 0.43
Boulder (no./sinkhole) -0.24 0.02
Photosynthetically active radiation (μmol/sec/m2) -0.19 0.19
Decaying log cover (%) -0.18 0.24
Boulder cover (%) -0.13 -0.29
Logs (no./ m2) -0.12 -0.29
Snags (no./ m2) 0.04 -0.40
Total ground layer cover (%) 0.11 0.31
Rock cover (%) 0.13 -0.36
Summer canopy cover (%) 0.13 -0.26
Levins index of vertical diversity -0.08 0.18
Total shrub cover (%) 0.03 -0.10
Log diameter (cm; average) 0.05 -0.02
Snag decay stage (average) 0.17 -0.18
Snag diameter (cm; average) 0.19 -0.20
Soil pH 0.23 -0.11
Leaf litter cover (%) 0.25 0.07
Soil moisture (%) 0.25 -0.01
Log decay stage (average) 0.25 -0.28
Slope angle (%) 0.31 -0.49
Bryophyte cover (%) 0.37 -0.47
Tree canopy cover (%) 0.41 -0.42
Soil organic matter (%) 0.44 -0.37
Leaf-litter depth (cm; average) 0.51 -0.71
2010 K.E. Francl, C.R. Faidley, and C.J. Small 43
sinkhole was surrounded by mature forest versus an open-canopied early
successional field) was a strong driver in predicting salamander densities.
Our NMS ordination results confirmed these predictions for salamander
habitat preferences. Specifically, salamander capture success was greater
in habitats with increased soil moisture and moisture-enhancing parameters
Figure 3. Non-metric multidimensional scaling (NMS) ordination of mean salamander
capture success for (A) Northern Slimy Salamanders and (B) Eastern Red-backed
Salamders at 26 survey sites at the Selu Conservancy. Symbol size indicates relative
capture success at each site. Open circles represent grassland sinkholes; closed
circles represent forest sinkholes. Asterisks represent upland control sites (See Fig. 4 for
44 Southeastern Naturalist Vol. 9, No. 1
(e.g., tree canopy, bryophyte cover, soil organic matter) and increased
habitat cover (e.g., leaf litter, standing snags)—habitat variables indicative
of later successional, forested habitats. Given that sites surrounded by
or partly defined as an early successional habitat (e.g., fields and thickets)
consistently captured fewer salamanders, we believe that isolation from a
continuous mature forest may inhibit Red-backed and Slimy Salamander
habitation. Future studies incorporating additional upland control sites and
field sites at known distances to contiguous forest tracts may help to better
explain these findings.
We found that salamander capture success did change across rounds,
as success markedly declined as the summer temperatures increased. This
trend is typical in salamander research, as salamanders tend to burrow
deeper into the soil as ambient temperatures rise (Heatwole 1962). However,
we did not find that these changes in capture success were directly
in line with soil moisture or soil temperature on a species-specific basis.
Our assumed lack of dependence on soil moisture (only significant when
both species’ capture rates were examined jointly) contradicts a number of
studies that emphasize the importance of this habitat measure (e.g., Grover
1998, Heatwole 1962, Marsh and Beckman 2004). It is possible that
the range in soil moisture—and, indeed soil temperature, as well—simply
remained within a range of tolerance for both species, so that obvious species-
specific responses could not be detected.
Figure 4. Non-metric multidimensional scaling (NMS) ordination of mean salamander
capture success in 26 survey sites at the Selu Conservancy, Montgomery County, VA.
Vectors indicate the strength and direction of habitat variable correlations with NMS
ordination axes. Longer vectors indicate stronger correlations; correlation reaches its
maximum in the direction shown. Open circles represent grassland sinkholes. Closed
circles represent forest sinkholes. Asterisks represent upland control sites.
2010 K.E. Francl, C.R. Faidley, and C.J. Small 45
Our negative relationship between salamander capture success and total
vegetation volume contradicts previous research, as well. Indeed, Heatwole
(1962) found that the amount of vegetation positively influenced salamander
abundances—opposite of our findings. However, we believe that our
TVV metric reflected grassy sites surrounded by early successional areas—
perhaps an indirect indicator of sinkhole isolation rather than a direct
reflection of capture success. This isolation from contiguous forest tracts
may feed into research on edge effects by Marsh and Beckman (2004).
Their study examined the effects of Red-backed and Slimy Salamander
densities in relation to distance to forest (gravel) roads—roads similar to
those found at the Selu Conservancy. Marsh and Beckman (2004) found
differences in moisture regime and capture success at distances from roads
of 20–80 m into the forest. Given that the majority of our sites are within
80 m of a forest edge (meeting roads or early successional fields; Fig. 1),
the edge effect may be playing a strong role in our capture success and the
trends we discovered.
Our results suggest that the immediate surrounding landscape (e.g.,
sinkholes in a forest matrix vs. sinkholes isolated in a field matrix) affects
salamander capture success. Secondarily, microhabitat measures appear to
drive relative densities. Because of the variability in microhabitat measures
across sinkhole sites, it appears that forested sinkholes provide favored but
not unique habitats for these two species of salamanders. However, we emphasize
that understanding sinkholes’ full value to salamanders is a work in
progress, as we are examining trends as a snapshot in time.
To better understand how sinkholes are ecologically valuable, we plan
to build on this project, examining the effect(s) of historical land use on
salamander populations. Through our collection of over 400 salamander
tail samples from this study, genetic analyses may help us to understand
if these sinkholes served as refugia for forested salamanders in an historically
agriculturally dominated landscape (Leftwich 1992). As we expand
our study to sinkholes beyond the Selu Conservancy borders, we plan to
include sinkholes with differing past and present uses and varying lengths
of time since being part of a contiguous forest habitat. With these continued
surveys, we’ll determine if these sinkholes serve as sources of genetic
diversity on a larger scale.
We are grateful to the Virginia Herpetological Society for funding a portion of
this work. The Radford University Biology Department and the Office of Sponsored
Programs and Grant Management also provided funding. More than 10 Radford
University Biology undergraduate students assisted with salamander surveys and
vegetative field work.
46 Southeastern Naturalist Vol. 9, No. 1
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