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2012 SOUTHEASTERN NATURALIST 11(1):49–58
Light Bait Improves Capture Success of Aquatic
Funnel-Trap Sampling for Larval Amphibians
Stephen H. Bennett1,*, Jayme L. Waldron2, and Shane M. Welch2
Abstract - Aquatic funnel traps are a non-destructive means of surveying amphibians in
lentic habitats, particularly as compared to dip-net surveys that disturb aquatic vegetation
and the substrate, and affect the water column through increased turbidity. The objective
of this study was to examine the utility of glow stick-baited aquatic funnel traps
for larval amphibians, with a particular emphasis on ambystomatid larvae. We sampled
12 isolated ponds in the Mid-Atlantic Coastal Plain of South Carolina between April
and June 2010 and used detection/non-detection capture data to model the probability
of capturing larval amphibians in baited and un-baited funnel traps. Further, we used
count data (captures per trap) to examine whether glow stick-baited traps captured more
amphibian larvae than un-baited traps. We captured four Ambystoma species (A. mabeei,
A. opacum, A. talpoideum, and A. tigrinum) and tadpoles from the families Bufonidae,
Ranidae, and Hylidae in light-baited funnel traps. Captures of both Ambystoma larvae
and tadpoles were positively associated with light-baited traps, and we were 8.8 times
more likely to capture Ambystoma larvae and 5.7 times more likely to capture tadpoles
in glow stick-baited traps as compared to un-baited traps. Our results indicate that glow
sticks can greatly improve capture success of larval amphibians in funnel traps, and we
recommend their use as an active sampling method that is unbiased by surveyor experience
Surveys for new populations of protected Ambystoma species, as well as
effective monitoring of known populations and their breeding habitats, are
critically important conservation needs (Bevelheimer et al. 2008, Curtis and Paton
2010, Dodd and Dorazio 2004, Pechmann et al. 1991). The fossorial habits
of adult Ambystoma make both surveying and monitoring of adult populations
difficult (Bevelheimer et al. 2008, Palis 1997a ). Standard ambystomatid survey
and monitoring techniques for terrestrial habitats include visual searches, drift
fences with pitfall traps, and cover-board arrays; however, these methods are
only effective at capturing adults or newly metamorphosed salamanders during
migrations (Palis 1997a, Shaffer et al. 1994, Skelly and Richardson 2010).
Drift fences with pitfall traps can be costly to construct and require constant
monitoring to avoid salamander mortality (Bishop et al. 2006, Palis 1997b).
Cover-board arrays do not require constant monitoring, but are typically not as
effective at capturing ambystomatids (Houze and Chandler 2002) as other methods.
These techniques tend to be better suited for use in long-term monitoring
1South Carolina Department of Natural Resources, 100 Assembly Street, Columbia, SC
29201. 2Biological Sciences, 706 Coker Life Sciences, University of South Carolina,
Columbia, SC 29208 *Corresponding author - firstname.lastname@example.org.
50 Southeastern Naturalist Vol. 11, No. 1
of plethodontid salamanders, but neither is well suited for large-scale, shortterm
surveys to document Ambystoma distributions.
The genus Ambystoma contains 32 species, nine of which occur in, with
three limited to, the Southeastern United States: A. bishopi Goin (Reticulated
Flatwoods Salamander), A. cingulatum Cope (Frosted Flatwoods Salamander),
A. jeffersonianum Green (Jefferson Salamander), A. mabeei Bishop (Mabee’s
Salamander), A. maculatumn Shaw (Spotted Salamander), A. opacum Gravenhorst
(Marbled Salamander), A. talpoideum Holbrook (Mole Salamander),
A. texanum Matthes (Smallmouth Salamander), and A. tigrinum Green (Tiger
Salamander). Several of these species are afforded some federal and state protection,
or a measure of conservation concern within or throughout their range
(Table 1). All Ambystoma species occurring in the Southeast share certain
life-history traits to varying degrees. Adults are typically fossorial, inhabiting
uplands ranging from mesic mixed hardwood forests to xeric pine-dominated
sandhills (Martof 1968, Means 1996, Palis 1996, Petranka 1998, Semlitsch and
Wilbur 1988, Shoop 1960). Adult salamanders migrate to breeding sites from
October through March, depending upon the species. Breeding sites include a
variety of wetlands, ranging from ephemeral ponds to relatively permanent wetlands
(Anderson and Williamson 1973, Anderson et al. 1971, Walls and Altig
1986). Typically, these salamanders select breeding sites that are fish-free wetlands,
despite the range of wetland types (Gibbons and Semlitsch 1991, Hardy
and Anderson 1970, Petranka 1998).
Successful short-term surveys for species distributions can be accomplished
when temporal and spatial patterns of ambystomatid life history are considered.
Ambystomatid larvae are fully aquatic, spending several months—or longer for
paedomorphic salamanders—in breeding ponds prior to metamorphosis. Thus,
breeding ponds contain high relative abundances of these species, making this
aquatic stage of their life cycle well suited for surveys that attempt to establish
species presence. This consideration is important given that adults are largely
Table 1. Federal and state protection status of Ambystoma species that occur in the southeastern
Coastal Plain. States offering conservation status are indicated in parentheses. R = species is listed
throughout its geographic range, T = threatened, E = endangered, NR = not ranked, NSR = Not ranked
in southeastern United States, and SPC = species of special concern or in need of management.
Species Federal State Source
A. bishopi E(R) E (FL, GA) FFWCC 2010, Jenson and
Stevenson 2009, USFWS 2009
A. cingulatum T(R) T (AL, FL, GA) FFWCC 2010, Jenson and
Stevenson 2009, USFWS 2009
A. mabeei NR T (VA) Mitchell and Reay 1999
A. maculatum NR NSR
A. opacum NR NSR Levell 1997
A. talpoideum NR SPC (NC, TN) Lannoo 2005
A. tigrinum NR E (DE, MD); SPC (NC, SC) Lannoo 2005
2012 S.H. Bennett, J.L. Waldron, and S.M. Welch 51
fossorial and difficult to detect. Common aquatic sampling methods include dipnet
sweep samples, pull-seine nets, and aquatic funnel traps (Adams et al.1998,
Bishop et al. 2006, Ghioca and Smith 2007, Wilson and Pearman 2000).
In recent years, considerable effort has been directed toward surveying and
monitoring of the federally threatened Flatwoods Salamander (A. cingulatum)
and the federally endangered Reticulated Flatwoods Salamander (A. bishopii).
Aquatic sampling of Flatwoods Salamander breeding sites is an accepted method
to document presence (Bishop et al. 2006, Palis 1996). Because Ambystoma larvae
are nocturnal feeders and retreat to dense vegetation during the day, active
diurnal surveys that use nets and seines may have limited success. This issue is
further compounded when larvae occur at low densities (Bishop et al. 2006, Palis
1997a, Sekerak et al. 1996), requiring intensive dip-net sampling to maximize
Bevelheimer et al. (2008) determined that dip-net sampling was more effective
at capturing Flatwoods Salamander larvae than un-baited aquatic funnel
traps. When standardized by time, dip-net sampling was 5–10 times more effective
than passive traps (Bevelheimer et al. 2008). Despite the relative success of
the technique, intensive dip netting can disturb aquatic vegetation, substrate, and
affect the water column through increased turbidity (S.H. Bennett, pers. observ.).
Bishop et al. (2006) determined that sampling efforts for Flatwoods Salamander
larvae required forty-five 1-m net sweeps for each larva captured. Thus, surveys
using 0.5-m-wide nets could be expected to disturb approximately 28 m2 of pond
habitat per larva collected. The actual amount and severity of disturbance depends
on an individual’s sampling technique and the degree to which sampling
is temporally replicated. At best, dip-net surveys will result in minimal damage
along sampling transects, but more severe, localized damage may result from
multiple sweeps in areas with dense vegetation. The negative effects of localized
habitat damage are likely two-fold. First, damage to habitats used for foraging
and cover could affect survival. Secondly, increased turbidity potentially affects
detection probability, making it problematic to assess occupancy and abundance
from monitoring data. Thus, it is important that biologists use effective sampling
techniques that maximize capture probability while minimizing habitat damage
due to surveys.
Grayson and Roe (2007) suggested that photochemical glow sticks (light
sticks) provide an effective bait for salamander larvae. These authors compared
adult and larval amphibian capture success between glow-stick baited and unbaited
aquatic funnel traps at one pond and determined that glow sticks were
effective at increasing the capture rate of aquatic amphibians, primarily adult Notophthalmus
viridescens Rafinesque (Eastern Newt) and larval Rana catesbeiana
Shaw (Bullfrog). The objective of this study was to further examine the utility
of glow-stick baited aquatic funnel traps for amphibian surveys, with particular
emphasis on aquatic ambystomatid larvae. This information is important given
the conservation needs of several ambystomatid salamanders.
52 Southeastern Naturalist Vol. 11, No. 1
We sampled 12 isolated ponds in the South Carolina Coastal Plain in Jasper,
Hampton, Bamberg, Orangeburg, and Dorchester counties. All ponds were isolated
temporary wetlands, but encompassed a range of morphologies, origins,
and past disturbances. Ponds ranged from approximately 0.1 to 18.6 ha and
supported aquatic habitats ranging from open-canopied, grass-sedge dominated
ponds to closed-canopied ponds characterized by Taxodium ascendens L. (Pond
Cypress) and Nyssa sylvatica Marshall (Black Gum). Study ponds were surrounded
by various terrestrial habitats, including pine plantations, pine-savanna
woodlands, and agricultural areas (e.g., food plots maintained on wildlife management
We used aquatic funnel traps (commercially available as minnow traps)
with 0.48-cm2 white-plastic mesh to assess the effectiveness of glow-stick
baiting for nocturnal sampling of amphibian larvae in ponds. Our sampling
protocol was intended to 1) replicate expert-biased sampling used to survey
ambystomatid larvae by focusing on aquatic habitats likely to be targeted by
dip-net surveys, 2) provide for the safety of trapped animals and their habitats,
and 3) ensure equivalent application of glow stick-baited and un-baited
traps among pond habitats. Specifically, we deployed an even number of
traps per pond (range = 10–24) to ensure a balanced sampling design, i.e.,
each pond was sampled with an equal number of baited and un-baited traps.
Traps were deployed in a linear manner and placed at approximate 5-m intervals.
We deployed more traps in larger ponds and in ponds with more diverse
habitat structure (e.g., open water, grass-sedge, shrub-thicket). We alternately
baited traps with 15.2-cm green glow sticks (premium glow stick; Windy City
Novelties, Vernon Hills, IL), which ensured independence among baited and
un-baited traps in each pond, i.e., baited traps were separated by approximately
10 m. We placed traps parallel to the pond edge in water depths ranging
from 15–23 cm, which ensured that trap entrances were submerged while
allowing a portion of the trap to protrude above the water surface. This trap
position provided captured animals with access to air. Glow sticks were estimated
to function for eight hours, but we commonly observed illumination
twelve hours after activation. We deployed traps for one night at each pond
between April and June 2010; all traps were deployed in the late afternoon and
were retrieved the following morning. We recorded the number of amphibian
larvae captured per trap and identified all Ambystoma larvae to species. We
retained up to two individuals of each species from each pond for vouchers
and released the remaining larvae at the point of capture.
We used traps as sampling units and trap type (i.e., glow stick versus no
glow stick) as the predictor variable in two separate analyses of capture data.
First, we examined capture success using conditional logistic regression. We
2012 S.H. Bennett, J.L. Waldron, and S.M. Welch 53
used a binary response (i.e., 1 = capture, 0 = no capture) to model the probability
of capturing larval amphibians in baited and un-baited funnel traps. We
stratified data by pond to account for lack of independence among observations
from the same pond and to control for differences among ponds, e.g., species
composition and amphibian diversity. Thus, conditional logistic regression allowed
us to model capture probability while accounting for variation in larval
amphibian assemblages. Secondly, we used negative binomial regression to analyze
count data of captures per trap. This analysis was included in addition to
logistic regression to examine whether glow stick-baited traps captured more
amphibian larvae than un-baited traps. We used the generalized estimating
equations (Liang and Zeger 1986) to analyze the pond-correlated data. In both
analyses, we ran models separately for captures of tadpoles and Ambystoma
larvae. All analyses were performed using SAS 9.2.
Our trapping efforts yielded captures of four Ambystoma species: Mabee’s
Salamander, Marbled Salamander, Mole Salamander, and Tiger Salamander.
We captured Ambystoma larvae in 57 of 82 (69%) baited traps versus 29 of
82 (35%) un-baited traps. Similarly, we captured tadpoles, including species
from the families Bufonidae, Ranidae, and Hylidae, in 72% of baited traps and
56% of un-baited traps. Conditional logistic regression models indicated that
we were more likely to capture Ambystoma larvae (β = 1.09 ± 0.23, χ2 = 22.01,
df = 1, P < 0.0001) and tadpoles (β = 0.88 ± 0.29, χ2 = 8.96, df = 1, P < 0.01)
in traps that were baited with glow sticks than in un-baited traps. Odds ratios
indicated that we were 8.83 times more likely to capture Ambystoma larvae and
5.78 times more likely to capture tadpoles in glow-stick baited traps as compared
to un-baited traps.
Negative binomial regression (df = 1, χ2 = 7.45, P < 0.01) coefficients indicated
that counts of Ambystoma larvae (β = 0.83 ± 0.19, Z = 4.41, P < 0.0001)
and tadpoles (β = 0.60 ± 0.18, Z = 3.39, P < 0.001) were positively associated
with glow stick-baited traps. On average, we captured 2.35 (SD = 3.56, range
= 0–17) Ambystoma larvae and 10.66 (SD = 18.14, range = 0–120) tadpoles in
traps that were baited with glow sticks (n = 82) versus 1.02 (SD = 2.39, range
= 0–14) and 5.85 (SD = 11.55, range = 0–58) in un-baited traps (n = 82), respectively
Aquatic funnel traps are a non-destructive means of surveying amphibians in
lentic habitats. A major advantage of funnel trapping is that capture success is
unbiased by surveyor skill level and experience (Adams et al. 1998); however,
funnel traps can provide unpredictable results that necessitate the use of many
traps to effectively survey habitats, which incurs additional time and monetary
costs (Adams et al. 1998). Because glow sticks improve funnel-trap sampling
54 Southeastern Naturalist Vol. 11, No. 1
efficiency (Grayson and Roe 2007, this study), their use likely offsets these
disadvantages by reducing the number of traps required for surveys. This study
demonstrated that glow sticks increase the probability that funnel traps capture
tadpoles and Ambystoma larvae and increase the number of individuals that are
captured per trap.
Surveys for rare amphibians must rely on methods that enhance detection
probability (MacKenzie et al. 2005). Recent comparisons (e.g., Bevelheimer et
al. 2008) between dip-net and un-baited funnel-trap surveys demonstrated that
dip netting was up to ten times more effective than funnel traps at capturing
ambystomatid salamander larvae, particularly those of the federally protected
Flatwoods Salamander. We did not incorporate dip-net surveys in this study,
but we were nearly nine times more likely to capture Ambystoma larvae in glow
stick-baited traps than in un-baited traps. Our findings suggest that the increased
capture efficiency associated with the addition of glow sticks potentially balances
comparisons of the two techniques, or at least places funnel-trap sampling within
the range of effectiveness of dip-net sampling. Thus, we suspect that surveys
for rare Ambystoma species will benefit from including glow stick-baited funnel
traps in sampling protocols.
Figure 1. Average counts of tadpoles and ambystomatid salamander larvae captured
per trap, i.e., unbaited (n = 82) and glow-stick-baited funnel traps (n = 82). Data were
analyzed using negative binomial regression.Traps were deployed in 12 upland isolated
wetlands in the South Carolina Coastal Plain, 2010.
2012 S.H. Bennett, J.L. Waldron, and S.M. Welch 55
Our ambystomatid capture success likely reflected larval activity patterns.
Most ambystomatid larvae actively feed at night and retreat to protective cover
(e.g., vegetation and leaf litter) to feed during the day (Petranka and Petranka
1980). Although un-baited funnel traps passively capture larvae, glow-stick
baited funnel traps appear to actively sample nocturnal foragers that are attracted
to light. Dip-net surveys are typically used to conduct diurnal searches for larvae
that are hiding in cover, requiring multiple sweeps through vegetation clumps
to dislodge secretive species from their cover. To date, little attention has been
given to the effects of dip-net surveys on pond microhabitat, even though extensive
dip netting visibly disrupts pond microhabitats via increased turbidity and
altered vegetation structure. Although the degree to which this activity affects
pond microhabitat has not been described, suitable breeding ponds for rare species
are lacking, and remaining habitats are vulnerable to alterations (Bennett and
Nelson 1991, Semlitsch 2003). Thus, we recommend using glow-stick baited funnel
traps in lieu of dip-net sampling for amphibian monitoring to reduce damage
to wetland habitats, particularly when surveying for rare species.
Potential biases associated with glow-stick baiting may affect the results
of trapping efforts in amphibian surveys. For example, larval fish studies have
reported that light traps target individuals based on size (Marchetti et al. 2004),
age (Hernandez and Lindquist 1999), and taxonomic group (Marchetti and Moyle
2000). Further, issues related to pond turbidity, which potentially affects the ability
of larval amphibians to see light, could make it difficult to standardize capture
data (Marchetti et al. 2004). Turbidity is influenced by factors, such as season and
water depth, that make it difficult to assess the volume of water that is sampled
by light traps (Marchetti et al. 2004). These issues should only be a concern when
density and abundance estimates are required, and should not affect efforts to
assess presence. Additional research would prove valuable in identifying biases
and quantifying their effects, providing greater accuracy in density and relative
abundance estimates for amphibians,
Glow sticks can greatly improve capture success of larval amphibians in funnel
traps, at least within the environmental contexts studied here. Although the
behavioral mechanisms are unclear, positive phototaxis functionally changes
funnel traps from passive to active amphibian samplers. Because glow stick-baited
funnel traps have many advantages and are less likely to suffer from surveyor
skill-level biases, we recommend their use as an active sampling method. A costbenefi
t comparison of dip-net sampling versus glow stick-baited funnel trapping
is required to develop fully informed sampling protocols that maximize capture
success while minimizing habitat destruction.
We thank M. Martin and W. Kalinowsky for assistance with field work. We thank
J. Cantrell, T. Rainwater, and the South Carolina Department of Natural Resources for
logistical support and housing.
56 Southeastern Naturalist Vol. 11, No. 1
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