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Using Glow Sticks to Increase Funnel Trap Capture Rates for Adult Vernal Pool Amphibians

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Herpetological Review 48(3), 2017
544 TECHNIQUES
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Using Glow Sticks to Increase Funnel Trap Capture
Rates for Adult Vernal Pool Amphibians
Amphibians are declining around the globe, and causes
include climate change, disease, over-exploitation, and
habitat loss (Kiesecker et al. 2001; Stuart et al. 2004). In the
United States, vernal pools are ephemeral wetlands that
are critical habitat to dozens of North American amphibian
species (Petranka 1998; Lannoo 2005). Given that they dry
on a regular basis, they provide prime breeding grounds free
of fish predation (Karraker and Gibbs 2009). However, vernal
pools and their surrounding habitat are particularly vulnerable
to habitat loss because they are not protected by federal law
(Semlitsch and Bodie 1998; Gibbons 2003). Even if the wetlands
themselves are preserved, many species migrate to these pools
from great distances, making them sensitive to land-use change
in surrounding upland areas (Gibbs and Shriver 2005; Harper
et al. 2008). Salamander and anuran populations have also
declined in areas like U.S. national parks despite having both
wetland and upland habitat protected (Adams et al. 2013). With
such complex drivers of population decline, many species that
require vernal pool habitat are often designated as endangered,
threatened, or species of conservation concern in the states
they occur (e.g., 50% of Ambystoma in the northeatern U.S.;
Mitchell et al. 2006). Determining and monitoring population
status has become increasingly important, and many resources
have been devoted to improving amphibian monitoring: e.g.,
the United States Geological Survey Amphibian Research and
Monitoring Initiative (armi.usgs.gov), products from Partners
in Amphibian and Reptile Conservation (Graeter et al. 2013).
However, monitoring changes in species’ populations is
dependent on effective sampling techniques; thus, improving
sampling techniques is a critical component to monitoring and
conserving species.
Many pond-breeding amphibians are highly cryptic for
most the year. Monitoring efforts have consequently focused
on the breeding season when migrating adults and egg masses
are conspicuous at vernal pool breeding grounds (Miller
and Grant 2015; Davis et al. 2017). Despite the availability of
adults for sampling, egg masses are usually surveyed as an
index of adult breeding females instead (Crouch and Paton
2000; Miller and Grant 2015, Amburgey et al. 2017). To capture
adults, established survey techniques fall broadly into two
categories: active sampling where surveyors capture animals
and passive sampling where animals encounter traps. The most
frequently used methods include drift fences with pitfall traps,
aquatic funnel traps, visual encounter surveys (VES), and dip-
net surveys (Heyer et al. 1994; Hutchens and DePerno 2009;
Willson and Gibbons 2009). Each method varies considerably
in the amount of effort and material required. Drift fences are
passive arrays usually deployed by completely encircling vernal
pools with fencing and pitfall traps to ensure a near-census
of migrating breeding adults (Dodd 1991; Crouch and Paton
2000; Gibbons et al. 2006; Grayson et al. 2011). The amount of
resources needed to install, maintain, and monitor drift fences
is high, so drift fence studies are generally restricted to one or
a few vernal pools (Dodd 1991; Gibbons et al. 2006; Grayson
et al. 2011). Active sampling methods, such as VES and dip-
net surveys, are versatile and require less effort and material
to conduct surveys; however, outcomes are often biased by
observer skill, and they frequently damage essential habitat
(Heyer et al. 1994; Grant et al. 2005; Sutherland 2006; Bennett
et al. 2012).
Unlike active surveys where encountering and capturing an
available animal is mainly dependent on the quality of surveyor,
MICHAEL ANTONISHAK
DAVID MUÑOZ*
DAVID MILLER
435 Forest Resources Building, Ecology Program,
Department of Ecosystem Science and Management,
Penn State University, University Park, Pennsylvania 16802, USA
*Corresponding author; e-mail: djm516@psu.edu
Herpetological Review 48(3), 2017
TECHNIQUES 545
passive surveys are dependent on traps being available, on
animals encountering them, and on animals being caught and
retained (Luhring et al. 2016). As amphibians move terrestrially
to wetland breeding sites, drift fence surveys ensure high
encounter rates by restricting access to the pool via the trapping
array. An alternative passive method is to capture amphibians
in the aquatic environment itself. For vernal pools, aquatic
funnel traps (Fig. 1A) have commonly been used to study
larval amphibians or adult newts (Heyer et al. 1994; Buech and
Egeland 2002; Wilson and Pearman 2010; Bennett et al. 2012).
Funnel traps offer advantages for monitoring efforts because
they allow for rapid deployment and retrieval—compared to
drift fences—and eliminate biases and damaging methods
common to active sampling methods. In this study, our goal
is to improve the efficacy of aquatic funnel traps for capturing
adult breeding amphibians. In particular, aquatic funnel traps
have much lower encounter rates than drift fences by nature of
their design. Using lures, we hope to increase encounter rates,
and thus capture rates for adult amphibians moving in and
around vernal pools during breeding season. Past studies found
that baiting traps with glow sticks increased the capture rates of
larval amphibians 2 to 8 times compared to funnel traps with
no lure (Grayson and Roe 2007; Bennett et al. 2012). No studies
have tested the effectiveness of glow stick lures on the capture
rates of adult, vernal pool-breeding amphibians. We specifically
focus on the adult life stage because life history suggests adults
play the most critical role in population persistence (Stearns
1992; Petranka 1998). Capturing adults also make techniques
like mark recapture feasible, providing important estimates
of demographic parameters like abundance and survival to
improve conservation decisions (Williams et al. 2002; Nichols
2014). Captures in aquatic funnel traps have also been shown
to linearly scale with adult amphibian population density,
suggesting captures are a potential index of adult population
size (Wilson and Pearman 2010).
Here we experimentally test the efficacy of glow stick lures
in increasing captures of Jefferson’s Salamander (Ambystoma
jeffersonianum), the Spotted Salamander (Ambystoma
maculatum), the Eastern Red-spotted Newt (Notophthalmus
viridescens), and the Wood Frog (Lithobates sylvaticus). We
predict that the glow sticks will increase captures by providing
a visual stimulus that draws adult amphibians to the traps. By
increasing the efficacy of aquatic funnel traps with a simple
glow-stick lure, aquatic funnel traps can be a more powerful
tool for monitoring efforts.
Methods
Study Site.—We monitored pools in State Game Lands 176 in
Centre County, Pennsylvania, USA (40.778715°N, 78.006278°W ),
which are managed by the Pennsylvania Game Commission.
The study area is mixed deciduous forest that contains a dense
network of vernal pools. Within this network, we surveyed
twelve pools that were part of a long-term monitoring study.
These pools were spatially organized into three clusters with
each cluster containing four pools. The pools vary in size (mean
perimeter length: 66 m ± 25 m SD, range: 40–120 m) but have
similar habitat characteristics: no aquatic vegetation, leaf-litter
bottoms, mixed deciduous upland habitat. Each pool dries mid
to late summer in most years. Surveys for this experiment were
conducted from 31 March 2015 to 9 April 2015, and traps were
continuously deployed during surveys.
Fig. 1. A) The aquatic funnel trap with float (labeled with trap name). The right trap must be adjusted to eliminate gap between trap halves.
B) Deployed aquatic funnel trap baited with glow stick treatment. C) Internal view of aquatic funnel trap with 2.54 cm trap opening by
which animals entered. Shown with adult Ambystoma jeffersonianum.
PHOTOS BY D. J. MUÑOZ
Herpetological Review 48(3), 2017
546 TECHNIQUES
Amphibian Sampling.—Unaltered aquatic funnel traps
(Frabill®, Plano Molding Company, Illinois, USA) were used to
survey for salamanders. The traps are torpedo shaped and are
constructed of black vinyl coated steel with 6.35-mm mesh (Fig
1A). They measure 420 cm long with a 19 cm diameter. They
have two 2.54-cm cone-shaped openings on either side allowing
animals to enter with limited chances of exiting (Fig 1C). A
waterproof foam float (FOAMULAR®, Owens Corning, Toledo,
Ohio, USA) placed inside the trap raised the unit partially above
water to prevent drowning of air-breathing animals. A total of
seventy-nine traps were deployed each night, and traps were
positioned every 10 m along the pool perimeter, resulting in four
to twelve traps per pool depending on size. Traps were placed
along the pool’s edge in deep enough water that everything
but the float was submerged (typically less than 1 m from
pond’s edge; Fig. 1B). Openings were oriented perpendicular
to the pond’s edge and were attached to the bank with rope
so they would not drift. Every night, half of the traps in each
pool were systematically assigned to the treatment or control,
with treatment alternating between traps. The treatment was
a 15.24 cm activated green glow stick placed inside the funnel
trap (The Glow Company Ltd., Doncaster, United Kingdom).
Because movements from upland habitats mainly occur at
night (Petranka 1998), glow sticks were activated between 1600
h and 1800 h, and all glow sticks emitted light throughout the
night until researchers checked them the next day. To reduce
individual trap bias, treatment assignments were alternated
each trap night, meaning each night there were either N = 39
or N = 40 replicates of glow stick treatment. Each trap received
approximately the same number of trap nights with glow stick
or control (either 4 for 5 nights). Traps were checked daily, and
we recorded species, sex, and trap location of each capture.
Analysis.—We analyzed our captures (count data) using
a generalized linear mixed model with a negative binomial
distribution and log link function using the “glmmADMB”
package (Fournier et al. 2012) in the statistical software R (R
Core Team 2014):
where β represents fixed effect regression coefficients estimated
by the model and γ is a random intercept for individual traps.
The fixed effects included the site (specific pool), “migration
night”, and treatment. Because capture rates are greatly
influenced by the number of amphibians that chose to migrate
on a given night, we created a “migration night” indicator
variable (big migration night = 1; non-migration night = 0).
Because of this, we expected a migration night by treatment
interaction—the treatment is more effective on nights when
more amphibians have moved to the pools. No migration
night effect was predicted for N. viridescens given adults are
permanent residents of pools. We used a random effect for
individual trap to account for repeated measures and other
trap-specific variability. After initial analysis, fixed effects were
removed if there were insufficient data to estimate them. The
full set of effects was included for both sexes of A. maculatum
and male L. sylvaticus. Only a treatment effect was included for
female L. sylvaticus, and some sites were unable to be estimated
for A. jeffersonianum and N. viridescens (Table 1).
Results
Over the course of nine trapping nights, we captured 4935
amphibians (Table 2). Regression coefficients for treatment
were significant for all species (Table 3). Migration night was a
significant predictor of captures except for N. viridescens. No
species or sexes had a significant treatment by migration night
interaction. We converted our regression coefficient estimates
to log-odds ratios which show that glow sticks increased the
estimated mean number of captures of A. maculatum by 2.18–
3.64 times, A. jeffersonianum by 2.76–3.61 times, L. sylvaticus by
2.55–2.94 times, and N. viridescens by 3.22–6.47 times compared
to control traps (Fig. 2). Captures also varied significantly among
pools (results not reported). The random effect for trap absorbed
little variability, indicating traps performed similarly.
discussion
Monitoring species’ populations require effective sampling
techniques that provide informative data without commanding
too many resources (e.g., effort, materials). Monitoring vernal
pools is particularly important, as amphibian communities
in these systems are likely experiencing regional decline
(Mitchell et al. 2006; Adams et al. 2013). These systems are also
vulnerable to predicted increases in precipitation variability
Fig. 2. Log-odds ratio of captures (treatment vs. control) for the focal
species. AMMA = Spotted Salamander (Ambystoma maculatum),
AMJE = Jefferson’s Salamander (Ambystoma jeffersonianum), LISY =
Wood Frog (Lithobates sylvaticus), and NOVI = Eastern Red-spotted
Newt (Notophthalmus viridescens). Females are indicated in black
and males in grey. A value of 1:1 indicates no effect (dashed line).
Means are represented by squares, and segments are 95% confidence
intervals.
Herpetological Review 48(3), 2017
TECHNIQUES 547
under climate change, meaning monitoring data will become
only more important for informing conservation decisions
(Brooks 2004; Hayhoe et al. 2007; Anderson et al. 2015; Davis et
al. 2017). While most use of aquatic funnel traps has focused on
the larval stage of the amphibian (Heyer et al. 1994; Buech and
Egeland 2003), we demonstrate that aquatic funnel traps are a
helpful addition in the monitoring toolkit for adult, breeding
amphibians. We show that the use of commercially available
glow sticks significantly improve capture rates for four vernal
pool species. Mean captures of A. maculatum, A. jeffersonianum,
N. viridescens, and L. sylvaticus increased 2–6 times in glow
stick traps over control funnel traps. Captures were highest on
migration nights, but our results did not show a migration night
by treatment interaction, meaning glow sticks had consistently
higher capture rates throughout the breeding season. This
study adds to the growing body of literature that demonstrates
the utility of using glow sticks for increasing amphibian capture
rates, and the results of our study expand the utility of glow
sticks to adult Ambystoma spp. and L. sylvaticus (Grayson and
Roe 2007; Bennett et al. 2012).
The exact mechanism for adult amphibians to be attracted
to the glow sticks is unclear. For larval amphibians and
adult N. viridescens, light attracts prey, making traps a food
incentive (Bennett et al. 2012). When breeding at the vernal
pools, Ambystoma spp. and L. sylvaticus are thought not to
forage (Petranka 1998; Lannoo 2005). This suggests that the
glow sticks may merely provide a visual stimulus or may help
salamanders see movement of other individuals—either way,
likely increasing the encounter rate of amphibians with traps.
The glow sticks also increased captures of female amphibians,
table 3. Results from generalized linear mixed models. Mean model regression coefficients are
reported with standard error (±) and 95% confidence interval in parentheses. The treatment effect
is the impact of including an activated glow stick in a trap. A p-value < 0.05 is designated by an
asterisk (*). Migration night is an indicator of nights when large groups of animals migrate to the
pools. Interaction is the interaction term between treatment and migration night.
Species Treatment Migration night Interaction
A. maculatum 0.778 ± 0.387 1.78 ± 0.369 0.560 ± 0.465
(0.020, 1.54) * (1.06, 2.51) * (-0.350, 1.47)
A. maculatum 1.29 ± 0.392 3.47 ± 0.374 0.608 ± 0.461
(0.522, 2.06) * (2.73, 4.20) * (-0.295, 1.51)
A. jeffersonianum 1.28 ± 0.440 2.59 ± 0.521 -0.885 ± 0.656
(0.419, 2.15) * (-1.57, 3.61) * (-2.17, 0.400)
A. jeffersonianum 1.02 ± 0.355 2.70 ± 0.440 0.192 ± 0.579
(0.320, 1.71) * (1.84, 3.56) * (-0.942, 1.33)
L. sylvaticus 1.079 ± 0.405 NA NA
(0.285, 1.87) *
L. sylvaticus 0.938 ± 0.225 1.67 ± 0.226 0.015 ± 0.293
(0.498, 1.38) * (1.22, 2.11) * (-0.560, 0.590)
N. viridescens 1.87 ± 0.311 1.07 ± 0.560 0.340 ± 0.635
(1.26, 2.48) * (-0.024, 2.17) (-0.903, 1.58)
N. viridescens 1.17 ± 0.250 0.115 ± 0.598 0.930 ± 0.697
(0.681, 1.66) * (-1.06, 1.29) (-0.436, 2.30)
table 2. Pooled captures of females () and males () of our focal
species across 12 vernal pools from 31 March to 9 April 2015.
Species Captures
A. maculatum 291
A. maculatum 1944
A. jeffersonianum 121
A. jeffersonianum 423
L. sylvaticus 48
L. sylvaticus 1818
N. viridescens 148
N. viridescens 142
table 1. Summary of fixed effects included for each analysis (“Y”= yes
included, “N”= excluded). Parameters were excluded if not enough
data were available to reliably estimate them.
Species Site Migration Treatment Interaction
night
A. maculatum Y Y Y Y
A. maculatum Y Y Y Y
A. jeffersonianum N Y Y Y
A. jeffersonianum N Y Y Y
L. sylvaticus N N Y N
L. sylvaticus Y Y Y Y
N. viridescens N Y Y Y
N. viridescens N Y Y Y
Herpetological Review 48(3), 2017
548 TECHNIQUES
which may improve encounter, capture, and retention rates
of male amphibians during the breeding season (Wilson and
Pearman 2010). Because migration night affected captures,
it is likely movement into and out ponds leads to the highest
encounter and capture rates. If glow sticks are effective because
they act as a visual cue, their brightness, wavelength, and
environmental factors such as moonlight or turbidity may limit
their effectiveness (Grayson and Roe 2007; Chen et al. 2008;
Bennett et al. 2012).
There were clear differences between the number of
female and male amphibians captured in our aquatic funnel
traps (Table 2). Petranka (1998) reports observed sex ratios
(male:female) for A. maculatum (1.5:1–3.5:1), A. jeffersonianum
(1.5:1–3:1), and N. viridescens (0.7:1–2.6:1) which are lower than
our observed Ambystoma spp. sex-ratios but similar to our N.
viridescens sex-ratio: 6.7:1, 3.5:1, and 0.96:1, respectively. Sex-
ratios for L. sylvaticus range from 1:1 to 12.3:1 (Berven 1990).
Our observed sex-ratio was 37.9:1. Without further data, it is
challenging to know if these values, for all species, represent the
true population structure or if they reflect sex-biased catchability
and retention. Males of migrating species arrive earlier than
females (unpublished data) and are likely to have higher
encounter rates if they wait along the perimeter for arriving
females (where traps are located). Egg masses are oviposited
near the periphery of these ponds (Petranka 1998; C. L. Davis,
pers. comm.), so females should also have high encounter rates
with the traps. Despite having potentially equal encounter rates,
male courtship behavior (seeking females, repeated contact and
gestures once found) and male-male competition may translate
into males having higher capture rates. To breed successfully
in a male-biased population, males may have more incentive
to explore and enter traps. Females may not have to move far
for males to begin courting them. Regardless, inclusion of glow
sticks significantly increased captures of both sexes.
Although this study shows that glow sticks significantly
increase the capture rates of breeding amphibians in vernal pools,
the benefit comes at an increased cost of $0.52 USD per trap
night, in addition to increased waste in form of spent glow sticks.
These higher costs may be avoided if glow sticks are optimally
deployed only on migration nights (versus the whole breeding
season)—maximizing the number of captures and minimizing
costs. Ensuring traps are fully functional also improves retention
rates. In one instance, two A. maculatum were stuck between
two trap halves and the trap had to be adjusted with plyers (e.g.,
Fig. 1A right). The 6.55-mm mesh size on the traps was also
large enough that some N. viridescens would get stuck trying to
escape. While glow sticks improve encounter and capture rates,
appropriate trap sizes and trap condition may improve these and
retention rates (Luhring et al. 2016). Trap entrances were large
enough that gravid Ambystoma spp. and Lithobates sylvaticus
would not be deterred from entering the trap.
Future research into glow stick lures should focus on
determining which aspect of the glow stick leads to increased
capture rates: visual cues, food cues, or potentially chemical
cues. Salamander eyes are sensitive to green and blue light, but
other species may be receptive to different wavelengths of glow
sticks (Chen et al. 2008). It would also be beneficial to understand
which aspects of passive sampling are impacted by glow sticks.
The visual cue may improve encounter rates, but altering other
trap features (e.g., size) may better help capture and retention
rates of amphibians (Luhring et al. 2016). By continuing to
improve the efficacy of aquatic funnel traps, monitoring efforts
can have a versatile tool for gathering high quality data on adult,
vernal pool amphibians without the intensive efforts of drift
fence surveys or the biases of active sampling techniques.
Acknowledgments.—We thank the Miller Applied Population
Ecology lab for their help collecting and entering this data. Specifi-
cally, we thank Staci Amburgey, Courtney Davis, and Eric Teitsworth.
Chris Schalk, Thomas Luhring, and an anonymous reviewer im-
proved the quality of this manuscript. Procedures were approved
by Pennsylvania State University IACUC 45187 and surveys were
permitted by Pennsylvania Fish and Boat Commission Permit 2015-
01-120. This material is based upon work supported by the National
Science Foundation Graduate Research Fellowship under Grant No.
DGE1255832. Any opinion, findings, and conclusions or recommen-
dations expressed in this material are those of the authors and do
not necessarily reflect the views of the National Science Foundation.
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