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Agricultural co-management aims to promote environmental sustainability while maintaining food-safety standards. Amphibians, especially treefrogs, are known to enter fields of fresh produce intended for human consumption, which raises concerns for food safety and quality. We evaluated the effects of modifications to reduce the scalability of exclusion barriers on the fence-crossing behavior of Pacific treefrogs (Hyliola regilla) from lettuce fields in the Salinas Valley of California during January to May 2019. From small-scale field experiments, we found that fences modified with a horizontal lip at the top prevented all treefrogs from successfully crossing over. Fences incorporating sandpaper were also effective at deterring treefrog crossings compared to fences made of fiberglass square mesh (window screen) and polypropylene fabric (silt fence). The fence design that inhibited the most treefrog crossings-aluminum flashing with a horizontal lip-was a modification to commercially available material that is feasible to install at large scales, is durable over long periods of time, and can help to exclude additional nuisance wildlife, such as small mammals and lizards. Our findings provide quantitative evidence on the efficacy of an exclusion barrier constructed with off-the-shelf material that can be easily modified to improve the co-management of amphibians and agriculture.
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Wildlife Society Bulletin 45(2):305311; 2021; DOI: 10.1002/wsb.1168
From the Field
Evaluating Exclusion Barriers for Treefrogs
in Agricultural Landscapes
Department of Biology, Coe College, 1220 1st Avenue NE, Cedar Rapids, IA 52402, USA
MICHELLE L. GREEN, Department of Biological Sciences, University of South Florida Saint Petersburg, 140 7th Avenue South, Saint Petersburg,
FL 33701, USA
JONATHAN K. WARNER, Texas Parks & Wildlife Department, 10 Parks and Wildlife Drive, Port Arthur, TX 77640, USA
PAUL C. DAVIDSON, Department of Agricultural and Biological Engineering, University of Illinois UrbanaChampaign,
1304 West Pennsylvania Avenue, Urbana, IL 61801, USA
ABSTRACT Agricultural comanagement aims to promote environmental sustainability while maintaining
foodsafety standards. Amphibians, especially treefrogs, are known to enter elds of fresh produce intended
for human consumption, which raises concerns for food safety and quality. We evaluated the eects of
modications to reduce the scalability of exclusion barriers on the fencecrossing behavior of Pacic
treefrogs (Hyliola regilla) from lettuce elds in the Salinas Valley of California during January to May 2019.
From smallscale eld experiments, we found that fences modied with a horizontal lip at the top prevented
all treefrogs from successfully crossing over. Fences incorporating sandpaper were also eective at deterring
treefrog crossings compared to fences made of berglass square mesh (window screen) and polypropylene
fabric (silt fence). The fence design that inhibited the most treefrog crossingsaluminum ashing with a
horizontal lipwas a modication to commercially available material that is feasible to install at large
scales, is durable over long periods of time, and can help to exclude additional nuisance wildlife, such as
small mammals and lizards. Our ndings provide quantitative evidence on the ecacy of an exclusion
barrier constructed with otheshelf material that can be easily modied to improve the comanagement of
amphibians and agriculture. © 2021 The Authors. Wildlife Society Bulletin published by Wiley Periodicals
LLC on behalf of The Wildlife Society.
KEY WORDS Amphibian, California, comanagement, exclusion barrier, Hyliola regilla, modied fence, Pacic
treefrog, Salinas Valley.
When applied appropriately in agriculture, comanagement
techniques provide methods for growers to maintain food
safety and quality standards while simultaneously promoting
wildlife conservation and sustainability (Wild Farm
Alliance 2016). Following a 2006 outbreak of Escherichia coli
O157:H7 in spinach (Jay et al. 2007), the leafygreen in-
dustry sought methods to protect consumers from future
risk of contact with microbial pathogens (Nguyenthe
et al. 2016). Although growers and producers considered
consumer safety as top priority, the majority of growers also
feel personal responsibility for the protection of the water,
environment, and biodiversity on their farms (Beretti and
Stuart 2008, Lowell et al. 2010). Comanagement oers an
overall approach that growers can use to minimize microbial
hazards and conserve wildlife and other natural resources
through actions that are sciencebased, adaptive,
collaborative, commodityspecic, and site specic
(Lowell et al. 2010: 50). The leafygreen industry continues
to promote comanagement science because wildlife that use
agricultural areas can pose a threat to food safety
(Mandrell 2009, 2011; Langholz and JayRussell 2013).
The diversity of animals linked to foodborne illnesses
(JayRussell 2013, Erickson 2016) makes it dicult for
growers to mitigate wildlife contaminations because there is
no single solution to exclude all taxa from agriculture elds
(Rice 2014). Therefore, sciencebased comanagement that
is siteand commodityspecic can provide innovative ways
to eectively reduce contamination risks while protecting
wildlife and natural environments.
The irrigation used for leafy greens in Californias Salinas
Valley (Hansona et al. 1997), which is the largest growing
region for leafy green vegetables in the United States
(United States Department of Agriculture [USDA], 2014),
provides moisture for local amphibians in an otherwise arid
Received: 19 June 2020; Accepted: 4 February 2021
Published: 8 April 2021
This is an open access article under the terms of the Creative Commons
AttributionNonCommercialNoDerivs License, which permits use and
distribution in any medium, provided the original work is properly
cited, the use is noncommercial and no modications or adaptations
are made.
Hughes et al. Treefrog Fence Modications 305
region. Frogs, consequently, are frequently observed in the
aboveground reservoirs and drainages associated with
agriculturebased irrigation in the region. Even though
amphibians may not pose a serious contamination risk, they
have not been entirely ruled out as a potential vector or
reservoir for some human pathogens, such as Salmonella
bacteria, near crop elds (JayRussell and Doyle 2016).
Amphibians can also cause signicant economic losses to
farmers by ending up in prepackaged produce in the hands
of the consumer (Hughes et al. 2019). Furthermore, if they
are detected by foodsafety or foodquality professionals
during inspections, large areas of produce elds can be
labeled unsellable, and the product will often be destroyed.
Additionally, the 2013 foreign material manual from the
USDA considers frog parts or whole animals found in food
products as foreign material that is highly objectionable,
harmful, and unt for consumption(USDA 2013: 4).
In response, growers may use a comanagement barrier
strategy in an attempt to exclude amphibians from sensitive
growing areas. Because previous barriers were nonspecic
and temporary (e.g., silt fences), they proved inadequate to
mitigate intrusions by treefrogs, which have been observed
climbing directly over silt fences shortly after installation
(D. Hughes, Coe College, personal observation).
Because of the myriad of risks that amphibians pose to
leafygreen production in the Salinas Valley (e.g., Hughes
et al. 2019), there is a need to explore novel comanagement
practices. Modied fences could be a powerful exclusion
measure for reducing intrusions into crop elds that
improves the comanagement of treefrogs and agriculture.
Here, we evaluated the eectiveness of fence modications
to impede fencecrossing behavior in Pacic treefrogs
(Hyliola regilla), the most commonly identied species in
consumerbased incidents (Hughes et al. 2019). We tested
4 fence designs in microcosm eld experiments with the aim
to provide a more eective comanagement practice to
mitigate treefrog intrusions into elds that contain fresh
produce intended for human consumption.
We studied Pacic treefrogs (Anura: Hylidae: Hyliola
regilla) in the Salinas Valley, Monterey County, California,
USA. Known as Americas salad bowl (Anderson 2000), the
Salinas Valley was the largest region for growing leafy green
vegetables in the United States, producing more than 70%
of the nations lettuce annually (USDA 2014). Agriculture
in the region consisted of large, industrialized monocultures
with advanced irrigation systems, high fertilizer input, and
contractbased farm labor (FitzSimmons 1986). The region
contained a unique combination of fertile soil and suitable
climate for yearround cultivation (Stuart 2010). In coor-
dination with leafy green industry cooperators, we selected
two reservoirs in the southern area of the Salinas Valley
(near King City, Monterey County, California) that had
histories of treefrog intrusions. The aboveground wetlands
were reservoirs that stored water to irrigate the nearby
lettuce elds.
Frog Sampling
We captured individuals for fence trials with passive and
active trapping methods as part of a larger study on treefrog
intrusions into produce elds across the Salinas Valley
(Hughes et al. 2020). To intercept treefrogs, drift fences
made of silt fencing were installed with paired pitfall
traps around the two wetland reservoirs (Gibbons and
Semlitsch 1981, Todd et al. 2007). Pitfall traps consisted of
5gallon buckets embedded in the soil. A hole was cut in a
series of lids to create a rim (8 cm) at the top of each bucket
to deter treefrogs from climbing out. A separate series of
whole lids were used to cover buckets between sampling
bouts. Holes were drilled into the bottom of buckets for
drainage and a sponge was placed in the bucket for mois-
ture. The buckets were opened during monthly 5day in-
tervals from January to May 2019. When a sampling session
was over, whole lids were secured on the buckets. We de-
ployed 25 polyvinyl chloride (PVC) pipes (3.81 cm diameter
and 60 cm long) as treefrog retreats (Boughton et al. 2000),
which were elevated othe ground by fastening them
to a standalone stake or a post along the fence
(Myers et al. 2007). The bottom of the pipe was sealed to
allow water to ll and a small hole was drilled 6 cm from the
bottom of the pipe for drainage. We deployed 5 wooden
cover boards (0.9 m ×0.9 m) at on the ground around
each reservoir. We spaced all traps (pitfalls, pipes, and
boards) at approximately even intervals around reservoirs
dependent upon their size. During sampling bouts, all
traps were checked once every 24 hr. Treefrogs were
captured during daily trap visits and held in plastic con-
tainers (22 cm ×17 cm ×10 cm) with moist paper towels
and air holes until fence trials, which began in the evening
of each capture day.
Fence Trials
To assess the eectiveness of fence designs for hindering
fencecrossing behavior in treefrogs, we constructed ex-
perimental pens using PVC pipes and window screening
(1.5 m ×1.5 m ×1.5 m) (Fig. 1A,B). The tops of the pens
were covered with wood boards and rocks were placed in the
outer areas of the pens as refuge for treefrogs to prevent
desiccation (Boyle et al. 2019). We temporarily installed
two outdoor enclosures near one of the study reservoirs.
Each experimental fence design was built in a square
(0.9 m ×0.9 m) and placed in the center of the screenedin
arena for trials (Fig. 1A,B). All freestanding test fences
were initially constructed at the same height (0.9 m tall). To
run overnight trials, we placed treefrogs in the center of an
experimental fence shortly after sundown (17002000 hr)
and a wood cover was placed on the top of the outer pen.
The following morning (06000900 hr), we counted the
number of treefrogs in the outer test arena (escaped) and
those that remained in the experimental fence (did not
escape). Each treefrog was tested once in these escaperoom
fence experiments and all individuals were held for less
than 24 hr. After testing, treefrogs were marked using a
306 Wildlife Society Bulletin 45(2)
unique toeclip combination for later recognition (Donnelly
et al. 1994) and released at their site of capture.
Fence Designs
We compared 4 fence designs for reducing the fence
crossing behavior in treefrogs. We used 2 aluminumbased
fences (more permanent) and 2 meshbased fences (more
temporary). First, we tested sandpaper (320grit) glued to
aluminum ashing (Fig. 1C) because laboratory experi-
ments demonstrated that the asperity size (i.e., surface
roughness) of this material disrupts the liquidadhesive
properties of treefrog toepads and can induce slippage at low
angles while climbing (Crawford et al. 2016). Second, we
tested the eect of adding a 10cm horizontal lip at the top
of aluminum ashing (Fig. 1D) because previous studies
have shown that an overhanging edge at the top of a bucket
can limit amphibian escapes (Mazerolle 2003). Third, we
tested a fence made of windowscreening material (ber-
glass square mesh) supported by galvanized poultry wire
(Fig. 1E) because these fencing materials are often em-
ployed as barriers for deer and pigs in the Salinas Valley.
Fourth, we used a contractorgrade silt fence made of
polypropylene fabric (Fig. 1F) as a control because it is the
most commonly installed barrier for treefrogs around
Figure 1. Outdoor pens for microcosm exclusion experiments and experimental fence designs: (A,B) Field enclosures made of PVC pipes and window
screening, (C) Aluminum ashing axed with 320grit sandpaper, (D) Aluminum ashing with a 10cm horizontal lip at the top, (E) Windowscreen
material with chickenwire support, and (F) Polypropylene silt fence used as a control. Fence trials were conducted from January to May 2019 in the Salinas
Valley, Monterey County, California, USA.
Hughes et al. Treefrog Fence Modications 307
agricultural water sources in the Salinas Valley and has been
used in research as an inexpensive, temporary drift fence for
amphibians (Zug et al. 2001, Malone and Laurencio 2004).
Statistical Analysis
We compared the percentage of treefrogs that escaped per
trial between the 4 fence designs using ANOVA. We used
Ztests for 2 samples to compare frequency distributions for
the proportion of treefrogs that escaped per trial between
dierent fence designs. Mean proportions of escaped
treefrogs are presented with ±1 standard deviation and
sample ranges. We recognized signicance at P<0.05. We
used Excel 2016 (Microsoft Inc., Redmond, Washington,
USA) to organize data and R (v.3.6.2, R Core Team 2019)
to generate graphics and conduct analyses.
From 32 independent trials ( ̅
=8±2 trials per fence type,
range =711 trials per fence type) using 257 individuals
=8±3.2 treefrogs tested per trial, range =113 treefrogs
tested per trial), we found that the proportion of treefrogs
that escaped diered by fence type (F
=9.95, P<0.001).
Fences modied with a 10cm horizontal lip at the top were
the most eective design at preventing successful fence
crossings, with 0 treefrogs escaping during all trials (Fig. 2).
Fences that integrated 320grit sandpaper were also eective
at reducing fence crossings, with just 5 treefrogs escaping
across all trials ( ̅
=7.9% ±20.9%, range =055.6% es-
caped per trial). The other two fences were less eective at
inhibiting crossings: windowscreen fences with 36 treefrogs
that escaped ( ̅
=55.8% ±35.3%, range =083.3% escaped
per trial) and control silt fences with 37 treefrogs that
escaped ( ̅
=55.1% ±30.9%, range =0100% escaped per
Wildlife exclusion fencing is rapidly becoming a standard
tool employed by conservation practitioners, ranchers, and
land managers to redirect animal movements (Glista
et al. 2009, Knight et al. 2011, Markle et al. 2017), but there
is no onesizetsall solution, so each fence needs to be
designed with the biology of the target species in mind
(BaxterGilbert et al. 2015, Dillon et al. 2020). For
amphibians, there have been few experiments that test the
ecacy of dierent fence designs for their exclusion, with
most fences developed for redirecting nonclimbing species
into ecopassages (e.g., tunnels and culverts) to reduce
road mortality (Dodd et al. 2004, Grilo et al. 2010,
Beebee 2013). A previous study on barrier eectiveness
tested against two nonclimbing frog species (Ranidae:
Lithobates clamitans and L. pipiens) found that plastic fences
with heights of 0.9 m prevented all but one frog from
crossing over, whereas a height of 0.3 m blocked nearly 80%
of frogs and 0.6 m blocked more than 95% (Woltz
et al. 2008). In a series of laboratory experiments, Dodd
(1991) found that 3 treefrog species (Hylidae: Acris gryllus,
Hyla femoralis, and Pseudacris ocularis) successfully crossed
over a 0.35 m tall piece of aluminum ashing deployed in-
side a small container, with 180 individuals of 370 tested
(48.6%) crossing the barrier. We found that the addition of
a horizontal lip to the top of a fence made of aluminum
ashing prevented all tested treefrogs from crossing
over in microcosm eld experiments. This fence design may
be generally useful across taxa because a recent study, using
a similar experimental protocol, also found that barriers
Figure 2. Comparison of exclusion experiments showing the percentage of Pacic treefrogs (Hyliola regilla) that escaped per fence trial with bars
representing standard errors. An escape was dened as a treefrog found in the main fence arena after an overnight trial. Similar letters above fence types
indicate no signicant dierence based on pairwise comparisons. Sample sizes for treefrogs and trials per fence type are presented below letters. Lip:
aluminum ashing with a 10cm horizontal lip at the top. Sandpaper: aluminum ashing axed with 320grit sandpaper. Control: polypropylene silt fence.
Screen: windowscreen with chickenwire support. Fence trials were conducted from January to May 2019 in the Salinas Valley, Monterey County,
California, USA.
308 Wildlife Society Bulletin 45(2)
with a lip were the most eective at preventing ratsnakes
from climbing over (Macpherson et al. 2021).
Although all the fences that we tested inhibited some
fencecrossing incidents by treefrogs, the aluminumashing
fence with a horizontal lip was the most successful. The
modied aluminumashing fence is also the most feasible
for scaling up to a larger area among those tested because
the second most successful barrier requires adhering
320grit sandpaper to a long section of fence, which is time
consuming, requires maintenance after installation, and
can be cost prohibitive. The horizontallip design is a
straightforward modication to overthecounter aluminum
ashing, a material that has been used in numerous multi
year studies of breeding amphibians because it requires little
maintenance, is relatively inexpensive, and can be easily
installed (e.g., Pechmann et al. 1991, Semlitsch et al. 1996,
Todd et al. 2007). For stability and to prevent passage un-
derneath aluminum ashing, we recommend burying 10 cm
into the soil, attaching stakes to the back with metal ties,
and adding wood planks to the face so that the ties will not
wear through the fence (Fig. 3). Similar designs that in-
corporated horizontal lips for exclusion barriers have been
developed for commercial use by wildlife companies
(e.g., Animex:, and some have even
been deployed in conservation areas (e.g., Gleeson and
Gleeson 2012). However, modied fences do not appear to
have been thoroughly assessed for their eectiveness as an
amphibian barrier. Novel approaches to redirect amphibian
movements (e.g., Buxton et al. 2018) will not succeed
unless an evidencebased framework is adopted (Schmidt
et al. 2020) in which techniques are quantitatively
and rigorously evaluated to determine if they achieved their
intended outcome.
There is increasing recognition of the importance of
comanaging wildlife and agriculture to attain a stable co-
existence with biodiversity while promoting sustainable
economic growth (Treves et al. 2006, Karp et al. 2015).
Incidences of foodborne illness linked to the consumption
of wildlifecontaminated produce, however, can impede
progress in comanagement (Stuart 2009, 2011). Because
the sampling of human pathogens in wild frogs has not been
robust, either geographically or quantitatively (Ferens and
Hovde 2011, Gorski et al. 2013), growers cannot eliminate
frogs as a potential source of microbial disease, thus they
need access to rigorously tested, easy to implement and
maintain, comanagement options on their farms (Olimpi
et al. 2019). Our evaluation of exclusion barriers for tree-
frogs showed that modications to fences can be applied in
an agricultural context to improve the comanagement of
wildlife and agriculture. Previous attempts to exclude
Figure 3. Specications of the optimal fence design (aluminum ashing with a 10cm horizontal lip at the top) for inhibiting fencecrossing behavior in
Pacic treefrogs (Hyliola regilla) and photographs of ineld installations. Fence trials were conducted from January to May 2019 in the Salinas Valley,
Monterey County, California, USA.
Hughes et al. Treefrog Fence Modications 309
treefrogs from agricultural elds in the Salinas Valley were
unsuccessful, in part, because amphibian biology was not
considered when designing the barrier, nor was an evidence
based framework implemented. Silt fences, for example,
were used widely but had not been tested to make sure that
their installation led to a reduction in treefrog intrusions.
Farmers are now equipped with quantitative evidence that a
modied barrier will impede fencecrossing behavior in a
common treefrog species and some additional wildlife, in-
cluding small species of rodents, lizards, and other frogs
(Hughes et al. 2020). The widespread adoption of this
barrier design in the Salinas Valley would benet both
amphibian conservation and the freshproduce industry by
providing an evidencebased solution to reduce treefrog
intrusions in agricultural landscapes.
We thank the Center for Produce Safety for funding and
continued support of our project. Funding for our project
was made possible by the USDAs Agricultural Marketing
Service through grant AM170100XXXXG011 and the
California Leafy Greens Research Program. Animals were
handled under approved protocols from the California
Department of Fish and Games Scientic Collection
Permit (SCP # 13909) and the University of Illinoiss
Institutional Animal Care and Use Committee Protocol
(IACUC # 18081). We would also like to thank D. Donner
(Associate Editor), A. Knipps (Editorial Assistant), and
anonymous reviewers for their helpful comments, which
improved the manuscript.
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One of the deadliest roads in North America for species at risk fragments a marsh-lake ecosystem. To reduce road mortality, stakeholders installed >5 km of exclusion fencing along a southwestern Ontario, Canada, causeway in 2008–2009. Between 2012 and 2014, 7 culverts were installed to provide safe crossings. We evaluated the success of these mitigation strategies by 1) comparing results of road surveys conducted 5 years before and 5 years after fencing installation; and 2) monitoring use of culverts by turtles using motion-activated cameras at culvert openings and stationary antennas placed to detect movements of passive integrated transponder (PIT)-tagged turtles (68 Blanding's turtles [Emydoidea blandingii] and 30 spotted turtles [Clemmys guttata]). We also radio-tracked 30 Blanding's turtles to measure culvert use in relation to home ranges. Turtle and snake abundance was 89% and 53% lower, respectively, in completely fenced road sections than in unfenced sections; abundance was 6% and 10% higher, respectively, between partially fenced and unfenced sections. After mitigation, locations where we found reptiles on the road were associated with fence ends, underscoring the importance of fence integrity and ineffectiveness of partial fencing as a mitigation strategy. We confirmed use of culverts by Blanding's turtles, northern map turtles (Graptemys geographica), snapping turtles (Chelydra serpentina), and midland painted turtles (Chrysemys picta). Through radio-tracking, we determined that male and female Blanding's turtles home ranges overlapped with different segments of the causeway. We recommend that stakeholders emphasize ensuring fence integrity and continuity, limiting impact of edge effects, and conducting a comprehensive monitoring program