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We integrated road maps, traffic volume data, and pool locations in a modeling study to estimate the potential effects of road mortality on populations of pool-breeding spotted salamanders (Ambystoma maculatum Shaw). Population projections based on spotted salamander life tables imply that an annual risk of road mortality for adults of >10% can lead to local population extirpation; mitigation efforts (tunnels, road closures, and other measures) should seek to reduce road mortality rates to below this threshold. For central and western Massachusetts, we estimated that salamanders would be exposed to at least this threshold level of risk at 22–73% of populations (assuming a 100 vs. 500m migration distance, respectively). We conclude that road mortality can be a significant source of additive mortality for individual spotted salamanders in many parts of the species’ range. Efforts to prevent such mortality by transportation planners are likely warranted strictly on a biological basis in areas with road densities >2.5km per km2 of landscape and traffic volumes >250 vehicles/lane/day within the migration range of a breeding population of spotted salamanders.
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Can road mortality limit populations of pool-breeding amphibians?
James P. Gibbs
* and W. Gregory Shriver
College of Environmental Science and Forestry, State University of New York, 350 Illick Hall, 1 Forestry
Drive, Syracuse, NewYork 13210, USA;
U.S. National Park Service, Marsh-Billings-Rockefeller National
Historic Park 54 Elm Street, Woodstock, VT 05091, USA; *Author for correspondence (e-mail:; phone: +1-315-470-6764; fax: +1-315-470-6934)
Received 9 September 2003; accepted in revised form 9 July 2004
Key words: Ambystoma maculatum, Conservation, Demography, Pool-breeding amphibian, Road mor-
tality, Spotted salamander
We integrated road maps, traffic volume data, and pool locations in a modeling study to estimate the
potential effects of road mortality on populations of pool-breeding spotted salamanders (Ambystoma
maculatum Shaw). Population projections based on spotted salamander life tables imply that an annual risk
of road mortality for adults of >10% can lead to local population extirpation; mitigation efforts (tunnels,
road closures, and other measures) should seek to reduce road mortality rates to below this threshold. For
central and western Massachusetts, we estimated that salamanders would be exposed to at least this
threshold level of risk at 22–73% of populations (assuming a 100 vs. 500 m migration distance, respec-
tively). We conclude that road mortality can be a significant source of additive mortality for individual
spotted salamanders in many parts of the species’ range. Efforts to prevent such mortality by transportation
planners are likely warranted strictly on a biological basis in areas with road densities >2.5 km per km
landscape and traffic volumes >250 vehicles/lane/day within the migration range of a breeding population
of spotted salamanders.
Road mortality of amphibians is a worldw ide
conservation concern (e.g., W yman 1991; Ashley
and Robinson 1996; Hels and Buchwald 2001),
and the United States is no exception. About one-
fifth of the land area of the conterminous United
States experiences some ecological effect of roads
and traffic (Forman 2000), an effect that is
increasing in concert with the expansion of the
road network and increase in traffic volumes
(National Research Council 1997).
Amphibians are one taxonomic group likely
sensitive to this expansion because they frequently
are killed on roads, particularly during the highly
synchronized overland migrations many pool-
breeding species undertake annually (Wyman
1991). Mortality of dispersing juveniles, as well as
adults, during the post-breeding period also occurs
and could be significant, but is less well recognized,
in large part because the phenomenon is more
diffuse in time and space and therefore more
difficult to study.
Amphibian migration is primarily nocturnal,
when traffic vo lumes are highly reduced relative to
their daytime rates (about 80% of traffic volume
occurs during the day: Festin 1996). Moreover,
amphibian demography, particularly that of anurans
Wetlands Ecology and Management (2005) 13: 281–289 Ó Springer 2005
DOI 10.1007/s11273-004-7522-9
(frogs and toads), is characterized by extraordi-
nary potential for population increase (e.g., Ber-
ven 1990). Thus, it is possible that amphibian road
mortality is an unfortunate yet demographically
insignificant phenomenon. The sparse research
conducted on the topic to date, however, suggests
Most research on the effects of road mortality on
amphibians has entailed simple tallies of numbers of
individuals killed at particular road crossing sites, in
some cases yielding surprisingly large counts. For
example, road-kill counts along a 3.6 km section of a
two-lane paved causeway in Ontario, Canada over
two seasons yielded >32,000 individual amphibians
(Ashley and Robinson 1996). Wyman (1991)
reported average mortality rates of 50.3 to 100% for
hundreds of salamanders attempting to cross a
paved rural road in New York State, USA.
Attempts to translate such counts of road-killed
animals into population-level assessments of the
consequences of road mortality are few. Hels and
Buchwald (2001) estimated that 10% of amphibian
populations near roads were killed on roads
annually at their study area in Denmark. Carr and
Fahrig (2001) concluded that road mortality exerts
more negative effects on populations of vagile
species of frogs than on sedentary (largely aquatic)
species in Ontario, Canada. In the same region,
Fahrig et a1. (1995) demonstrated that local frog
populations decreased with increasing traffic vol-
ume. Only Gittins (1983), studying co mmon toads
(Bufo bufo) in Great Britain, concluded that road
mortality had a minimal effect on populations.
Transportation policy makers are becoming
increasingly aware of the issue of wildlife mortality
and predisposed to addressing it (Forman et al.
2002). Unfortunately, we do not yet know whether
road mortality is sufficient to pose a significant risk
to the viability of local or perhaps even regiona l
amphibian populations. Moreover, the few, site-
specific studies of amphibian road mortality con-
ducted to date have not provided the type of
information needed to advance planning efforts.
Particularly useful would be studies that identify
threshold combinations of road density and traffic
volume that lead to unacceptably high levels of
road mortality in amphibian populations.
We implemented a modeling approach first
described by Gibbs and Shriver (2002) to examine
the magnitude of road mortality on pool-breeding
amphibians. The modeling approach integrates
maps of actual road networks, data on traffic
volumes, and simulated animal movements to
estimate the effects of road density and traffic
volume on animal demography. We used spotted
salamanders (Ambyst oma maculatum Shaw) as a
model organism because (1) they are wi despread in
the eastern United States, (2) they are frequently a
target of efforts to mitigate amphibian road mor-
tality, and (3) their demography, characterized by
extended generations times, and slow locomotion
suggest that they would be among the amphibians
most sensitive to additive sources of adult mor-
tality, such as that generated by roads (Wyman
1991; Petranka 1998).
Estimating mortality in relation to traffic volume
The probability that a salamander would be killed
as it attempted to cross a road, p
, was esti-
mated based on an equation adapted from Hels
and Buchwald (2001):
¼ 1 e
where N = traffic volume in vehicles/min,
a = width of the kill-zone on a road (m), and
v = velocity (m/min) of the salamander moving
through the kill-zone. The kill-zone was estimated as
two times tire width (0.5 m total) per lane plus two
times total salamander body length (snout-to-tail
tip, see Hels and Buchwald 2001; data on mean adult
length obtained from Petranka 1998). A conserva-
tive value was used for v, salamander velocity, that
is, 1 m/min, based on our estimates of 12 adult A.
maculatum crossing a road (see next paragraph);
these animals averaged 1.3 m/min (±0.63 SD).
Assessing the validity of mortality estimates
We assessed the validity of our mortality estimates
in relation to traffic volume by measuring actual
salamander mort ality caused by fatal encounters
with vehicles on roads between March 15 and May
1 in 2003. We conducted surveys on New York
State Route 91 at Labrador Hollow (UTM 18
414312E, 4737775N [NAD27]) in Apulia, New
York. At this site, salamanders descend from the
forested slopes of Morgan Hill State Forest and
migrate across the highway en route to breeding
pools in the Labrador Hollow Unique Area. We
constructed 21, 3-m- long drift fences, spaced
approximately 47 m apart, on the breeding-pool
side of a 1 km length of highway (Figure 1). Drift
fences wer e located 1 m from the road and angled
10° toward the road. Each drift fence was outfitted
with one pitfall trap located in the center of the
drift fence. Pitfall traps were metal cans 0.5 m
deep and 7.5 cm in circumference. On six nights
with sufficient precipi tation to facilitate salaman-
der movement, we monitored salamander cross-
ings and vehicle passage. Observers repeatedly
walked along the highway from fence 1 to 21 (a
‘survey’ of ca. 30 min) and counted and removed
live and dead (or injur ed and presumably dying)
animals. Live and dead salamanders were tallied
(1) along segments of the highway between fences,
and (2) from the segments of the highway facing
drift fences where live animals captured in the drift
fence pitfalls were included in the count. This
sampling design provided two road crossing mor-
tality estimates for salamanders (calculated as
dead animals captured/total animals captured).
We estimated ‘tr ue’ mortality using salamanders
captured on each survey in segments of the road
facing drift fences and in drift fence pitfalls only
(‘true’ because all salamanders would either be
killed on the highway or captured in drift fences).
We also estimated an ‘inflated’ mortality using
road captures only (‘inflated’ mortality because all
animals killed were cou nted, but some live animals
escaped unnoticed between passes). Vehicle pas-
sage during surveys was recorded to the nearest
second to later estimate traffic volume during a
given survey period.
Estimating mortality in relation to road density
Annual road-associated mortality (d
) in spot-
ted salamanders was estimated as
¼ 1 ð1 p
where p
is the probability a salamander is
struck by a vehicle on any given road crossing (see
above) and n
is the number of road cross-
ings an individual salamander undertakes annu-
ally. The equation is derived from binomial
probabilities (Zar 1984 : 370–375) and estimates
the likelihood that a salamander is struck (and
presumably killed) during its cumulative road
crossings each year.
Figure 1. Sampling design used to estimate rates of road mortality of adult spotted salamanders in relation to traffic volume at
Labrador Hollow, Apulia, New York, March–May 2003.
We used spatially explicit sim ulations of move-
ments of individual salamanders on round-trip
movements between upland territories and breed-
ing pool areas to estimate the relationship between
and road density based on actual road
networks (Figure 2). Salamander movements were
simulated in landscapes within a 7224 km
of central and western Massachusetts (Figure 3),
which encompassed the Connecticut River Valley
as well as portions of the Berkshire Mountains. All
geographic data were obtained from MassGIS
( and
analyzed with ArcView 3.2 software and associ-
ated extensions. We randomly chose 500 breeding
pools within this area (from the data layer created
based on the survey conducted by Burne [2001]),
and counted the average number of times a single
salamander would have to cross a road on eight,
roundtrip, linear movements (to the N, NE, E, SE,
S, SW, W and NW) oriented upon the breeding
pool (Figure 2). Two dispersal distances were
evaluated 100 and 500 m away from the pool
to bracket the low and high estimates of average
adult salamander movement distances (125 m)
reported by Semlitsch (1998). Road density in a
corresponding circle of radius 100 or 500 m of
each breeding pool was also calculated by dividing
the total length of roads present by the area of the
circle (as km roads/km
of landscape). From these
data (n = 500 pairs of road density vs. salaman-
der road crossing frequency for each dispersal
distance) the relationship between road density
and average n
was estimated via least-
squares linear regression analysis (with intercepts
forced through zero).
Demographic significance of road mortality
We estimated mortality for each of the 500
breeding pools assessed in Massachusetts, using
the observed road density surrounding each site as
well as a statew ide, night-adjusted traffic volume
estimate. This value was estimated to be 160
vehicles/lane/day, based on the number of all
motor vehicles registered in Massachusetts multi-
plied by the annual miles traveled by an average
vehicle in the United States in 1998 divided by the
lane ‘mileage ’ in the stat e (FHA 1999). Traffic
volumes were reduce by 80% to conform to the
Figure 2. Example of simulated patterns of movement by an individual adult spotted salamander moving on 8 round-trips (straight
lines) from a breeding pool (open dot) and crossing (filled dots) roads (solid lines). In this case the average frequency of road crossing
was 1.75 (average of 0, 0, 0, 2, 2, 2, 4 and 4). Road density was subsequently estimated within a circular area of radius = migration
distance and centered upon the same pool. Simulated movements were subsequently replicated 500 times for two movement distances
(100 and 500 m away from pools).
proportion of overall traffic volume that occurs
between 1800 and 0600 h (Festin 1996), the peak
period of the salamanders’ nocturnal movements
(Petranka 1998).
We evaluated the potential significance of
additive road mortality to spotted salamander
population viability by integrating published esti-
mates of vital rates into a population projection
model that estimated the number of adults (N
a given year (t) as:
¼ N
þ N
where (N
) is the number of eggs produced in a
given year (t):
¼ N
u and
= adult annual survival rate, u
= average
eggs produced per individual, r
= survival rate
from egg to metamorphosis, r
= survival rate of
juveniles through their first winter, K
= the car-
rying capacity of larval habitat.
The mod el reflects limitation of adult popula-
tions through density dependent process in the
larval stage, typical of most pool-breeding
amphibians (see Vonesh and de la Cruz 2002).
Based on Petranka’s (1998) synopsis of the
scientific literature on spotted salamander biol-
ogy, annual adult survival rate, r
, was estimated
at 0.7, survival rate of egg to metamorphosis was
estimated at 0.04, and r
= survival rate of
juveniles through their first winter was estimated
at 0.6. The clutch size parameter, u, incorporates
number of eggs per mass (ca.80), number of egg
masses laid per female per year (ca.2), and annual
breeding probability (ca.0.38), all halved to reflect
contributions by both males and females. Last,
starting populations were set at 10,000 eggs and
100 breeding adults (an approximately stable age
distribution) and K
, carrying capacity of the
larval habitat, was set at 10,000. Under this sce-
nario, a spotted salamander population was
projected over 25 years under varying levels of
adult survival (r
= 0.4, 0.5 and 0.6), each
reflecting varying levels of additive road mortality
in relation to the scenario of no road mortality
= 0.7).
Estimating mortality in relation to traffic volume
Mortality estimates were highly and positively
correlated with volume of traffic that passed dur-
ing the same interval: p
= 1.537*vehicles/min
0.0048, R
= 0.927, Figure 4).
Assessing the validity of mortality estimates
We co unted 330 live an d dead spotted sala-
manders on road crossing surveys. Mortality
rate at drift fences (‘true’ mortality rate) was
estimated at 19% (16 dead/70 live) whereas
mortality estimated from captures on the road
between drift fences (‘inflated’ mortality rate)
was 25% (61 dead/183 live). We used road
mortality estimates from 5 surveys along the
road during which we collected >10 animals
between drift fences, reduced these estimates by
6% (the inflation factor between true and
inflated estimates), and found that field esti-
mates of mortality were nearly identical
(Figure 4) to those predicted by the equation of
Hels and Buchwald (2001).
Figure 3. Area of central and western Massachusetts, USA,
where salamander movements were simulated in relation to
roads (upper). Circles represent the 500 randomly chosen
breeding pools mapped by Burne (2001) used in the analysis.
Arrow on map of the United States (lower) indicates location of
Estimating cross ing frequency in relation to road
The relationship between road density (RD) and
expected road-crossing frequency (n
(Figure 5) was linea r and positive for two migr a-
tion distances, with slope increasing but model fit
declining as dispersal distance increased: 100 m,
= 0.0658*RD (R
= 0.81) and 500 m,
= 0.356*RD (R
= 0.54).
Demographic significance of road mortality
Based on our probability model that integrated
traffic volume and road density, we estimated that
for the 500 breeding pools assessed in Massachu-
setts, the median road mortality rate was 17%
annually for animals moving 100 m and 37% for
animals moving 500 m. Population projections
indicated that a spotted salamander population
with a starting number of eggs = 10,000 (at a
breeding pool’s carrying capacity) and starting
number of adults = 100 would (after achieving a
stable age distribution) attain an equilibrium adult
population size of about 200 after 20 years
(Figure 6). Additive road-associated mortality of
10% woul d lead to a stable adult population,
albeit at a lower equilibrium level, of about 80
individuals (Figure 6). Road mortality levels of 20
and 30% would lead to population extirpation
(Figure 6) within 25 years. Thus, a threshold rate
Figure 4. Relationship between road mortality predicted on the basis of traffic volume, salamander rate of locomotion, and sala-
mander length (based on Hels and Buchwald 2001) vs. that estimated in the field during five surveys at Labrador Hollow, Apulia, New
York, in 2003.
Figure 5. Relationship between road density and estimated road crossing frequency for spotted salamanders moving at two distances
(100 and 500 m).
of somewhere between 10 and 20% likely occurs
above which population declines owing to road
mortality would be expected to ensue.
Can road mortality limit populations of pool-
breeding amphibians? These analyses imply that
road mortality could be a significant source of
additive mortali ty for spotted salamanders in
central and western Massachusetts. Moreover, our
population projections, which integrate these road
mortality estimates with the demography of the
spotted salamander, indicate that road mortality
alone could lead to local population extirpation in
many areas. Our estimates are somewhat conser-
vative insofar as they consider only mortality
during breeding migrations whereas some move-
ments do occur outside the breeding season and
may further expose adults and migrating juveniles
to road mortality.
Notably, our population projections suggest
that the ‘dose-response’ curves between sizes of
salamander populations and levels of road mor-
tality are not a simple linear relationship. Scenar-
ios for patterns of local population decline with
increasing exposure to road mortality are likely
quite complex in amphibians and low levels of
road mortality may have little effect on salaman-
der populations. This is because many population
processes in amphibians are the result of density-
dependent interactions in the larval stage (e.g.,
Berven 1990). Thus, initial road-ca used declines in
the adult population may be partially compen -
sated for by increased juvenile recruitment, as
reduced input of eggs results in less larval crowd-
ing, and hence higher rates of metamorphosis (e.g.,
Vonesh and de la Cruz 2002). Once egg input of
adults drops below that needed for saturation
of larval habitats, however, the number of meta-
morphs produced and the number of breeding
adults will be linearly and positively related. Thus,
we expect the relationship between the intensity of
road mortality and populations of spotted sala-
manders is one in which salamander populations
remain stable in the face of increasing road mor-
tality and then go into rapid decline after a
threshold has been crossed, a pattern largely borne
out by the outputs of the population model
(Figure 6). Further research is needed to under-
stand these processes and estimate critical thresh-
olds of road mortality on salamander population
Although we have validated a critical compo-
nent of the model, that is, road mortality in rela-
tion to traffic volume (Figure 4), substa ntial
uncertainties remain with the relationship between
road crossing frequency and road density. In
particular, our analysis assumed that salamanders
traverse the landscape in a linear fashion and cross
any road encountered. That salamanders navigate
completely independently of roads, however, is
naive. In fact, roadbeds and ditches may physically
Figure 6. Projections of a single spotted salamander population over 25 years under scenarios of adult survival = 0.7 (typical estimate
of adult survival in areas without heavy road mortality) vs. road-mortality reduced levels of adult survival = 0.6, 0.5 and 0.4. The
model reflects limitation of adult populations through density dependent processes in the larval stage, that is 10,000 eggs, with starting
population of 100 adults and 10,000 eggs, with the population achieving a stable age distribution between years 1 and 5.
prevent road crossings and the salamanders
themselves may avoid crossing roads. Some sala-
manders may remain on road surfa ces for
extended periods, either arrested physiologically
by debilitating compounds associated with salts
used to de-ice road surfaces or to linger upon the
warmer surfaces for purposes of thermoregulation.
Detailed behavioral studies of movements of
individual salamanders in relation to roads would
greatly clarify these issues and suggest ways to
model them more accurately. W e expect, for
example, that high levels of road mortality such as
those estimated do indeed occur where salaman-
ders must cross roads to reach vital habitats (e.g.,
isolated breeding ponds that limit population
recruitment in an area such as our study site in
Labrador Hollow) that are separated by roads
from adult habitat on uplands (e.g., Wyman 1991).
However, aversions to road crossing, coupled with
options for habitat choices, would deflate our
mortality esti mates to an unknown degree.
It must be emphasized that this study was per-
formed at a regional spatial scale to examine the
question of whether road mortality could lead to
the extirpation of regional A. maculatum popula-
tions. Mortality at any particular site could be
dramatically greater or less than what we estimate
depending on local road densities and traffic vol-
umes. For example, daily traffic volumes on ‘main
roads’ and other arteries in this part of the United
States can be on the range of 1000–10,000 vehicles
per day. Some restricted access highways carry
10,000–100,000 vehicles/day (FHA 1999). Roads
with such traffic volumes are wholly lethal to any
migrating salamander.
The applicability of our findings to other
amphibians and regions is unclear. Frogs and
toads move more quickly across roads and are
less vulnerable to road mortality (Wyman 1991).
Frogs may be more mobile, however, particularly
during the post-breeding period (e.g., Carr and
Fahrig 2001) and hence may undertake road
crossings at greater frequency over an entire
season. Frogs are also more active diurnally,
when traffi c volumes are dramatically higher
(Festin 1996). Even for other salamanders, vari-
ation in life hist ory may make application of
these results tenuous. For example, the red-spot-
ted newt (Notophthalmus viridescens) wanders for
years prior to sexual maturation during which
time it is likely highly vulnerable to road mor-
tality. Notably this species is among the most
sensitive to habitat fragmentation (Gibbs 1998)
and chronic road mortality may contri bute to this
sensitivity. Conversely, plethodontid salamanders
may be more limited in their movements than
Ambystoma and hence less vulnerable. From a
geographical perspective, these results are likely
applicable to other regions that are urbanized to
a similar degree as the region studied in central
Massachusetts (for exampl e, much of the north-
eastern United States excluding northern New
England and Northern New York).
To convert our analyses into policy relevant
information, we identified the combinations of
traffic vo lume and road density that produce a
predicted level of 25% or more additive mortality
associated with roads (Figure 7). Based on this
analysis, we estimated that combinations road
densities >2.5 km per km
of landscape and
traffic volumes >250 vehicles/lane/day within the
dispersal and migrati on range of a particular
breeding population of spotted salamanders
could generate demographically signi ficant mor-
tality levels (Figure 6). Although interactions
among salamander movement, road mortality,
and population persistence remain unclear, it is
apparent that conservation planning should
accommodate local movements and breeding
migrations of salamanders if local and perhaps
even regional populations are to remain secure.
Moreover, if efforts are successful in limiting
rates of traffic-caused mortality to <10% (See
Figure 6) of all individuals attempting to cross
Figure 7. Combinations of road density and daily traffic vol-
ume that produce an estimated level of annual road-associated
mortality of 25% in adult spotted salamanders migrating to two
different distances (100 and 500 m).
roads during migration circuit to a particular
pond, e.g., by tunnel construction, road closure,
or physically transporting individuals, then those
efforts are likely warranted to stave off local
population extirpation.
We are grateful to R. Brooks and P. Paton for
inviting us to the Vernal Pools Symposium at The
Wildlife Society Conference in Burlington,
Vermont, in 2003, thereby motivating us to
undertake this analysis. We are also thank them
for insightful comments and helpful editing, which
was similarly provided by two anonymous
reviewers. Many undergraduate and graduat e
students at SUNY-ESF kindly assisted with road
surveys at Labrador Hollow, particularly Joel
Strong, Debra Endr iss, Nancy Karraker, Sara
Ashkannejhad, Stephanie Smith, and Matthew
Young. Mary Clements of the New York State
Department of Transportation arranged permis-
sion and material support for us to conduct the
road surveys, for which we are grateful.
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... Some amphibians require vernal pools for successful breeding (i.e., obligate species), whereas others use vernal pools for breeding in addition to other habitats (i.e., facultative species). Because of the extensive, dense road system in the U.S., especially in the Northeast, amphibians are often killed on roads during their migration to vernal pools (LeClair et al., 2021) and this mortality is expected to have negative impacts on local populations (Gibbs and Shriver, 2005;Sterrett et al., 2018). A contiguous transition between upland forests and vernal pools, uninterrupted by roadways, is essential in the reproductive success of amphibians (Compton et al., 2007). ...
... Initially downloaded as lines, the widths of roads are imposed as buffers following different widths according to the category of road. Roads follow a hierarchy in which those roads carrying more traffic are wider and pose a greater threat to amphibian migration (Gibbs and Shriver, 2005). In the scope of this statewide study, the traffic volumes of roads are not available. ...
... Roads are responsible for mass mortality of between 50 and 100% of migrating amphibians (Wyman, 1991), and are thus emphasized in the application of vernal pool preservation scores in this work. Other work has similarly emphasized the role of roads in inhibiting amphibian migration (see Compton et al., 2007), and empirical studies of amphibian road mortality call for the study of road networks and densities in evaluating migratory landscapes such as those in the vicinity of vernal pools (Gibbs and Shriver, 2005;Mazerolle, 2004;Wyman, 1991). The other factors evaluated here are the distribution of upland forests and dispersal of amphibians to neighboring vernal pools. ...
Vernal pools are required habitat of pond-breeding amphibians, yet their legal protections in the United States are not established, leaving vernal pools vulnerable to development and habitat fragmentation. Seasonally, amphibians migrate to breed in vernal pools. Roads and upland forest loss can jeopardize that migration, resulting in mortality. Vernal pools surrounded by fewer roads and more upland forest are of greater preservation priority in the management of amphibian populations. This study presents a statewide preservation prioritization of New Jersey's 13,594 vernal pools mapped in a previous work. The prioritization builds on that work by calculating a preservation score based on a combination of six characteristics regarding habitat fragmentation in the migratory area surrounding a vernal pool. The scores are calculated at a range of migration distances to account for a variety of amphibian species, and it is found that the different distances make little difference to preservation scores because of the overall small scale at which amphibians migrate. Results are presented that show clear parts of the state that should be prioritized for vernal pool preservation, mainly in areas where fewer roads lead to less development and habitat fragmentation. Areas of preservation priority are further refined using geographic clusters which show assemblages of vernal pools with favorable migratory areas. A special class of vernal pools has migratory areas that are uninterrupted by roads. These vernal pools and the assemblages of favorable migratory areas are often in preserved lands, but those that are not should receive special attention in both the field confirmation of amphibian habitat and subsequent protective measures, which can vary from conservation easements to citizen science interventions in sustaining local amphibian populations. Furthermore, surrounding states that also have a significant amount of vernal pools may be able to use this prioritization framework for preservation and conservation.
... Two important factors related to the species are motility (D'Amico et al. 2015) and the ability to avoid or escape from vehicles, with animals that run less oftensuch as amphibians-being more likely to be run over than others with a greater capacity of reaction to a vehicle-such as birds or some mammals (Mazerolle et al. 2005). Within an animal group, the risk of being hit will depend on the time of year, either because the species has different requirements depending on it, as is the case of the dependence of water sources in some amphibians and birds (Gibbs and Shriver 2005), or due to the variation of the activity periods, as may be the case of some reptiles with brumation, some mammals 1 3 42 ...
... Page 2 of 11 with hibernation, or amphibians with aestivation (Garriga et al. 2017;Fernández-López et al. 2022). In the latter, the probability of being run over increases in rainy periods and, particularly, after sunset, when they reach their maximum activity (Gibbs and Shriver 2005). In addition, the effects of weather conditions on roadkill risk can also be indirect, for example, by influencing the presence of possible prey on roads (Barrientos and Bolonio 2009). ...
... In the first place, detectability, since sampling has always been carried out during the daytime, and the carcasses of animals run over at night may not remain on the road the next day. As the period of greatest activity of amphibians is at night, and especially when precipitation and humidity are high (Gibbs and Shriver 2005), it is possible that at the time of sampling, either the carcasses have been run over by a lot of cars and are not visible anymore or have been predated (Mazerolle et al. 2005). In addition, the study year was dry for the area (only 187 mm 3 of precipitation), which could reduce the activity of these vertebrates and, consequently, their roadkill mortality (Jakob et al. 2003). ...
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Due to rapid human expansion in the last century, wildlife roadkill is becoming a concerning threat to biodiversity and human safety. The frequency of roadkill events depends on factors related to specific traits of the road—tortuosity or the presence of fences, among others—and the animal ecology—such as activity patterns, reproductive season, or thermoregulation. These, in turn, are related to environmental factors, with seasonal variations. Here, we assessed roadkill mortality of terrestrial vertebrates over the year. To do this, we sampled 10 road sections (of 3 km, by walk) in the south of Spain for a full year, registering the carcasses of run-over vertebrates. Then, we analysed the spatiotemporal patterns of roadkill events for the four vertebrates’ classes and the effects of road traits (presence of fence, tortuosity, distance to water point) and environmental variables (mean temperature and precipitation). Mammals suffered the highest mortality by roadkill (45.72%). The frequency of collisions was independent of tortuosity, presence of fences, and precipitation, while mean temperature significantly increased the probability of collision of mammals, birds, and reptiles. There was a seasonal effect in the number of collisions, which spatial pattern depended on the class of vertebrates. All this leads us to conclude that, to reduce the impact caused by roadkill mortality on wildlife, we need specific measures to be taken timely in each critical place and for each vertebrate group.
... Our results suggest that hoop nets could support the study of migration in amphibians and, when set near roads, could mitigate road mortality. Understanding migratory ecology is critical to conservation (Gibbs and Shriver 2005;Russell et al. 2005;Glista et al. 2008), and access to breeding sites promotes persistence of amphibian populations (COSEWIC 2012;Smith and Green 2005). Future studies could discriminate between migratory and non-migratory movements by deploying nets both during and outside migratory periods. ...
... Our methods did not allow quantification of road-associated mortality but capture of thousands of toadlets adjacent to a highway that represents a major mortality source for toads of all age classes (Ohanjanian 1997;Dulisse and Boulanger 2014;Dulisse 2015), and their relocation to upland habitat for overwintering likely supported the survival of captured individuals. Use of hoop nets to mitigate road mortality could be valuable given the threat roads pose to toad (Bull 2009;Matsuda et al. 2006) and other amphibian populations (DeMaynadier and Hunter 2000; Gibbs and Shriver 2005;Puky 2006;Glista et al. 2008;Beebee 2013). If replicated and quantified, this method could represent an alternative to road surveys in which technicians detect animals on roads, and transport them safely across, which often occur at night to accommodate nocturnal travel (Sinsch 1990; Schmetterling and Young 2008; Petrovan and Schmidt 2019). ...
... Large population sizes presumably increase the number of potential dispersers, allowing for genetic connectivity levels above previous expectations based solely on the results from individual dispersal kernels, while at the same time, large population sizes downplay the differentiating effect of genetic drift, lowering genetic distances between large populations while increasing those between small ones (Masel, 2011). While the effect of the road as a genetic barrier is limited in these species, it still has an impact on adult mortality rates (Gibbs & Shriver, 2005;Schmidt & Zumbach, 2008 were recaptures from alternate sides of the road ever detected. As in other urodeles, movement frequency of T. pygmaeus was much lower than in anurans, but this was not reflected in a marked genetic structure as in other Caudata analysed (P. ...
In the face of habitat loss, preserving functional connectivity is essential to maintain genetic diversity and the demographic dynamics required for the viability of biotic communities. This requires knowledge of the dispersal behaviour of target species, which can be modelled as kernels, or probability density functions of dispersal distances at increasing geographic distances. We present an integrative approach to investigate the relationships between genetic connectivity and demographic parameters in organisms with low vagility focusing on five syntopic pond‐breeding amphibians. We genotyped 1056 individuals of two anuran and three urodele species (1732–3913 SNPs per species) from populations located in a landscape comprising 64 ponds to characterize fine‐scale genetic structure in a comparative framework, and combined these genetic data with information obtained in a previous 2‐year capture–mark–recapture (CMR) study. Specifically, we contrasted graphs reconstructed from genomic data with connectivity graphs based on dispersal kernels and demographic information obtained from CMR data from previous studies, and assessed the effects of population size, population density, geographical distances, inverse movement probabilities and the presence of habitat patches potentially functioning as stepping stones on genetic differentiation. Our results show a significant effect of local population sizes on patterns of genetic differentiation at small spatial scales. In addition, movement records and cluster‐derived kernels provide robust inferences on most likely dispersal paths that are consistent with genomic inferences on genetic connectivity. The integration of genetic and CMR data holds great potential for understanding genetic connectivity at spatial scales relevant to individual organisms, with applications for the implementation of management actions at the landscape level.
... Furthermore, ditches with decantation pits or other drainage elements that often accompany linear infrastructure to conduct runoff can serve as deadly pitfall traps for amphibians (Bi et al., 2020). The strong effect of roadkill on amphibians is related to the slow pace of their terrestrial movement and the inability to perceive traffic risks (Gibbs and Shriver, 2005). More importantly, when migrating between breeding sites, foraging terrestrial grounds, and hibernacula, they often cross roads (Matos et al., 2012). ...
We test a forecasting strategy to identify potential hotspots of amphibian roadkill, combining the spatial distribution of amphibians, their relative risk of collision with vehicles and data on road density in Spain. We extracted a large dataset from studies reporting road casualties of 39 European amphibian species and then estimated the ‘relative roadkill risk’ of species as the frequency of occurrence of casualties for each amphibian and standardized by the range of distribution of the species in Europe. Using a map with the spatial distribution of Spanish amphibians at a spatial resolution of 10 × 10 Km squares, we estimated the ‘cumulative relative risk of roadkill’ for each amphibian assemblage as the sum of risk estimates previously calculated for each species. We also calculated the total length of roads in each square (road density). Finally, combining all layers of information, we elaborated a forecasting map highlighting the potential amphibian roadkill risk across Spain. Our findings are relevant to suggest areas that should be focused on at more detailed spatial scales. Additionally, we found that the frequency of roadkill was unrelated to the evolutionary distinctiveness score and conservation status of amphibian species, while was positively correlated with their distribution range.
... Carcasses may be removed from the location of the collision by the traffic or by scavengers, thus leading to uncertainty in the roadkill observation data. Thirdly, since currently existing large-scale data sources on road traffic are limited, the environmental variables representing the 'hazard' component of amphibian roadkill risks only included road density, although other road and traffic characteristics were found important in ex-plaining amphibian roadkill in many previous studies [81,[87][88][89]. In addition, our study was based on a cross-sectional design where amphibian roadkill and environmental variables were all assumed to be temporally static. ...
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Roads have major impacts on wildlife, and the most direct negative effect is through deadly collisions with vehicles, i.e., roadkill. Amphibians are the most frequently road-killed animal group. Due to the significant differences between urban and rural environments, the potential urban-rural differences in factors driving amphibian roadkill risks should be incorporated into the planning of mitigation measures. Drawing on a citizen-collected roadkill dataset from Taiwan island, we present a MaxEnt based modelling analysis to examine potential urban-rural differences in landscape features and environmental factors associated with amphibian road mortality. By incorporating with the Global Human Settlement Layer Settlement Model—an ancillary human settlement dataset divided by built-up area and population density—amphibian roadkill data were divided into urban and rural data sets, and then used to create separate models for urban and rural areas. Model diagnostics suggested good performance (all AUCs > 0.8) of both urban and rural models. Multiple variable importance evaluations revealed significant differences between urban and rural areas. The importance of environmental variables was evaluated based on percent contribution, permutation importance and the Jackknife test. According to the overall results, road density was found to be important in explaining the amphibian roadkill in rural areas, whilst precipitation of warmest quarter was found to best explain the amphibian roadkill in the urban context. The method and outputs illustrated in this study can be useful tools to better understand amphibian road mortality in urban and rural environments and to inform mitigation assessment and conservation planning.
... Both habitat types need to be functionally connected to facilitate amphibian dispersal and to ensure the persistence of local populations (Sinsch, 2014;Hamer et al., 2015). Roads constructed between terrestrial and aquatic habitats can prevent amphibians from accessing these habitat resources or result in high numbers of individuals being killed by traffic, causing population declines (Fahrig et al., 1995;Hels and Buchwald, 2001;Gibbs and Shriver, 2005). High road mortality rates often occur when amphibians are undertaking seasonal movements, such as during spring migrations to breeding ponds (Elzanowski et al., 2009;Brzeziński et al., 2012). ...
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Road traffic often inflicts higher mortality rates on amphibians than other vertebrates, especially where roads bisect migration pathways. To facilitate safe movements by amphibians between non-breeding and breeding habitats, under-road tunnels are being increasingly installed together with barrier fencing or walls. However, few observational studies have correlated aspects of road mitigation placement and design with amphibian population sizes. Here, we assessed the effectiveness of 13 under-road tunnels (ten cylindrical and three square-shaped) along a two-lane sealed road in northern Hungary positioned between upland forest habitat and a floodplain containing breeding ponds. Amphibian count surveys at tunnels and along road transects above tunnels were conducted at night during the spring migration period from 2009 to 2012. We detected a total of seven amphibian species, with the common toad (Bufo bufo) representing > 90% of individuals counted. Using community N-mixture modelling, we found that tunnels with larger-sized entrances and tunnels positioned near other tunnels had higher amphibian abundance. We also found that road mortality was higher above tunnels closest to breeding ponds for some species. Moreover, tunnel usage rates and road mortality rates were far lower and higher, respectively, than other studies that assessed similar species along European roads. These results imply that barrier walls and fencing were largely ineffective at directing amphibians towards the tunnels and were not preventing amphibians from accessing the road surface. Our results demonstrate the importance of placement and design in the usage of under-road tunnels by amphibians but underscore the need to maintain barrier fences and walls to reduce road mortality rates and connect amphibian habitats.
... Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society. (Bujoczek et al., 2011), spotted salamander (Ambystoma maculatum) (Gibbs & Shriver, 2005) and turtles (Congdon, Dunham & van Loben Sels, 1994). It is likely that, in many cases, road mortality is neither completely additive nor compensatory. ...
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In light of rapidly expanding road networks worldwide, there is increasing global awareness of the growing amount of mammalian roadkill. However, the ways in which road mortality affects the population dynamics of different species remains largely unclear. We aimed to categorise the demographic parameters in mammalian populations around the world that are directly or indirectly affected by road mortality, as well as identify the most effective study designs for quantifying population-level consequences of road mortality. We conducted a comprehensive systematic review to synthesise literature published between 2000 and 2021 and out of 11,238 unique studies returned, 83 studies were retained comprising 69 mammalian species and 150 populations. A bias towards research-intensive countries and larger mammals was apparent. Although searches were conducted in five languages, all studies meeting the inclusion criteria were in English. Relatively few studies (13.3%) provided relevant demographic context to roadkill figures, hampering understanding of the impacts on population persistence. We categorised five direct demographic parameters affected by road mortality: sex-and age-biased mortality, the percentage of a population killed on roads per year (values up to 50% were reported), the contribution of roadkill to total mortality rates (up to 80%), and roadkill during inter-patch or long-distance movements. Female-biased mortality may be more prevalent than previously recognised and is likely to be critical to population dynamics. Roadkill was the greatest source of mortality for 28% of studied populations and both additive and compensatory mechanisms to roadkill were found to occur, bringing varied challenges to conservation around roads. In addition, intra-specific population differences in demographic effects of road mortality were common. This highlights that the relative importance of road mortality is likely to be context specific as the road configuration and habitat quality surrounding a population can vary. Road ecology studies that collect data on key life parameters, such as age/stage/sex-specific survival and dispersal success, and that use a combination of methods are critical in understanding long-term impacts. Quantifying the demographic impacts of road mortality is an important yet complex consideration for proactive road management.
... The lack of movement could be an adaptation to the urban environment to avoid mortality by roadkill, one of the major threats to amphibian wildlife worldwide [63][64][65]. Furthermore, an overall decrease in activity could also lower energy expenditure, which might be advantageous for toads in urban environments, usually offering lower prey abundance [66]. ...
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Advancements in tracking technologies provide an increasingly important tool in animal monitoring and conservation that can describe animal spatial behavior in native habitats and uncover migratory routes that otherwise may be difficult or impossible to map. In addition, high-resolution accelerometer sensors provide powerful insights into animal activity patterns and can help to identify specific behaviors from accelerometer profiles alone. Previously, such accelerometers were restricted to larger animals due to size and mass constraints. However, recent advances make it possible to use such devices on smaller animals such as the European green toad (Bufotes viridis), the focus of our current study. We deploy custom made tracking devices, that consist of very-high-frequency transmitters and tri-axial accelerometers, to track toads in their native urban environment in Vienna (Austria). A total of nine toads were tracked, ranging from three to nine tracking days per individual during the post-breeding season period. We demonstrate that our devices could reliably monitor toad movement and activity during the observation period. Hence, we confirmed the predominantly nocturnal activity patterns and recorded low overall movement at this urban site. Accelerometer data revealed that toads exhibited brief but intense activity bursts between 10 pm and midnight, resting periods during the night and intermittent activity during the day. Positional tracking alone would have missed the major activity events as they rarely resulted in large positional displacements. This underscores the importance of and value in integrating multiple tracking sensors for studies of movement ecology. Our approach could be adapted for other amphibians or other animals with mass constraints and may become standard monitoring equipment in the near future.
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Road mortality is suspected to have contributed to widespread population declines in turtles in the United States, a country with exceptionally high turtle diversity. We examined the issue through a modeling study that integrated road maps and traffic-volume data with simulated movements of (1) small-bodied pond turtles, (2) large-bodied pond turtles, and (3) terrestrial and semiterrestrial (“land”) turtles. Our model predicted that road networks typical of the northeastern, southeastern, and central regions have the potential to limit land-turtle populations and, to a lesser extent, populations of large-bodied pond turtles. Nowhere are populations of small-bodied pond turtles likely threatened regionally by road mortality. We conclude that the demographic traits of some turtles, in combination with their mobility, may jeopardize population persistence within road networks typical of the eastern and central United States.
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During a 7-yr population on the wood frog, Rana sylvatica, the breeding population size fluctuated by a factor of 10 and juvenile production by a factor of 100. Variation in the adult population among years was largely due to variation in juvenile recruitment. Annual net replacement rates (R"o) varied from 0.009 to 7.49. Survivorship curves (calculated using the number of eggs deposited as the initial point) showed that most variation in the proportion of individuals surviving to adulthood was due to variation on larval survival; juvenile and adult survival was relatively constant among years. Male and female survival did not differ. Because females matured a year later than males, on average 2.3 times as many males as females from a given clutch survived to breed. This difference accounted for the observed male-biased sex ratio in breeding choruses. Premetamorphic survival and size at metamorphosis were negatively correlated with the number of eggs deposited. Length of larval period was positively correlated with number of eggs deposited. Survival was higher among juveniles that metamorphosed early and were large at metamorphosis. Larger juveniles matured earlier and were also larger as adults. The population appeared to be regulated through density-dependent factors affecting larval survival, larval size, and time of metamorphosis. Adult population size also negatively affected total clutch volume. Mean monthly rainfall positively affected adult survival.
(1) As the population size breeding in the lake remained fairly constant at around 6300 males and 2300 females over a 4-year period (1978-1981), it appeared to be under some form of density-dependent control. The survival rate between years was approximately 0.52 for males and 0.40 for females, and survival appeared to be independent of age. (2) About 3000 males and 1400 females entered the breeding population each year; the lower number of females being explained by a delay of one year in maturation. This delay and the higher mortality rate of females produced a 3: 1, male: female, sex ratio at the lake. Emigration to nearby breeding sites was low and individuals appeared to breed every year. (3) There was a high level of deformities which did not appear to have a marked effect on survival. The effect of road casualties on the population was minimal.
In view of an extensive road system, abundant and rapidly growing vehicular traffic, and a scattered literature indicating that some ecological effects of roads extend outward for >100 m, it seems likely that the cumulative ecological effect of the road system in the United States is considerable. Two recent studies in The Netherlands and Massachusetts ( U.S.A.) evaluated several ecological effects of roads, including traffic noise effects, and provide quantitative evidence for a definable “road-effect zone.” Based on the approximate width of this asymmetric convoluted zone, I estimate that about one-fifth of the U.S. land area is directly affected ecologically by the system of public roads. I identify a series of assumptions and variables suggesting that over time this preliminary estimate is more likely to rise than drop. Several transportation planning and policy recommendations, ranging from perforating the road barrier for wildlife crossings to closing certain roads, offer promise for reducing this enormous ecological effect.
Vehicular traffic can be a major source of mortality for some species. Highly vagile organisms may be at a disadvantage in landscapes with roads because they are more likely to encounter roads and incur traffic mortality. To test this prediction, we assessed the population abundance of two anuran species of differing vagility, the leopard frog ( Rana pipiens, more vagile) and the green frog ( Rana clamitans, less vagile), at 30 breeding ponds. Traffic density, an index of the amount of potential traffic mortality, was measured in concentric circles radiating from the ponds out to 5 km. We conducted multiple linear regressions relating population abundance to traffic density, pond variables, and landscape habitat variables and found that leopard frog population density was negatively affected by traffic density within a radius of 1.5 km. There was no evidence that the presence of vehicular traffic affected green frog populations. These results suggest that traffic mortality can cause population declines and that more vagile species may be more vulnerable to road mortality than less vagile species.
This books presents papers given at the April 1989 conference Global Climate Change and Life on Earth: Evidence, Predictions and Policy. In general, the text is written to provide understanding for non-professionals, but there is considerable variation. Each chapter is virtually independent.