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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
1,
* and W. Gregory Shriver
2
1
College of Environmental Science and Forestry, State University of New York, 350 Illick Hall, 1 Forestry
Drive, Syracuse, NewYork 13210, USA;
2
U.S. National Park Service, Marsh-Billings-Rockefeller National
Historic Park 54 Elm Street, Woodstock, VT 05091, USA; *Author for correspondence (e-mail:
jpgibbs@syr.edu; 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
Abstract
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
2
of
landscape and traffic volumes >250 vehicles/lane/day within the migration range of a breeding population
of spotted salamanders.
Introduction
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
otherwise.
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).
Methods
Estimating mortality in relation to traffic volume
The probability that a salamander would be killed
as it attempted to cross a road, p
killed
, was esti-
mated based on an equation adapted from Hels
and Buchwald (2001):
p
killed
¼ 1 e
Na=v
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
282
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
road
) in spot-
ted salamanders was estimated as
d
road
¼ 1 ð1 p
killed
Þ
n
crossings
where p
killed
is the probability a salamander is
struck by a vehicle on any given road crossing (see
above) and n
crossings
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.
283
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
n
crossings
and road density based on actual road
networks (Figure 2). Salamander movements were
simulated in landscapes within a 7224 km
2
region
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
(http://www.state.ma.us/mgis/massgis.htm) 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
2
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
crossings
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).
284
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,t
)in
a given year (t) as:
N
a;tþ1
¼ N
a;t
r
a
þ N
e;t
r
m
ðK
l
N
e;t
Þ=K
l
r
j
where (N
e,t
) is the number of eggs produced in a
given year (t):
N
e;tþ1
¼ N
a;t
r
a
u and
r
a
= adult annual survival rate, u
m
= average
eggs produced per individual, r
m
= survival rate
from egg to metamorphosis, r
j
= survival rate of
juveniles through their first winter, K
l
= 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
a
, was estimated
at 0.7, survival rate of egg to metamorphosis was
estimated at 0.04, and r
j
= 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
l
, 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
a
= 0.4, 0.5 and 0.6), each
reflecting varying levels of additive road mortality
in relation to the scenario of no road mortality
(r
a
= 0.7).
Results
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
killed
= 1.537*vehicles/min
0.0048, R
2
= 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
Massachusetts.
285
Estimating cross ing frequency in relation to road
density
The relationship between road density (RD) and
expected road-crossing frequency (n
crossings
)
(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,
n
crossings
= 0.0658*RD (R
2
= 0.81) and 500 m,
n
crossings
= 0.356*RD (R
2
= 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).
286
of somewhere between 10 and 20% likely occurs
above which population declines owing to road
mortality would be expected to ensue.
Discussion
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
persistence.
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.
287
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
2
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).
288
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.
Acknowledgments
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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... Roads negatively impact multiple life stages of amphibians (i.e., waterbody salini cation via road salt, tra c noise impacting frog mating calls, etc.) (Taylor and Goldingay, 2010;Coelho et al., 2012;Beebee, 2014;Kioko et al., 2015). Temperate amphibians generally have two distinct life stages: an aquatic egg/larval stage and a terrestrial (or semi-terrestrial) adult stage (Gibbs and Shriver, 2005). Therefore, most amphibians require a habitat matrix that includes both wetlands and upland habitats to persist (Bradford, 1983;Lamoureux and Madison, 1999;Joly et al., 2003). ...
... Fragmentation due to roads can lead to extreme mortality events when amphibians cross roads to breed and return to the uplands on rainy nights in the spring, summer and fall (Mazerolle, 2004;Sterrett et al., 2018). This mortality can be severe enough to cause local extirpations (Gibbs and Shriver, 2005). ...
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Roads pose significant threats to wildlife populations worldwide, leading to habitat fragmentation and high mortality rates among various species. Mitigation strategies such as wildlife underpasses have been implemented to alleviate these impacts, yet few studies have assessed their effectiveness before and after implementation. We conducted a case study in the northeastern United States to evaluate the efficacy of a wildlife underpass complex in mitigating amphibian road mortality. The study area encompassed a 1.3 km stretch of road, where two underpasses were constructed to facilitate amphibian passage. Through a comprehensive survey spanning five years pre-construction and seven years post-construction, we collected data on amphibian mortality and environmental factors. Linear mixed-effects models were used to assess changes in mortality rates before and after underpass construction using a before-after control-impact design. Our findings indicate a substantial reduction in mortality across the entire amphibian community and for non-arboreal amphibians within treatment areas post-construction. While arboreal amphibian mortality decreased, the difference was not statistically significant. The underpasses effectively facilitated amphibian movement, with observed usage by various species, including arboreal individuals. Overall, our study provides empirical evidence of the effectiveness of wildlife underpasses in reducing amphibian road mortality, highlighting them as a potentially important conservation action. These findings underscore the significance of incorporating underpass structures into transportation planning and infrastructure development to mitigate negative impacts on wildlife populations. Moreover, our study contributes valuable insights for future research and informs policy initiatives aimed at enhancing habitat connectivity and safeguarding vulnerable amphibian populations in environments bisected by roadways.
... However, mortality may be higher than reported, and mass mortality events more frequent, as detection of such events required at least seven repeated surveys for a certainty of 85% (Hallisey et al. 2022). Unfortunately, a mortality rate over 10% may lead to local extirpations, and 22-73% of the populations in central and western Massachusetts may exceed this threshold (Gibbs and Shriver 2005). Applying this criterion, several of the populations we reviewed are likely imperiled by mortality on roads (Appendix). ...
... Paved roads can greatly influence salamander populations, with several papers raising concerns about potential extirpation of populations due to road and urban development (Barry and Shaffer 1994; Gibbs and Shriver 2005;Stranko et al. 2010; Bieri and Leonard 2019). Terrestrial salamanders were lower in number, stream salamanders occurred less frequently, egg masses were smaller, and overall salamander diversity was lower near paved roads (Diller and Wallace 1994;Gibbs 1998;Foster et al. 2004;Porej et al. 2004;Bowles et al. 2006;Miller et al. 2007;Price 77 Page 8 of 18 ...
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Context Road expansion has raised concerns regarding road effects on wildlife and ecosystems within the landscape. Salamanders, critical ecosystem components and bioindicators, are vulnerable to road impacts due to habitat loss, migrations, and reliance on stream health. Systemic reviews considering the effects of different road types on salamanders are lacking. Objectives We summarize 155 studies of road effects on salamanders, including paved, unpaved, and logging roads, hiking trails, railroads, and powerlines. We examine trends in road type, study area, and impacts on salamanders; summarize current knowledge; and identify knowledge gaps. Methods We used Web of Science for literature searches, completed in January 2023. We reviewed and summarized papers and used Chi-squared tests to explore patterns in research efforts, research gaps, and impacts on salamanders. Review Roads had negative effects on salamanders through direct mortality, damaging habitat, and fragmenting populations. Traffic and wetland proximity increased negative impacts in some studies; abandoned logging roads showed negative effects. Positive effects were limited to habitat creation along roads. Habitat creation and under-road tunnels with drift fencing were effective mitigation strategies. Non-passenger vehicle roads were critically understudied, as were mitigation strategies such as bucket brigades and habitat creation along roads. Conclusions With road networks expanding and salamander populations declining, managers must account for road effects at landscape scales. The effects of non-paved roads on salamanders are poorly understood but critically important as such roads are frequently located in natural areas. Managers should incorporate mitigation strategies and work to reduce road impacts on vulnerable wildlife.
... This movement often brings them across roads, heightening their vulnerability to vehicular collisions (Hels & Buchwald, 2001;Fahrig et al., 1995). Studies in other regions, such as Europe and North America, consistently show that amphibians suffer high mortality rates in road-adjacent habitats, underscoring that this is a widespread conservation issue in biodiversity-rich areas (Beebee, 2013;Gibbs & Shriver, 2005). ...
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The rapid expansion of road networks presents significant challenges to wildlife, particularly in biodiversity hotspots like the Mudumalai Tiger Reserve (MTR) in Tamil Nadu, India. This study evaluates the ecological impact of roads on wildlife within the MTR by analysing roadkill data collected over ten months across three road segments: an interstate highway, a state highway, and a secondary road. A total of 343 roadkill incidents were recorded, spanning 42 species, with reptiles (30.32%) and amphibians (27.41%) being the most affected groups. Among the road segments, the interstate highway exhibited significantly more roadkills compared to the state and secondary roads, especially for reptiles and amphibians. Roadkill rates were higher in Dry Deciduous Forest habitats than in Dry Thorn Forests. Notably, the Three-striped Palm Squirrel, Jungle Babbler, and Garden Lizard were identified as the most susceptible species in their respective taxa. This study underscores the urgent need for targeted mitigation measures, such as wildlife crossings and speed restrictions, to reduce road-related mortality in MTR and similar landscapes across India, thereby contributing to biodiversity conservation efforts.
... This analysis also indicates which roads could be considered for road signs, road crossing brigades, seasonal road closures, and road tunnels/bridges. Roads with > 250 cars/day have been documented as being essentially impenetrable to Ambystoma maculatum (Gibbs and Shriver 2005). With 65% of breeding ponds across the range of SCLTS having a road with > 1000 cars/day within migration distance, road mortality constitutes a major threat to species persistence. ...
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Many organisms with complex life cycles rely on both terrestrial and aquatic habitats to survive, which increases their susceptibility to habitat fragmentation as they require access to sufficient amounts of both habitats and connectivity between them. Amphibians are particularly susceptible to fragmentation and are declining globally. We conducted the first range-wide geospatial analysis for the federally endangered Santa Cruz Long-toed Salamander (SCLTS; Ambystoma macrodactylum croceum ) to address the impacts of land use change and habitat fragmentation as barriers to recovery. First, we used data from an extensive drift fence array to determine migration distances of SCLTS. We then used these calculated distances to determine the amount of suitable and accessible habitat around all current breeding ponds as well as those being considered as potential release sites. Land use changes have reduced the amount of suitable upland habitat within migration distance of SCLTS breeding ponds by 34% across the range. Habitat fragmentation due to roads has further reduced uplands by another 12% and sea level rise projected by 2060 reduces it another 14%, leaving only 40% of potential terrestrial habitat suitable, accessible, and unflooded. Based on a population viability analysis (PVA) developed for the congeneric California tiger salamander, this would render only 24% of SCLTS breeding populations viable in the long term based on terrestrial habitat quality. This range-wide assessment provides guidance on which breeding populations should be targeted for land use restoration and experimental road crossing structures, and which potential breeding sites should be prioritized for release of captive-bred animals.
... Roads with a few thousand cars per day, per lane, become nearly impossible to cross for slower moving wildlife, such as herpetofauna (Gibbs and Shriver 2002). The additive mortality caused by traffic can significantly affect long-term population dynamics of turtles and amphibians (Congdon et al. 1993, Congdon et al. 1994, Gibbs and Shriver 2005. ...
... For example, the association between movement patterns and habitat fragmentation in mammalian taxa is a well-documented conservation and management issue (Crooks 2002, Gardiner et al. 2018, Tucker et al. 2018, Wattles and DeStefano 2013. Similarly, road kills of turtles during nesting season (Gibbs and Shriver 2002, Haxton 2000, Piczak et al. 2019) and amphibian mortality during spring migration (Fahrig et al. 1995, Gibbs andShriver 2005) have directly resulted in local population declines. As others have noted, characterizing local-scale movement patterns, such as variation in migration and home ranges, is important when developing effective conservation plans and making management decisions (Hagani et al. 2021. ...
... Many factors such as temperature, precipitation and wetlands near roadsides affecting the activity and behavior of amphibians, increase the mortality rate of amphibians on roads (Ashley et al., 1996;Červinka et al., 2015;Özcan and Özkazanç, 2020). Asphalt and concrete road surfaces absorb heat, so amphibians use roads to sunbathe and regulate their body temperature (Andrews and Gibbons, 2005;Gibbs and Shriver, 2005). Because many amphibian species are small and move slowly, making it difficult for drivers to see them and this increases the risk of being killed on the roads (Mazerolle, 2005;Arpacık, 2022). ...
... The breeding season is the critical period for amphibians when the number of individuals exposed to roadkills increases rapidly (Orłowski et al., 2008). For amphibians that migrate to their areas of breeding, road mortality recorded at grades changing from 19% (Gibbs & Shriver, 2005) to 98% (Hels & Buchwald, 2001). The highest peaks are generally recorded during spring (the starting season for breeding migration) under temperate zone circumstances. ...
Article
The Polish Roadkill Observation System (PROS) database, a large dataset of roadkills collected between 2000 and 2022 in Poland, was used. We calculated the total length for each road type and the main type of environment around the wildlife-vehicle collision (WVC) event, in a grid of 10 × 10 km (e.g., spatial unit). We explored the spatial congruence in WVCs among amphibians, reptiles, birds and mammals across the country, using spatially explicit correlation based on the Mantel tests. We used a) Generalized Linear Mixed Models to investigate the association between WVC and the type of dominant environment and animal group, and b) Generalized Boosted Regression Models to investigate, separately for each animal group, the association between WVC and the length and type of road in each spatial unit. A total of 19,846 roadkills were recorded in Poland, involving 28,952 individuals from different animal species: 14 amphibians, 8 reptiles, 133 birds and 52 mammals. The spatial distribution of roadkill events in the country was mainly clustered around the biggest cities. Hotspots were concentrated near cities (Warsaw, Krak´ow, Rrzesz´ow) and in areas known for high biodiversity. Coldspots - relatively smaller than hotspots - were areas characterized by a high density of housing infrastructure, with lower naturality and a predominance of single roadkill casualties. A higher spatial congruence in WVC was found between birds and mammals (71 %) than between the other animal groups. Overall, the animal group less congruent with the other groups was amphibians (13 %), while birds were most congruent with all groups. We discussed some advantages and drawbacks when working with non-systematic survey datasets of roadkills. Finally, we recommended including roadkill clusters of multiple animal groups (hotspots) in strategies for mitigating wildlife-vehicle collisions but also considering more specific strategies, which can combine the type of environment and roads, concerning each animal group.
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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.
Article
(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.
Article
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.
Article
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.
Book
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.