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ORIGINAL PAPER
Amphibian road mortality in Europe: a meta-analysis
with new data from Poland
Andrzej Elzanowski &Joanna Ciesiołkiewicz &
Mirella Kaczor &Joanna Radwańska &Radosław Urban
Received: 27 December 2007 /Revised: 6 June 2008 / Accepted: 16 June 2008 / Published online: 17 July 2008
#Springer-Verlag 2008
Abstract While the increasing vehicular traffic is widely
suspected to play a role in the worldwide amphibian popu-
lation decline, the research of amphibian road mortality is
scarce, fragmented, fraught with methodological problems,
and largely inconclusive. As the first attempt at a synthesis, we
analyzed all available data on amphibian mortality in Europe
and combined them with four previously unpublished surveys
conducted by us. Based on our recalculation of road-kill
counts in terms of species-specific road-kill recordability, we
conclude that, in lowland Central Europe, the common toads,
Bufo bufo, are the most common victims of vehicular traffic
in suburban landscapes, while the common frogs, Rana
temporaria and Triturus newts, prevail in rural landscapes.
The green frogs also tend to be more frequent in rural areas.
Common tree frogs, Hyla arborea, are unexpectedly rare in
the road-kill record despite their terrestrial and migratory habits.
In consideration of problems with obtaining accurate amphib-
ian population estimates, we further propose the road kills-to-
spawners ratio (R/S) as a working measure of the impact of
road mortality on a local population. While the R/Sratio may
not reflect the losses to an entire local amphibian population,
it is free of the errors of whole-population estimates, which
are notoriously difficult for amphibians. When corrected for
species-specific road-kill recordability, most results suggest
that the impact of roads on newts may have been under-
estimated and that the impact on common frog populations
may be higher than on those of common toads.
Keywords Amphibian conservation .Road ecology .
Habitat fragmentation
Introduction
The negative impact of road mortality on local populations of
at least some amphibian species (van Gelder 1973; Fahrig et
al. 1995;HelsandBuchwald2001; Cooke and Sparks 2004;
Mazerolle 2004;Pelletetal.2004; Gibbs and Shriver 2005)
implicates the rapid increase of traffic intensity in many parts
of the World as a factor of the global amphibian population
decline (Puky 2006). However, the quantification of this
impact on populations of any particular species is a complex
task that requires substantial research effort far beyond road-
kill counts and taking into account the relative roles of
natural mortality, especially its density-dependent mecha-
nisms (Berthoud and Müller 1986;HelsandBuchwald2001;
Harper and Semlitsch 2007). This is probably why there are
very few, only three in Europe (van Gelder 1973;Helsand
Buchwald 2001; Cooke and Sparks 2004), conclusive studies
specifically of the impact road-crossing mortality (rather than
roads in general) on amphibian populations.
In this paper, we provide new data on amphibian road
mortality from four widely different habitats of Lower Silesia,
a southwestern province of Poland, and review the published
data on amphibian road mortality in Europe with an attempt to
make them better comparable and thus more useful to assess
the impact of road mortality on amphibian populations.
Having performed breeding censuses in all four study sites,
we calculated the populational impact of road mortality in
terms of a road kills-to-spawners ratio (R/S).Ourworkpoints
Eur J Wildl Res (2009) 55:33–43
DOI 10.1007/s10344-008-0211-x
Electronic supplementary material The online version of this article
(doi:10.1007/s10344-008-0211-x) contains supplementary material,
which is available to authorized users.
Communicated by H. Kierdorf
A. Elzanowski (*):J. Ciesiołkiewicz :M. Kaczor :
J. Radwańska :R. Urban
Department of Zoology, University of Wrocław,
21 Sienkiewicz Street,
50-335 Wrocław, Poland
e-mail: elzanowski@biol.uni.wroc.pl
to the need of methodological improvements in amphibian
road mortality studies and additional basic research in am-
phibian population biology.
Materials and methods
We analyzed all available (the total of 14) published reports
with any usable quantitative information on amphibian road
mortality in Europe (Table 1), the majority of them coming
from Central Europe. A literature search on Zoological Record
did not reveal other reports except for several qualitative and
more or less casual observations.
As far as warranted by the monitoring methods, we re-
calculated both others’and our own road-kill counts using
species-specific correction factors, that is, the ratios of all
casualties to the those found by routine monitoring. Following
Hels and Buchwald (2001), we used 1.88 for common toads,
Bufo bufo, 3.15 for brown frogs, Rana temporaria and Rana
arvalis,1.5forcommonspadefoots,Pelobates fuscus,and
15 for the Triturus newts. For other species, we used ten-
tative correction factors, 5 for common tree frogs, Hyla
arborea, 4 for Bombina toads, 3 for the green frogs, Rana
lessonae,Rana ridibunda,andRana kl. esculenta,and2for
the green toads, Bufo viridis, which were interpolated from
Hels and Buchwald’s(2001) experimental values by taking
into account body size, visibility, and palatability. Although
they may prove too conservative, especially for the tree frogs
and Bombina toads, their use was unavoidable given the
variation in methodology of published road mortality studies.
We studied amphibian mortality on four secondary, two-lane
roads in four widely different landscapes of Lower Silesia, SW
Poland for two consecutive years (including two breeding
seasons). All four surveyed road sections were selected because
of the known abundance of amphibians in their environs. Two
of them cross rural areas, one a mosaic suburban landscape and
the other a street in a small town. Vehicular traffic on all four
was in the range of 400–500 vehicles/day. All surveys were
performed by walking one road side there and the other back.
The four surveyed road sections were as follows:
Chelmski Pond (in the Bory Dolnośląskie conifer forest
complex, near ChełmŻarski village of 90 inhabitants,
Lubsko county, Żary district, Lubuskie province, 51°47′N,
14°54′E). The 500-m road section runs between pastures
and meadows on one side and a pine forest surrounding a
large, 41-ha pond (reaching to 30 m from the road
shoulder) on the other. The surveys were performed three
to four times per week between 6:00 and 7:00 A.M. for
over1year(9Marchto9November2001and1Feb-
ruary to 15 July 2002).
Stobrawa Forest (Kluczbork county, Opole province,
50°56′N, 18°06′E). The 500-m road section crosses
hydrophilic broad-leaved woodland with the predom-
inance of the alders, Alnus glutinosa, and birches,
Betula verrucosa, and a 0.8-ha pond located some 300
from the road and connected by a creek to a roadside
ditch. The surveys were performed three to four times
per week at sunrise for over 1 year (1 March 2000 to
15 June 2001).
Wrocław suburb (the valley of Bystrzyca river, 51°07′N,
17°02′E). The 1,800-m road section crosses a mosaic
landscape of a small river valley with broadleaved woods
including fragments of natural riverside woodland Salici-
Populetum, a park, and interspersed fields (mostly aban-
doned cropland), eight pools and small ponds (0.16–
1.0 ha, jointly 2.1 ha) with lush hydrophilic vegetation,
and a narrow creek (1–1.5mwide)crossingtheroad.
The closest pond is located 200 m from the road, but
flood waters come much closer to the road in the spring.
The surveys were performed every other day throughout
the amphibian migration season (March to May) and
one to two times a week in other months between 6:00
and 9:00 A.M. for over 2 years (1 October 2002 to 30 June
2004) including two breeding seasons.
Town of Lubsko (Żary district, Lubuskie province, 51°
46′N, 14°56′E). The 200-m street section in the
outskirts of a small town (16,000 inhabitants) lies
adjacent to a school playing ground with a 25-m
2
fire-
emergency concrete reservoir overgrown by water
plants (Schoenoplectus lacustris,Typha angustifolia,
Lemna minor). Thirty-seven counts of green toad
casualties were performed in two breeding seasons
(1 May to 17 July), 16 in 2001, and 21 in 2002.
Our censuses of spawning anurans were performed pri-
marily by counting individuals, spawn clumps, and strings
and by registering calls of green frog males. Most difficult
were the counts of newts breeding in Stobrawa Forest: The
newts were captured three times per season by two persons in
the pond, but their numbers in the ditch could be only
estimated.
Results
Road-kill densities
The recorded absolute road-kill densities (Table S1) are clear-
ly not comparable because of differences in sampling meth-
ods and the length of transects and thus can only provide
orders of magnitude of amphibian road-crossing mortality. In
the amphibian fatality hotspots, primarily in rural areas, road-
kill densities amount to thousands per kilometer per year
over short road sections and those in suburban areas are in
the range of 100–500 km
−1
year
−1
. The two highest am-
34 Eur J Wildl Res (2009) 55:33–43
Table 1 European studies of amphibian road mortality (in chronological order) with the four roads surveyed in this study at the bottom
Site/area, road length, traffic
density, and authority
Survey period, frequency and
method [breeding season (BS)]
Habitats/landscape Species
a
in the order of
decreasing road-kill frequency
Nijmegen area,
“Overasseltse en hatertse
vennen”nature reserve,
Holland, 1.5 km, 336
vehicles/day (9.4 vehicles/h
during toad migration)
(van Gelder 1973)
1 BS (22.II–15.V), continuous
recording every night “until no
or hardly any animals were
seen”
Woodland (unspecified) Only B.b. studied
Białowieża Forest, E Poland,
17 km, no traffic data
(Wołk1978)
19 counts (13.V–24.IX),
mornings, by motorcycle,
30 km/h, after removing or
marking older casualties on
the preceding morning
Natural mixed forest with
2 creeks
R.t.,B.b.
Denmark, 15,224 km various
roads, from highway to
secondary, no traffic data
(Hansen 1982: Table 1)
One-time surveys over 3 years,
on moped, “very slowly”
Diverse habitats B.b,BF(R.d. and R.a.)
b
,B.v.,
T.c.,R.e
Berlin woods, Germany,
4.7 km, max. 380 (at one end)
to 2,000 vehicles/day (Miech
1988)
10 years, irregularly, 2–3 times
a week at variable day time,
by bike or car, occasionally
by foot
Broadleaved and conifer forest,
7 pools, one supplied by a creek
B.b., R.t., T.v.,GF,T.c., R.a.
Osnabrück area, Germany,
8.5 km across woodland with
2,659 vehicles/day and fields
with 780 vehicles/day
(Fuellhaas et al. 1989)
1 incomplete year, every 5 days,
no day time information,
by foot
6.1 km across fields, 2.4 km
across woodland
B.b.,R.t.,S.s.,T.a.,T.c.,GF
Pieniny National Park,
S Poland, no road and traffic
data (Rybacki 1995)
42 days (27 spring, 15 fall),
irregular surveys most counts
on one road side only, then
multiplied by 2, otherwise
methods and times unspecified
Various (in part thermophilic)
habitats of the Pieniny Klippen
Belt and its surroundings
B.b.,R.t.,B.v.,T.mo.,T.a.,
Bo.v.,S.s.,T.c.,T.v.
Sromowce Wyżne, Dunajec
Valley, Pieniny Mts, S Poland,
2 km, 60–120 vehicles/h in
the morning (Zamachowski
and Plewa 1996)
2 incomplete BS (IV-V), morning
counts (0700–0900 hours) by
unspecified methods
Between a new reservoir and
an open field with ditches and
vernal pools
R.t.,T.a.,B.b.,T.mo.,T.v.,
B.v.,S.s.
Warta river delta, Słońsk
Nature Reserve, W Poland,
11.3 km, no traffic data
(Bartoszewicz 1997)
1 year, by bike or car, twice a
week, usually on two
consecutive (!) work days
A mosaic of mostly open habitats
including marshland
BF and GF, B.b.,B.v.
Western France (Department
Vendée), 68.2 km of a new
road (opened preceding fall),
“200–450 vehicles/km”
(Lodé 2000)
c
1 incomplete season (7.5 months,
IV-XI), 33 weekly surveys by
car at <40 km/h and by foot
Unspecified B.b. (56.7% of all amphibians)
and 4 unspecified anurans,
T.h. (28.4% of all
amphibians) and 3
unspecified salamandrids
Djursland Peninsula, N
Denmark, 0.6 km,
3,200 vehicles/day
(Hels and Buchwald 2001)
altogether 6 months: incomplete
BS (IV-V) and VIII of 2
consecutive years
Farmland with small woods and
numerous ponds
BF, B.b.,Tv. and T.c.,P.f.
b
Wielkopolska landscapes,
W Poland, ca. 1,500 km,
no traffic data (Rybacki and
Krupa 2002)
2–3 morning surveys in one
spring (IV–V), presumably
by car
Various habitats in 11 protected
landscape areas
B.b.,B.v.,R.a.,R.t.,P.f., and
single Bo.b. (misnamed as
Bo.v.), H.a.,R.e.,R.r.
Oleśnica fish farm, Chodzież
county, Wielkopolska
Province, W Poland, 1,137 m,
1 BS (16.III–16.VI), by
unspecified means, every 2 days
afternoon (1500–1600 hours)
420 m across mixed woodland
with some ponds and an inn;
700 m across gardening lots and
T.v. ,B.b.,R.t.,R.a.,R.e.,Bo.b.,
P.f.,T,c.,R.l.,B.v.
Eur J Wildl Res (2009) 55:33–43 35
Table 1 (continued)
Site/area, road length, traffic
density, and authority
Survey period, frequency and
method [breeding season (BS)]
Habitats/landscape Species
a
in the order of
decreasing road-kill frequency
around 500 vehicles/day
(36 vehicles/h between 1400
and 1800 hours) (Rybacki and
Domańska 2004)
fields with many ponds; a creek
crossing the road and flowing to
the ponds
Zielona Góra woods,
W Poland, 5.8 km, 7,000–
8,500 vehicles/day (Najbar
et al. 2006)
2 years (altogether 1,900 surveys
on 437 days), 2–10 (mean 4.3)
times a day, regularly morning
(0600–0800, 1400–1600, and
1700–2300 hours); mostly by
car, occasionally by foot
Pine and mixed forests, 4 creeks
expanding into ponds
B.b.,R.t.,B.v.,P.f.,T.v.,R.e.,
T.a.,T.c., and R.a
Wrocław Plain ca. 55 km
2
around Wrocław, Dolny Śląsk
(Lower Silesia) Province,
48.8 km, 350–10,500 (mostly
350–479) vehicles/day
(Orłowski 2007)
Over 2 years including 2 BS,
thrice a week 15.III-30.IX,
twice in the remaining periods,
by car (20–50 km/h) and
occasionally (through migration
season) by foot on rain-free
afternoons
Arable land (92%) with very
little forest (1.6%) and some
built-up areas, and 47 pools and
ponds (total area 6.7 ha) within
200 m of the road
Only B.b. studied
Portalegre District, S Portugal,
26 km, 5,000 vehicles/d
(Ascensao and Mira 2006)
c
2 years, 54 surveys every
2 weeks, by car 20 km/h
Mostly agro-forestry (montado),
some pastures, meadows fields
and olive groves
B.c. 36%, P.c. 25%, B.b. 14%,
S.s. 12.5%, P. w. 8%, D.g.
1.4%, T.ma. 0.9%, H.m.
0.4%, R.p. 0.4%, A.c. 0.2%
Chełmski Pond, SW Poland,
0.5 km, 450 vehicles/day
(this study)
Over 1 year including 2 BS, 3–
4 per week, 6–7 a.m., by foot,
there on one road side and back
on the other; see also “Material
and methods”
Between pastures and meadows
on one side and a pine forest
surrounding a large pond on the
other; see also “Material and
methods”
R.t.,B.b.,R.a.,GF(R.e and
R.l.), T.c ,P.f.,Bo.b.,H.a.
Stobrawa Forest, SW Poland,
0.5 km, 400 vehicles/day
(this study)
Over 1 year including 2 BS, 3–
4 per week starting at sunrise,
by foot, there on one road side
and back on the other; see also
“Material and methods”
Hydrophilic broad-leaved
woodland, a pond some 600
from the road, a creek
connecting the pond to the
roadside ditch; see also
“Material and methods”
R.t.,B.b.,T.v. ,GF(R.e and R.l)
Wrocław suburbs, SW Poland,
1.8 km, 450 vehicles/day (this
study). See Ciesiołkiewicz
et al. (2006) for a map
Over 2 years including 2 BS,
every other day in BS, 1–
2 times a week in remaining
periods, 0600–0900 hours, by
foot, there on one road side and
back on the other; see also
“Material and methods”
Mosaic landscape of a small river
valley, with broadleaved woods,
8 small ponds and a creek; see
also “Material and methods”
B.b.,T.t.,GF,R.a.,H.a.
Town of Lubsko, SW Poland,
0.2 km, 500 vehicles/day
(this study)
2 BS, altogether 37 counts by
foot, there on one road side and
back on the other; see also
“Material and methods”
A small town street adjacent to a
school playing ground with a
pool
B.v
a
Species abbreviations: A.c. Alytes cisternasii, B.b. Bufo bufo, B.c. Bufo calamita, BF brown frogs, Bo.b. Bombina bombina, Bo.v. B. variegata,
B.v. Bufo viridis, D.g. Discoglossus galganoi,GFgreen frogs, H.a. Hyla arborea, H.m Hyla meridionalis, P.c. Pelobates cultripes, P.f. Pelobates
fuscus, P.w. Pleurodeles walti, R.a. Rana arvalis, R.d. Rana dalmatina, R.e. Rana kl. esculenta, R.l. Rana lessonae, R.p. Rana perezi, R.r. Rana
ridibunda, R.t. Rana temporaria, S.s. Salamandra salamandra, T.a. Triturus alpestris, T.c. Triturus. cristatus, T.h, Triturus helveticus, T.ma.
Triturus marmoratus, T.mo. Triturus montandoni, T.v. Triturus vulgaris
b
The distribution of frogs in Denmark is very patchy (E. Buchwald, pers. comm.). Rana dalmatina and, to a lesser extent, Rana arvalis, replace R.
temporaria in some parts of Denmark such the Isle of Lolland, which was surveyed by Hansen (1982). By contrast, R.. temporaria is common in
Djursland Peninsula but green frogs are absent there (Hels and Buchwald 2001).
c
Studies were not included in further quantitative comparisons because of the monitoring methods (especially too low frequency) and
incomparable species composition (largely unspecified for Western France).
36 Eur J Wildl Res (2009) 55:33–43
phibian road mortality records, at Sromowce Wyżne and the
Oleśnica fish farm (Table S1), are at the expense of single
taxa, common frogs and Tri turus newts, respectively. The
mass mortality at Sromowce Wyżne may have been a
composite result of setting an ecological trap by separation
of the wintering from spawning sites (Zamachowski and
Plewa 1996) and disorientation of amphibians following the
flooding of a part of the Dunajec river valley (Rybacki
2002). In addition, in upland areas, the common frogs may
be more prone to daytime migration than other species com-
monly observed on European roads (Berthoud and Müller
1986). Mass mortality of Triturus newts at the Oleśnica fish
farm is apparently caused by their mass migration along a
creek (Rybacki and Domańska 2004), as was the case in a
location in England (Evans 1989). A similar situation occurs
at Stobrawa Forest, where the newts enter the road from a
roadside ditch that receives a creek flowing from a pond. The
third highest mortality figure comes from a 0.5-km stretch of
road at Chelmski Pond where amphibians come to breed in a
pond surrounded by a pine forest while their feeding habitat
is across the road. This is a typical ecological trap as most
amphibians that breed in the pond have to cross the surveyed
road, which separates them from feeding habitats and runs
very close to the pond.
Species composition
Twenty-six amphibian species have been recorded dead on
European roads (Table 1), including 18 out of some 20 am-
phibian species with ranges across the continental Europe
and eight endemic Iberian species from Portugal (Ascensao
and Mira 2006). Most of published records come from
Central Europe and prove fairly consistent in terms of
relative species frequencies (Table 2): The most frequent are
common toads, common frogs (as the main or only com-
ponent of brown frogs), and, after the correction for road-kill
recordability, the Triturus newts (of various species, depend-
ing on the habitat and geographic area). Common toad
road kills are abundant in most landscapes, although their
numbers show considerable variation as do their populations
(Günther and Geiger 1996). They prevail in suburban areas
and the only two studied mountainous locations, the Pieniny
National Park (Table 1) and the Sudetes in SW Poland
(Baldy 2002). In contrast, the common frogs and, after cor-
rections for recordability, the Triturus newts prevail in rural
areas (including seminatural wilderness) where they may
outnumber both brown frogs and common toads (three
sites) or at least the toads (one site). In suburban sites, the
newts and brown frogs are by an order of magnitude less
frequent than in rural areas. The absence of newts in the
record from Bialowieża forest, Warta river delta, and
Wielkopolska landscapes is probably due to fragmentary
sampling (Table 1).
The green frog road kills are more frequent in rural than in
suburban sites (Table 2,Fig.1). The green frogs are absent or
rare over large parts of Denmark including the Djursland
Peninsula, and the pool frog (R. lessonae), the most migra-
tory of European green frogs (Günther 1996a), does not
occur there at all. In contrast, their absence in the Białowieża
Forest record is due to the lack a nearby pond and/or frag-
mentary sampling.
Other species including green toads, common spadefoots,
Bombina toads, and common tree frogs occurred only in
some habitats but not in others, the tree frogs being clearly
the least frequent among road kills. The only site where the
treefrogsperishedonroadinappreciablenumbersisChełmski
Pond, where the top numbers of other amphibian also were
killed.
Road kills-to-spawners ratios
The road kills-to-spawners (R/S) ratios obtained in our study
are fairly consistent (Table 3) considering that the estimates
they are based on were obtained by a different person in
each of the three major sites. The R/Sratios for common
toads (0.23–0.40) are comparable to that projected from van
Gelder’s data (0.39) in Holland if the whole stretch of road
were used by traffic (Table 3). However, those for common
frogs (0.40–0.66) are three to four times higher than those
recorded in Denmark (Hels and Buchwald 2001). The
highest is the R/Sratio obtained for common newts in
Stobrawa Forest after the correction of road-kill numbers.
At this site, the number of spawners must have been more
than halved even if the actual population size were twice
our estimate. Local green frog and Bombina toad popula-
tions were affected only by the ecological trap at Chełmski
Pond, which may also be the case at Oleśnica fish farm (no
population estimates are available from this site). Not un-
expectedly, a small town (Lubsko) population of green toads
incurred heavy losses and may have been nearly halved by
moderate street traffic.
Discussion
One approach is to establish the mortality of road-crossing
migrants (henceforth referred to as the road-crossing mortal-
ity) and determine the fraction of an entire population that
crosses the road. The road-crossing mortality has been re-
corded either by continuous counting of all individuals on a
road, both dead and alive (van Gelder 1973), or by a com-
bination of continuous counting and road fencing (Kuhn
1987; Gibbs and Shriver 2005). The road-crossing mortality
may equal or approach the road mortality of the entire local
breeding population only if all or most breeding adults have
to cross a road. This is the case when a breeding site is
Eur J Wildl Res (2009) 55:33–43 37
Table 2 Species shares (in %) in the European record of amphibian road kills including three of the four roads surveyed in this study (the town of Lubsko and other single-species sites excluded)
Site (R, rural; S, suburban) Totals %UN %NS %B.b. %BF %GF %B.v.%P.f. %Bo. %H.a.
A C A C A C A C ACA C ACAC AC
Oleśnica fish farm (R) 903 6,832 15 34 67 16 4.0 18 7.5 7.4 3.0 0.3 0.1 4.2 0.8 5.5 2.9 0.0
Chełmski Pond (R) 820 3,114 14 8 30.3 21 10 36 30 12 9.3 0.0 7.0 2.6 1.8 1.9 1.3 1.8
Stobrawa Forest (R) 363 1322 27 5 22 19 10 45 39 3.0 2.0 0.0 0.0 0.0 0.0
Bialowieża Forest
a
(R) 919 2714 nd 0.0 15.5 10 84.5 90 0.0 0.0 0.0 0.0 0.0
Djursland Peninsula (R) 204 900 0.0 14 47 17 7.0 59 42 0.0 0.0 10 4.0 0.0 0.0
Wrocław suburbs (S) 465 938 8 0.0 81 76 9 14 1.3 1,9 0.0 0.0 0.0 0.2 0.4
Zielona Góra woods (S) 3,017 5,967
b
1.2 0.5 3.5 91 87 2.9 4.6 0.2 0.4 2.3 2.3 1.7 1.3 0.0 0.0
Osnabrück area (S) 298 678 nd 2.3 15.5 91 75 6.4 8.9 0.3 0.4 0.0 0.0 0.0 0.0
Berlin woods (S) 2,478 5,828 9 1.6 11 73 58 16 21 1.1 1.4 0.0 0.0 0.0 0.0
Wielkopolska landscapes 2,009 4,044 41 0.0 42 39 7.4 11.5 +
c
6.2 6.0 3.3 2.5 +
c
+
c
Pieniny Natn. Park 1,485 3,901 nd 3.7 21 63 45 17 21 0.0 15 12 0.0 1.0 1.6 0.0
Sromowce Wyżne 16,688 61,344 nd 4.5 18.5 1.0 0.5 94.5 81 0.0 0.02 0.01 0.0 0.0 0.0
Denmark 1,463 3,349 1.7 1.8 12 81 66.5 12.5 17 0.6 0.8 2.3 2.0 0.0 0.0 0.0
Actual figures (A) have been corrected (C) using road-kill recordability conversion factors (see “Materials and methods”). Column abbreviations as in Table 1footnote except UN unidentified, NS
Salamandridae
a
The mean from 19 days was used for a 6-month season on the assumption that higher mortality at the peak of breeding season is approximately compensated by lower mortality in the early spring
and late autumn. This is most probably an underestimate (and so is the corrected figure) as the observer may have missed other amphibian species (at least newts). This study was excluded from
the species percentage comparisons.
b
The correction factors may possibly be too high for this study because of frequent monitoring which, however, started hours after sunrise and was done mostly by car.
c
A few individuals (unspecified number)
38 Eur J Wildl Res (2009) 55:33–43
completely encircled by roads with lethal traffic (probably a
rare situation) or when the hospitable terrestrial habitats lie
across the road to a body of water that is encircled by in-
hospitable habitat but provides the only breeding site in the
area. In most cases, only a fraction of the population crosses
the surveyed section of the road, and thus the road-crossing
mortality (i.e., mortality calculated for this fraction only) is
higher than the road mortality of the entire population, which
can be estimated by complex modeling as employed by Gibbs
and Shriver (2005).
Another most common and easier practicable approach in
assessing the impact of road-crossing mortality on a local
population is through comparing annual estimates of road-kill
numbers and the entire local population. However, obtaining
these two realistic estimates from raw counts is anything but
straightforward, an issue that only begins to be addressed in
the literature. The numbers of road kills recorded in the ma-
jority of studies, including ours, constitute only a fraction of
the actual road kills because cadavers quickly disappear from
the road as a result of scavenging and mechanical destruction
by vehicles (Hansen 1982; Hels and Buchwald 2001;Slater
2002;Najbaretal.2006;Langenetal.2007) and because a
fraction of those that remain on the road is overlooked both
in surveys by foot (Hels and Buchwald 2001) and from
motor vehicles, the latter technique being grossly inaccurate
specifically for amphibians (Slater 2002;Langenetal.2007;
see also Puky 2006). Therefore, short of continuous counting
of all individuals crossing a road, the actual numbers of road-
kills must be estimated using appropriate correctionfactors for
their recordability (i.e., the ratios of actual to recorded num-
bers of road kills). Road-kill recordability is the product of
cadaver’s chances to be preserved (for a certain time) and its
chances to be noticed by the observer using an appropriate
survey technique. Since it clearly depends on body size,
mechanical properties, coloration, and palatability, it is clearly
species-specific and so must be correction factors as experi-
mentally demonstrated by Hels and Buchwald (2001)for
Tritu rus newts, common spadefoots, common toads, and
brown frogs. Unfortunately, no experimental correction fac-
tors are available for frequent traffic victims from other
species. As a result, it remains largely unknown what species
are actually most affected by road mortality rather than best
represented in road-kill counts except that sedentary species
that live in or close to their breeding waters all year round are
expectedly less vulnerable than mobile, migratory species
(Carr and Fahrig 2001).
Hels and Buchwald’s(2001) recordability corrections fac-
tors, which were obtained for routine monitoring by foot every
day at dawn, are conservative for both our and the majority of
published studies that employed much less accurate road-kill
counting methods (Table 1). All our surveys were done on foot
in the morning hours (although only in Stobrawa forest really
at dawn), by walking on one road side there and on the other
back, but only every 2–3 days, which substantially increased
the chance of removal and destruction of road kills prior to our
monitoring. The road-kill recordability must be much lower (if
at all calculable) for afternoon surveys, since most amphibians
are killed at night, and for monitoring from a motor vehicle,
which seems grossly inaccurate even if conducted by two
persons in a car, an observer and a driver (Slater 2002;Langen
et al. 2007).
Species differences
The common toads and common frogs are the most abundant
and ecologically versatile anurans, and both species often
breed in the same ponds (Schlüpmann and Günther 1996;
Babik and Rafiński 2001; Carrier and Beebee 2003;Cooke
and Sparks 2004). Both species undertake relatively long
migrations (up to 2 km). However, common toads are much
slower than common frogs (Hels and Buchwald 2001), which
should make them more vulnerable to road death. And yet,
the common frog populations are at least as much affected
as, and, after corrections for recordability, probably even more
NS B.b. BF GF
0
20
40
60
80
100
% share
R rural
S suburban
a
NS B.b. BF GF
0
20
40
60
80
100
% share
R rural
S suburban
b
Fig. 1 The median shares (%) of newts and salamanders (NS),
common toads (B.b.), brown frogs (BF), and green frogs (GF) in all
amphibian roadkills from four rural (Oleśnica fish farm, Chełmski
Pond, Stobrawa Forest, and Djursland Peninsula) and four suburban
(Wrocław suburbs, Zielona Góra woods, Osnabrück area, and Berlin
woods) areas: athe actual counts; bestimates corrected for record-
ability. The median values for GF in bare 2.5 for rural and 0.9 for
suburban areas
Eur J Wildl Res (2009) 55:33–43 39
affected by traffic than common toad populations (Table 3).
This result agrees with observations that common frog popu-
lations have been decimated in the urban areas of Germany
(Schlüpmann and Günther 1996) and reduced in the built-up
areas with high road densities in the Alps (Landmann et al.
1999), although specifically the role of traffic alone has not
been determined.
The moor frogs are much less frequent than common frogs
in most road-kill records, except for the results of two to three
surveys across the Wielkopolska landscapes (Rybacki and
Krupa 2002). The moor frogs as a species are clearly less
abundant than are common frogs, although this may not ex-
plain the entire difference, as moor frogs were moderately
abundant in the surrounding habitats in Stobrawa Forest and
Wrocław suburbs (at least 100 in a pond approximately 130 m
from the road) and yet nearly absentfrom the road-kill record.
Intriguingly, road density has had less than expected effect on
moor frog populations in The Netherlands (Vos and Chardon
1998) in contrast to a strong negative impact on common
frogs (Reh and Seitz 1990, Landmann et al. 1999). As com-
pared to moor frogs (Günther and Nabrowsky 1996), com-
mon frogs disperse to a wider spectrum of terrestrial feeding
habitats, which, in combination with the strong philopatry with
respect to the spawning sites (Schlüpmann and Günther 1996),
may account for their higher traffic vulnerability.
The rarity of European tree frogs in the road-kill record is
somewhat unexpected because this species is widespread and
locally abundant throughout most of continental Europe and
considered fairly common in Poland (Głowaciński and
Rafiński 2003). Pellet et al. (2004) showed some impact of
both road density and traffic intensity on their populations in
Switzerland, and a related species, Hyla meridionalis,has
been recorded in small numbers on the roads of Portugal
(Ascensao and Mira 2006).
Rural vs. suburban landscapes
Our review of road-kill recording methods employed in
various studies (Table 1) suggests great and incalculable
differences of accuracy. Furthermore, diverse combinations
of provided and missing data made various studies compa-
rable in some but not in other respects; hence, usually only
subsets of studies listed in Table 1could be used for our
comparisons. However, our comparisons of the relative shares
of species in the lowland record (Table 2) show an-order-of-
magnitude differences between rural areas with the preva-
lence of common frogs and newts and suburban areas with
the prevalence of common toads (Fig. 1). Unfortunately, the
small sample size of four for each suburban and rural areas,
which results from the low comparability of published
studies, does not warrant the employment of statistical tests.
The differences in absolute road-kill densities between ru-
ral and suburban areas (Table S1) are certainly exaggerated by
the different average length of surveyed road sections, 5.2 km
Table 3 Impact of road mortality on European amphibian populations in terms of road kills-to-spawners ratios as based on actual (A) and
corrected (C) road-kill numbers, with spawning aggregation estimates below
Site/area Newts B.b. R.t. GF B.v. P.f. Bo.
ACACACA CACACAC
Chełmski Pond
(this study)
No data 0.21 0.40 0.22 0.66 0.08 0.24 ––0.09 0.34
400 700 600 70
Stobrawa Forest
(this study)
0.09 1.35 0.13 0.23 0.13 0.40 0.02 0.06 –––
110 T.v. 350 650 280
Wrocław suburbs
(this study)
–0.15 0.29 0.19 0.61 0.003 0.01 –––
1610 180 910
Djursland Pen.
a
(Hels and
Buchwald 2001)
no data –0.04 0.13 ––0.06 0.09 –
3245 352
Nijmegen area
b
(van Gelder 1973)
–0.18 –– – ––
280 ♀
Wrocław Plain
c
(Orlowski 2007)
–0.10 0.19 –– – ––
5500
Town of Lubsko
(this study)
–––– >0.4 >0.8 ––
80
a
After corrections for recordability, the fractions of adult brown frogs and spadefoots killed annually by traffic were each about 10% (or,
respectively, 7–21% and 5–25%).
b
Because of continuous registering of all toads there is no need to apply a correction factor. The fraction of the entire female population of 310
killed on the road section used by vehicles was 15.8%. However, if the entire road encircling the spawning site were used, the annual loss to the
entire population would be 31.3% and the R/Sratio 0.39.
c
Two to three counts per season by unspecified methods in 46 bodies of water within 200 m from a road
40 Eur J Wildl Res (2009) 55:33–43
for the suburban areas but only 0.7 km for the rural areas
where more of mortality hot spots have been selected. How-
ever, the striking differences in species shares (Table 2)are
unlikely to be substantially affected by road-section length.
For example, two of the suburban areas, Wrocław suburbs i
Zielona Góra woods, also were selected for high amphibian
abundance and yet yielded road-kill species shares similar to
the other two suburban sites (Table 2). We therefore suggest
that an-order-of-magnitude difference in the absolute num-
bers of green frog road kills (Table S1) to an extent reflects
their greater share in the rural compared to suburban areas,
which is consistent with their shares being highest in three
rural areas (Table 2).
A straightforward explanation for the species differences in
amphibian road mortality between rural and suburban land-
scapes is that it reflects differences in relative species abun-
dance, which is what is assumed in British monitoring studies
of amphibian decline (Cooke and Sparks 2004). In a full
agreement with this explanation, differences in the coloniza-
tion rates of pools by three most common amphibian species
in The Netherlands are related to their abundance (Laan and
Verb o o m 1990). It follows that monitoring amphibian road
mortality may prove to be the most effective method of
detecting overall population trends in addition to determining
the impact of vehicular traffic.
Impact on local populations
In order to estimate the impact of traffic on a local breeding
population, one has to first know the population size. In a
model study, Hels and Buchwald (2001) counted all
individuals entering and leaving a body of water and thus
could precisely calculate losses to the entire populations of
brown frogs and common spadefoots. In practice of
extensive rather than intensive amphibian road mortality
studies, only a count of spawning individuals (usually at the
peak of a breeding season) can usually be obtained and used
as a proxy for the entire population. However, the number of
spawners may provide a fair approximation of the entire
local breeding population only for those amphibians, es-
pecially for some green frogs, that stay in or close to their
breeding waters throughout the breeding season, although
natural mortality in spawning aggregations has to be taken
into account if the additive impact of another mortality factor
(road traffic) is to be estimated. For other amphibian species,
such as the common toads and Bombina toads (Günther and
Schneeweiss 1996) and common tree frogs (Pellet et al.
2007), the number of spawning individuals (or chorus mem-
bers) is at any time substantially different from the real size
of a local population. Even the maximum number of spawn-
ers is only a fraction of the breeding population, which varies
among species depending on their biology and behavior
(Cooke 2000). For example, female common toads do not
breed annually, leave water immediately after spawning, and
the number of leaving females is much lower than the
number of incomers due to their high mortality in breeding
aggregations (Günther and Geiger 1996).
If the population structure of an amphibian species were
well known, it would be possible to estimate the entire
population size from the maximum spawner count by using an
appropriate probabilistic model. We thus propose to explicitly
acknowledge that all that can be reliably calculated in most
studies of the populational impact of road mortality is the
road-kills-to-spawners ratio (R/S), with the number of road-
kills per one year in the numerator and the number of
spawning individuals in the denominator. Although both
figures are usually only estimated in most resource-limited
studies, they can be recorded by counts if enough effort is
made (e.g., by employing the combined drift-fencing and
pitfall-trap technique). By contrast, the censuses of all
individuals (including non-breeders) dispersed on land are
barely feasible, making the estimates of entire local popula-
tions, e.g., by the addition of 10% (Orlowski 2007), arbitrary
and thus prone to errors. By avoiding these compounding
errors, the R/Sratios make various estimates of traffic impact
on the same or closely related species (with similar biology
such the Trit urus newts and the three members of the Rana
esculenta hybrid complex) much better comparable between
studies. We therefore believe that it is better to use plain R/S
ratios that are comparable for the same or similar species and
bear potentially calculable errors rather than as hoc “correc-
tions”for the entire local population.
There are surprisingly few breeding censuses of amphib-
ians in Europe (Table 3). In the Nijmegen area of Holland,
van Gelder (1973) registered all breeding common toads
coming and leaving the spawning “fens”by continuous
counting and counted their spawn strings. In the Djursland
Peninsula of Denmark, Hels and Buchwald (2001)per-
formed a precise count of breeding common frogs by com-
plete drift fencing of five ponds and counting spawn clumps
and calling males. Our censuses are clearly much less
accurate than those used in the model studies by van Gelder
(1973) and Hels and Buchwald (2001) and produce only
estimates rather than precise figures subject to statistical
evaluation. And yet, they seem to provide informative ap-
proximations inasmuch as we used the mean values from
two breeding seasons, and the R/Sratios based on our es-
timates (Table 3) are fairly consistent between the three
major multispecies breeding sites (Chełmski Pond, Stobrawa
Forest, and Wrocław suburbs), although each was studied by
a different person.
The R/Sratios overestimate the impact of road-crossing
mortality on the entire population inasmuch as spawners do
not represent entire local populations. However, we use them
to assess this impact for two reasons. First, in the absence of
probabilistic models for realistic estimates of the entire
Eur J Wildl Res (2009) 55:33–43 41
population, the R/Sratio is the only currently available mea-
sure for quantifying this impact and thus allowing for future
corrections once such models are available. Second, for road
mortality studies of the same species and even closely related
species with similar biology (such as Tr iturus newts), the R/S
ratios are certainly better comparable than whole-population
estimates based on various ad hoc assumptions. In addition,
the overestimations of whole-population mortality stemming
from the R/Sratios may, to a degree, be compensated by
common underestimations of road-crossing mortality result-
ing from applying too low recordability correction factors.
In terms of R/Sratios, the impact on local common toad
populations is roughly comparable in Holland (van Gelder
1973), England (Cooke and Sparks 2004), and Poland
(Table 3). In contrast, our figures for common frogs are
three to four times higher than those obtained in Denmark
(Hels and Buchwald 2001). The common frog populations
are more affected than those of common toads in Chelmski
Pond, Stobrawa Forest, and Wrocław suburbs, even though
in the latter, the number of common frog road kills is only
one fourth of the number of common toad road kills.
The impact of road mortality on the local population of
common newts in Stobrawa Forest is likely to be devastating.
The newts may incur comparable or even higher mortality at
Oleśnica fish farm, Djursland Peninsula, and Chełmski Pond
where no spawning aggregation estimates are available. In
fact, road mortality of newts may be much higher than nor-
mally recorded at most sites, as suggested by the 85% share of
newts among all amphibians crossing a road in Luxemburg
(Proess 2003).
Relatively low losses to green frog populations are cer-
tainly related to their more sedentary life around waters as is
the case of American green frogs, Rana clamitans, which are
much more sedentary and thus much less affected by road
traffic than leopard frogs, R. pipiens (Carr and Fahrig 2001).
However, some green frogs, especially juvenile Rana kl.
esculenta, also explore terrestrial habitats (Günther 1996b)
and R. lessonae undertake regular overland migrations
(Günther 1996a). This explains why at least some green
frog populations are vulnerable to ecological trap situations,
as exemplified by Chełmski Pond where 24% of the spawning
population is destroyed by traffic annually (in the absence of
another body of water, the spawning population of green frogs
may comprise most of the local breeding population). Despite
relatively low road mortality, the numbers of their road kills
are by an order of magnitude higher in the rural compared to
suburban areas (Table 3), suggesting a strong pressure of
urban agglomerations on green frog populations.
Future research suggestions
In terms of future research, our results indicate a needfor basic
and applied studies to lay a foundation for standardized and
informative amphibian road mortality surveys. In the way of
basic research, the most needed are studies of species-specific
population structure and dispersion, which would allow
relating the size of spawning aggregations to the entire local
populations and thus make realistic conversions of R/Sratios
into road mortality figures for entire populations. In terms of
applied studies, the most needed is a continuation of work on
differential road-crossing mortality (with many basic ram-
ifications into amphibian locomotion) and road-kill record-
ability, as started by Hels and Buchwald (2001), which
would help to develop hitherto nonexistent minimum stand-
ards for amphibian road mortality studies. With appropriate
standards and accurate recordability coefficients, these
studies will also be useful for monitoring population trends,
at least among common species.
Acknowledgments We thank E. Buchwald (National Forest and
Nature Agency, Copenhagen), A. Zywicka for a pilot study of the area
in Wrocław suburbs, and an anonymous reviewer for his exquisite
criticism and corrections.
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