Eur J Wildl Res (2009) 55:33^3 DOI 10.1007/sl0344-008-0211-x ORIGINAL PAPER Amphibian road mortality in Europe: a meta-analysis with new data from Poland Andrzej Elzanowski • Joanna Ciesiolkiewicz • Mirella Kaczor • Joanna Radwanska • Radoslaw 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 population 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at a synthesis, we analyzed all available data on amphibian mortality in Europe and combined them with four previously unpublished surveys conducted by us.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 amphibian population estimates, we further propose the road kills-to-spawners ratio (RJS) as a working measure of the impact of road mortality on a local population. While the R/S ratio 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-Electronic supplementary material The online version of this article (doi:10.1007/sl0344-008-0211-x) contains supplementary material which is available to authorized users. Communicated by H. Kierdorf A. Elzanowski (El) ■ J. Ciesiolkiewicz • M. Kaczor • J. Radwanska • R. Urban Department of Zoology, University of Wroclaw, 21 Sienkiewicz Street, 50-335 Wroclaw, Poland e-mail: elzanowski@biol.uni.wroc.pl 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; Hels and Buchwald 2001; Cooke and Sparks 2004; Mazerolle 2004; Pellet et al. 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 mechanisms (Berthoud and Miiller 1986; Hels and Buchwald 2001; Harper and Semlitsch 2007). This is probably why there are very few, only three in Europe (van Gelder 1973; Hels and 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). Our work points Springer 34 Eur J Wildl Res (2009) 55:33^3 to the need of methodological improvements in amphibian road mortality studies and additional basic research in amphibian 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 recalculated 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, 1.5 for common spadefoots, Pelobates fuscus, and 15 for the Triturus newts. For other species, we used tentative correction factors, 5 for common tree frogs, Hyla arborea, 4 for Bombina toads, 3 for the green frogs, Rana lessonae, Rana ridibunda, and Rana kl. esculenta, and 2 for 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 Dolnoslaskie conifer forest complex, near Chelm Zarski village of 90 inhabitants, Lubsko county, Zary 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 over 1 year (9 March to 9 November 2001 and 1 February 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 predominance 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). Wroclaw 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 abandoned cropland), eight pools and small ponds (0.16-1.0 ha, jointly 2.1 ha) with lush hydrophilic vegetation, and a narrow creek (1-1.5 m wide) crossing the road. 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 (Zary district, Lubuskie province, 51c 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-m2 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 primarily 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 SI) are clearly not comparable because of differences in sampling methods 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- Springer Eur J Wildl Res (2009) 55:33^3 35 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 Survey period, frequency and Habitats/landscape Species3 in the order of density, and authority method [breeding season (BS)] 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) Bialowieza Forest, E Poland, 17 km, no traffic data (Wölk 1978) Denmark, 15,224 km various roads, from highway to secondary, no traffic data (Hansen 1982: Table 1) Berlin woods, Germany, 4.7 km, max. 380 (at one end) to 2,000 vehicles/day (Miech 1988) Osnabrück area, Germany, 8.5 km across woodland with 2,659 vehicles/day and fields with 780 vehicles/day (Fuellhaas et al. 1989) Pieniny National Park, S Poland, no road and traffic data (Rybacki 1995) Sromowce Wyzne, Dunajec Valley, Pieniny Mts, S Poland, 2 km, 60-120 vehicles/h in the morning (Zamachowski and Plewa 1996) Warta river delta, Slonsk Nature Reserve, W Poland, 11.3 km, no traffic data (Bartoszewicz 1997) Western France (Department Vendee), 68.2 km of a new road (opened preceding fall), "200-450 vehicles/km" (Lode 2000)c Djursland Peninsula, N Denmark, 0.6 km, 3,200 vehicles/day (Hels and Buchwald 2001) Wielkopolska landscapes, W Poland, ca. 1,500 km, no traffic data (Rybacki and Krupa 2002) Olesnica fish farm, Chodziez county, Wielkopolska Province, W Poland, 1,137 m, 1 BS (22.II-15.V), continuous recording every night "until no or hardly any animals were seen" 19 counts (13.V-24.IX), mornings, by motorcycle, 30 km/h, after removing or marking older casualties on the preceding morning One-time surveys over 3 years, on moped, "very slowly" 10 years, irregularly, 2-3 times a week at variable day time, by bike or car, occasionally by foot 1 incomplete year, every 5 days, no day time information, by foot 42 days (27 spring, 15 fall), irregular surveys most counts on one road side only, then multiplied by 2, otherwise methods and times unspecified 2 incomplete BS (IV-V), morning counts (0700-0900 hours) by unspecified methods 1 year, by bike or car, twice a week, usually on two consecutive (!) work days 1 incomplete season (7.5 months, IV-XI), 33 weekly surveys by car at <40 km/h and by foot altogether 6 months: incomplete BS (IV-V) and VIII of 2 consecutive years 2-3 morning surveys in one spring (IV-V), presumably by car 1 BS (16.III-16.VI), by unspecified means, every 2 days afternoon (1500-1600 hours) Woodland (unspecified) Natural mixed forest with 2 creeks Diverse habitats Broadleaved and conifer forest, 7 pools, one supplied by a creek 6.1 km across fields, 2.4 km across woodland Various (in part thermophilic) habitats of the Pieniny Klippen Belt and its surroundings Between a new reservoir and an open field with ditches and vernal pools A mosaic of mostly open habitats including marshland Unspecified Farmland with small woods and numerous ponds Various habitats in 11 protected landscape areas 420 m across mixed woodland with some ponds and an inn; 700 m across gardening lots and Only B.b. studied R.t., B.b. B.b, BF (R.d. and R.a.f, B.v, T.c, R.e B.b., R.t, T.v., GF, T.c, R.a. B.b., R.t, S.S., T.a., T.c, GF B.b., R.t, B.v., T.mo., T.a., Bo.v., S.S., T.c, T.v. R.t, T.a., B.b., T.mo., T.v., B.v, S.s. BF and GF, B.b., B.v. B.b. (56.7% of all amphibians) and 4 unspecified anurans, T.h. (28.4% of all amphibians) and 3 unspecified salamandrids BF, B.b., Tv. and T.c, P.f.b B.b., B.v, R.a., R.t., P.f., and single Bo.b. (misnamed as Bo.v), H.a., R.e., R.r. T.v, B.b., R.t.,R.a., R.e., Bo.b., P.f., I.e.. R.I., B.v. Springer 36 Eur J Wildl Res (2009) 55:33^3 Table 1 (continued) Site/area, road length, traffic Survey period, frequency and Habitats/landscape Species3 in the order of density, and authority method [breeding season (BS)] decreasing road-kill frequency around 500 vehicles/day (36 vehicles/h between 1400 and 1800 hours) (Rybacki and Domanska 2004) Zielona Gora woods, W Poland, 5.8 km, 7,000-8,500 vehicles/day (Najbar et al. 2006) Wroclaw Plain ca. 55 km around Wroclaw, Dolny Slask (Lower Silesia) Province, 48.8 km, 350-10,500 (mostly 350-479) vehicles/day (Orlowski 2007) Portalegre District, S Portugal, 26 km, 5,000 vehicles/d (Ascensao and Mira 2006)c Chelmski Pond, SW Poland, 0.5 km, 450 vehicles/day (this study) Stobrawa Forest, SW Poland, 0.5 km, 400 vehicles/day (this study) Wroclaw suburbs, SW Poland, 1.8 km, 450 vehicles/day (this study). See Ciesiolkiewicz et al. (2006) for a map Town of Lubsko, SW Poland, 0.2 km, 500 vehicles/day (this study) 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 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 2 years, 54 surveys every 2 weeks, by car 20 km/h 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" 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" 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" 2 BS, altogether 37 counts by foot, there on one road side and back on the other; see also "Material and methods" fields with many ponds; a creek crossing the road and flowing to the ponds Pine and mixed forests, 4 creeks expanding into ponds 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 Mostly agro-forestry (montado), some pastures, meadows fields and olive groves Between pastures and meadows on one side and a pine forest surrounding a large pond 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" Mosaic landscape of a small river valley, with broadleaved woods, 8 small ponds and a creek; see also "Material and methods" A small town street adjacent to a school playing ground with a pool B.b., R.t, B.V., /'/.. /.'... A'.,.. T.a., T.c, and R.a Only B.b. studied B.c. 36%, P.c. 25%, B.b. 14%, S.S. 12.5%, P.w. 8%, D.g. 1.4%, T.ma. 0.9%, Km. 0.4%, R.p. 0.4%, A.c. 0.2% R.t, B.b., R.a., GF (R.e and «./.), T.c, P.f., Bo.k, H.a. R.t, B.b., T.V., GF (R.e 0.4 >0.8 (this study) 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/S ratio 0.39. c Two to three counts per season by unspecified methods in 46 bodies of water within 200 m from a road affected by traffic than common toad populations (Table 3). This result agrees with observations that common frog populations have been decimated in the urban areas of Germany (Schlupmann and Gunther 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 explain the entire difference, as moor frogs were moderately abundant in the surrounding habitats in Stobrawa Forest and Wroclaw suburbs (at least 100 in a pond approximately 130 m from the road) and yet nearly absent from 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 compared to moor frogs (Gunther and Nabrowsky 1996), common frogs disperse to a wider spectrum of terrestrial feeding habitats, which, in combination with the strong philopatry with respect to the spawning sites (Schlupmann and Gunther 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 Springer locally abundant throughout most of continental Europe and considered fairly common in Poland (Glowacinski and Rafinski 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 comparable in some but not in other respects; hence, usually only subsets of studies listed in Table 1 could 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 prevalence 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 rural and suburban areas (Table SI) are certainly exaggerated by the different average length of surveyed road sections, 5.2 km Eur J Wild! Res (2009) 55:33^3 41 for the suburban areas but only 0.7 km for the rural areas where more of mortality hot spots have been selected. However, 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, Wroclaw 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 numbers of green frog road kills (Table SI) 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 landscapes is that it reflects differences in relative species abundance, 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 colonization rates of pools by three most common amphibian species in The Netherlands are related to their abundance (Laan and Verboom 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, especially 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 (Gunther and Schneeweiss 1996) and common tree frogs (Pellet et al. 2007), the number of spawning individuals (or chorus members) is at any time substantially different from the real size of a local population. Even the maximum number of spawners 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 (Gunther 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 populations, e.g., by the addition of 10% (Orlowski 2007), arbitrary and thus prone to errors. By avoiding these compounding errors, the R/S ratios make various estimates of traffic impact on the same or closely related species (with similar biology such the Triturus 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 "corrections" for the entire local population. There are surprisingly few breeding censuses of amphibians 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) performed a precise count of breeding common frogs by complete 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 approximations inasmuch as we used the mean values from two breeding seasons, and the R/S ratios based on our estimates (Table 3) are fairly consistent between the three major multispecies breeding sites (Chelmski Pond, Stobrawa Forest, and Wroclaw suburbs), although each was studied by a different person. The R/S ratios 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 Springer 42 Eur J Wildl Res (2009) 55:33^3 population, the R/S ratio is the only currently available measure 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 Triturus 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/S ratios may, to a degree, be compensated by common underestimations of road-crossing mortality resulting from applying too low recordability correction factors. In terms of R/S ratios, 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 Wroclaw 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 Olesnica fish farm, Djursland Peninsula, and Chelmski Pond where no spawning aggregation estimates are available. In fact, road mortality of newts may be much higher than normally 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 certainly 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 (Giinther 1996b) and R. lessonae undertake regular overland migrations (Giinther 1996a). This explains why at least some green frog populations are vulnerable to ecological trap situations, as exemplified by Chelmski 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 need for 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/S ratios 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 ramifications into amphibian locomotion) and road-kill record-ability, as started by Hels and Buchwald (2001), which would help to develop hitherto nonexistent minimum standards 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 Wroclaw suburbs, and an anonymous reviewer for his exquisite criticism and corrections. References Ascensao F, Mira A (2006) Spatial patterns of road kills: a case study in Southern Portugal. In: Irwin CL, Garrett P, McDermott KP (eds) Proceedings of the 2005 international conference on ecology and transportation. 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