Although rare species are often the focus of conservation measures, more common species may experience similar decline and suffer from the same threatening processes. We tested this hypothesis by examining, through an information-theoretic approach, the importance of ecological processes at multiple scales in the great crested newt Triturus cristatus, regionally endangered and protected in Europe, and the more common smooth newt, Lissotriton vulgaris. Both species were similarly affected by the same processes, i.e. suitability of aquatic and terrestrial components of their habitat at different scales, connectivity among breeding sites, and the presence of introduced fish. T. cristatus depended more on water depth and aquatic vegetation than L. vulgaris. The results show that environmental pressures threaten both common and rare species, and therefore the more widespread species should not be neglected in conservation programs. Because environmental trends are leading to a deterioration of aquatic and terrestrial habitat features required by newt populations, populations of the common species may follow the fate of the rarest species. This could have substantial conservation implications because of the numerical importance of common species in ecosystems and because commonness could be a transient state moving towards rarity. On the other hand, in agreement with the umbrella species concept, targeting conservation efforts on the most demanding species would also protect part of the populations of the most common species.
Citation: Denoël M, Perez A, Cornet Y, Ficetola GF (2013) Similar Local and Landscape Processes Affect Both a Common and a Rare Newt Species. PLoS ONE 8(5): e62727. https://doi.org/10.1371/journal.pone.0062727
Editor: Benedikt R. Schmidt, Universität Zurich, Switzerland
Received: December 27, 2012; Accepted: March 25, 2013; Published: May 3, 2013
Copyright: © 2013 Denoël et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: MD is a Research Associate at the F.R.S. – Fonds de la Recherche Scientifique (Belgium). GFF was funded by a scholarship of the University of Milano-Bicocca. This research benefited from a public grant of Service Public de Wallonie, Contrat de rivière du sous-bassin hydrographique de la Vesdre, F.R.S. – FNRS (Crédit au chercheur 1.5.040.10) and Fonds Spéciaux pour la Recherche (C11/23 - University of Liège). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
An important question in conservation biology is whether sympatric rare and common species can be similarly affected by habitat change . Because they are more abundant or have a broader range of distribution, common species are often believed to be not threatened. Until recently, they have also attracted less attention from ecologists, a consequence of their less preoccupying conservation status . In this perspective, they were thought to indirectly take advantage of the protection of rare species through the umbrella species concept, even though this would require a sufficient distribution overlap and similar minimum requirements as the rarest species . However, because of their numerical importance in ecosystems, and consequently their large contribution to the global biomass, it is now recognized that status changes of common species may have important consequences . Furthermore, over geological times and space, commonness is only a transient situation .
The conservation of rare species can allow the conservation of common species if they are similarly affected by threatening processes . In such situations, the rarest or most threatened species are expected to be more severely affected by environmental processes than the more common ones. Identifying the threatening processes is challenging , but the formulation of a priori hypotheses on ongoing processes, followed by the application of information-theoretic statistical models, explicitly testing these hypotheses, can greatly help the identification of threatening processes based on distribution patterns .
Amphibians are a valuable group in which to examine these questions as they are one of the most threatened classes of organisms worldwide, but also because much attention has been paid to the rarest species , . Common amphibian species also face population declines, such as the common toad (Bufo bufo) in Europe and the northern leopard frog (Lithobates pipiens) in many states of the USA , . In newts, several conservation programs (e.g. Life, Natura 2000) have focused on the great crested newt Triturus cristatus, an emblematic species protected under the Habitat Directive Annex 2 – (Figure 1A). In contrast, the smooth newt Lissotriton vulgaris (Figure 1B) is much less protected, in part because of its assumed commonness. However, reports of regional decline suggest that it could also be affected by environmental pressures –.
Both pictures show males from a pond in Pays de Herve (Belgium) and are representative of a rare and emblematic (A) and a more common and less protected (B) species.
Previous research on crested and smooth newts has improved our knowledge on the ecological requirements of these species, but also raised new questions (see e.g. , , , ). In particular, most research undertaken on both crested and smooth newts living in sympatry occurred in areas where they remained widely distributed and where both species were equally frequent in ponds (see Table 1 for details , –). The situation remains to be clarified in more modified agricultural landscapes where the crested newt is much rarer than the smooth newt. This pattern is typical of Western Europe, where modernization of agricultural practices and urbanization of natural lands has resulted in a decline of pond-breeding amphibians , . Therefore, this situation may be representative of very large areas of Europe in the near future. Two previous studies fit this pattern, but have suggested that new surveys targeting sympatric crested and smooth newts, including a larger set of variables, are needed to identify processes acting at various scales , . Specifically, a number of previous studies considered landscapes within a radius of 400–500 m from ponds, whereas finer-grain studies suggest that more detailed, shorter-range analyses may also be valuable . Among the wide range of pollutants that are toxic to amphibians, laboratory studies have evidenced the risk of water pollution by nitrogenous compounds as found in fertilizers and urban water discharges , but until now no field studies have assessed their detrimental effect on newt distribution. This could be particularly relevant in periurban and agricultural areas dominated by cattle grazing. Past landscapes (i.e. historical land use) have also not yet been examined in newts, although they could also affect these species today .
The objective of this study was to identify the major processes (Table 2) that could threaten these two newt species at different scales in an agricultural landscape that has been affected by habitat change over the last few decades . We hypothesize that both species rely similarly on environmental conditions, thus validating the umbrella species concept, that they are simultaneously affected by multiple processes, and that the rarest species could be declining more rapidly because of greater sensitivity to habitat degradation.
Materials and Methods
The capture permit was authorized by the Ministère de la Région Wallonne (Division de la Nature) and issued on 19th February 2008 on the basis of ethics approval of the field study on newt ecology by Conseil Supérieur Wallon de la Conservation de la Nature.
Study Area and Sampling Procedures
This study was conducted in Pays de Herve, an agricultural area of eastern Belgium at the border of Germany and the Netherlands. The sampling areas were chosen from the known distribution of the crested newt . We surveyed 74 ponds during the newt reproductive season (March–June 2008) (Figure 2). The surveys took place in the 12 ponds historically inhabited by this species (1990–2004), in all ponds within a 500-m radius around these ponds, and also within a 500-m radius around these new sets of surrounding ponds (Figure 2). Surrounding ponds were located using recent topographic maps (Institut Geographic National [IGN], 1∶20,000, published in 1999–2000), colour orthoimages (IGN: Direction Générale de l’Agriculture, 0.5-m resolution, 2006), and by field observations.
Blue circles: ponds, yellow patches: studied localities (based on historical presence of Triturus cristatus): 1, En Géliveau; 2, Hansez; 3, Haute Rafhay; 4, Stoki; 5, Margarins; 6, Vogelsang; 7, Blanc Baudet; 8, Gut Benesse Hof; 9, Hof Krompelberg; 10, Gemerhet; 11, Corney; 12, Harbenden. Geographic coordinates: Belgian Lambert Grid (expressed in km).
To obtain an index of newt abundance, each pond was sampled by dip-netting (40×35 cm dip-net with a 1.8 m pole) . All ponds were visually screened for newts at the time of dip-netting but no more species were found in this way. The small size of ponds allowed covering the entire surface of the ponds several times. Deep zones were additionally sampled with large nets (5 to 10 m2 seines). This sampling design was particularly adapted to reduce escape possibilities during netting. Censuses ended after several unsuccessful nettings in various areas of the ponds, including open and vegetated parts. This method has the advantage of sampling all micro-habitats and thus gives comparable values across ponds since all ponds were surveyed similarly. Previous studies showed that the detection probability with this type of removal estimate is very high  and that using dip-nets is an adequate method for sampling European newts . Because previous studies highlighted that amphibians, including newts, can leave water temporarily during the breeding season , , our method did not aim to determine the total adult population, but rather to approach the size of the aquatic adult population as closely as possible at any given time. By sampling all habitats, including pond vegetation and banks, hidden newts can also be captured. Previous studies have shown that blind dip netting such as done in the present study does not give lower abundance estimates during daytime than during night-time . Although it is possible that one species was missed in some ponds, the absence of a record indicates that the species is very rare in that pond or suggests that this would at best be a “sink” or transient habitat , . Previous studies have shown that newt abundance is correlated with habitat quality . We therefore believe that our approach is sufficiently robust to evaluate the association between species and environmental variables. We took into consideration only adults because we sought to determine the index of abundance similarly across ponds. There were no ponds in which we found larvae and no adults (qualitative checks were carried out at other times during the study period). All amphibians were handled with wet gloves during sampling. All material was washed and disinfected after every visit to a pond.
Pond and Landscape Traits
We measured several environmental variables representing five major processes that can determine newt distribution (Table 2). Five variables describing pond features were measured in situ during the newt census (Table 2). The maximum water depth and macrophyte cover were assessed in the field, whereas the pond surface area was assessed either in the field or obtained through aerial photo interpretation. The presence of introduced fish (both native and exotic to Belgian fauna, but all outside their natural habitat) was determined by dip-netting and seining, visual observations, and interviewing local owners. These fish are often locally invasive. Oxygen was measured with an oximeter (Hach Lange Multi HQ40d). To evaluate water pollution, a specific visit of each pond was made to gather water samples, which were preserved at 4°C and directly transported to the laboratory for chemical analyses. Sampling took place within 2 days in June. The concentration of three nutrients, mostly caused by water pollution (orthophosphates, nitrites and ammonium) was evaluated through colorimetric analysis using, respectively, blue Molybedne, de Griess and Berthelot reagents.
The number of ponds within 100 and 500 m of the focal pond was recorded as a measure of the pond’s present-day connectivity (Table 2). These values were chosen because radio-tracking studies showed that 50% of movements occurred within 100 m, whereas 500 m is usually considered to cover most movements , . Connectivity was available for the present-day period only, because not all ponds were recognizable on historical maps. To evaluate landscape composition, shape files were drawn from recent orthoimages (IGN – DGA, 2006) and historic aerial images (IGN, 1947–1954) in ArcGis 9.3 (Esri, Redlands, CA, USA) to represent land cover limits (i.e. forest, croplands, gardens, buildings and ponds). First-order polynomial functions were adjusted using ground control points selected on the scanned historical aerial images and on the 2006 reference orthoimages. Topographical maps and field visits helped create the landscape layers.
Three out of five “processes” were represented by multiple environmental variables (Table 2), which were strongly correlated to each other. Including correlated variables may bias the regression results; preliminary models including the original variables showed high values of the variance inflation factor, indicating that multicollinearity affected these models. We therefore used principal component analysis (PCA) to summarize variables in a lower number of uncorrelated components. PCA was performed using the correlation matrix and variable scaling. To keep the roles of these five processes distinct, we performed separate PCAs for the variable set representing each one (Table 2). Extracted components were rotated (Varimax rotation) to improve interpretation. The PCA was run over pond features, present-day and historical pond landscapes at both the 100 and 500 m radius. Extracted components explained 67%, 71%, 85%, 72%, and 74% of the total variance. The correlation between the original variables and the extracted components is shown in Table 2.
We used an information-theoretic approach, based on Akaike’s information criterion (AIC), to identify the processes and the spatial scales most likely to affect the abundance index of the two newt species , . We analysed relationships between newt abundance and environmental features using generalized linear models (GLMs), assuming a quasi-Poisson error distribution to take into account overdispersion. First, we built GLMs considering all possible combinations of the variables (either environmental variables or PCA components) representing the five processes (Table 2). For each model, we calculated the quasi-AIC corrected for small sample size (Q-AICc) . A model was not considered as a “candidate model” if a simpler, nested model had a lower Q-AICc . Furthermore, for each process, we considered only one spatial scale at a time, meaning if we included connectivity at the 100 m scale, we did not include connectivity at the 500 m scale and vice-versa. For each candidate model i, we then calculated the Q-AICc weight wi, which is the probability that a given model is the best one, given the set of candidate models considered . We also reported significance values of variables included in the best models, to facilitate the interpretation of the models and of the role played by predictors . Using Q-AIC instead of Q-AICc would not change the results (for both species the best models would remain the same). Errors were not spatially autocorrelated (for all best models, Moran’s I <0.05, P>0.2). None of the candidate models showed multicollinearity (for all models and all variables, variance inflation factor <5). Conditional partial regression plots were built using the visreg package . Finally, we used an unequal variance t-test to compare the features of ponds inhabited by the rarest species or only by the common species .
Triturus cristatus was found in 16% of the ponds (n = 12 out of 74) within the known area of presence of the species. Six out of the 12 ponds inhabited by T. cristatus were not the same as the ones detected in the 2004 study. The average number of adults detected in inhabited ponds (± SE) was 7±3; in the most populated pond, we detected 32 adults. L. vulgaris was found in 45% of the ponds (n = 33 out of 74). In ponds inhabited by L. vulgaris, the average number of adults detected was 24±5 and the maximum number detected in a given pond was 123.
T. cristatus was not found in 6 of the 12 historical sites and L. vulgaris in 2 out of the 10 historical sites. One of the ponds had disappeared because of the construction of a railway. The others were still present. L. vulgaris was observed in one pond where it was not detected during the past census. The new survey allowed the addition of six new populations of T. cristatus in sites that were not surveyed in the previous study.
The model that most likely explained the distribution of T. cristatus (i.e. the model with rank 1 in Table 3) suggests that this species is influenced by present landscape features (scale, 100 m), past landscape (scale, 500 m), fish presence, local features and connectivity (scale, 100 m) (Tables 3 and 4a). Three further models showed weight greater than 0.1. All the models with weight greater than 0.1 included local features, connectivity and present and past landscape composition (Table 3). Fish presence was included in three out of the four models with weight greater than 0.1. Examination of individual variables included in the best AIC model showed that T. cristatus was associated with large and deep ponds with abundant aquatic vegetation but without fish, surrounded by a high number of wetlands within 100 m, in landscapes that are currently open and that had low urbanization in the past (Figure 3, Table 4a).
Panels represent conditional partial regression plots, based on the best selected model for both Triturus cristatus (grey bands and full lines) and Lissotriton vulgaris (green bands and interrupted lines). The “number of wetlands” were square-root transformed values; the other variables are components extracted by PCAs: see methods for more details. Shaded areas represent 95% confidence bands.
The best model for L. vulgaris (i.e. the model with rank 1 in Table 5) was very similar to the best model for T. cristatus (Table 3). Lissotriton vulgaris was related to present landscape features (scale, 100 m), past landscape (scale, 500 m), fish presence, local features and connectivity (scale, 100 m). Two further models showed weight greater than 0.1. All models with high support were similar, being subsets of the best model (Table 5). All models with weight greater than 0.1 included local features, connectivity and present and past landscape composition. Examination of variables included in the best AIC model showed that smooth newts were associated with large, deep and less polluted ponds with abundant aquatic vegetation, surrounded by a high number of wetlands within 100 m, in landscapes that are currently open and that had low urbanization in the past (Figure 3; Table 4b).
The distribution of T. cristatus was nested within the distribution of L. vulgaris, as the smooth newt was detected in 11 out of the 12 ponds with crested newt. We therefore compared the features of ponds hosting both species with those of ponds with L. vulgaris only. Ponds with both species were significantly different for components 2 and 3 of the PCA run over pond features (t26.663 = −3.161, P<0.01 and t27.045 = −2.566, P<0.05, respectively), indicating that these ponds were larger and deeper, with more aquatic vegetation than those with L. vulgaris only (Table 6). The other environmental variables were not significantly different between the two groups of ponds (all P>0.11; Table 6).
Commonness and Rarity
The analysis of ecological processes involved in the distribution of T. cristatus and L. vulgaris showed that, despite a difference in commonness, both species share similar responses to environmental features. As shown by the best AIC models (Table 4), all the processes tested had a similar influence on both species: their abundance was related to the same environmental variables, and the effect was in the same direction for both species. This indicates that conservation actions should consider multiple factors to adequately protect these species. The suitability of pond features (water quality, macrophytes, water depth), the absence of introduced fish species, the connectivity among sites, and the composition of present-day and historical landscapes were important for newt distribution. These processes are mostly determined by anthropogenic activities and are acting in a direction that is unfavourable for newt persistence, both in the study area and in most modern agricultural landscapes. It can therefore be expected that without action plans, both the rare and the “common” species will quickly decline. These results support recent evidence that not only the rarest, but also the apparently most common amphibians are at risk , , , . Commonness patterns should not be overlooked in ecological research as commonness is only a transient state, which means that what is common today may be rare tomorrow .
The use of surrogate species has been a major tool in conservation planning, but has also received criticism as several conditions need to be met for applying conservation actions efficiently. Recent analyses showed empirical evidence for this concept, but more data are needed on amphibians . Both newt species used similar habitats and responded similarly to disturbance, which supports the potential efficiency of the umbrella species concept in amphibians . The similarity within several processes acting at multiple scales argues even more for simultaneous protection of common and rare species. Moreover, the rarest species responded more strongly to environmental changes than the most common species. Newt populations often exist within networks of meta-populations or patchy populations – and, in some cases, observed changes of occupancy may be part of extinction and colonization dynamics that are characteristics of meta-populations ; for instance, L. vulgaris was detected in one pond where it was not found 10 years before. Nevertheless, for T. cristatus the decline persists because of the overall loss of suitable habitats and, under situations of continuous loss of habitat quality, colonizations cannot compensate extinctions , . Evaluating changes in occurrence across time gave coherent results with these patterns: in the recent surveys T. cristatus was not found again in half the ponds, whereas L. vulgaris was still found in all but two ponds. Such a focus has broad applications as both species are sympatric over a large part of their distribution ranges. However, targeting T. cristatus populations would favour only a small fraction of L. vulgaris populations, as they are six times more numerous than T. cristatus populations in the study area . These results are confirmed with significant trends at a broader scale that showed that T. cristatus is more localized and less widespread than L. vulgaris . In fact, actions on the ponds inhabited by the more common species could also help the recovery of T. cristatus in nearby populations as this species is favoured by a high density of ponds. On the other hand, our analysis considered species of newts typical of open landscapes, while responses to environmental modifications may be even more complex if the whole amphibian community is considered , , . This means that the umbrella species approach should be used with care only once ecological requirements are sufficiently known.
Ecological Processes: Shared and Specific Patterns across Studies
Previous studies on the ecology of crested and smooth newts considered heterogeneous sets of environmental variables and spatial scales , , –, ,  (Table 1). Each of these studies highlighted determinants of newt distribution and thus improved our knowledge in terms of conservation management. As outlined by Zanini et al. , there is geographic variation of underlying ecological processes and thus different results can be found in contrasted regions. For instance, Hartel et al.  indicate that traditional management of the landscapes studied in Romania was the basis of the relative unimportance of landscape determinants. The present study was conducted in an area where rarity was more pronounced than in other studies (but see ; Table 1). The results confirm previous knowledge on these species but also show that in altered landscapes, multiple processes are acting simultaneously on newt distribution. Indeed, all the processes considered contribute to explaining the distribution of the two species. In addition, considering different spatial scales and water pollution provided new insights into the anthropogenic pressure on natural populations.
Towards Effective Conservation Measures
Although this study targeted areas that were known to be inhabited by a rare newt, both study species were absent from two-thirds of the ponds. The fact that this pastoral landscape hosts a high density of ponds  must therefore be balanced by their limited suitability for newt breeding. Although only one pond had disappeared since the last survey period (1990–2004), several ponds showed signs of future disappearance (i.e. shallow depth, eutrophication, partial destruction), suggesting a higher rate of pond loss over the long term, as shown in other studies , .
In terms of suitability of aquatic sites, ponds need to be restored to maintain a high water depth (i.e., at least 1 m in such agricultural lands) while avoiding fish introduction, as permanent ponds are more likely to sustain fish. Habitat restoration programs, including pond creations for T. cristatus, have proved to be successful and should be followed over wide areas . Fish have been shown to be particularly detrimental to newts , , –. However, smooth and crested newts could also coexist with fish , , . Here, introduced fish were found in 16% of ponds. Although most of them could not predate adult amphibians, they can eat eggs and larvae . This could explain why we only found two populations of T. cristatus and four populations of L. vulgaris coexisting with fish. In these cases of cohabitation, newt abundance was very limited (maximum two T. cristatus and 17 L. vulgaris detected). Large fish were not found in coexistence with newts, except in one population of L. vulgaris where fish were not numerous. The highest susceptibility of crested newts to fish may be due to the more pelagic behaviour of its larvae , , but more work is needed to understand the mechanisms of coexistence between newts and fish . Because amphibian resilience is possible after fish removal , this management action should be included in conservation plans. Aquatic vegetation provides support and protection for the eggs and shelter for the newts , , – and should be favoured, but without excess, as ecological succession would lead to pond disappearance . Sources of pollution should be identified and managed to avoid run-off of pollutants. Too few studies have integrated pollution by fertilizers and domestic run-offs and how they affect amphibians , although laboratory experiments have shown their direct effects on larval stages . The high concentration of pollutants found in ponds within this study and their relation to absence or low abundance of newts show that the landscape studied is heavily polluted. Buffer zones should therefore be designed around each pond to reduce the risk caused by the use of fertilizers and building water discharges.
Previous studies have shown that woods or scrubs should be maintained near breeding ponds as they provide an adequate space for vital activities such as feeding outside the reproductive season, but also for aestivation and wintering , , . The preferential emigration from ponds is often toward forests instead of open landscapes in both T. cristatus and L. vulgaris , and the occupied ponds are usually only at a few hundred metres from forests , , : these arguments further support the importance of forests. However, both T. cristatus and L. vulgaris typically breed in ponds located in open landscapes. The differences between the effect of past and previous landscapes showed that terrestrial processes are complex and would require specific investigations. Telemetry and capture-mark-recapture studies, such as those conducted by Jehle  are needed to understand terrestrial requirements in both traditional and modernized landscapes.
Connectivity, often highlighted in newt research , , is not only important at large scales around core ponds, but also in the vicinity given that the number of ponds within a radius of 100 m had a significant effect on both species in the present study. This confirms results of radio-telemetry showing that most individuals remain very close to the breeding ponds . Viability prediction models highlighted that T. cristatus populations harbouring more than 40 adults may have a lower risk of extinction in case of isolation . The usual number of adults detected in all the populations studied was below this value, emphasizing the importance of maintaining landscapes with high pond density.
In conclusion, commonness should not be neglected in conservation management and the adequacy of surrogate species should be evaluated to ensure that what is common now does not become rare. In the current perspective of amphibian decline, multiple stressors should be considered together to allow efficient conservation programs.
We are very grateful to C. Clark for her constructive comments on the manuscript, to M. Calcoen, Q. Pirotte, V. Jourdan, P. Wegnez and C. Zanchi for their field help, to J. Tonneau for his laboratory support with the chemical analyses, to S. Liégeois for her support with the Service Public de Wallonie, to C. Willemarck for providing information on Bassin de la Vesdre and to the municipalities and private owners who provided access to their ponds. This is AFFISH (Applied and Fundamental Fish Research Center) publication number 2.
Took the pictures: MD. Provided GIS support: YC. Conceived and designed the experiments: MD. Performed the experiments: AP MD. Analyzed the data: MD GFF AP. Wrote the paper: MD AP YC GFF.
- 1. Rannap R, Lohmus A, Briggs L (2009) Niche position, but not niche breadth, differs in two coexisting amphibians having contrasting trends in Europe. Diversity Distrib 15: 692–700.
- 2. Gaston KJ (2011) Common Ecology. Bioscience 61: 354–362.
- 3. Roberge JM, Angelstam P (2004) Usefulness of the umbrella species concept as a conservation tool. Conserv Biol 18: 76–85.
- 4. Gaston KJ (2010) Valuing Common Species. Science 327: 154–155.
- 5. Lambeck RJ (1997) Focal species: a multi-species umbrella for nature conservation. Conserv Biol 11: 849–856.
- 6. Lindenmayer DB, Manning AD, Smith PL, Possingham HP, Fischer J, et al. (2002) The focal-species approach and landscape restoration: a critique. Conserv Biol 16: 338–345.
- 7. McIntire EJB, Fajardo A (2009) Beyond description: The active and effective way to infer processes from spatial patterns. Ecology 90: 46–56.
- 8. Wake DB, Vredenburg VT (2008) Are we in the midst of the sixth mass extinction? A view from the world of amphibians. Proc Nat Acad Sci USA 105: 11466–11473.
- 9. Stuart SN, Chanson JS, Cox NA, Young BE, Rodrigues ASL, et al. (2004) Status and trends of amphibian declines and extinctions worldwide. Science 306: 1783–1786.
- 10. Johnson PTJ, McKenzie VJ, Peterson AC, Kerby JL, Brown J, et al. (2011) Regional decline of an iconic amphibian associated with elevation, land-use change, and invasive species. Conserv Biol 25: 556–566.
- 11. Bonardi A, Manenti R, Corbetta A, Ferri V, Fiacchini D, et al. (2011) Usefulness of volunteer data to measure the large scale decline of “common” toad populations. Biol Conserv 144: 2328–2334.
- 12. Jehle R, Thiesmeier B, Foster J (2011) The crested newt. A dwindling pond-dweller. Bielefeld, Germany: Laurenti-Verlag. 152 p.
- 13. Gent T (2001) The conservation of the great crested newt Triturus cristatus in the UK. Rana 4: 295–305.
- 14. Edgar PW, Bird DR (2005) Action plan for the conservation of the great crested newt Triturus cristatus species complex in Europe. Strasbourg, France: Council of Europe.
- 15. Maletzky A, Kyek M, Goldschmid A (2007) Monitoring status, habitat features and amphibian species richness of crested newt (Triturus cristatus superspecies) ponds at the edge of the species range (Salzburg, Austria). Ann Limnol - Int J Limnol 43: 107–115.
- 16. Arntzen JW, Kuzmin S, Beebee T, Papenfuss T, Sparreboom M, et al.. (2008) Lissotriton vulgaris. IUCN Red List of Threatened Species Version 2011–1.
- 17. Denoël M (2012) Newt decline in Western Europe: highlights from relative distribution changes within guilds. Biodivers Conserv 21: 2887–2898.
- 18. Meyer A, Zumbach S, Schmidt B, Monney J-C (2009) Les amphibiens et les reptiles de Suisse. Bern, Switzerland: Haupt Verlag.
- 19. Schmidt B, Zumbach S (2005) Lista rossa della specie minacciate in Svizzeria. Anfibi. Bern: UFFAP & Karch.
- 20. Joly P, Miaud C, Lehmann A, Grolet O (2001) Habitat matrix effects on pond occupancy in newts. Conserv Biol 15: 239–248.
- 21. Hartel T, Schweiger O, Öllerer K, Cogalniceanu D, Arntzen JW (2010) Amphibian distribution in a traditionally managed rural landscape of Eastern Europe: Probing the effect of landscape composition. Biol Conserv 143: 1118–1124.
- 22. Denoël M, Ficetola GF (2008) Conservation of newt guilds in an agricultural landscape of Belgium: the importance of aquatic and terrestrial habitats. Aquat Conserv 18: 714–728.
- 23. Beebee TJC (1985) Discriminant analysis of amphibian habitat determinants in South-East England. Amphibia-Reptilia 6: 35–43.
- 24. Cooke AS, Frazer JFD (1976) Characteristics of newt breeding sites. J Zool, Lond 178: 223–236.
- 25. Skei JK, Dolmen D, Ronning L, Ringsby TH (2006) Habitat use during the aquatic phase of the newts Triturus vulgaris (L.) and T. cristatus (Laurenti) in central Norway: proposition for a conservation and monitoring area. Amphibia-Reptilia 27: 309–324.
- 26. Curado N, Hartel T, Arntzen JW (2011) Amphibian pond loss as a function of landscape change - A case study over three decades in an agricultural area of northern France. Biol Conserv 144: 1610–1618.
- 27. Boothby J (1998) Foreword: Ponds - a search for significance. In: Boothby J, editor. Ponds and pond landscape of Europe. Lancashire, UK: Pond Life Project. 1–9.
- 28. Jehle R (2000) The terrestrial summer habitat of radio-tracked great crested newts (Triturus cristatus) and marbled newts (T. marmoratus). Herpetol J 10: 137–142.
- 29. Egea-Serrano A, Tejedo M, Torralva M (2011) Behavioral responses of the Iberian waterfrog, Pelophylax perezi (Seoane, 1885), to three nitrogenous compounds in laboratory conditions. Ecotoxicology 20: 1246–1257.
- 30. Piha H, Luoto M, Merila J (2007) Amphibian occurrence is influenced by current and historic landscape characteristics. Ecol Appl 17: 2298–2309.
- 31. Denoël M (2004) Répartition, habitat et conservation des amphibiens du Pays de Herve (Belgique). Bull Soc Herpétol France 111–112: 49–77.
- 32. Denoël M, Lehmann A (2006) Multi-scale effect of landscape processes and habitat quality on newt abundance: implications for conservation. Biol Conserv 130: 495–504.
- 33. Wilbur HM (1997) Experimental ecology of food webs: complex systems in temporary ponds. Ecology 78: 2279–2302.
- 34. Arntzen JW (2002) Testing for equal catchability of Triturus newts by dip netting. J Herpetol 36: 272–276.
- 35. Wagner N, Pellet J, Lötters S, Schmidt BR, Schmitt T (2011) The superpopulation approach for estimating the population size of ‘prolonged’ breeding amphibians: Examples from Europe. Amphibia-Reptilia 32: 323–332.
- 36. Kopecký O, Vojar J, Denoël M (2010) Movements of Alpine newts (Mesotriton alpestris) between small aquatic habitats (ruts) during the breeding season. Amphibia-Reptilia 31: 109–116.
- 37. Denoël M, Andreone F (2003) Trophic habits and aquatic microhabitat use in gilled immature, paedomorphic and metamorphic Alpine newts (Triturus alpestris apuanus) in a pond in central Italy. Belg J Zool 133: 95–102.
- 38. Perret N, Pradel R, Miaud C, Grolet O, Joly P (2003) Transience, dispersal and survival rates in newt patchy populations. J Anim Ecol 72: 567–575.
- 39. Van Buskirk J (2005) Local and landscape influence on amphibian occurrence and abundance. Ecology 86: 1936–1947.
- 40. Semlitsch RD (2008) Differentiating migration and dispersal processes for pond-breeding amphibians. J Wildl Manage 72: 260–267.
- 41. Ficetola GF, Padoa-Schioppa E, De Bernardi F (2009) Influence of landscape elements in riparian buffers on the conservation of semiaquatic amphibians. Conserv Biol 23: 114–123.
- 42. Richards SA (2008) Dealing with overdispersed count data in applied ecology. J Appl Ecol 45: 218–227.
- 43. Richards SA, Whittingham MJ, Stephens PA (2011) Model selection and model averaging in behavioural ecology: the utility of the IT-AIC framework. Behav Ecol Sociobiol 65: 77–89.
- 44. Symonds MRE, Moussalli A (2011) A brief guide to model selection, multimodel inference and model averaging in behavioural ecology using Akaike’s information criterion. Behav Ecol Sociobiol 65: 13–21.
- 45. Burnham KP, Anderson DR (2002) Model selection and multimodel inference: a practical information-theoretic approach. New York: Springer Verlag.
- 46. Breheny P, Burchett W (2012) visreg: Visualization of regression models. R package version11–1http://CRANR-projectorg/package=visreg.
- 47. Ruxton GD (2006) The unequal variance t-test is an underused alternative to Student’s t-test and the Mann-Whitney U test. Behav Ecol 17: 688–690.
- 48. Houlahan JE, Findlay CS, Schmidt BR, Meyer AH, Kuzmin SL (2000) Quantitative evidence for global amphibian population declines. Nature 404: 752–755.
- 49. Branton M, Richardson JS (2011) Assessing the value of the umbrella-species concept for conservation planning with meta-analysis. Conserv Biol 25: 9–20.
- 50. Ficetola GF, De Bernardi F (2004) Amphibians in a human-dominated landscape: the community structure is related to habitat features and isolation. Biol Conserv 119: 219–230.
- 51. Griffiths RA, Sewell D, McCrea RS (2010) Dynamics of a declining amphibian metapopulation: Survival, dispersal and the impact of climate. Biol Conserv 143: 485–491.
- 52. Jehle R, Wilson GA, Arntzen JW, Burke T (2005) Contemporary gene flow and the spatio-temporal genetic structure of subdivided newt populations (Triturus cristatus, T. marmoratus). J Evol Biol 18: 619–628.
- 53. Hanski I, Gaggiotti OE (2004) Ecology, genetics and evolution of metapopulations. San Diego, USA: Elsevier Academic Press.
- 54. Ficetola GF, Siesa ME, Manenti R, Bottoni L, De Bernardi F, et al.. (2011) Early assessment of the impact of alien species: differential consequences of an invasive crayfish on adult and larval amphibians. Diversity Distrib: 1141–1151.
- 55. Denoël M, Ficetola GF (2007) Landscape level thresholds and newt conservation. Ecol Appl 17: 302–309.
- 56. Gustafson DH, Andersen ASL, Mikusiński G, Malmgren JC (2009) Pond quality determinants of occurrence patterns of great crested newts (Triturus cristatus). J Herpetol 43: 300–310.
- 57. Zanini F, Pellet J, Schmidt BR (2009) The transferability of distribution models across regions: an amphibian case study. Diversity Distrib 15: 469–480.
- 58. Wood PJ, Greenwood MT, Agnew MD (2003) Pond diversity and habitat loss in the UK. Area 35: 206–216.
- 59. Rannap R, Lohmus A, Briggs L (2009) Restoring ponds for amphibians: a success story. Hydrobiologia 634: 87–95.
- 60. Leu T, Luscher B, Zumbach S, Schmidt BR (2009) Small fish (Leucaspius delineatus) that are often released into garden ponds and amphibian breeding sites prey on eggs and tadpoles of the common frog (Rana temporaria). Amphibia-Reptilia 30: 290–293.
- 61. Dolmen D (1982) Zoogeogeography of Triturus vulgaris (L.) and T. cristatus (Laurenti) (Amphibia) in Norway, with notes on their vulnerability. Fauna Norv, ser A 3: 12–25.
- 62. Denoël M, Džukić G, Kalezić ML (2005) Effect of widespread fish introductions on paedomorphic newts in Europe. Conserv Biol 19: 162–170.
- 63. Denoël M, Ficetola GF, Ćirović R, Radović D, Džukić G, et al. (2009) A multi-scale approach to facultative padomorphosis of European newts in the Montenegrin karst: distribution pattern, environmental variables and conservation. Biol Conserv 142: 509–517.
- 64. Hartel T, Nemes S, Cogalniceanu D, Ollerer K, Schweiger O, et al. (2007) The effect of fish and aquatic habitat complexity on amphibians. Hydrobiologia 583: 173–182.
- 65. Monello RJ, Wright RG (2001) Predation by goldfish (Carassius auratus) on eggs and larvae of the eastern long-toed salamander (Ambystoma macrodactylum columbianum). J Herpetol 35: 350–353.
- 66. Braz E, Joly P (1994) Microhabitat use, resource partitioning and ecological succession in a size-structured guild of newt larvae (g. Triturus, Caudata, Amphibia). Arch Hydrobiol 131: 129–139.
- 67. Schmidt BR, Van Buskirk J (2005) A comparative analysis of predator-induced plasticity in larval Triturus newts. J Evol Biol 18: 415–425.
- 68. Winandy L, Denoël M (2013) Cues from introduced fish alter shelter use and feeding behaviour in adult alpine newts. Ethology 119: 121–129.
- 69. Vredenburg VT (2004) Reversing introduced species effects: Experimental removal of introduced fish leads to rapid recovery of a declining frog. Proc Nat Acad Sci USA 101: 7646–7650.
- 70. Miaud C (1994) Role of wrapping behavior on egg survival in three species of Triturus (Amphibia, Urodela). Copeia: 535–537.
- 71. Miaud C (1993) Predation of newt eggs (Triturus alpestris and T. helveticus) - Identification of predators and protective role of oviposition behavior. J Zool 231: 575–582.
- 72. Sztatecsny M, Jehle R, Schmidt BR, Arntzen JW (2004) The abundance of premetamorphic newts (Triturus cristatus, T. marmoratus) as a function of habitat determinants: An a priori model selection approach. Herpetol J 14: 89–97.
- 73. Hartel T, Nemes S, Ollerer K, Cogalniceanu D, Moga C, et al. (2010) Using connectivity metrics and niche modelling to explore the occurrence of the northern crested newt Triturus cristatus (Amphibia, Caudata) in a traditionally managed landscape. Environ Conserv 37: 195–200.
- 74. Ficetola GF, Marziali L, Rossaro B, De Bernardi F, Padoa-Schioppa E (2011) Landscape-stream interactions and habitat conservation for amphibians. Ecol Appl 21: 1272–1282.
- 75. Schabetsberger R, Jehle R, Maletzky A, Pesta J, Sztatecsny M (2004) Delineation of terrestrial reserves for amphibians: post-breeding migrations of Italian crested newts (Triturus c. carnifex) at high altitude. Biol Conserv 117: 95–104.
- 76. Marnell F (1998) Discriminant analysis of the terrestrial and aquatic habitat determinants of the smooth (Triturus vulgaris) and the common frog (Rana temporaria) in Ireland. J Zool, London 244: 1–6.
- 77. Malmgren JC (2002) How does a newt find its way from a pond? Migration patterns after breeding and metamorphosis in great crested newts (Triturus cristatus) and smooth newts (T. vulgaris). Herpetol J 12: 29–35.
- 78. Halley JM, Oldham RS, Arntzen JW (1996) Predicting the persistence of amphibian populations with the help of a spatial model. J Appl Ecol 33: 455–470.