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Acta Oecologica 110 (2021) 103713
Contents lists available at ScienceDirect
Acta Oecologica
journal homepage: www.elsevier.com/locate/actoec
Population trends of four species of amphibians in western France; results
from a 15 year time series derived from road mortality counts
Roger Meek
Institute for Development, Ecology, Conservation and Cooperation, Via G. Tomasi di5 Lampedusa 33, I-00144 Rome, Italy
A R T I C L E I N F O A B S T R A C T
Keywords: Reports of declining amphibians are a major concern for conservation given their role as indicators of envi
Amphibians ronmental change. This includes potentially impacting on the ecology other species and highlights the impor
Population changes tance of monitoring long-term changes in amphibian populations. The core difficulty is gathering long term data
Road mortalities
sets that as well as being time consuming, may be costly and impractical for logistical reasons. Road mortalities
West France
have value in this respect since they are highly visible and have been frequently used as a metric to monitor
changes in a variety of animal populations. In this paper they have been employed as proxies to estimate long-
term population changes in four European species of amphibians. From January 2005 to December 2019 a total
of 747 amphibian mortalities were recorded, the majority of which were urodeles. Most individuals were adults
that entered roads during the annual migration period. Long term annual counts fluctuated widely in all species
and peaked during the 4-year period from 2009 to 2012 with second period of high numbers in Triturus mar
moratus and Hyla arborea during 2018 and 2019. Long-term population trends were evaluated using regression
analysis of the logarithmic (loge) transforms of annual counts against year as independent variables, which were
then tested against a hypothetical 0 regression coefficient, indicative of population stability. The results indicated
long-term stability in T. marmoratus and H. arborea but potential declines in Lissotriton helveticus and Pelophylax
lessonae, the latter showing metapopulation extinctions followed by recolonisation. Stepwise regression of po
tential climate drivers in amphibian numbers suggested a 2-year lag of rainfall during October and December
were potential factors involved in population change.
1. Introduction Road mortalities have long been implicated in amphibian population
decline (Beebee, 2013) but they have also been widely employed as
During the previous decades amphibian biologists have documented proxies to examine population changes in amphibians (e.g. Meyer et al.,
sudden or rapid population shifts or declines in amphibians (e,g, 1998; Meek, 2018), snakes (Capula et al., 2014; Rugiero et al., 2013),
Blaustein et al., 1994; Blaustein et al., 2011; Meyer et al., 1998; Hou lizards (Meek, 2020) and mammals (e.g. Mallick et al., 1998; Baker
lahan et al., 2000; Green, 2003) with a series of multifaceted in et al., 2004; Widenmaier and Fahrig, 2006; Battisti et al., 2012; D’Amico
teractions being proposed as factors driving the changes (e.g. Collins and et al., 2018). As metrics road mortalities give only an index of abun
Storfer, 2003). These include chaotic population dynamics due to time dance but have value in that they are highly visual on road surfaces
lags in density dependent responses (e.g. Wilbur, 1990), climatic effects minimising detection problems (Beebee and Griffiths 2005; Beckmann
and habitat alterations by humans (e.g. Hanski, 1998; Alford and and Shine, 2014) and if carcasses are removed they represent indepen
Richards, 1999; Carlson and Edenhamn, 2000; Hartel, 2005; Gollmann dent replicates and hence avoid double counting and autocorrelation.
et al., 2002) and disease (e.g. Ariel et al., 2005). Amphibians are now The long-term data series they generate facilitates predictions of how
considered bio-indicators of ecosystems where major population populations change over time, which is key information for establishing
changes may seriously impact on other species, for instance snake a valid conservation strategy (e.g. Ferri et al., 2017). Further insights can
populations (Zipkin et al., 2020), highlighting the importance of moni be achieved if several sympatric species are monitored simultaneously
toring long-term trends. This is especially relevant in fragmented land enabling comparison of inter-specific trends, and when sourced from
scapes, where species that operate through metapopulation dynamics fragmented habitats they provide a degree of understanding of trends in
can be seriously impacted (Levins, 1970). areas that are a major concern for amphibian populations.
E-mail address: rogermeek85@aol.com.
https://doi.org/10.1016/j.actao.2021.103713
Received 17 August 2020; Received in revised form 25 January 2021; Accepted 6 February 2021
Available online 27 February 2021
1146-609X/© 2021 Published by Elsevier Masson SAS.
R. Meek Acta Oecologica 110 (2021) 103713
Despite the importance of long-term data sets such data are still, Vendee, western France, The locality is dominated by agriculture that
perhaps not surprisingly, rare in the literature probably a consequence had experienced little or no major changes in land use during the study
of the long-term effort and funding required. This paper presents results period from 2005 to 2019. The general climate is mild oceanic with
of an ongoing long-term monitoring program to determine population mean annual air temperature 13–14 ◦ C. July–August are the hottest and
trends in four species of European amphibians; Hyla arborea, Pelophylax driest months (mean ≈ 22 ◦ C) with high precipitation from October until
lessonae, Lissotriton helveticus and Triturus marmoratus (Fig. 1). Two February. During summer when rainfall is low it is normal for all but the
species, H. arborea and L. helveticus, are known to undergo major pop largest water bodies to dry up.
ulation shifts with apparent declines in areas of Europe (eg Stumpel, Life history cycles of L. helveticus and T. marmoratus are in general
1987a,b; Denoel et al., 2005; Pellet et al., 2006) whilst P. lessonae is similar (Arnold and Ovenden, 2002). Both species arrive at ponds from
known for unusual population dynamics, including population crashes terrestrial habitat between November and January with courtship and
and low reproductive success (e.g. Sjögren, 1991; Bressi, 1995; Zahn, reproduction taking place almost immediately with egg laying from the
1996). beginning of February before returning to terrestrial habitats from late
The present study was carried out in a fragmented landscape in May/June. During the summer terrestrial phase T. marmoratus appar
Vendee, western France where extensive modification of the environ ently remain inactive below ground with several also being found in
ment as a result of agricultural activities, including use of agrochemi drains (Jehle, 2000 & pers. observations). The green frog P. lessonae
cals, are a normal annual activity (Meek, 2018). Agricultural activities in spends the summer months around ponds and other water bodies but is
fragmented landscapes are two major factors identified in amphibian inactive in winter hibernating at the bottom of ponds. The tree frog
declines (e.g. Blaustein et al., 1994; Battisti et al., 2016). In the study H. arborea appears near ponds in March/April with spawn clumps found
area amphibian migration to breeding ponds occurs mostly from from late April to June. They remain close to ponds during the summer
October through to December in urodeles or during late summer due to months at distances of around 12 m from the pond edge mostly in shaded
pond drying or in early winter for hibernation in ponds in P. lessonae. areas but also in partial sunny locations (Meek, 2011). Longer distance
Hence samples represent mostly breeding adults. movement commences around mid-August with calling until November.
The key questions addressed in this study are 1) do sympatric am Examples of each species are shown in Fig. 1.
phibians show similar long term population trends and is there evidence Mortalities were collected on roads (distance ≈16 km) between
for population declines? 2) to what extent do numbers fluctuate on an wetland areas close to the villages of St Denis du Payre, and Chasnais.
annual basis? 3) are there long term changes in population size char The distance between the two is approximately 6 km (see Meek, 2012 for
acteristics ? 4) is there evidence for climate effects on amphibian a schematic view). Surveying for road mortalities commenced 2005 and
numbers? were made four and six times per month throughout each year. Normally
surveys were undertaken at mid-week and weekends. This was by a
2. Methods single observer on a bicycle at a speed of around 5–10 km/h. Tests of
comparison of carcass detection showed that detection increased when
The study area (46◦ 27′ N; 1◦ 53′ W) is a fragmented landscape in using a bicycle compared to motorcar or motorcycle. When found each
Fig. 1. Examples of study species; H. arborea (A), P. lessonae in amplexus (B), T. marmoratus female depositing eggs (C) and L. helveticus (D).
2
R. Meek Acta Oecologica 110 (2021) 103713
individual was measured for snout to vent length (S.V.L.) with an esti variables evaluated at α = 0.05. Inclusion in a forward construction
mated maximum error of 5% depending on carcass condition. Each model was by introducing each significant variable at a time then adding
located carcass was removed to avoid double counting. Road traffic each new candidate variable incrementally to the model. If a new var
volume increased slightly during the period 2005 to 2019 with counts iable candidate reduced the significance level it was omitted. It is
showing means of 11.1–60.2 vehicles per hour depending on the road appreciated that independent variable selection in arbitrary but the
(Meek, 2012). Low road mortality counts in P. lessonae were supported method is reasonable when the number of candidate variables is not
by additional data from pond presence including during spring summer large (Gotelli and Ellison, 2004).
2020 giving 16 years for this metric. Five large ponds were selected and Climatic information has been sourced from the nearest weather
regularly checked for P. lessonae presence. The criterion for ‘pond station at La Rochelle-Le Bout Blanc, which is approximately 35 km from
presence’ was defined as a minimum of 15 frogs at each pond and that the study locality (https://www.infoclimat.fr/climatologie/annee/
larvae or spawn should be present indicating reproduction. 2000/la-rochelle-le-bout-blanc/valeurs/07315.html).
3. Results
2.1. Statistical analysis
A total of 747 amphibians were found on roads with urodeles the
To evaluate long-term population regression analysis was applied to
most frequently found; L. helveticus n = 281, T. marmoratus n = 265 and
annual counts as the dependent variable after transformation to loga
H. arborea n = 154 and P. lessonae 47. All species were found throughout
rithms (Loge) and treating year as the independent variable. This gives,
the year but monthly distributions were negatively skewed in
logeN = b + m±ε*year L. helveticus; z = 2.86, P = 0.004, T. marmoratus z = 2.82, P = 0.005 and
P. lessonae z = 1.16, P = 0.04, reflecting the periods of migration to
Where logeN, represents road mortalities, m the regression coeffi breeding ponds but although trends were similar in H. arborea the result
cient and b the y-intercept and ε white noise error. The latter in was not significant, z = 0.61, P = 0.54. Monthly distributions are shown
corporates measurement error and random variation (Gotelli and in Fig. 2 along with broken lines derived from theoretical expected
Ellison, 2004). In P. lessonae mortality counts and pond presence had 4 monthly numbers under a hypothesis of equality of monthly counts
years with 0 counts and hence in both the dependent variable was using:
treated as logeN+1. The null hypothesis is that logeN (or logeN+1) is
stable when m = 0; significant departures from m would indicate pop Expected = 1/N1*N2
ulation change with positive or negative regression coefficients repre
where N1 is number of months and N2 the total sample size for each
senting population increase or decrease respectively. Departures from
species. Multiplying or dividing expected numbers by the actual
the 0 regression coefficients were evaluated using t-tests at n-2 d.f.
numbers can find monthly deviations from the expected. Monthly
(Bailey, 1995). Data were drawn from all roads pooled for each year
presence on roads increased with increasing rainfall in L. helveticus and
giving 15 data sets.
T. marmoratus r = 0.8 and 0.79 respectively (both P = 0.002) but
Since unusually high or low year counts could potentially have an
monthly rainfall was not significant in H. arborea (r = 0.50, P = 0.09) or
inordinate effect on m, tests for influence function (Gotelli and Ellison,
P. lessonae (r = 0.24, P = 0.45).
2004) were carried out to estimate the potential errors of the true
Pooled annual SVL’s distributions between 2005 and 2019 indicated
regression coefficients using Jackknifing (Sahinler and Topuz, 2007).
the distributions of urodeles and P. lessonae were significantly skewed
This non-random method produces repeatable results by systematically
toward larger individuals (D’Agostino skewness test; T. marmoratus g1 =
removing one-year data sets from the sample with regression analysis
− 0.69, z = − 3.68, P = 0.0002; L. helveticus g1 = − 0.59, z = − 3.54, P =
re-applied to the jackknifed sets giving a series of pseudo-m values. The
0.0003, P. lessonae g1 = 2.08, z = 3.43, P = 0.0005) (Fig. 3). The minor
means of the pseudo m coefficients were then compared against the true
positive skew towards smaller individuals in H. arborea did not differ
coefficients.
significantly from a Gaussian distribution (g1 = 0.01, z = 0.069, p =
Homogeneity in annual counts were evaluated by Leven’s test for
0.945). ANOVA with Tukey Pairwise indicated significant differences
equality of multiple variances. This is less sensitive to departures from
between yearly S.V.L in all species (Table 1) with greatest fluctuations in
normality and considers the distances of the observations from their
L. helveticus (Fig. 4). Less variation in annual S.V.L. was found in
sample medians. The test is also robust for smaller samples (Box and
T. marmoratus and H. arborea. The presence of a series of zero counts
Jenkins, 1976) and rejects equality of variance when,
from 2015 to 2018 in P. lessonae probably signify true zeros rather than
W > Fα, k-1, N-k
measurement error and hence represent a local extinction event (Gotelli
and Ellison, 2004).
where W is the test statistic, Fα, k-1, N-k the upper critical value of the F- Wide annual changes in numbers were observed in all species with
distribution k-1 and N-k degrees of freedom with significance α at P = long-term variance greatest in L. helveticus and T. marmoratus. In general
0.05. Post hoc tests were with Tukey HSD. annual counts peaked during the 4-year period from 2009 to 2012 when
Long-term inter-population correlation of trends between species 50.1% of all mortalities were recorded. A second period of high numbers
were assessed using the Pearson correlation coefficient with α = 0.05. was recorded in 2018 and 2019, which formed 17.9% of the total
Tests for skewness in S.V.L. distributions were made using the D’Ag sample. The test for homogeneity of annual variance between species
ostino skewness test (g1) using z –scores against a null hypothesis of S = was rejected (Leven’s = 3.2, p = 0.03, multiple comparisons test, P =
0, (D’Agostino et al., 1990). 0.005) with post hoc Tukey indicating a significant difference between
To test for potential climatic effects on population changes in annual L. helveticus and P. lessonae (Tukey HSD = 4.005). All species, except
counts stepwise regression was applied to selected climatic variables as T. marmoratus, showed years with outliers. The results are shown in
independent or predictor variables and compared to annual amphibian Fig. 5.
counts as the response or dependent variable. Climatic variables were Regression analysis of long-term numbers gave positive coefficients
long term monthly and total annual rainfall along with annual temper that were not significantly different from 0 in T. marmoratus and
atures during the study period. These data were further analysed by H. arborea suggesting that despite wide annual fluctuations long-term
lagging the climate data to 1, 2 and 3 years previous to annual counts for numbers were generally stable. Jackknife analysis identified a strong
comparisons. Initial analysis was by applying regression to each inde influence function in H. arborea in 2018 that increased the positivity of
pendent variable against the response variable for inclusion in the final the regression coefficient. In L. helveticus and P. lessonae the coefficients
model (Gotelli and Ellison, 2004) with significance for the independent
3R. Meek Acta Oecologica 110 (2021) 103713
Fig. 2. Histograms of monthly presence indicating differences in movement onto roads in T. marmoratus and L. helveticus during October into January compared to
the anurans.
30 30
T. marmoratus L. helveticus
25 25
20 20
Frequency
15 15
10 10
5 5
0 0
30 40 50 60 70 80 90 10 20 30 40 50
14
25
H. arborea 12
P. lessonae
20
10
Frequency
15 8
6
10
4
5
2
0 0
20 30 40 50 60
20 30 40 50 60 70 80 90 100 110
S.V.L (mm) S.V.L. (mm)
Fig. 3. Histograms showing SVL distributions in the four study species. See text for details.
were negative suggesting possible long-term declines in L. helveticus but high), T. marmoratus (2006 low) and L. helveticus (2010 & 2012 high).
not necessarily in P. lessonae (see below and Discussion). The regression The long-term trends are shown in Fig. 6A and B.
coefficients are shown in Table 2 along with the outcomes from the Long-term annual number trends were closely correlated between
jackknife analysis, which were in good agreement with the true co T. marmoratus and L. helveticus and H. arborea but not between
efficients. Jackknifing identified high influence functions in P. lessonae L. helveticus and H. arborea. No long-term trends were found between
for 2010 road counts (in 12 of 15 jackknifes) and 2019 data for pond P. lessonae and the remaining species. The full results can be found in
presence (in 11 of 15 Jackknifes), which could potentially give a Table 3 in addition to comparisons with other small sympatric am
disproportionate leverage function on the true long-term regressions. phibians and lizards – see Discussion.
Less impacted were the outcomes for H. arborea (2006 low and 2017 If generation time is assumed to be approximately 2–3 years the
4R. Meek Acta Oecologica 110 (2021) 103713
Table 1 with highest rainfall but if long winter dry periods occur this may impact
Intra-specific comparisons of annual S.V.L. ranges of road-killed amphibians on migratory movement to water bodies. Absence of significance of
using ANOVA and post hoc Tukey Pairwise Comparison tests. See text for further annual temperatures, lagged or otherwise, is perhaps an expected result
details. given less thermal dependency in amphibians compared to reptiles.
Ranges of annual means (m. (F) df P However, stepwise regression has been subject to criticism as potentially
m.) ANOVA oversimplifying the results (e.g. Roecker, 1991) suggesting further cli
T. marmoratus 52.5–70.0 2.14 14, 0.01 matic data and time series extensions are needed to detect potentially
271 spurious statistical effects (e.g. Lukacs et al., 2010). Additionally, in
L. helveticus 21.8–38.5 8.74 11,R. Meek Acta Oecologica 110 (2021) 103713
Table 2
Regression coefficients (m) relating annual mortality or pond presence counts between 2005 and 2019 along with results of Jackknife analysis (m JK). For m the
±values are standard errors of the coefficients. For the jackknife ± are the standard deviations of the pseudo-regression coefficients. Right column identifies years with
influence functions either low L or high H.
m ± (SE) t P m JK ± (SD) n Influence F
L. helveticus − 0.10 0.06 1.8 0.10 − 0.09 0.02 15
T. marmoratus 0.06 0.007 8.5R. Meek Acta Oecologica 110 (2021) 103713
(Stumpel, 1987a,b; Stumpel and Hanekamp, 1986). Pellet et al. (2006) some insight into potential stochastic exogenous influences on long term
suggested that in Switzerland both intrinsic and extrinsic factors are trends, for instance rainfall patterns and temperatures. In the present
involved in driving population change including log-linear density study the results varied, for example there were significant correlations
dependence and 2 year rainfall lags (Friedl and Klump, 1997) that between annual numbers of H. arborea and T. marmoratus and the agile
precipitation during the reproductive season impacts on larvae survi frog Rana dalmatina (Meek, 2016 and unpublished data) and between
vorship and recruitment. and T. marmoratus and B. spinosis (Table 3).
The marbled newt T. marmoratus showed greater population stability This study supports the notion that road mortalities represent a
with least annual variance and SVL variation. This species has a lifespan valuable metric to quantify amphibian road mortality suggesting that in
of around 9 years, at least in females (Jakob et al., 2003), employs toxic addition to representing independent replicates the trends represent
(tetrodotoxin) skin secretions as a defence mechanism (Griffiths, 1996) samples of breeding individuals and a proxy for population change.
with limited movement to summer home ranges (Jehle, 2000) all of Short-term data sets may reveal important demographic information (e.
which may contribute to low natural and road mortalities and popula g. size or age structure) but they are less useful in verifying the key in
tion stability. The extraordinary changes in numbers and presence formation required for conservation strategies. Computer simulations
observed in the green frog P. lessonae have been reported previously in have indicated that detecting a true population decline increases with
other regions (e.g. Sjögren, 1988, 1991; Zahn, 1996). Low road mor the duration of the study period (Scherer and Tracey, 2011); for instance
tality counts compared to the other species likely involve strong site a short-term study might have concluded the absence of P. lessonae after
fidelity and hence limited longer distance movement during much of the 2015 was a fatal extinction event. Additionally, annual counts were
year. However, mark-release-recapture studies have indicated lower than statistically expected for most of the 15-year time series in all
P. lessonae may travel distances of up to 15 km (Tunner, 1992), which species (66% in P lessonae to 73% in L. helveticus). This generally agrees
must facilitate pond recolonisation. A mass migration event in with the comment of Alford and Richards (1999) that population de
P. lessonae was recorded in the study area during the autumn of 2010, clines could outnumber population increases without indicating overall
the year of highest road mortality counts, when over several days large decline in population (but see Green, 2003).
numbers were observed moving nocturnally. High densities at ponds Given the above comments it is clear that continued population
and a period of high rainfall possibly drove this event and probably monitoring in the study locality is essential to significantly improve
represent a recolonisation. Pond presence, although of value, does not understanding of long-term population change especially in P. lessonae
give detailed estimates of numbers, especially during periods of high and L. helveticus, species that are potentially at risk. The limited time
abundance when counting is difficult or impossible. Lack of agreement frames when most mortalities (e.g. seasonal) occur must facilitate con
was mostly between mortalities and pond presence previous to 2010 servation efforts. For example public information signs warning of road
when the numbers were very high, uncountable and hence inexact, but crossing amphibians that have been erected in the UK for common toads
even with this statistical anomaly the two data sources are correlated have been shown to be effective (e.g. Beebee, 2013). Underground
long term (r = 0.64, P = 0.01) and congruent in showing declines from crossings on known main migration routes installed on the A83 auto
2011/2012 followed by total absence from 2015 to 2018 and limited route in France (Mougey, 1996) represent simple mitigation measures
recovery in 2019/2020. This suggests metapopulation extinctions fol that could have a critical impact on the long term viability of amphibian
lowed by recolonisation, known in other areas of Europe for Pelophylax populations.
(e.g. Sjögren, 1988, 1991; Zahn, 1996), as is low or complete repro
ductive failure (Bressi, 1995; Bressi, 1998). For example, Zahn (1996) Declaration of competing interest
found that ponds in Upper Bavaria occupied by P. synkl.esculenta were
generally characterised by low reproductive success whilst in the The author declares that he has no known competing financial in
Netherlands it was as low as 12% (Stumpel and van der Voet, 1998). terests or personal relationships that could have appeared to influence
Strong site fidelity during the summer months probably increases the work reported in this paper.
vulnerability to Procambarus clarki, an alien crayfish that consumes all
stages of amphibian development, especially spawn and larvae, and is
Acknowledgements
known to have major impacts on amphibian populations in Europe and
elsewhere (Kats and Ferrer, 2003; Ficetola et al., 2011; Nunes et al.,
Comments by two reviewers and editor Dr Luca Luiselli made valu
2013). This species is capable of reaching epidemic proportions in the
able suggestions to improve the original submitted manuscript.
study area as it is in other regions (Souty-Grosset et al., 2016; Meek,
2018).
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