Effects of the non-native amphibian species Discoglossus pictus on the recipient amphibian ...

17 Pages·2013·0.61 MB·English

BiolInvasions(2013)15:799–815
DOI10.1007/s10530-012-0328-4
ORIGINAL PAPER
Effects of the non-native amphibian species Discoglossus
pictus on the recipient amphibian community: niche overlap,
competition and community organization
Alex Richter-Boix • Nu´ria Garriga •
Albert Montori • Marc Franch •
Olatz San Sebastia´n • Dani Villero •
Gustavo A. Llorente
Received:13October2011/Accepted:27August2012/Publishedonline:5September2012
(cid:2)SpringerScience+BusinessMediaB.V.2012
Abstract Thepaintedfrog,Discoglossuspictus,was a laboratory evaluation of the effects of competition
introduced into the Iberian Peninsula 100 years ago between the non-native and the two native species
and its distribution has steadily increased since then. withwhichitoverlapsmostcommonly:Bufocalamita
We studied the effects of this non-native amphibian and Pelodytes punctatus. The presence of D. pictus
species on the native ones inthe northeasternarea of larvaereducedthesurvival,bodymassandactivityof
the peninsula. We compared amphibian assemblages B. calamita, and increased time to metamorphosis.
inregionswithandwithoutD.pictustoestimateniche Furthermore, D. pictus showed the highest consump-
overlapbetweenspecies.Additionally,wecarriedout tion rate while P. punctatus showed the lowest. One
possible consequence of these competitive interac-
tionsisanalterationofspeciesco-occurrencepatterns
A.Richter-Boix intherecipientcommunityonaregionalscale.Inthe
DepartmentofPopulationBiologyandConservation non-invadedarea,thecheckerboardscore(C-score)of
Biology,UppsalaUniversitet,Norbyva¨gen18D,
co-occurrence indicated that the community was
75236Uppsala,Sweden
structured, whereas the standardized C-score in the
e-mail:[email protected]
invaded area indicated that the community did not
A.Richter-Boix(cid:2)M.Franch(cid:2)O.SanSebastia´n differ significantly from having a random structure.
Fundacio´ Emys,CarreteraSantaColoma,12,
Theseresultssuggestthatcompetitionbetweennative
17421Riudarenes,Girona,Spain
andnon-nativespeciescancauserecipientcommuni-
N.Garriga(&)(cid:2)A.Montori(cid:2)M.Franch(cid:2) tiestobecomelessstructured.
O.SanSebastia´n(cid:2)G.A.Llorente
DepartamentdeBiologiaAnimal(Vertebrats),Facultatde
Keywords Invasivespecies(cid:2)Competition(cid:2)
Biologia,UniversitatdeBarcelona,AvingudaDiagonal,
Communityresponse(cid:2)Co-occurrence
643,08028Barcelona,Spain
e-mail:[email protected]
O.SanSebastia´n
Introduction
SociedaddeCienciasAranzadi,Departamentode
Herpetolog´ıa,Zorroagagaina,11,E-20014SanSebastia´n,
Spain The effects of invasions by non-native species are
often complex, which makes their impact on a
D.Villero
recipient community difficult to predict (Williamson
Grupd’EcologiadelPaisatge,CentreTecnolo`gicForestal
1999; Ficetola et al. 2007; Osunkoya et al. 2011).
deCatalunya,CarreteradeSantLlorenc¸ deMorunys,
km2,Solsona,25280Lleida,Spain Several ecological interactions between non-native
123
800 A.Richter-Boixetal.
andnative species may beinvolved, andtheir effects mechanismsofinvasionandimpactarenotnecessar-
may be apparent to a greater or lesser degree ily linked (Hayes and Barry 2008), a non-native
(Kiesecker et al. 2001; Blaustein and Kiesecker species that spreads widely is more likely to affect
2002; Kats and Ferrer 2003). The various levels of multiplenativespeciesandleadtoareductionintheir
impact a non-native species may have on a native populations(OrtegaandPearson2005).
community are related to the capacity of the non- Because of the widely recognized importance of
native species to invade and the invasion stage it has competition in amphibian communities during their
reached (Blackburn et al. 2011). Many studies have larval phase (e.g. Alford 1999), several studies have
assessed the determinants of invasiveness (Van Kle- evaluated the possible impact of competition from
unen et al. 2010) to elucidate why some species non-native tadpoles on native tadpoles (Kupferberg
succeedandothersfailinestablishingthemselvesand 1997;Crossland1998;KieseckerandBlaustein1998;
spreading over a range into which they have been Smith 2005;Knightet al. 2009;Crosslandand Shine
introduced, but quantifying the consequences of 2010; Rebelo et al. 2010) and their effects on
invasion on native communities is much more diffi- community structure; although only a few of these
cult. However, the negative impact of biological studies were carried out on arid or Mediterranean
invasions on communities and ecosystem dynamics amphibian communities (Woodward 1982; Richter-
is considered one of the primary threats to global Boix et al. 2007a; Escoriza and Boix 2012). The
biodiversity (IUCN 2000; Mack et al. 2000; Clavero competitiveness ofthedifferentspeciescan bequan-
and Garc´ıa-Berthou 2005; D’Amore et al. 2010). tified using tadpole performance measures such as
Amphibians are the most threatened animal group larval period, size at metamorphosis and mortality.
under IUCN Red List criteria (IUCN 2010). 29 % of Pre-metamorphic traits such as timing of and size at
the world’s amphibian species are threatened with metamorphosis influence post-metamorphic perfor-
extinction (IUCN 2010) and at least 43.2 % of all manceandfuturefitnessofamphibians(Altweggand
described species are currently experiencing popula- Reyer 2003). If non-native species substantially out-
tiondecline(Stuartetal.2004).IntheMediterranean perform co-occurring natives, then we can expect
basin, there is a high level of endemism (64 %) and serious widespread consequences for the community
25.5 %oftheamphibianspecies arethreatened (Cox structure (Daehler 2003). On the other hand, if non-
etal.2006).Habitatloss,pollutionandinvasivealien nativespeciesrarelyoutperformco-occurringnatives,
species have the largest impact on Mediterranean oriftheirfitnessisonlymarginallysuperiortothatof
amphibiancommunities(Coxetal.2006).Mediterra- the natives, then the effects on the community are
nean ecosystems have a long history of biological likely to be minor. Nevertheless, even if non-native
invasions (Di Castri 1990; Rebelo et al. 2010) and species are not very competitive, competition in the
theirtemporarywetlandsareespeciallythreateneddue community may persist due to other factors, such as
to invasions and habitat destruction (Geiger et al. theabilityofaspeciestocolonizeunoccupiedniches
2005; Ficetola et al. 2011; Lillo et al. 2011), which rapidly (Lodge 1993; Rejmanek and Richardson
haveanimpactonamphibianpopulations. 1996).
Althoughthereareafewexceptions(e.g.Rhinella The aim of the present study was to examine the
marina,LithobatescatesbeianusandEleutherodacty- effects of competition between the non-native
lus coqui), the invasion dynamics of amphibian amphibian species Discoglossus pictus and native
species and the effects on recipient amphibian com- amphibiansinthenortheasternIberianPeninsula.The
munities have been little studied (Kupferberg 1997; specificobjectiveswere:(1)toquantifytheecological
Beard and Pitt 2005; Ficetola et al. 2007). This is breadthduringthelarvalphaseofeachspeciesinthe
presumably because of the perception that most of communityinordertoestimatenicheoverlapbetween
these species do not pose severe ecological or species;(2)toevaluate,inlaboratoryexperiments,the
economicthreats(KrausandCampbell2002).Recent competitive interactions and mechanisms between
studies show that high-impact invaders among D.pictusandthenativespecieswithwhichitoverlaps
amphibiansandreptilestendtoshowlowerestablish- mostfrequentlyinnature;and(3)toevaluatewhether
ment success rates in native communities than mam- thepresenceofD.pictusaltersspeciesco-occurrence
malsorfish(RicciardiandCohen2007).Althoughthe patternsonaregionalscalebycomparingcommunity
123
Effectsofthenon-nativeamphibianspecies 801
organization in a non-invaded region with that of an northeastern Spain. It has expanded about 140 km
invaded region. We hypothesized that if the invasive towards the south and north, and about 60 km to the
speciesoverlapswithsomenativespeciesinbreeding west, in favorable areas (Fig. 1). The introduced
sitesandcompeteswiththem,thenthese interactions species appears to be abundant, both in terms of
couldleadtochangesinthecommunitystructure. populations andindividuals,andcontinuestoexpand
itsrangeinbothFranceandSpain(FradetandGeniez
2004;Montorietal.2007).Anoveralldispersionrate
Materialsandmethods of 1.53 km/year ± 0.8 km (Montori et al. 2007) has
been calculated, which is lower than other invasive
Studysystem amphibian species such as R. marina (Phillips et al.
2007; Urban et al. 2008). Recently, new human-
Discoglossus pictus is native to North Africa and mediatedintroductionsoutsideitsrangeofexpansion
Sicily(Zangarietal.2006)andisthoughttohavebeen havebeendescribedalongtheSpanishMediterranean
introduced from Algeria to Banyuls-sur-Mer (Pyr- coast(FradetandGeniez2004;Franchetal.2007).
e´ne´es-Orientales department, France) in 1906 (Win- The area of northeastern Spain occupied by
trebert1908),fromwhereitcolonizedsuitableareasof D. pictus is a typical Mediterranean region, with hot
northeastern Spain and southeastern France (Martens anddrysummers,mildwintersandtworainyseasons:
and Veith 1987). In the invaded areas, amphibian one in spring and one in autumn (Mart´ın Vide and
speciesrichnessissimilartothatofnon-invadedareas Olcina 2001). The combination of climate, ground
(Cheylanetal.1987;Llorenteetal.1995;Pascaletal. composition, and pond size and depth gives rise to a
2003),butwithgreateroverlap(Montorietal.2007), rangeofpotentialbreedingareasforamphibians,from
which could modify the amphibian community ephemeralrainpoolstopermanentwaterbodies.The
structure. nativeamphibiancommunityinthiszonecomprisesa
SinceDiscoglossuspictuswasintroducedinBany- totalof10species:threeurodela(Salamandrasalam-
uls-sur-Mer, it has expanded to a continuous range andra, Triturus marmoratus and Lissotriton helveti-
of over 7,000 km2, from southeastern France to cus) and seven anuran species (Alytes obstetricans,
Fig.1 Distributionof
Discoglossuspictus:native
range(darkgrey)and
introducedrange(square).
Thehigherresolutionmap
representstheregionof
northeasternSpainwithboth
studyareas(blackspots).
Thearrowindicateswhere
thespecieswasintroduced
(Banyuls-sur-Mer)
123
802 A.Richter-Boixetal.
Pelodytes punctatus, Pelobates cultripes, Bufo cala- round, with many invertebrate predators but without
mita, Bufo bufo, Hyla meridionalis and Pelophylax fish; and (4) permanent ponds where fish are the top
perezi). predators.Usingthisdata,weconstructedapresence–
absence matrix for the different species at each site,
Fieldsurveymethods:ecologicalbreadth which were first assigned to one of the four habitat
inbreedinghabitats categoriesdescribed.
For the analysis of niche overlap, the similarity of
Inthespringof2005and2006,wesearchedforwater habitat use (calculated using the pond categories
body habitats in a region within the current range of previously explained) between two species was
D. pictus, which is well documented (Montori et al. calculated using the symmetric niche overlap coeffi-
2007), and in regions outside its range (Fig. 1). The cient (Pianka 1973). Pianka’s coefficient was mea-
first citations of D. pictus in the former region were sured using percentage use of each pond category by
from1995,andthisentireregionwasfullycolonized eachpairofspeciesfollowingtheformula:
by1997(SIARE2008).Theregionspresentthesame Pnp p
species richness and cover similar areas. Breeding O ¼O ¼ i ij ik
jk kj qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
siteswereidentifiedbythepresenceofeggs,tadpoles Pnp2Pnp2
i ij i ik
or metamorphic individuals, which were detected by
where p and p represent the proportions of the ith
conducting dip-netting and egg searches (Richter- ij ik
pondusedbythejthandkthspecies.Themeanniche
Boix et al. 2007a). The sites surveyed spanned the
overlapofthecommunitieswastestedstatisticallyby
range of aquatic breeding habitats in the area. Sam-
performing10,000MonteCarlorandomizationsofthe
pling time periods were established on the basis of
original data to create pseudo-communities, using
preliminarydataandtookaccountoftemporaldiffer-
EcoSim v7.72 (Gotelli and Entsminger 2007). We
ences in breeding activity between species (Richter-
applied the randomization algorithm RA2 (Lawlor
Boix et al. 2006), thereby ensuring that all species
1980)tothedata,withnichebreadthrelaxedbutwith
were detected. All the sites were surveyed once or
the zero structure of the matrix retained (Gotelli and
twice each year during the breeding season, and the
Entsminger2007).Thisalgorithmismorerealisticfor
surveyswereperformedbytwoexpertobservers.The
our data because the larval life-history traits of the
duration of active sampling and number of dip-nets
species studied impose constraints on their resource
varied depending on pond size following standard
utilization. For example, some species never use
techniques(Heyeretal.1994).Allamphibianspecies
ephemeral pools because their larval periods are
were identified in the field and returned to the water.
longerthanthewaterpermanenceofthehabitat.Since
Several studies performed in northeastern Spain
notallbreeding-pondhabitatsareequiprobable(some
provide us with good information about the distribu-
habitat categories are very common and others very
tion and pond breeding habits of the amphibian
rare), the observed utilization matrix and simulated
communities (Llorente et al. 1995; Pleguezuelos
matriceswerescaledrelativetothenumberofpondsin
1997; Pleguezuelos et al. 2002; SIARE 2008).
each category we sampled, with the user-defined
Although the gradient of permanence is continuous,
functionavailableinEcoSim(GotelliandEntsminger
we categorized the ponds by hydroperiod and top
2007).
predator (detected by visual survey and dip-netting).
Based on previous studies (Wellborn et al. 1996;
StocksandMcPeek2003;VanBuskirk2003;Richter- Competitionmeasuredinlaboratoryexperiments
Boix et al. 2007a) we described four categories: (1)
ephemeralpoolsthatdryupwithinweeks,containing ExperimentI:intraspecificandinterspecificeffects
water for less than 2 months, and with few or no onlarvallife-historytraits
predators;(2)temporarypondsthatfloodinthespring
and autumn rainfall, and hold water until the end of In 2006, we conducted laboratory experiments to
spring or early summer, containing water for more measure the effects of competition on the non-native
than2 months,usuallywithsmallinvertebratepreda- species and the two native species with the highest
tors; (3) permanent ponds containing water all year estimated niche overlap in nature: B. calamita and
123
Effectsofthenon-nativeamphibianspecies 803
P.punctatus.Theexperimentswereperformedinthe tadpoles (swimming or feeding). Activity was
laboratory at the University of Barcelona under the recorded three times a day by conducting focal
same photoperiod conditions (12D:12L) and temper- observations during the second and third weeks of
atures of 21–23 (cid:3)C. The competitiveness of the the experiment, with a total of 40 records for each
species was determined from pairwise experiments, tank. The timing of metamorphosis, defined as the
with the effects of D. pictus on the native species stagewhenthetailhasbeencompletelyabsorbed,was
calculated by comparing the performance of each determined by daily checks at the beginning of the
specieswhenrearedalone(intraspecificcompetition) experiment and twice-daily checks during metamor-
with their performance when reared together (inter- phosisclimax.Atmetamorphosis,wetbodymasswas
specific competition). The experimental design con- measured(tothenearest 0.1 mg)andthegrowthrate
sisted ofeighttreatments, each replicatedfour times: estimated as the mass gained each day during the
(1) a low-density, single-species treatment for each larval period [mass at metamorphosis divided by
species,with15tadpolesperexperimentalunit(three larvalperiod(g/days)].Survivalwasexpressedasthe
low-density treatments: D. pictus, B. calamita and proportionoflarvaethatcompletedmetamorphosisin
P. punctatus, with four replicates per species and each experimental tank. In the analysis, tank means
therefore12tanks);(2)ahigh-density,single-species wereusedinordertoavoidpseudoreplication.Before
treatment for each species, with 30 tadpoles per unit analysis, larval period, mass at metamorphosis and
(three high-densitytreatments:D.pictus,B.calamita growth rate were log transformed, and survival to
andP.punctatus,withfourreplicatesperspeciesand metamorphosis and tadpole activity were arcsin
12tanks);and(3)interspecifictreatmentsbetweenthe transformed. To analyze the intraspecific and inter-
invasiveD.pictusandthetwonativespecies,with15 specific effects of each species, we used multivariate
tadpolesofeachspecies(twotreatmentsorcombina- analyses of variance (MANOVA) for the three traits
tions:[D.pictus ? B.calamita;D.pictus ? P.punct- together (larval period, mass at metamorphosis and
atus],fourreplicatesandeighttanks). growth rate). When statistically significant effects
The experimental units consisted of plastic tanks were detected, successive ANOVAs were conducted
filledwith30Lofdechlorinatedtapwater.Thewaterin for each variable. Since we wanted to test whether
each tank was changed every week to prevent the competitionbetweenspecieswassymmetricorasym-
accumulation of lethal levels of metabolic products. metric, and because the baseline responses of the
The tadpoles were fed three times a week with species differed in the intraspecific treatments, we
commercial rabbit pellets (SANKY PIBAR. S.A., standardized the absolute changes in responses to
Barcelona,Spain).Allthe treatmentsconsistedofthe facilitate interspecific comparisons. We applied the
same amount of food per week, which was gradually standardizedmodeldevelopedbyMorinandJohnson
increased throughout the experiment from approxi- (1988).Standardizedintraspecificeffectswerecalcu-
mately4–5 gofpelletsatthebeginningto15 gatthe latedbysubtractingthemeanofthefourreplicatesof
end of the experiment, in accordance with tadpole the low-density treatment from the values for each
growth and food requirements. We used a variable individual in the high-density treatment, divided by
number of clutches from the area containing the thesubtractedvalue.Standardizedinterspecificeffects
invaded community for each species (four for for each species were calculated by subtracting the
D. pictus, four for B. calamita and five for mean of the four replicates of the species at high
P. punctatus). We mixed tadpoles from different densityfromtheindividualvaluesforthesamespecies
clutches in a tank and used a sample of this mixture in the interspecific treatment, divided by the mean
inallexperimentalunitsinordertoreducethechance subtracted value (Morin and Johnson 1988). Differ-
of any genetic bias. The tadpoles were added to the ences between the species for intraspecific and inter-
experimental tanks when they reached the develop- specific effects were tested with MANOVAs and
mentalGosnerstage25(Gosner1960). consecutive ANOVAs for the three continuous traits
During the experiment, tadpole behavior was and with generalized linear models (GLZ) for survi-
monitored by recording the number of individuals vorship and activity proportions. All statistical tests
still alive in the tank and the proportion of active werecarriedoutusingStatisticasoftware.
123
804 A.Richter-Boixetal.
ExperimentII:speciesconsumptionrateestimation Analysisofthecommunitystructureintheinvaded
andnon-invadedareas
In spring 2007, we conducted an experiment to
measure the consumption rate of the species used in We compared the structure of invaded and intact
the previous experiment. Three spawns from each amphibian communities to evaluate whether the
species were collected and transported to the presenceofD.pictusalterstheco-occurrencepatterns
laboratory in Barcelona. In the laboratory the of species. For this analysis, we created a presence–
clutches were mixed. When embryo development absence matrix using the survey data. In the matrix,
was complete at Gosner stage 25, a total of 25 eachcolumnrepresentsabreedingpond,andeachrow
tadpoles were taken at random and raised individ- representsadifferentnativeamphibianspecies.Newts
ually in 1-L plastic containers. We added a rigid and anurans can compete with each other in the
mesh 2–3 cm from the bottom of the container to terrestrial phase (juveniles and adults), but the study
prevent tadpoles from eating their own feces. For only focused on the larval overlap where the biotic
2 weeks, each tadpole was fed with small pieces of interaction between the groups is a predator–prey
Sera(cid:4) Spirulina tabs placed in a small Petri dish on interaction and not a competition interaction as
the mesh. After 2 weeks, individuals were removed presupposedinco-occurrencestudies.Aslarvalnewts
from the water with a glass screen and positioned on and tadpoles do not compete for the same food
an absorbent cloth to remove excess water. Their resourceswithinthepond(Anderson1967;Wassersug
wet mass was then weighed to 0.1 mg accuracy. The 1980), we excluded the three urodela from this
procedure took less than 30 s and did not affect the analysis. We used the Guild Structure Module in
tadpoles adversely. The day before this procedure EcoSim (Gotelli and Entsminger 2007) to compare
was carried out, the tadpole food was removed to patternsofanuranco-occurrencebetweentheinvaded
ensure that their guts were empty before being andthenon-invadedregions.Weusedthedistribution
weighed (Savage 1952). After weighing, the tad- rangeofD.pictusinnortheasternSpain(Montorietal.
poles were replaced individually in the same 2007) to classify breeding ponds as invaded or non-
containers for a further five days. During this invaded. D. pictus itself was not included in this
period, the tadpoles were fed on Spirulina Tabs analysisofspeciesoccurrencebecauseitwasusedto
that had previously been dried and weighed. Every define the two regions (Sanders et al. 2003). We
2 days, any uneaten food was removed carefully and appliedStoneandRoberts’(1990)checkerboardscore
placed on a small pre-weighed glass Petri dish, then (C-score) as a quantitative index of co-occurrence.
replaced by a new weighed piece of Spirulina Tab. This index measures the degree to which species
When tadpole feces were detected in the Petri dish, co-occur, but is not as stringent as other indices
they were carefully pipetted out to make sure that because it does not require perfect segregation
the tadpoles only fed on the pieces of Spirulina Tab between species (Gotelli 2000). We constructed two
during these 5 days. After the fifth day, any uneaten presence–absence submatrices, for the invaded and
food was removed and the tadpoles were starved for non-invaded communities, and we calculated the
1 day before being weighed. Uneaten food was C-score for each submatrix and the variance in these
dried and weighed. Using these data, we calculated two C-scores. The larger this variance, the more the
the consumption rate (‘‘Food ingested’’/time (days)) twocommunitiesmaydifferintheirpatternofspecies
for each tadpole (Richardson 2002), where: ‘‘Food co-occurrence. The observed variance in C-scores
ingested’’ = (amount of food given) - (amount of between the two presence–absence submatrices was
food uneaten). The consumption rate for each compared with the 10,000 variances created through
tadpole was divided by its initial body mass randompartitionsofthedata.Iftheobservedvariance
(measured at the beginning of the experiment). waslargerthanexpectedbychance,theco-occurrence
Differences in the consumption rate between species patterns in the two submatrices could be considered
were analyzed using ANOVA. Previously, the statisticallydistinct(Sandersetal.2003).Tocompare
consumption rate corrected for body mass was log C-scores from the randomized matrices, we calcu-
transformed in order to conform to the assumptions lated the standardized C-score for each matrix as
of normality. (v –v )/SD , where v is the observed index,
obs sim sim obs
123
Effectsofthenon-nativeamphibianspecies 805
v is the mean index of the 10,000 randomized Competitionlaboratoryexperiments
sim
matrices, and SD is the standard deviation of the
sim
randomizeddata(GotelliandArnett2000). Effectsofnon-nativespeciesonnativespecies
We also repeated the niche overlap analysis inde-
pendently for each region, including only anuran Discoglossus pictus had a different effect on the two
species and excluding D. pictus, to study whether native species (Fig. 3). The presence of D. pictus
niche overlap of native species was similar in both reduced the survivorship of B. calamita tadpoles
invadedandnon-invadedregions.Theparametersfor (F = 28.659; P\0.001), but not of P. punctatus
2,9
thenicheoverlaptestwerethesameasthoseexplained (F = 3.339; P = 0.082) (Fig. 3a). In addition,
2,9
above. We computed the mean niche overlap coeffi- the presence of D. pictus reduced the activity of
cients for each species and the standard error for the B. calamita (F = 6.135; P = 0.021) (Fig. 4a).
2,9
invadedandnon-invadedregion. Consequently,B.calamita tadpolestransformed later
and were significantly smaller than those under
the intraspecific treatment (Fig. 3b–d). Meanwhile,
Results thepresenceofnon-nativetadpolesdidnotaffectthe
activity (F = 2.715; P = 0.119) (Fig. 4a), growth
2,9
Nicheoverlap ordevelopmentalratesofP.punctatus.Thepresence
of D. pictus did not significantly affect body mass or
From2005to2006,atotalof333pondsweresurveyed: the other traits analyzed which were similar to the
138intherangeofDiscoglossuspictusand195outside results for P. punctatus under the intraspecific treat-
therangeofdistributionofthisnon-nativespecies.Of ment(Fig. 3b–d).
these 333 potential breeding sites, most were ephem- The presence of native tadpoles also reduced the
eral and temporary ponds (Fig. 2). The non-native survivorshipofD.pictus(F = 10.780;P = 0.001)
3,12
species D. pictus was detected in 64.3 % of the sites (Fig. 3a). However, their presence had no effect on
surveyedinsideitsrangeofdistributionandshoweda eitheractivity(F = 2.455;P = 0.113)(Fig. 4a)or
3,12
clear preference for ephemeral and temporary ponds. life-historytraits(Table 2a).
Like D. pictus, the native species were not uniformly
distributedinallavailablewaterbodiesandshoweda Differencesbetweenspecies
preference for certain categories of water bodies
(Fig. 2). In the pairwise niche overlap calculations, An increase in the density of conspecifics generated
theindexwasgreaterthan0.5formostpairsofspecies similarresponsesinthespecies.Inallcases,mortality
(Table 1). D. pictus overlaps most with the native increased with intraspecific competition but without
B.calamitaandP.punctatusinbreedinghabitats.The significantdifferencesbetweenspecies(F = 0.944;
2,9
valuesofthenicheoverlapindexbetweenD.pictusand P = 0.424)(Fig. 5a).BothD.pictusandB.calamita
these two native species were higher than 0.99 (the increased activity, while P. punctatus reduced it
index ranges from 0 to 1). The three species were (Fig. 4a), even though there were no significant
commonlysyntopicinephemeralandtemporaryponds differences in the activity response compared to
(Fig. 2), and co-occurred at 19 % of the sites where intraspecific competition either (F = 2.117; P =
2,9
D. pictus was found. At the community level, the 0.176). With regards to the life-history traits, there
simulations indicated that neither the observed mean wereintraspecificeffects,buttheonlysignificantdif-
niche overlap [mean observed = 0.699; simulated ferences were in mass at metamorphosis (Table 2b).
indices = 0.681; P (obs[expected) = 0.363] nor ThemassatmetamorphosisofP.punctatuswaslower
the observed variance [observed variance = 0.053; thantheconspecificeffectsrecordedforD.pictusand
simulated variances = 0.042; P (obs[expected) = B. calamita (Fig. 5c), suggesting strong density
0.181] were statistically significant. The null model effectsforP.punctatus.
suggests that niche overlap in this community in the When we compared behavioral and life-history
category of breeding pond is random, with the detec- reactionstointerspecificcompetitionbetweenspecies
tion of all pairwise species combinations possible after calculating standardized values, we could see
withinthearea. that interspecific competition was asymmetric. The
123
806 A.Richter-Boixetal.
Fig.2 Speciespondpreference(mean±SE)andnumberand Lissotriton helveticus; Aob: Alytes obstetricans; Dpi: Discog-
percentage of ponds of the different categories. Dashed lines lossus pictus; Ppu: Pelodytes punctatus; Pcu: Pelobates
limit the overlap zone with the non-native species. Species cultripes; Bca: Bufo calamita; Bbu: Bufo bufo; Hme: Hyla
whichoverlapmorewithD.pictusareshownaswhitecircles. meridionalis;Ppe:Pelophylaxperezi
Ssa:Salamandrasalamandra;Tma:Triturusmarmoratus;Lhe:
Table1 Nicheoverlapindexbasedonbreedinghabitatsofamphibianspeciesinthestudyarea
Species Tma Lhe Aob Dpi Ppu Pcu Bca Bbu Hme Ppe
Ssa 0.819 0.712 0.790 0.327 0.353 0.691 0.305 0.863 0.779 0.796
Tma 0.567 0.947 0.288 0.312 0.825 0.261 0.931 0.874 0.989
Lhe 0.715 0.581 0.634 0.685 0.506 0.792 0.820 0.466
Aob 0.568 0.593 0.955 0.533 0.984 0.981 0.924
Dpi 0.997 0.775 0.994 0.568 0.674 0.285
Ppu 0.788 0.984 0.598 0.704 0.299
Pcu 0.754 0.935 0.965 0.818
Bca 0.529 0.631 0.272
Bbu 0.984 0.898
Hme 0.835
TheitalicizedvaluesindicatetheoverlapindexofD.pictuswithalltheotherspecies.Thehighestvaluesinbold
Ssa:Salamandrasalamandra;Tma:Triturusmarmoratus;Lhe:Lissotritonhelveticus;Aob:Alytesobstetricans;Dpi:Discoglossus
pictus; Ppu: Pelodytes punctatus; Pcu: Pelobates cultripes; Bca: Bufo calamita; Bbu: Bufo bufo; Hme: Hyla meridionalis; Ppe:
Pelophylaxperezi
interspecific differences in survivorship were almost presence of individuals of other species (F =
3,12
significant. Bufo calamita suffered high mortality in 28.712;P\0.001)(Fig. 4b).Othernegativeeffects
the presence of D. pictus, whereas P. punctatus and of interspecific competition on B. calamita were a
D.pictushadalowerreductioninsurvivorshipinthe longerlarvalperiod,asmallersizeatmetamorphosis
presence ofotherspecies(F = 3.472;P = 0.051) andslowergrowth(Table 2b;Fig. 5b–d).B.calamita
3,12
(Fig. 5a).Asimilarpatternwasobservedintheother suffered stronger effects than the other two species,
traitsmeasured:B.calamitareduceditsactivitywhen demonstrating the poor competitiveness of this
competing with the non-native species, whereas the species and the asymmetry of competition in this
activity of the other species was affected less by the system.
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Effectsofthenon-nativeamphibianspecies 807
Fig.3 Mean response and standard error for each species to B?D=B. calamita with D. pictus, P?D=P. punctatus
intra-andinterspecificcompetition:asurvivaltometamorpho- with D. pictus. Labels A and B indicate Scheffe´ post hoc
sis, b larval period, c mass at metamorphosis, d growth rate. differences between treatments. The post hoc differences are
Intraspecific interactions: L low density, H high density; betweentreatmentswithdifferentletters.Theabsenceoflabels
Interspecific interactions: D?B=Discoglossus pictus with indicatesnodifferencesintheposthocanalysis
Bufo calamita, D?P=D. pictus with Pelodytes punctatus,
Speciesconsumptionrateestimation Analysisofthecommunitystructureintheinvaded
andnon-invadedareas
Theresultsofthesecondexperimentshowedthatthe
species differ in their consumption rates (F = The community co-occurrence patterns differed
2, 62
180.52;P[0.001).TheconsumptionrateofD.pictus between the non-invaded area and the invaded area.
wassignificantlyhigherthanthatofP.punctatus.The Inthenon-invadedarea,theC-scorewassignificantly
consumption rate of B. calamita was intermediate higher than expected by chance, with a standardized
betweentheothertwospecies(Fig. 6). C-scoreof3.43,whereasintheinvadedareaitfellto
123
808 A.Richter-Boixetal.
Fig.4 Meanandstandarderroroftherelativeeffectofintra- b relative effect of competition to activity. Labels A, B and
and interspecific competition for each species: a activity as C indicate Scheffe´ post hoc inter-specific differences. Intra
proportion of tadpoles swimming or feeding. Labels A and intraspecificinteraction.SeeFig.3forabbreviations
B indicate Sheffe´ post hoc differences between treatments,
1.18anddidnotdiffersignificantlyfromrandom.The differences in habitat preferences between regions
analysis of variance of the co-occurrence index (B. calamita: v2 = 11.591; P = 0.003; P. punctatus:
between the two areas showed an observed variance v2 = 9.329;P = 0.009).
index larger than expected by chance (observed
variance index = 154,903.10; simulated variances =
102,780.30;P(obs[expected) = 0.036)(see‘‘Mate- Discussion
rialsand methods’’ section). Species segregation was
largerinthenon-invadedareathanintheinvadedarea The invasion process has been divided into several
for almost all the species, although the overlap stagesthatthenon-nativespeciesneedstocompleteto
differencesbetweenareaswerelessthanthestandard succeed in its invasion (Van Kleunen et al. 2010;
error for each area. The results indicate that the two Blackburn et al. 2011). According to the unified
regions are not significantly different, but there is a framework described by Blackburn et al. (2011),
tendency for species segregation to decrease in the D.pictushasreachedthespreadstagebyovercoming
presenceofthenon-nativespecies(Table 3). allthebarriersandshouldbereferredtoasaninvasive
These differences in species segregation between species. The present study demonstrates that the
theinvadedandnon-invadedareawerealsoreflected establishment of D. pictus in its introduced range
in a change of pond use by species, especially altersthenativeamphibiancommunities,possiblyby
B. calamita and P. punctatus. Both species showed a increasingcompetitionatthelarvalstage.
reduction in use of ephemeral ponds in the invaded Larvalamphibiancommunitiesarestructuredbyan
areawithrespectthenon-invadedarea(from58.97to ecological gradient of habitats defined by water
50 % for B. calamita and from 51.35 to 36.36 % for duration, competition and predation risk (Semlitsch
P. punctatus). Otherwise, use of temporary ponds et al. 1996; Wellborn et al. 1996; Pechmann et al.
increased in the invaded region (from 28.21 to 1989, 2001). These traits of breeding-ponds, which
42.31 % for B. calamita, and from 32.43 to 57.58 % determine the success of several species across the
for P. punctatus). Both species showed significant hydroperiodgradient,affectspeciesindifferentways
123