Amphibia-Reptilia 29 (2008): 235-244

Factors affecting repatriation success of the endangered Italian agilefrog (Rana latastei)

Daniele Pellitteri-Rosa1,*, Augusto Gentilli1, Roberto Sacchi1, Stefano Scali2, Fabio Pupin1,

Edoardo Razzetti3, Franco Bernini1, Mauro Fasola1

Abstract. Repatriations and translocations are among the most frequent actions deployed for conservation of amphibiansall over the world. However, very few studies have analysed the effectiveness of these actions. The aims of this study werei) to measure the success of repatriations of Rana latastei carried out within a conservation project in Lombardy (NorthernItaly) from 1999 to 2001, and ii) to analyse how the environmental factors affected repatriation success. Eighteen ponds weresurveyed from February to April 2006, for an overall of 45 field surveys. Each pond was carefully checked for frogs andegg masses. We classified each pond as newly excavated or pre-existing according to habitat management actions and wemeasured 16 variables concerning micro- and macro-habitat characteristics, human disturbance and presence of predators.Rana latastei was found in 33.3% of monitored sites and the repatriation outcome of tadpole release did not differ betweennewly excavated and non managed ponds. Habitat variables influenced the success of repatriations, which increased in siteswith higher percentage of woodlands. Finally, repatriation outcome was also negatively influenced by human disturbance andpredator occurrence.

Keywords: amphibians, conservation, habitat management, Northern Italy, Rana latastei, repatriation.

Introduction

Repatriations and translocations are practicescommonly used for the conservation of amphib-ians and reptiles all over the world (Bloxamand Tonge, 1995; Fischer and Lindenmayer,2000; Marsh and Trenham, 2001; Trenham andMarsh, 2002; Semlitsch, 2003; King et al.,2004), particularly for endangered species (re-viewed in Dodd and Seigel, 1991; Reinert,1991; Andren and Nilson, 1995). Several au-thors have been sceptical about the effective-ness of these practices (Burke, 1991; Doddand Siegel, 1991; Cunningham, 1996; Daszaket al., 1999; Moritz, 1999; Ficetola and DeBernardi, 2005a). For example, in their reviewof factors affecting the success rate of reloca-tions and repatriations, Dodd and Seigel (1991)

1 - Dipartimento di Biologia Animale, Università di Pavia.P.za Botta 9, I-27100 – Pavia, Italy

2 - Museo Civico di Storia Naturale di Milano, CorsoVenezia 55, I-20121 – Milano, Italy

3 - Museo di Storia Naturale, Università di Pavia. P.za Botta9, I-27100 – Pavia, Italy*Corresponding author; e-mail: masterfauna@unipv.it

cautioned against unsuitable habitats for tad-poles development as well as against humandisturbance, predation and high dispersion rateof individuals from release sites. Cunningham(1996) and Daszak et al. (1999) focused onthe risk of transmission of diseases, and Moritz(1999) argued that preserving ecological andevolutionary processes through habitat restora-tion is more effective than any practice involv-ing translocation. On the other hand, other au-thors underlined that habitat management mightbe efficiently supported by repatriations, par-ticularly when dealing with species or popula-tions living in altered landscapes (Gariboldi andZuffi, 1994; Madsen et al., 1996; Edgar et al.,2005). For example, the introduction of few for-eign individuals in a small population sufferinginbreeding depression can increase its geneticdiversity and the breeding output of the females(Madsen et al., 1999). Despite the debates onthis topic, few studies have analyzed the envi-ronmental and structural factors that may af-fect the success of repatriations (but see Vander Sluis et al., 1999; Razzetti and Scali, 2002).The main habitat management practices used in

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236 D. Pellitteri-Rosa et al.

North America and Europe have been restora-tion projects, such as the reclaiming of formerlydrained wetlands or the creation of new ponds(Mitsch et al., 1998; Mierzwa, 2000; Scoccianti,2001).

The Italian agile frog (Rana latastei) is athreatened species endemic to the Po basin(Northern Italy and Southern Switzerland) andIstria (Slovenia). It suffered from habitat de-pletion, human disturbance and water pollution(Barbieri and Bernini, 2004; Barbieri and Maz-zotti, 2006), is listed in the annex II of theHabitat Directive, and is considered vulnerable(VU) in the 2006 IUCN Red List of Threat-ened Species. The Italian agile frog has been thetarget of several conservation plans (Grossen-bacher et al., 2002; Ferri et al., 2004; Ferri andZavagno, 2005).

From 1999 to 2001, the Regione Lombardia(the regional administration responsible for na-ture conservation in Lombardy, Northern Italy)promoted a conservation project for this speciesin six Natural Parks. Damaged and poorly pre-served ponds were restored mainly by litter re-moval and digging and new ponds were exca-vated in suitable areas in order to increase thenumber of reproductive sites. Finally, tadpoleswere repatriated (for the purposes of this paperwe adopted the terminology proposed by Doddand Seigel, 1991) in newly excavated ponds andin all natural pre-existing ponds not previouslyused by the species for breeding (Gentilli et al.,2002; Scali et al., 2002). All repatriations werecarried out during 2000 and repeated again in2001 (Gentilli et al., 2002). In this study, we car-ried out a complete census of 18 ponds whereItalian agile frog tadpoles were released, in or-der to i) assess the success of the repatriations,and ii) analyse the environmental and struc-tural factors affecting the repatriation outcome.Since repatriations involved both managed andunmanaged ponds, we also evaluated whetherhabitat management increased the outcome ofrepatriations.

Materials and methods

Study area

The 18 surveyed ponds were located in six Regional Parksof Lombardy (Parco Lombardo Valle del Ticino, ParcoAgricolo Sud Milano, Parco Adda Sud, Parco Serio, ParcoOglio Sud, Parco Mincio): thirteen ponds were newly exca-vated, while five ponds pre-existed and were non managed(fig. 1).

The Po plain is one of the most intensely human-dominated landscapes of Italy, where habitats for amphib-ians are isolated (Ficetola and De Bernardi, 2004). Frag-ments of natural wetlands outside the existing protected ar-eas are very scattered, and are at risk of disappearing in thenear future. Therefore, all parks considered in this study pre-serve the last natural wetlands of Lombardy plain, and arecharacterized by a mosaic of natural (i.e., dry gravel river-beds, broadleaf woods dominated by oaks, alders, poplarsand wet meadows) and human-associated habitats (i.e., riceand cereal crops and rural small towns). Moreover, the rurallandscape is crossed by a web of many irrigation canalsedged with trees and brambles, and traditionally used asboundaries between fields. The mean elevation of the sixparks ranges from 20 to 200 m a.s.l. Temperature rangesfrom 0-3◦C in January to 22-24◦C in July, and rainfall (600-700 mm/year) occurs mainly during spring and autumn (Pi-lon, 2004).

Field methods

Surveys were carried out during the breeding period ofthe Italian agile frog in our study area, from Februaryto April 2006 (Barbieri and Bernini, 2004; Barbieri andMazzotti, 2006). Each pond was repeatedly checked forthe presence of frogs and eggs up to the detection ofthe species or to a maximum of six surveys; ponds weretherefore surveyed on average 2.5 times, and we reliablycould exclude that non detection of Italian agile frogs couldbe due to lack of detection (MacKenzie et al., 2002). Duringdaylight, we searched the entire perimeter of each pond, andwe recorded the presence of egg masses. After dusk, wesearched for adult frogs using a head-lamp, and we detectedthe presence of adult males hearing the breeding calls usingan hydrophone (Dolphin Ear, frequency range 7-22 000 Hz).

For each pond, we coded habitat management actionsas newly excavated and pre-existing, and we measured16 variables (see table 1 for description) concerning 6micro-habitat variables (including structural features andmicro-habitat measures as percentage of cover within a50 m radius), 8 macro-habitat variables measured as per-centage of cover within a 500 m radius, human distur-bance (presence/absence), and the occurrence of predators(presence/absence) such as crayfishes, fishes or ducks. Themacro-habitat variables were measured using a GIS soft-ware (ArcGis 8.0) and a digital map (scale 1 : 10 000) ofthe land use of the Lombardy (DUSAF, Use Destination ofAgricultural and Forest Soil, a project of the Regione Lom-bardia and of the regional agency of agricultural and forestservices, ERSAF, 2003).

We considered a repatriation to be successful in a pondwhen we found at least one egg mass and/or more than oneadult calling male.

Factors affecting repatriation success of the endangered Italian agile frog 237

Figure 1. Location of the 18 study ponds (square: excavated, circular: not managed) within the 6 Parks: 1 Parco LombardoValle del Ticino, 2 Parco Agricolo Sud Milano, 3 Parco Adda Sud, 4 Parco Serio, 5 Parco Oglio Sud, 6 Parco Mincio.

Statistical methods

We used logistic regression models in order to address thepotential effects of habitat variables, human disturbance andoccurrence of predators on the repatriation outcome. As oursample size was similar to the number of measured vari-ables, the logistic models were formulated by only a sub-setof independent variables so to avoid over-parametrizationand Freedman’s paradox (Bissonette, 1999; Anderson andBurnham, 2002). For these analyses, we first calculated acorrelation matrix (Spearman correlation coefficient) amongindependent variables with the aim of identifying the sub-sets of independent non-correlated variables. Therefore, dis-crete models were developed, each including only 3 inde-pendent variables chosen among the less correlated ones,in a way that every habitat variable was considered at leastonce (Merli and Meriggi, 2006). Inference from models wasmade according to the Information-theoretic approach (An-derson et al., 2000; Anderson and Burnham, 2002; Maze-

rolle, 2006). This strategy allows to select the best modelfitting the data with the minimum number of relevant vari-ables, by weighting models within a set of candidate modelsusing the Akaike weights (Burnham and Anderson, 2002).In case where Akaike weights are lower than 0.90, there isnot definitive best model, and the better strategy is to com-pute a model averaged estimate of the parameters of interestacross all the models (i.e., the “multi model inference”, An-derson et al., 2000; Mazerolle, 2006). Therefore, for each ofthe selected models, we computed the modified Akaike’s In-formation Criterion (AICc; Sugiura, 1978), differences withthe minimum AICc (�i) and Akaike weights (wi) and weranked models according to these indices. The relative im-portance of predictor variables were measured, as resultedfrom the best models, by the sum of the models Akaikeweights (

∑w) where each variable appeared (Burnham and

Anderson, 2002). As in our set of candidate models, for eachvariable there was not the same number of models with or

238 D. Pellitteri-Rosa et al.

Table 1. Variables measured at each pond.

Variables Code

Micro-habitat variablesPond’s area (rank transformed) AREADepth (cm) DEPTHRegime – Water permanence (transitory or permanent) REGShore incline (all horizontal, horizontal and inclined, all inclined) INCL% of pond covered by submerged vegetation (0%, 1-50%, 51-100%) SUBVEG% of riparian vegetation along the shoreline (0%, 1-50%, 51-100%) RIPVEG

Macro-habitat variables% of water zone within 500 m of radius around the pond WAT% of woodland within 500 m of radius around the pond WOOD% of poplar within 500 m of radius around the pond POP% of natural vegetation within 500 m of radius around the pond NAT% of meadow within 500 m of radius around the pond MEAD% of barren zone within 500 m of radius around the pond BAR% of agricultural zone within 500 m of radius around the pond AGR% of urban areas and infrastructure within 500 m of radius around the pond URB

Human disturbanceAgriculture, tourism, pollution, transit, noise (presence/absence) HUM

PredatorsFish, crayfish, ducks, turtles (presence/absence) PRED

without it, we divided the∑

w by the number of models thevariable entered, thus obtaining a mean w (w̄). To quantifythe effects of the predictor variables the β̂ (standardized par-tial regression coefficients) were weighted and averaged on

the models obtaining ˆ̄β. The unconditional sampling vari-

ance (var ˆ̄β) was computed for each predictor (Burnham andAnderson, 2002). Finally, the separate effect of managementactions on the repatriation outcome was assessed by a χ2

test (Fisher’s exact test).

Results

Repatriations were successful in only 6 out ofthe 18 study ponds (33.3% of the sites; see ta-ble 2 for details). Human disturbance, mainlytourism and traffic, was detected in 88.9% ofthe sites (16 out of 18), and potential predators,mostly fish and introduced red swamp cray-fish (Procambarus clarkii), but also ducks (in2 sites) and the exotic pond slider (Trachemysscripta), were found in 38.9% of the sites (7 outof 18).

Effect of habitat variables

Spearman correlation analysis among micro-and macro-habitat variables, human disturbanceand occurrence of predators (table 3) lead to

build 12 different models taking into account allthe original 16 variables (table 4).

Comparison among the multiple regressionmodels showed that repatriation outcome wasbest predicted (the minimum loss of Kullback-Leibler information; Anderson et al., 2000) bythe model including percentage of woodland,pond’s shore incline and occurrence of preda-tors, which showed the minimum AICc. Othertwo models, both of them including percentageof woodland, substantially supported the for-mer. The first included also pond’s shore inclineand percentage of barren zone (�AICc = 0.323)and the second pond’s depth and human dis-turbance (�AICc = 2.166). The other modelshad a �AICc greater than 2.5 and the cumula-tive Akaike weights smaller than 0.1 suggestinga slight contribution to the repatriation successprediction.

The relative importance of woodland andshore’s incline as predictive variables was em-pirically supported by the mean Akaike weightsfor the models where these variables appeared(w̄ = 0.236 for woodland and w̄ = 0.300 forshore’s incline; table 5). The effect of these vari-ables on the repatriation outcome was largely

Factors affecting repatriation success of the endangered Italian agile frog 239

Table 2. Presence of Italian agile frog at the 18 study ponds.

Site Park Presence n of egg masses n of calling �� n of adults

Boffalora Adda Sud no 0 0 0Villa Pompeiana Adda Sud yes 2 0 0Riazzolo 1 Agricolo Sud no 0 0 0Riazzolo 2 Agricolo Sud no 0 0 0Lacchiarella Agricolo Sud yes 1 0 0Muzzetta Agricolo Sud no 0 0 0Bosco in città Agricolo Sud no 0 0 0Pasturago Agricolo Sud no 0 0 0Le Grazie 1 Mincio no 0 0 0Le Grazie 2 Mincio no 0 0 0Bertone Mincio yes 14 0 0Marcaria 1 Oglio Sud no 0 0 0Marcaria 2 Oglio Sud no 0 0 0Lanca Romano 1 Serio yes 5 0 1Lanca Romano 2 Serio no 0 0 0Vignolo 1 Ticino yes 3 8 4Vignolo 2 Ticino yes 2 4 3Vignolo 3 Ticino no 0 0 0

positive (table 5). The other variables whichseem to be relevant are the occurrence of preda-tors (w̄ = 0.137) and the human disturbance(w̄ = 0.053), both with negative effect (table 5),while percentage of barren zone (w̄ = 0.105)and pond’s water permanence (w̄ = 0.052),showed a positive effect on the repatriation out-come (table 5).

Effect of habitat management actions

Four out of the six ponds with positive out-come were newly excavated, while the othertwo were not managed; this difference was notstatistically significant (χ2 = 0.138, df = 1,P = 0.56).

Discussion

Our survey shows that repatriations of the Ital-ian agile frog were only partially successful,since the species was only present at 33.3%of the sites. This result seems quite low, butis in line with the figures (11-44%) of suc-cess rates for translocation plans all over theworld (Razzetti and Scali, 2002). Reasons forlow success rate of repatriations projects arequite heterogeneous and strictly related to the

environmental requirements of the species andthe project realization (reviewed in Razzetti andScali, 2002). In our study case the low suc-cess rate might be related to habitat featuresof selected sites and also to the non completeremoval of factors determining local extinc-tion of the species. Moreover, the presence ofa metapopulation might influence the repatri-ation outcome, but in our study area all siteswere so much isolated that we can exclude theirnatural colonization; indeed Italian agile frogsare highly philopatric, and move within veryshort ranges (100-150 m) around the native sites(Lanza, 1983; Bernini et al., 2000). For the samereason we can also exclude the possible effectsof outbreeding depression (Sagvilk et al., 2005).

Moreover, our data suggest that repatriationswere not more effective in pre-existing sitesthan in newly excavated ponds. Nevertheless,the excavation of new ponds could be the onlysolution if natural ponds are lacking or highlycompromised because of the worsening of envi-ronmental conditions due to permanent absenceof water, changing of vegetation or urban andagricultural growth. In all such situations, thecreation of new ponds is the last chance to in-crease the survival probability for amphibians,thus their probability of discovering and colo-

240 D. Pellitteri-Rosa et al.

Tabl

e3.

Cor

rela

tion

mat

rix

(Spe

arm

anco

effic

ient

)am

ong

habi

tatv

aria

bles

mea

sure

din

the

18po

nds.

Woo

dlan

dPo

plar

Nat

ural

Mea

dow

Bar

ren

Agr

icul

tura

lU

rban

Wat

erPo

nd’s

Dep

thSu

bmer

ged

Rip

aria

nSh

ore

Hum

anPr

edat

ors

vege

tatio

nzo

nezo

near

eas

perm

anen

cear

eave

geta

tion

vege

tatio

nin

clin

edi

stur

banc

e

Wat

erzo

ne−0

.63

−0.2

70.

680.

320.

480.

120.

190.

02−0

.34

−0.3

20.

590.

350.

01−0

.32

0.26

Woo

dlan

d0.

21−0

.48

−0.3

4−0

.20

−0.4

0−0

.31

−0.2

30.

510.

01−0

.71

−0.5

20.

150.

10−0

.25

Popl

ar0.

120.

30−0

.11

−0.4

5−0

.60

−0.0

5−0

.17

−0.2

3−0

.41

0.04

−0.2

90.

26−0

.36

Nat

ural

vege

tatio

n0.

550.

39−0

.21

−0.0

10.

39−0

.41

−0.3

20.

370.

49−0

.31

−0.0

3−0

.01

Mea

dow

0.02

−0.4

8−0

.08

0.18

−0.5

1−0

.18

0.16

0.47

−0.5

7−0

.10

0.38

Bar

ren

zone

0.08

−0.4

30.

290.

35−0

.11

0.31

0.36

−0.2

5−0

.17

−0.3

7A

gric

ultu

ralz

one

0.45

0.02

0.14

0.34

0.43

0.09

0.45

−0.2

00.

08U

rban

area

s−0

.09

−0.3

20.

150.

220.

090.

43−0

.07

0.52

Wat

erpe

rman

ence

0.10

0.09

0.07

0.23

−0.6

20.

50−0

.16

Pond

’sar

ea0.

20−0

.25

−0.3

5−0

.07

0.20

−0.1

4D

epth

−0.1

8−0

.18

0.27

0.00

0.12

Subm

erge

dve

geta

tion

0.32

−0.1

3−0

.56

0.18

Rip

aria

nve

geta

tion

−0.1

9−0

.19

−0.1

6Sh

ore

incl

ine

−0.3

70.

00H

uman

dist

urba

nce

−0.0

8

Factors affecting repatriation success of the endangered Italian agile frog 241

Table 4. Candidate multiple regression models and model inference criteria for the Italian agile frog repatriation outcomeand the 16 variables in the 18 ponds.

Habitat variables R2 Maximized N. AICc �AICc wlog-likelihood parameters

WOOD + INCL + PRED 0.300 −8.021 5 24.042 0.000 0.324WOOD + BAR + INCL 0.286 −8.182 5 24.364 0.323 0.276WOOD + DEPTH + HUM 0.205 −9.104 5 26.208 2.166 0.110SUBVEG + REG + MEAD 0.178 −9.409 5 26.817 2.776 0.081AREA + REG + PRED 0.136 −9.897 5 27.794 3.753 0.050NAT + PRED + HUM 0.109 −10.204 5 28.408 4.366 0.037AGR + WAT + REG 0.082 −10.523 5 29.047 5.005 0.027MEAD + BAR + SUBVEG 0.080 −10.538 5 29.077 5.035 0.026MEAD + SUBVEG + URB 0.080 −10.541 5 29.082 5.040 0.026POP + RIPVEG + DEPTH 0.048 −10.910 5 29.82 5.778 0.018URB + RIPVEG + DEPTH 0.023 −11.193 5 30.386 6.345 0.014AGR + BAR + HUM 0.020 −11.227 5 30.454 6.412 0.013

Table 5. Multi model inference on models parameters andrelative importance of the 16 variables for the Italian agilefrog repatriation outcome.

Predictor w̄ ˆ̄β var ˆ̄βINCL 0.300 0.66 2.19WOOD 0.236 1.84 36.84PRED 0.137 −0.21 0.64BAR 0.105 0.66 10.18HUM 0.053 −0.07 0.10REG 0.052 0.08 0.10

nizing new sites is reduced. Since our surveywas carried out six years after the first release,we cannot exclude that over a longer period newexcavated ponds will be also colonized.

The features of the habitat around the man-aged sites could be a first possible explanationfor the low success of the repatriations in ourstudy area, as vegetation is one of the mostimportant factors affecting amphibian distrib-ution (Ficetola and De Bernardi, 2004; Porejet al., 2004). Indeed, our results suggest thatrepatriations best performed in ponds associ-ated with preferred habitats by the Italian agilefrogs, i.e. mature forests (Bernini et al., 2004)and non disturbed habitat such as barren zonesand ecotones (Barbieri and Bernini, 2004). Bycontrast, repatriations generally failed outsideof frog’s preferred habitat, reflecting the highsensibility of the species to habitat depletion(Andreone and Luiselli, 2000). Thus most offailures may be due to the inclusion of several

sub-optimal sites within the original conserva-tion project. Indeed, cereal crops (maize andrice) and meadows – two of the most avoidedhabitats by the species-covered more than halfof the area around the ponds (47% and 10%respectively), while woodlands represented onaverage only the 18% (the mean percentage ofwoodland in successful ponds was 27%). Thisoccurred because site selection was made tak-ing into account not only habitat requirementsof the species, but also administrative and po-litical reasons concerning land properties, bud-gets, and interest of resident people for amphib-ian conservation.

A second possible factor resulting in low suc-cess of repatriations could be the high incidenceof human disturbance and predators, which bothentered as best predictors of the repatriation out-come. Indeed, human disturbance, such as agri-cultural practices, water pollution, tourism andtransit, as well the presence of predators, asthe red swamp crayfish and the pond slider, areamong the well known possible causes of am-phibian decline (Heyer and Muedeking, 1976;Newman, 1987; Cruz and Rebelo, 2005). Actu-ally, the removal of possible predators is a de-cisive factor for the success of repatriations be-cause of their severe impact on amphibian lar-val communities (Heyer et al., 1975; Azevedo-Ramos et al., 1999; Hero et al., 2001). Humandisturbance and occurrence of predators in our

242 D. Pellitteri-Rosa et al.

study area are so widespread, that the repatri-ation project carried out in 2000 was not ableto include ponds without those factors of dis-turbance. The high incidence of predators anddisturbance in our study area is also confirmedby the fact that several newly excavated ponds(38%) were also colonized by predators, and allbut two ponds suffer of human disturbance.

Finally, the positive effect of water regime onthe repatriation outcome confirms the above re-marks about the effect of habitat features on thisstenoecious species, i.e. repatriations best per-formed in permanent ponds where water perma-nence during all the breeding period allows thecompletion of the reproductive cycle. The pos-itive effect on repatriation outcome of pond’sshore incline might be related to the preferenceof the species for deep and permanent wetlands(Ficetola and De Bernardi, 2005b): shore in-cline may therefore be an effective predictor fordepth (and also better than the actual measureof depth) since water levels in our study areaare subjected to unpredictable fluctuations dueto industrial and agricultural employ of water,as well as rainfall.

In conclusion, the results of our work suggestsome relevant issues about the guidelines to fol-low for management and conservation of am-phibians. Despite in our study new populationswere established only in a minority of ponds,repatriations might represent an effective tool topromote local and regional populations of am-phibians, mainly in highly compromised land-scapes where barriers such as roads, urban areasand infrastructure would prevent natural colo-nization (Seigel and Dodd, 2002; Trenham andMarsh, 2002). Actually, this study pointed outthat characterization of environmental variablesshould be necessary prior to start repatriations.

Acknowledgements. We are very grateful to the SocietasEuropaea Herpetologica (SEH) which granted this projectfor the year 2006. We thank the staff of the Parks (ParcoLombardo Valle del Ticino, Parco Agricolo Sud Milano,Parco Adda Sud, Parco Serio, Parco Oglio Sud, Parco Min-cio) that facilitated our study, and the Regione Lombardiafor funding and collaboration. Finally, we would like to

thank Drs. Alberto Meriggi, Enrico Merli and Paolo Lam-berti for their essential support and suggestions on the sta-tistical analyses.

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Received: July 17, 2007. Accepted: December 15, 2007.