Predictive models of species distribution may be very useful for understanding actual distribution of elusive species, including several snakes. The southern smooth snake (Coronella girondica) is likely the most elusive snake species of peninsular Italy, and is therefore well-suited for predicting potential distribution studies. In this paper we predict the potential distribution map of this species in Italy by using MAXENT algorithm, that finds the probability distribution of maximum entropy that is constrained by considered ecological parameters. Presence data for Coronella girondica were gotten from CKmap. The potential distribution model of Coronella girondica showed a very good overall performance (AUC = 0.959), and indicated that high suitability areas correspond mainly to Tyrrhenian north and central regions, including Liguria, western Emilia-Romagna, Tuscany, Umbria, and northern Latium. Southern Italian regions were clearly unsuitable for this snake species. Overall, our study revealed that previous distribution maps indicating the occurrence of Coronella girondica in southern Italy and Sicily were poorly reliable. The conservation and management implications of our study are also addressed

Bombi, Pierluigi

Capula, Massimo

Luiselli, Luca

Salvi, Daniele

Firenze University Press: E-Journals

Acta Herpetologica 4(1): 7-13, 2009
Predicting elusiveness: potential distribution model  
of the Southern smooth snake, Coronella girondica, in Italy
Pierluigi Bombi1, Luca Luiselli2, Massimo Capula3, Daniele Salvi4
1 Via Maria Giudice 23, I-00135 Roma, Italy. Corresponding author. E-mail: bombi@uniroma3.it
2 F.I.Z.V. (Ecology) and Centre of Environmental Studies ‘Demetra s.r.l.’, via Olona 7, I-00198 Roma, 
Italy. E-mail: lucamlu@tin.it
3 Museo Civico di Zoologia, via Ulisse Aldrovandi 18, I-00197 Roma, Italy. E-mail: m.capula@
comune.roma.it
4 Dipartimento di Biologia ambientale, Università Roma Tre, viale Marconi 446, I-00146 Roma, Italy. 
E-mail: salvi@uniroma3.it
Abstract. Predictive models of species distribution may be very useful for under-
standing actual distribution of elusive species, including several snakes. The southern 
smooth snake (Coronella girondica) is likely the most elusive snake species of penin-
sular Italy, and is therefore well-suited for predicting potential distribution studies. In 
this paper we predict the potential distribution map of this species in Italy by using 
MAXENT algorithm, that finds the probability distribution of maximum entropy 
that is constrained by considered ecological parameters. Presence data for Coronella 
girondica were gotten from CKmap. The potential distribution model of Coronella 
girondica showed a very good overall performance (AUC = 0.959), and indicated that 
high suitability areas correspond mainly to Tyrrhenian north and central regions, 
including Liguria, western Emilia-Romagna, Tuscany, Umbria, and northern Latium. 
Southern Italian regions were clearly unsuitable for this snake species. Overall, our 
study revealed that previous distribution maps indicating the occurrence of Coronella 
girondica in southern Italy and Sicily were poorly reliable. The conservation and man-
agement implications of our study are also addressed. 
Keywords. Distribution modelling, elusive species, Serpentes, Colubridae, Italy.
INTRODUCTION
Analysis of many aspects of ecology and conservation may be helped by the use of pre-
dictive models of species distribution (Graham et al., 2004), including for instance model-
ling studies of invasive species spread (e.g., Thuiller et al., 2005), impacts of climate change 
(Thomas et al., 2004), and spatial patterns of species diversity (Guisan and Zimmermann, 
2000; Graham et al., 2006). Modelling potential distribution of a given species based on 
presence-only data is also a useful tool for predicting new localities of presence for rare and 
8 P. Bombi et alii
threatened species that are hardly found in the wild (Godown and Peterson, 2000; Engler 
et al., 2004), or for species that are simply very elusive and whose distribution is therefore 
little known (Guisan et al., 2006). The utility of these presence-only modelling methods is 
that they only require a set of known occurrences together with predictor variables such as 
topographic, climatic, edaphic, biogeographic and remotely sensed variables (Phillips and 
Dudik, 2008). The main advantage of presence-only data methods is that it is not necessary 
to confirm absence records, a big problem especially in elusive species.
Several snake species are genuinely elusive, and therefore their distribution may be 
somewhat underestimated and least known compared to other reptiles. For instance, in 
recent years it has been demonstrated that the true distribution of the snake Hemorrhois 
hippocrepis is considerably wider in Sardinia than previously suspected (e.g., compare maps 
in Razzetti and Bonini, 2006 with that in Bruno and Maugeri, 1977). Among the snake spe-
cies found in the Italian peninsula, the Southern smooth snake (Coronella girondica) is like-
ly the most elusive, and certainly one of the least known in terms of both ecology and dis-
tribution. Indeed, this snake spends much of the time below-ground or behind flat stones, 
and is active in the open only at night (Agrimi and Luiselli, 1994; Capula et al., 1995). As 
a consequence, its local distribution is often very little known (Razzetti and Bonini, 2006; 
Bologna et al., 2007). Coronella girondica is therefore a well-suited species for modelling its 
niche in order to find new suitable areas, also because there is circumstantial evidence that 
this species is declining over wide sectors of its current range (Bologna et al., 2007).
Our aim in this paper is to present a model of distribution for Coronella girondica in 
Italy, and to discuss the potential importance of distribution maps in terms of ecological 
and conservation purposes.
MATERIALS AND METHODS
Presence data for Coronella girondica were gotten from CKmap (Stoch, 2000-2005). The 
CKmap databank represents the largest, most authoritative, and most updated resource of faunistic 
knowledge in Italy, being composed by more than 500000 records regarding approximately 10000 
terrestrial and freshwater species (Ruffo and Stoch, 2005). CKmap data indicates the occurrence of 
each species within the cells of the Universal Transverse Mercator (UTM, 10 × 10 km) grid that 
intersect the Italian territory. The presence data was downscaled to the resolution of environmental 
predictors (e.g., Araújo et al., 2005) and implemented in the modelling procedure. In order to build 
a reliable model of habitat suitability, one distribution record from north-eastern Italy was excluded 
from the computations, as in recent studies it has been considered to be likely the result of an intro-
duction event (e.g., Lapini et al., 1999). 
Potential distribution of Coronella girondica in Italy was assessed by MAXENT algorithm 
(Phillips et al., 2004, 2006). In order to estimate the target probability distribution, MAXENT finds 
the probability distribution of maximum entropy that is constrained by considered ecological param-
eters. In the case of modelling ecological niches of species, these constraints consist of the values of 
those pixels at which the species has been detected (Phillips et al., 2004, 2006; Peterson et al., 2007). 
MAXENT models produce predictions in the form of real numbers between 0 and 100, representing 
cumulative probabilities of occurrence (Phillips et al., 2004, 2006; Peterson et al., 2007). MAXENT 
can combine predictors to manage over-fitting by regularizing factors (Phillips et al., 2006; Phillips 
and Dudík, 2008). We performed this algorithm by using the 19 climatic variables (Table 1), with a 
resolution of 30” of geographic degree, provided by WorldClim (Hijmans et al., 2004). 
9Modelling distribution of Coronella girondica in Italy
The reliability of the potential distribution model was assessed by Area Under Curve (AUC) 
criterion through a jack-knife procedure. This validation procedure compares the predicted values of 
habitat suitability assigned to presence and pseudo-absence data in the test subset by producing the 
Receiver Operating Characteristic (ROC) plots (Fielding and Bell, 1997) and deriving the relative 
AUC value (Faraggi and Reiser, 2002). The main positive feature of AUC consists of being a single 
threshold-independent measure for model performance (Fielding and Bell, 1997; Manel et al., 2001; 
Allouche et al., 2006; Peterson et al., 2007). An AUC value can be interpreted as the probability that 
a presence site, randomly chosen from the dataset, has a higher predicted value than an absence site 
(Elith et al., 2006, Phillips et al., 2006).
Models were run by using MAXENT 3.1 (Phillips et al., 2006), which was also utilized for the 
jack-knife procedure for the model validation.
RESULTS 
The potential distribution model of Coronella girondica showed a very good overall 
performance (AUC = 0.959; see Fig. 1). The potential distribution map (Fig. 2) showed 
that high suitability areas correspond mainly to Tyrrhenian north and central regions, 
Table 1. Climatic variables used to elaborate the models
Var no. Description
var 1 Annual Mean Temperature
Var 2 Mean Diurnal Range [Mean of monthly (max temp - min temp)]
Var 3 Isothermality [(var2 / var7) * 100]
Var 4 Temperature Seasonality (standard deviation * 100)
Var 5 Maximum Temperature of Warmest Month
Var 6 Minimum Temperature of Coldest Month
Var 7 Temperature Annual Range (var5 - var6)
Var 8 Mean Temperature of Wettest Quarter
Var 9 Mean Temperature of Driest Quarter
Var 10 Mean Temperature of Warmest Quarter
Var 11 Mean Temperature of Coldest Quarter
Var 12 Annual Precipitation
Var 13 Precipitation of Wettest Month
Var 14 Precipitation of Driest Month
Var 15 Precipitation Seasonality (standard deviation / mean)
Var 16 Precipitation of Wettest Quarter
Var 17 Precipitation of Driest Quarter
Var 18 Precipitation of Warmest Quarter
Var 19 Precipitation of Coldest Quarter
10 P. Bombi et alii
including Liguria, western Emilia-Romagna, Tuscany, Umbria, and northern Latium. 
These regions indeed correspond to the areas with the higher number of known presences 
(Razzetti and Bonini, 2006). Wide regions in both northern and southern Italy appeared 
completely unsuitable for Coronella girondica (Fig. 2). 
DISCUSSION
The most updated and reliable distribution map for Coronella girondica in Italy is giv-
en in Razzetti and Bonini (2006). These authors pointed out that several historical records 
are unreliable, and that the species is likely more common on the Tyrrhenian side of Italy 
than on the Adriatic side. Razzetti and Bonini (2006) map was considerably different from 
previous distribution maps for this species (Bruno and Maugeri, 1977), where Coronella 
girondica was claimed to be widespread also in the southern regions of Italy, i.e. Cam-
pania, Calabria, Basilicata, Puglia, and Sicily. Our modelling analysis clearly confirmed 
the reliability of Razzetti and Bonini (2006) map compared to Bruno and Maugeri (1977) 
map, indicating that the southern Italian regions are unsuitable for the presence of this 
Fig. 1. Receiver Operating Characteristic (ROC) plots for the potential distribution model of Coronella 
girondica in Italy. The “Area Under ROC Curve” is the AUC value.
11Modelling distribution of Coronella girondica in Italy
colubrid species. Thus, our study reveals that at least 14 presence records in Bruno and 
Maugeri (1977) map are unreliable. On the other hand, the potential distribution map giv-
en in this study mirrors widely the map drawn by Capula et al. (1995), which was built in 
consideration of the reproductive ecology characteristics of Coronella girondica that were 
considered inappropriate for the life in the southern Italian climates.
However, if we compare the map of true distribution given in Razzetti and Bonini 
(2006) with the potential distribution map given here, some differences also do exist. Indeed, 
we can stress that the potential range of this snake is much wider in Tuscany, Umbria and 
Latium than currently demonstrated. For instance, there are very few recent records for pres-
ence sites in Tuscany, Umbria, and Latium whereas the distribution suitability map indicat-
ed that wide sectors of these regions are very well suited for Coronella girondica. We would 
Fig. 2. Potential distribution model for Coronella girondica. Probability of presence increases from white 
to black.
12 P. Bombi et alii
therefore urge further accurate research in these regions to improve the knowledge of the 
local distribution of this species. In conservation terms, it is however necessary to stress that 
some areas that are depicted as potentially very good for this snake (for instance the areas 
surrounding Rome) are currently strongly altered due to human intervention on the natural 
habitat. Coronella girondica may have been therefore extirpated from these areas, and the 
remnant populations that are still eventually found should be carefully managed and pro-
tected in order to avoid further decline in these potentially important areas for the species. 
The well-suited areas in Tuscany and Umbria are likely less exploited than those in Latium, 
suggesting that the relatively few presence records should depend more on unsatisfying field 
research than on extirpation of the species from potentially good areas. It is, therefore, likely 
that Coronella girondica is not really threatened or in decline in these latter regions, but that 
its presence may have simply gone undetected in many sites due to its elusiveness accom-
plished with suboptimal field research by herpetologists. Nevertheless these hypotheses need 
to be confirmed by including human impact variables in the analysis.
In conclusion, we suggest that careful field research should be done especially in Tus-
cany and Umbria to improve the number of presence data for this species, and that spe-
cial conservation and management actions should be done in the suitable areas of Lat-
ium, where the suitability of the areas for Coronella girondica is high but where the human 
intervention on the habitat has been very intense. 
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Predicting elusiveness: potential distribution model of the Southern smooth snake, Coronella girondica, in Italy

Pierluigi Bombi

Luca Luiselli

Massimo Capula

Daniele Salvi

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Predicting elusiveness: potential distribution model of the Southern smooth snake, <i>Coronella girondica</i>, in Italy

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Predicting elusiveness: potential distribution model of the Southern smooth snake, Coronella girondica, in Italy

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