Behind wolf predation on wild ungulates: environmental factors influencing the distribution of kill sites in Northern Italy

Predation is a hierarchical process whereby predators are constrained to kill prey within the area they select while hunting. Therefore kill sites are not randomly distributed, rather where kill sites occur is a function of prey distribution and predictability and environmental factors that influence prey detection, access, or the success of an attack [1]. Wolves (Canis lupus) are considered generalist apex predators, preying mainly on wild ungulates. Being socially organized in packs, usually consisting of the breeding pair and their offspring, wolves roam within their exclusive territory and cooperate during the hunt. Wolves are well adapted for cursorial predation with chases ranging from 100 m to more than 5 km [2]. The aim of this research is to identify the main environmental factors influencing the distribution of wolf kill sites, so at the same time determine which factors influenced the vulnerability of prey once the hunt began. The study was carried out in Liguria (5343 km2 region in Northern Italy; Fig. 1) The wild ungulate community includes the wild boar (Sus scrofa) and the roe deer (Capreolus capreolus), widely distributed with high densities, the fallow deer (Dama dama) and the chamois (Rupicapra rupicapra) more localized. Moreover, the red deer (Cervus elaphus) has a sporadic presence along the boundaries of the region. Wolves reached the Ligurian Apennines in the late 1980s (the first illegally killed wolf was found in 1990) and the Ligurian Alps in the late 1990s (the first illegally killed wolf was found in 1997). The most recent research estimated the presence of minimum five wolf packs by non-invasive genetic sampling [3]. Using data collected through a monitoring project carried out between 2007-2014 [3], we delineated wolf range using the sampled wolf genotypes by a fixed kernel estimator. We considered all claimed and verified cases of wolf predation upon wild ungulates recorded during 2007-2016, reporting the preyed species and possibly some related information (sex, age, proportion of consumption). Around each kill site we defined a buffer corresponding to the potential hunting area of wolves. We used a width of 13 km, corresponding to the average travel distance of wolves during the night to go from dens or resting sites to hunting sites in Italy [4]. We compared the plots where kill sites were recorded and an equal number of random plots within the estimated wolf range. We formulated a habitat suitability model following an approach presence vs. availability by binary logistic regression analysis (BLRA, forward stepwise method); we tested the hypothesis that wolf choice of kill sites is influenced by the morphology and the land use of the area. In each plot, we measured from the Corine Land Cover III level and the Digital Elevation Models (DEM) the environmental variables used as covariates: four slope classes (range between 0° and >60°), road density, path density, forests (broad-leaved, coniferous, and mixed forests), urban and cultivated areas, scrublands, open areas (pastures and grasslands), and bare ground (rocks and areas with little or no vegetation cover). We considered even two-way interactions between covariates. We tested the model performance by the percentage of correct classifications of original cases, Nagelkerke’s R2, and receiver operating characteristic (ROC) curve analysis. We identified 74 distinct wolf genotypes, corresponding to 189 non-invasive DNA samples (98% faeces, 1% urines and 1% hairs), collected in the study area from 2007 to 2014. Wolf range had a total extent of 5068 km2. We mapped and digitized 62 wolf kill sites; among the preyed wild ungulates, we identified 23 roe deer, 18 fallow deer, 16 wild boars, and 5 chamois (Fig. 1). BLRA showed a negative effect of the road density, the urban areas, the mixed forests, and the medium slopes (20-40°), a positive effect of steep slopes (>60°), open areas, and bare ground, the latter without statistical significance (Tab. 1). The logistic model explained 56.4% of the variance of the response variable and correctly classified 78.2% of original cases, 82.3% of kill sites and 74.2% of control ones. The area under the ROC curve was significantly greater than that of a model that randomly classifies the cases (AUC=0.883±0.029; P<0.001). Wolves kill sites in Liguria were steep, open habitats (pastures and grasslands) far from roads and urban areas. Wolves tend to avoid areas with high road and human settlement densities, as they may be barriers to wolf movements and a cause of direct mortality both from vehicle collisions and illegal killing. Moreover, human disturbance associated with roads and urban areas may deter or interfere with wolves when attempting to kill prey, or afterward during carcass consumption. Wolves seemed to select steep slopes, probably because they may find a suitable habitat in terms of advantage during hunting activities. We found that hiding-cover levels were lower at kill sites than at random sites. Indeed, dense cover can affect the prey capacity to exploit refuges, thus enhance its chances of escaping an attack, and can increase the chance of detection the predator, 394 because of its noisier approach. From the predator point of view, in open habitats prey were easier to locate and catch. Wild ungulates mainly use open habitats during the night as feeding areas, because of the higher quality resources, and more closed habitats during the day, with less forage but a higher degree of shelter. Wild ungulates have to face a constant trade-off between the choice of better food patches and predation risk. This trade-off is mediated by the vigilance behaviour, which requires exclusive visual attention to scan the environment, thereby interrupting or slowing down foraging activity. Wolves usually take advantage of this wavering behaviour to start the rush. Moreover, wolves are mainly active from dawn to dusk and this is probably closely related to their hunting pattern, which matches with the activity patterns of wild ungulates. Overall, the environmental factors of the kill sites identified in this study are consistent with the cursorial hunting strategy of wolves.