Effects of predator recovery on prey and competitors: wolves, wild boar and red foxes

Large carnivores are recovering across Europe, providing significant modifications in the structure of trophic webs through restored interspecific interactions. Although apex predators have been generally reported to predate on ungulates and interfere with mesocarnivores, facilitative interactions may arise through prey carcasses providing increased foraging opportunities to scavenger species. Spatiotemporal responses in the behaviour of prey and competitors which persisted for long periods in absence of large carnivores would be expected, but information is still fragmentary and conflicting across studies. Concerning scavenging species, the potential for facilitative vs. suppressive interactions may vary spatiotemporally, but long-term information as well as on a fine temporal scale is scarce. Within this thesis, I considered the main apex predator of temperate ecosystems of Europe, i.e., the wolf Canis lupus, its main prey (i.e., the wild boar Sus scrofa) and its main potential competitor (i.e., the red fox Vulpes vulpes) and I assessed their spatio-temporal and trophic relationships in a mediterranean area recently recolonised by the large carnivore. I also took benefit of estimates of the temporal variation of predator and prey abundance to analyse their role in modulating interspecific interactions. Although the wild boar is often the main prey of the wolf, and interspecific killing by this predator on the red fox has been reported in literature, both the suid and the small canid are omnivorous scavengers potentially showing partial dietary overlap with the predator. Complex interactions may emerge, because the predatory action of the wolf can provide energetic foraging opportunities to the wild boar and the red fox through increased carcass availability. First, I concentrated on the wolf-wild boar interactions. The wild boar was confirmed as the most used prey by wolf in my study area, with an absolute occurrence in the diet ranging annually from 40% to 73% (n = 2849 scats; November 2017 to March 2023). However, previous long-term work in my study area, as well as a comparison of wild boar selection by the wolf between Maremma and an area including a different ungulate community (Chapter 2) revealed spatial, seasonal, and inter-annual differences in selection indices of this suid, suggesting that the wild boar may not be preferred when larger and easier prey are available (e.g., the red deer Cervus elaphus and the fallow deer Dama dama). In particular, an analysis of wolf prey selection conducted at a fine, seasonal, scale in Maremma across three full years indicated complementarity of the wild boar and the fallow deer, that played a switching primary role for the predator during the year (Chapter 3). Considering the high predatory pressure imposed on wild boar, I expected spatio-temporal antipredator responses to occur. Previous studied failed to detect them, although potential differences in the response between males and females were not addressed. Being the wild boar a polygynous, sexually-sized dimorphic species with females living in matriarchal groups with their offspring, I expected stronger responses in the latter than in males. By analysing camera trapping data (n = 21251 detections of wild boar, wolf and people), I found a divergence between sex classes, with more pronounced diurnal activity for females with offspring, especially in spring, indicating some degree of antipredator avoidance. I suggested the vulnerability related to the biological cycle as an important factor inducing the females to modify seasonally their behaviour. Nevertheless, the wild boar activity was predominantly nocturnal, which suggests that other antipredator responses, not yet addressed (e.g., defence in groups) played a major role. Moreover, responses towards humans were stronger than those to the wolf, indicating avoidance of people as a strong mechanism influencing the spatio-temporal modulation of the wild boar activity (Chapter 4). Eventually, I evaluated the trophic response of the wild boar to the wolf recovery, including the potential for increasing scavenging opportunities (Chapter 5). Diet analyses revealed that plant material composed c. 90% of the wild boar diet. Thirteen samples contained cervid hair out of 1261 faecal pellets analysed in 2023, while no hair of deer was detected in wild boar pellets prior to wolf recolonization (1991-1994: n = 3748 pellets analysed), indicating an increase scavenging activity by the wild boar, albeit in a small extent. Wild boar dominance in the wolf diet, and high wolf density, expectedly lead to relatively low availability of deer leftovers for scavengers, as well as stimulating fear reactions in wild boar which may avoid carcasses to reduce risk of encounters with predators. As a last step, I evaluated the wolf-fox interactions in a long-term, 6-year period during which wolf numbers increased from one to three packs (Chapter 6). Spatio-temporal association and trophic facilitation were reported by a previous study conducted in the first stages of the wolf recovery. I expected that this association would reduce throughout years along with increasing the wolf numbers, thus potential interference. Accordingly, the use of large ungulates by the red fox decreased along with the increase in wolf abundance ranging from c. 40% in 2017 to c. 6% in 2023 (n = 5150 scats), without major changes in wild ungulate densities throughout the years, suggesting lower trophic possibilities for the red fox in presence of three wolf packs. Contrarily to what expected, camera trapping records (n = 2619 wolf and n = 26195 fox detections) reported a high interspecific seasonal temporal overlap of activity between the two canids (Δ=0.70–0.94, 0-1 scale) that increased throughout years. A spatial association of the monthly detection rates of the wolf and the red fox was reported too and recurrent-event analyses supported attraction rather than avoidance, revealing a strong interspecific spatiotemporal association between the two canids. Results suggest a low potential for interference, with foxes keeping on considering carcasses of wolf prey as profitable food resources, although the actual benefit was reduced by increasing wolf abundance. Overall, results revealed complex interspecific dynamics between the wolf and the two scavenger species (i.e., the wild boar and the red fox) in this recently ecolonized area by the apex predator, the intensity of which can change throughout years. Future work should evaluate the potential consequences for processes acting at broader community-to-ecosystem scales. The ongoing recovery of the wolf across Europe can provide the opportunity for targeted assessments.