Problems of Synurbization—Wild Boar in the City

Abstract

This work addresses the problem of synurbization, with its causes and effects specified using the example of wild boar (Sus scrofa). It presents basic biological parameters of the species, including those that promote its synurbization—small habitat demands, omnivorism, as well as ecological, behavioral, and demographic flexibility. It also discusses intra-species transformations stemming from wild boar adaptation to the urban space and pinpoints habitat fragmentation, ecological restoration, and phenotypic flexibility as the underlying causes of people–wild boar interactions. These interactions are primarily negative because wild boars attack humans and domestic animals and cause many traffic accidents. An analysis of the literature included in this study shows that, unfortunately, there are currently no fully effective methods that could protect urban areas and their inhabitants from the threats posed by wild boars. In order for sustainable urban development policies to be effectively implemented, there is a need for intensive, holistic research and cooperation between experts in many fields: wildlife, economics, public health, sociology, ethics, psychology, and urban planning. The synurbanization of wild boars is a large and growing social problem, but from an ecological perspective, there is a need to take action and develop methods to mitigate human/wild animal conflicts, not only from a human perspective. A one-sided view and action can be a threat to many animal species.

1. Introduction

Today, the number of city residents is approximately 4 billion, accounting for 55% of the global population (79.1% in developed countries). The world’s population is estimated to increase significantly by 2050, with 68% of the population being urban area residents (86.6% in developed countries) [1]. Urban population growth is observed primarily in Europe, North America, and Latin America, representing approximately 70% of the global population who inhabit urban areas [1,2]. Urbanization and economic agglomeration exert positive effects on economic growth and the spatial spillover effect.

The impact of urbanization on economic growth varies across regions and is determined by geographic location, availability of resources, and local economic conditions [3]. Geographic location is completely connected with the natural environment, and the related habitats represent the basic spatial domain in which various species of animals, plants, and other organisms live, grow, are born, and reproduce. In the contemporary world, transformations are observed in terms of the structural and functional characteristics of natural habitats, ending up in either their fragmentation or loss. The fragmentation of large and interconnected natural habitats into smaller and separated areas results in the loss of their primary properties and functions. This modification is triggered by, among other things, the extension of areas used for agricultural purposes, the development of urban agglomerations, including conurbation, and road network expansion [4]. All of these transformations induce synantropization, i.e., the adaptation of wild animal species to the new environment, while synurbization, i.e., the adaptation of different species of wild animals to the conditions of urban areas, is a specific subcategory of this process [5]. The 21st century is, hence, deemed a period of intensified synantropization and synurbization of animals.

Anthropopressure and the spatial structure of the landscape, modified upon its effect, lead to the appearance of wild animal species in the environment of large urban agglomerations [6]. Expanded urbanized areas create an “ecological vacuum”, meaning vacant ecological niches [7], which are attractive to some wild species because of the availability of anthropogenic food and multiple shelters, lower predator pressure, higher temperatures, and more daylight [8]. Not all of them can successfully cope with the challenges of urbanization; however, some are capable of not only persisting but also thriving in urban environments by leveraging available anthropogenic resources [9]. Persistence in human-modified habitats necessitates adaptive changes in animal behavior, morphology, physiology, and genetic structure of the population [10,11].

Birds adapt to the urban ecosystem both physiologically (e.g., changes in stress hormone levels) and behaviorally (e.g., extended breeding seasons, altered foraging strategies). Great tits (Parus major) adjust their song frequency in noisy urban environments [12], while magpies (Gymnorhina tibicen), inhabiting the outskirts of urban areas in Australia, initiate reproduction earlier than their rural counterparts [13].

Small- and medium-sized mammals [14,15,16,17,18] and reptiles also inhabit urban agglomerations [19]. Bat responses to urbanization have been shown to be highly species-specific: some adapt readily to urban habitats, whereas others decline due to natural habitat loss [20,21,22]. Red foxes (Vulpes vulpes) successfully leverage anthropogenic food resources in cities [14], beech martens (Martes foina) utilize dens within buildings [15], and lizards (Anolis sagrei) exhibit altered exploratory behaviors [19].

Phenotypic flexibility facilitates the production of alternative phenotypes across diverse environmental conditions [23]. The rate of these phenotypic transformations is higher in human-dominated habitats than in less disturbed, natural environments [24]. Interactions of man with wildlife species [5,25] can have either positive outcomes (contact with nature is widely considered beneficial to our psyche) or negative ones (animals damage properties, participate in traffic collisions, and pose a threat to the health and lives of humans and companion animals) [5,26,27]. The underlying causes of conflict include competition for resources, i.e., food and space [28].

Mammals, like mice, foxes, and, above all, wild boars (Sus scrofa) [17,18,29,30], pose a huge problem in large European cites [5,31,32,33,34,35,36]. The scale of this phenomenon is immense, all the more so because the population of wild boar has been continuously increasing across the globe [37,38]. Urban wild boars often display little fear in encounters with humans and are regarded as the most conflict-prone species among wild animals [39].

The negative outcomes of human–wild boar interactions are the growing number of traffic accidents involving wild boars and their attacks on residents and/or pets. According to the goals of sustainable development, human settlements should be safe, including both transportation infrastructure and green spaces. Nonetheless, wild boar–vehicle collisions constitute a major problem in many countries worldwide [40]. Such incidents often result in human injuries, generating healthcare costs [41,42]. The costs of wild boar–vehicle collisions increased between 2003 and 2016 and are currently estimated at approximately EUR 9.66–12.31 million per year [42]. The number of wild boar attacks on humans and/or domestic animals has increased as well [43,44,45,46]. Interspecific and intraspecific disease transmission to humans and domestic animals is another emerging problem further burdening healthcare systems [47,48]. Ineffective management of wild boar populations and people–wild boar interactions results in escalating public and private expenditures [49].

Conflicts generated by wild boars thriving in urban and suburban areas are complex. Responses of authorities vary and are often exacerbated by ambivalent public attitudes toward their presence in the urban space [31,36]. One thing is certain: the efforts undertaken to mitigate human–wildlife conflicts are an integral part of wildlife management and conservation in urban habitats [39].

The aim of this work is to

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Present the human–wildlife interaction;

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Identify causes and effects of the synurbization phenomenon using the example of Eurasian wild boar;

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Present recommended means and viable methods for counteracting threats posed by wild boars to urban agglomerations and their residents.

2. Species Biology Wild Boar—Sus scrofa

The wild boar (Sus scrofa) is an even-toed mammal of the swine family, inhabiting all continents except Antarctica [50] (Figure 1).

As a eurybiotic species, it inhabits both semi-deserts and high mountain ecosystems; however, its largest populations are found in mixed forests with oak growing in lowlands [52]. The most important factor determining its distribution in the natural environment is the availability of water, food, and shelter [53].

Wild boar is a gregarious, territorial, and migratory species. It is also an opportunistic omnivore with highly developed senses as well as a polygamous species, whose females are monoestrous, multifetal, and multiparous. Habitat alterations and access to high-energy food resources have induced significant reproductive changes in wild boar populations, including accelerated female maturation. Young females now often produce their first litter within the first year of life, while older sows are capable of giving birth to and rearing two litters annually. On average, sows produce and raise 7–9 piglets per litter, with smaller litters (1–4 piglets) and exceptionally large litters (10–13 piglets) being comparatively rare. These reproductive shifts substantially increase population growth in certain regions and countries [54,55,56,57].

The real threat of wild boar to soils and ecosystems has been well documented [58], and the species has been recognized as a driver of disturbances in both biotic and abiotic landscape components [59,60]. Wild boar population expansion and the invasion of anthropogenic environments adversely affect local biodiversity [61,62,63]. Through rooting, wild boars modify geomorphological processes [64], disrupt soil structure stability [50], and impede vegetation regeneration [50,59,60,65,66]. On the other hand, they may also contribute positively by limiting pest abundance [67,68], accelerating nutrient cycling [69], and enhancing biological renewal and biodiversity preservation [70].

The destructive influence of wild boar has been confirmed in Australia, New Zealand, Tasmania, Pacific Islands, and both North and South America. In these regions, the species is widely considered a conflict animal and pest, and in North America and Australia, it is officially classified as invasive [56].

Wild boars are also deemed reservoirs for numerous epidemiologically and economically significant pathogens [71,72]. They are susceptible to a wide range of viral diseases, including African swine fever (ASF), as well as bacterial infections [73]. Additionally, they are hosts to a variety of parasitic species [57,74,75,76], thereby posing risks to domestic livestock, companion animals, and humans [77].

Furthermore, wild boars are sensitive to environmental pollution, which makes them valuable bioindicators [78,79,80,81]. Processes of contaminant absorption and bioaccumulation in tissues provide insights into the extent of environmental contamination in their habitats [82,83,84].

Muscle tissue and internal organs obtained from hunted wild boars may exhibit elevated concentrations of toxic elements such as lead, cadmium, arsenic, and mercury [85,86,87,88,89,90,91]. This finding necessitates the continuous monitoring of meat quality and careful selection of organs intended for consumption [92].

3. Species Expansion

Over the past two decades, wild boar populations have increased and expanded their range in human-dominated and urbanized landscapes [8,52]. The primary drivers of this phenomenon are the species’ high reproductive potential and exceptional ability to adapt to diverse environments [31,93].

Intensification of agricultural production has increased the protein and energy contents of crops, which, when eaten by wild boars, make them reach sexual maturity and the age of beginning reproduction earlier, and also increase their fecundity [94]. Wild boars have adapted their foraging strategies to large-scale monocultures of high-energy crops, particularly maize. Supplementary feeding practices in hunting grounds provide an additional dietary source of this crop. Its insulinogenic properties, combined with its contamination by Fusarium fungi producing zearalenone—a potent estrogenic mycotoxin—stimulate anabolic processes and disrupt reproductive functioning, leading to polyestrous cycles in females. As a consequence, wild boar populations have experienced rapid growth [95,96,97].

Climate change, including global warming, is also important in this respect [98], as it promotes the fruiting of oak and beech, whose fruits provide valuable, high-energy fodder for wild boars [99,100]. At the same time, global warming reduced the mortality of wild boars, especially the young ones [38]. Continued population growth has led to the saturation of natural habitats, which made the animals seek shelter and food elsewhere. They migrate to cities; to live, survive, and thrive in cities, they adjust their behavioral traits and utilize anthropogenic resources [101,102,103].

Studies on the spatial ecology of urban wild boar [32,104,105,106,107,108] have demonstrated that they occupy forested habitats but frequently venture into urban and suburban zones. They depend heavily on urban forests, which serve primarily as shelter and daytime resting sites. During nocturnal foraging, they often visit urban meadows, while rarely exploring agricultural fields within city limits. By restricting activity to nighttime hours, from dusk till dawn, wild boars minimize the risks associated with human proximity. This nocturnal activity may also be partly linked to the species’ sensitivity to heat stress. Daytime resting occurs in covered habitats, often near buildings and roads. Movement patterns in natural habitats typically involve relatively short distances, although over longer timeframes they may range from few to several dozen kilometers [107,108]. In contrast, urban boars exhibit reduced mobility and smaller foraging ranges during their nightly activity [32].

The reduction in the migratory behavior of wild boars in cities has resulted in their sedentary lifestyle, which in turn prolongs their reproductive season [34,109,110]. It also results in changes in their dietary patterns and exploratory behaviors but also increases intraspecific aggression. Animals tame with humans and adapt to their behavior, while the lack of migration, greater access to food, and lesser pressure from predators significantly reduce their natural selection, thereby increasing survival rate and longevity, ultimately leading to poorer health conditions. Urban life also triggers changes in the circadian rhythm, which prolongs and shows variations, mainly due to the exposure of animals to artificial light and staying in shelters when humans are most active [109,111]. But despite this, human–wild boar interactions occur.

Gene flow between populations from urbanized and non-urbanized areas is low, which is due to the fact that wild boars from urban areas remain close to the cities they live in [32]. This leads to genetic differentiation between such populations, and to naturally diminished genetic variability in populations that live only in urban areas [112]. Food availability strongly determines wild boar condition, as exemplified by populations inhabiting Barcelona, which are larger and heavier, and whose body condition index (BSC), as expressed by their body weight and body meat and fat contents, is higher compared to the wild boars living in nearby country parks, with particularly pronounced differences observed in this respect among adult females [113]. Studies conducted by the cited authors showed the presence of anthropogenic food in the digesta of wild boars from Barcelona, including in 60.2% of the animals analyzed in the spring–summer season and in 40.6% of the animals analyzed in the autumn–winter season. Higher triglyceride and lower creatinine concentrations were found in the serum of the animals living in the city, which may have resulted from the high-energy, fatty acid-rich anthropogenic diet. The intake of anthropogenic food can lead to animals becoming habituated to the presence of humans, while also causing changes in their phenotype and metabolic diseases. Most often, urban areas are invaded by packs, i.e., herds of females with offspring, while adult males are observed less frequently [31].

A comparison of the individual home range of wild boars in Krakow and the Bialowieza Forest showed that they were more than half the size in the city than in the forest complex [101]. The wild boars observed in Krakow traveled much longer distances, and their activity was more season-dependent compared to the individuals from the Bialowieza Forest. Wild boars are strongly associated with urban tree canopies, but they are also eager to use any urban greenery, even a small area, that will provide them with shelter [16,32]. In the face of anthropopression, wild boars modify their activity periods and lead a primarily nocturnal lifestyle in cities [101]. Cahill et al. [31] pointed out periods of increased wild boar activity in cities, including the months of summer drought in Mediterranean climates, and the months when snow and frost reduce their ability to root and forage for food in areas with colder climates. Marin et al. [32] noted that changes triggered by synurbization vary both between and among populations in different cities. At this point, however, it is impossible to create an exact picture of the synurbanized wild boar.

Efforts to manage the urban wild boar population increasingly focus on updating distribution data. These include bibliographic analyses, field surveys that aim to identify hotspots of road collisions and crop damage, as well as interviews with residents [105].

4. Wild Boar—Social Issue

The presence of wild boars in urban areas has become a social issue. Public discourse, particularly in online forums, often focuses on issues such as culling, crop damage, ASF, supplementary feeding, and aggressive encounters with humans. All of them have become the subject of sociological surveys [114].

As reported by Rancew-Sikora et al. [114], the discussions held on online platforms make it possible to distinguish two opposing groups among residents, whose views on the presence of wild boars in the city differ extremely. The community presenting an ecocentric view expresses compassion and concern for animals and care for their welfare. In their opinion, people should bear responsibility for the expansion of cities into wildlife habitat areas. They also express anti-hunting views and oppose lethal techniques (culling). In turn, residents with anthropocentric views focus on the welfare of wild animals, but at the same time express anger and fear resulting from their activity, the damage they make, and their attacks on residents and pets. They also express dissatisfaction with the performance of responsible services. They expect rapid and effective removal of wild boars from the city and support the use of lethal techniques. The results of various studies indicate that city residents generally choose solutions that appear to be non-lethal to animals, as they are unaware of the risks posed to humans and animals [5,36].

5. Reduction in the Urban Population of Wild Boars—Spatial Study

In order to reduce the population of wild boars in cities, recommendations are being implemented and activities are being undertaken to discourage them from leaving their natural habitats, restricting their access to anthropogenic food, and inhibiting their reproduction.

Bulking grounds are being prepared to encourage animals to stay in forests, as well as hunting plots and barrier strips in cooperation with managers of hunting districts in areas adjacent to the city. Reduction measures, including trapping and/or shooting, are being implemented. Attempts are also being made to implement contraception [36,38,115,116,117,118].

Measures to deter and discourage wild boars from occupying urban areas involve laying out special scent preparations. In addition, administrative units exchange information with residents, housing cooperatives, and managers of hunting districts, and migration routes and habitats of wild boars are also established via inspections of urbanized areas and local inspections. City officials recommend permanent fencing of plots of land, properties, and recreational areas; securing composting garbage cans, keeping trash containers in lockable trash pergolas, and securing them against being tipped over by animals; securing trash garbage cans against penetration by wild boars; and abandoning the planting of species that are a feeding base for wild boars in favor of deterrent plants [36,119,120]. To effectively reduce urban wild boar populations, it is essential to combine the aforementioned conventional management strategies with interdisciplinary approaches from various fields of science, as proposed in urban planning initiatives in Poznań and Gdynia [36].

The trapping of live individuals is carried out with respect to animal welfare, using drop nets, teleanesthesia, cage traps, night hunting, and hunting with a chase [115,121,122]. In some agglomerations, the trapping and shooting of wild boars are implemented together. The results indicate that most of the captured individuals are young animals, less than a year old [32,115]. Marin et al. [32] believe that the trapping of young animals probably reflects the demographics of the local population and the increased caution of older individuals.

The use of chemical and/or surgical sterilization of wild boars is under consideration as well; however, both these sterilization methods are difficult to implement in practice, as they require careful monitoring, trapping, and immobilization of the animals, which involves enormous costs and requires specialized personnel. The use of contraception in the case of wild boars requires more extensive research and has so far been implemented on a small scale. Contraception is seen as a method of proactive measures and seems not to be an optimal solution in the current situation of the threat of African swine fever (ASF) and the need for immediate population reduction [123]. A research group of Massei et al. [38] has been trying for years to develop methods to reduce the reproduction of wild animals considered troublesome for humans in urban settings. The potential measures must be species-specific and inaccessible to other animals. Such a solution seems effective and promising but, for the time being, is not provisioned in the legal regulations binding in Europe.

Socio-spatial conflicts escalate to varying degrees, and they can be assessed by analyzing spatial data on the location of events and behavior of wild boars. However, collecting such data is very labor- and cost-intensive, as it requires access to urban surveillance systems and systematic analysis of intervention reports. Dudzinska and Dawidowicz [124] proposed a method aiding the assessment of the risk of wild boar invasions and socio-spatial conflicts in cities using large multimedia and descriptive datasets from social media, i.e., crowdsourcing data. The method was tested in Poland in the city of Olsztyn. It enabled the identification of types of socio-spatial conflicts involving wild boars in the city and confirmed the usefulness of crowdsourcing data.

In response to growing public concerns, spatial studies have been conducted in various European cities [124] with the aim of reducing wild boar population sizes and monitoring their presence within urban agglomerations. From an ecological perspective, conflicts between humans and wild animals pose a threat to many species, which is why there is a need to take action and develop methods to mitigate them for the benefit of not only humans but the animals as well [125]. In this context, ethnozoological research can serve as a tool for assessing various scenarios in which conflicts arise and identifying the factors responsible for them. Researchers emphasize that an effective population control plan in terms of the phenomenon of wild boar synurbization should combine several methods [116,117,123]. The selective trapping of individuals of a certain sex and age, reducing anthropogenic food resources, and modifying urban habitats to restrict access to shelters and hiding places are the most common measures taken against excessive wild boar populations in cities, but it should be remembered that the intensity of conflict management varies depending on social perceptions, politics, religious and cultural beliefs, and geographical region [126]. All these factors point to the complexity of the problem in terms of developing flexible tools to reduce conflicts between humans and wildlife, including between humans and wild boars.

6. Conclusions

The intensifying expansion of humans into areas adjacent to natural habitats leads to their degradation, deforestation, the establishment of new urban agglomerations, and the spatial growth of existing ones. These changes have brought the boundaries of forests and cities closer together. The wild boar has entered cities, colonized emerging urban ecological niches, and successively reproduced therein. Its excessively large population has triggered imbalance in urban ecosystems, aggravating conflicts with urban space residents and generating economic consequences, including particularly an increase in public expenses for covering damages. These losses pose a global challenge to authorities and biologists dealing with wild nature protection. Consequently, efforts to reduce urban wild boar populations have been undertaken, including selective culling, urban habitat modification, and restricting access to anthropogenic food sources. The presented literature review shows that there are currently no fully effective methods to safeguard urban areas and their residents from wild boar-related threats. Conflict management varies depending on social perceptions, politics, religious and cultural beliefs, and geographical region. It seems that in order for a sustainable urban development policy to be effectively implemented, there is a need for holistic research and the integration of interdisciplinary knowledge in the field of wildlife specialists, economy, public health, sociology, ethics, psychology, and urban planning.