1. Introduction
Urbanization and the expansion of transportation networks significantly transform landscapes, often detrimentally affecting wildlife. In urban areas, this growth leads to the fragmentation and displacement of natural habitats, which, in turn, isolates wildlife populations and increases the risk of Wildlife–Vehicle Collisions (WVCs) in urban areas [
1]. The reasons for transportation planners’ often limited focus on avoiding critical habitats are complex. Traditionally, road construction prioritizes human-centric factors such as economic efficiency, traffic flow, and connecting urban centers, while environmental concerns, especially those related to wildlife habitats, are secondary [
2,
3]. This oversight stems partly from a lack of comprehensive environmental impact assessments and a limited understanding of habitat fragmentation’s long-term ecological consequences [
4]. Additionally, the urgency of infrastructural demands often leads to expedited planning processes that overlook environmental concerns [
5], and a lack of collaboration between urban planners, ecologists, and conservationists exacerbates this issue [
6], resulting in road networks that are efficient for humans but harmful to wildlife habitats.
These planning shortcomings have serious implications. According to one study, approximately 1 million wildlife are killed in vehicle collisions in the United States each year [
7]. Roads cause not only immediate mortality but also lead to significant ecological disruptions, such as genetic isolation among wildlife populations [
8]. As habitats are fragmented by roads, these populations become segmented, leading to reduced genetic diversity and increased vulnerability to various environmental threats [
9].
Focusing on the bobcat (
Lynx rufus) in the San Jose region, we see the direct consequences. San Jose’s unique urban dynamics, typical of many rapidly developing urban centers in North America, provide a pertinent example of urban expansion impacting native wildlife habitats. Bobcats, native to this region and requiring extensive home ranges, are particularly vulnerable to urban encroachment and habitat fragmentation [
10]. These creatures serve as ecological linchpins in the San Jose area, playing a crucial role in maintaining the balance of the local ecosystem. As apex predators, bobcats help regulate the populations of smaller mammals and rodents, preventing overpopulation and its associated ecological imbalances [
11,
12]. This predation is essential for controlling the spread of diseases and maintaining a healthy and diverse ecosystem. Furthermore, their presence is indicative of a thriving natural habitat, signifying a well-functioning ecosystem [
13]. Therefore, the preservation of bobcats is not only essential for their own species’ survival but is also integral to maintaining the overall health and balance of San Jose’s urban environment, reflecting the interdependence of all species within this ecosystem.
To protect wildlife and enhance road safety, it is crucial to identify and implement suitable locations for wildlife crossings [
14,
15]. This challenge intertwines the welfare of both wildlife and humans, marking a pivotal moment in the development of San Jose’s urban landscape [
16]. The urgency of this task is highlighted by the high incidence of roadkill, which not only affects local ecosystems but also has global implications. Bobcats, as apex predators, are essential for maintaining the balance of the food chain, and their loss can lead to significant ecosystem disruptions. By strategically placing wildlife crossings, we can reduce these incidents, thus preserving the ecological integrity and biodiversity of the area [
17]. Moreover, this approach addresses a broader concern: the impact of infrastructure development on wildlife [
18]. Accurately evaluating and predicting wildlife crossing points is vital for mitigating habitat fragmentation and ensuring ecological connectivity, both locally and globally.
Moreover, the study of bobcat crossings is not confined to its immediate implications for this specific species. It stands as a prototypical example for addressing similar issues involving a multitude of wildlife species and urban infrastructure. Lessons learned from studying bobcats can be applied to inform the conservation efforts of other wildlife, ensuring that urban environments remain hospitable for diverse and resilient ecosystems [
19]. In this way, the research on bobcat crossings in San Jose transcends its immediate context and contributes to a broader understanding of how urbanization impacts wildlife and ecosystems [
19]. It underscores the urgent need for innovative solutions that balance the imperatives of urban development with the preservation of biodiversity and ecological integrity.
This article aims to illuminate the path towards harmonious coexistence between urban development and wildlife conservation, offering practical insights and solutions. We explore innovative urban planning approaches that integrate wildlife conservation, such as designing and implementing wildlife corridors and crossings specifically tailored for bobcats. By reducing wildlife–vehicle collisions, these solutions help preserve bobcat populations and maintain ecological balance. This study provides a blueprint for cities globally to navigate the challenges of urban expansion while safeguarding their natural habitats and the species that inhabit them.
5. Discussion
In this study, we explored the impact of human activities on bobcat habitat selection and suitability. Our analysis of the San Jose region, encompassing conservation areas such as Basking Ridge Conservation Area, Tulare Hill Ecological Preserve, and North Coyote Valley Conservation Area, indicated that major roads, including Highways 101 and 85, and urban land use, including the land between Highways 101 and 85, the silver leaf, and the town along the Monterey Road, called the Los Paseos significantly negatively impacted bobcat habitat suitability. These have led to habitat fragmentation and reduced connectivity within these suburban and natural landscape. Major roadways notably disrupt ecological continuity, severely disrupting the ecological environment and intensifying roadkill incidents, thereby restricting bobcat movement. Ruell et al. [
28] also highlighted the negative impacts of roads on habitat connectivity.
Comparing the different layers, from
Figure 2, we find that the interactions between various factors may influence suitability by affecting bobcat behavior. In our analysis, comparing the influence of roadkill incidents and traffic volume on habitat suitability, we found that while the general patterns of habitat suitability associated with roadkill incidents broadly align with those of traffic volume, the areas of highest impact for each do not overlap. Specifically, these factors differentially affect bobcat behaviors such as movement patterns, territorial range, and crossing frequency, with peak areas for roadkill incidents suggesting a higher risk to bobcats in certain regions [
84], while traffic volume peak areas may correspond to areas with restricted bobcat movements due to noise and continuous vehicle presence. The occurrence of roadkill incidents is not concentrated at the peaks of traffic flow but aligns more with the suitability in the movement patterns of bobcats. Firstly, it is evident that the cause of roadkill incidents is not just heavy traffic flow, but the result of the interaction between traffic flow and bobcat movement patterns. Additionally, comparing the bobcat movement maps with layers of vegetation, streams, land use, etc., we can see some patterns similar to bobcat activities. Areas rich in vegetation and far from urban areas show higher suitability and activity density. This also reflects to some extent that natural environmental factors like vegetation are attractors of bobcat activity. This observation also underscores the significance of natural environmental factors as determinants of bobcat activity, evident from the bobcat movement maps. The patterns depicted in these maps show a clear correlation between areas of dense vegetation and increased bobcat activity [
85]. Particularly, these areas, typically remote from urban settings, are not only characterized by higher habitat suitability but also by a greater density of bobcat activities. For example, areas such as Metcalf Motorcycle County Park and the portion southwest of the two preserves and the area between South San Jose have more dark green patches on the Suitability Map, demonstrating better suitability for bobcats. This is also an area where bobcat activity patterns are more intense (
Figure 3). At the same time, however, the fragmental distribution of vegetation has led to more frequent travelling of bobcats on roads in this area, as seen in the illustration of bobcat movement patterns, even on roads, where their movement trajectories cross roads and connect the two reserves (
Figure 2). This suggests that, aside from traffic flow, the natural landscape features, such as vegetation cover, significantly influence bobcat movements, attract their presence, and contribute to habitat preference, offering essential resources and sheltered breeding locations [
85]. Vegetation cover is an important factor in bobcat habitat selection, providing them with necessary food sources and secluded breeding grounds [
86].
Moreover, our analysis in
Figure 3 revealed a dichotomy in the attractiveness of vegetation cover for bobcats. While areas rich in vegetation are highly appealing to bobcats, their proximity to transportation infrastructure increases the risk of roadkill. This finding is supported by Schmidt et al. [
87], who indicated that bobcats near roads have a higher risk of roadkill. Cain [
88] also observed an increased likelihood of roadkill for medium-sized carnivores in highly suitable habitats along roadways.
Overall, our research emphasizes that although areas with rich vegetation offer higher suitability for bobcats, road traffic infrastructure and roadkill phenomena significantly diminish this suitability. Some studies have also shown that predators, such as bobcats, tend to cross primary and secondary roads within their range of habitat. An increase in the density of primary and secondary roads results in more bobcats crossing and a greater risk of roadkill injury [
84], due to their movement pattern. This is consistent with Serieys et al. [
17], who found that bobcats prefer densely vegetated coniferous forests as habitats. However, abundant vegetation does not always equate to high suitability scores. Overlapping analysis (
Figure 2) showed that roadkill incidents primarily occur on main roads, highly overlapping with areas frequently traversed by bobcats. This might be due to vegetation density providing cover but also potentially attracting bobcat activity, thereby increasing the risk of roadkill [
89]. Our study indicates that in addition to considering the individual impact of various factors in habitat suitability assessments, the interplay and potential negative effects of these factors must also be considered. Therefore, future research should aim to adjust and optimize the suitability assessment system to more comprehensively reflect the complex relationships between environmental factors. Especially in assessing habitat suitability, it is crucial to consider not only the independent effects of environmental factors but also how these factors interact and affect the behavior and survival of wildlife. Moreover, our study underscores the need for conservation strategies, particularly in urban and suburban areas. When planning and managing natural reserves, the potential negative impacts of roads and urban land use on wildlife habitats should be fully considered. For instance, increasing ecological corridors and wildlife crossings can reduce roadkill incidents and enhance habitat connectivity. Additionally, raising public awareness about wildlife conservation, particularly among residents in urban and suburban areas, can help reduce disturbances to wildlife habitats. To delve deeper into bobcat habitat suitability analysis, we selected four areas (
Figure 3) for case studies, comparing, learning, and applying our findings by overlaying suitability maps and roadkill points to analyze key points in depth.
Figure 3 shows that Segment A, being close to the town, has the lowest suitability score. Despite some vegetation, it is primarily urban greenery and close to houses. Segment B, near a water source, has vegetation only on the northeast side and is traversed by a flyover across the main road, resulting in good suitability. Segment C, with open views and belt-like vegetation on both sides, also crossed by a flyover, has slightly lower suitability compared to B. Segment D, farthest from the urban area, with broad views and some vegetation, also shows good suitability in the image. In conclusion, it is found that the suitability of different sections varies due to the different environmental landscapes in their surroundings. However, the same feature is that the habitat suitability is lower in the crossings where there is no advantage of natural resources such as vegetation and water. Whereas the abundance of favorable natural conditions and appropriate tunnel treatments will increase the suitability of the intersection location. In addition, the closer to the urban landscape, the lower suitability even though there are high quality vegetation and corridor bridge. For details, the comparison of the four sections reveals that areas with vegetation on both sides or near a water source have higher suitability, as clearly demonstrated by B and C. Additionally, in similar road environments, the comparison between B and C indicates that more abundant and favorable natural factors lead to higher suitability. This suggests that increasing vegetation around roads and in poorer corridors could be considered to reduce roadkill. Therefore, following this, we will conduct case studies and research based on the characteristics of different intersections (
Figure 4), exploring measures to enhance suitability in the area.
Section A is characterized by a complex interplay of transportation networks and natural environments. In such areas, the challenge lies in bobcats potentially wandering into high-risk roadkill zones due to the blurred boundaries between roads, human settlements, and natural habitats. Based on Section 1-1 in
Figure 4, railways may become their pathways, with animals using these linear structures for movement [
90]. The common approach to mitigate threats from trains to mammals is to install fences along railway lines, preventing wildlife from entering the railway area. Fences, typically made of metal or wood and sufficiently high to deter target species, are proposed by [
91]. However, fencing can restrict wildlife migration and habitat use, leading to habitat fragmentation and disruption of population genetic flow [
57]. They also involve high construction and maintenance costs, requiring regular monitoring and repair. To address this, Spanowicz et al. [
92] collected roadkill data in Canada and Brazil and designed an adaptive fencing plan, prioritizing high-risk areas to increase connectivity and reduce costs. Additionally, the authors of [
93] developed a warning device that alerts animals as trains approach, enhancing their attention and facilitating learning. Moreover, for Section A, where roads are flanked by rich vegetation and central medians covered with dense plants, management measures including regular trimming and clearing roadside vegetation have been proven effective in reducing roadkill incidents for various carnivores [
94,
95,
96]. Different species respond variably to the height and density of vegetation in road medians, influencing their road-crossing behavior [
96]. However, more research is needed on the impact of road medians on bobcat crossing behaviors to achieve integrated vegetation management around these areas. For such sections, physical barriers like higher fences in high-risk areas along railways and roads can prevent bobcats from entering these zones. Additionally, comprehensive measures like managing road vegetation can effectively reduce animal roadkill incidents. Roadway A is characterized by its complex transportation network intertwined with the natural environment. The challenge with this type of area is that bobcats may stray into areas of high roadkill risk because the boundaries between the road, habitat, and natural environment are not obvious. Railroads may become their walking paths because typically there is sparse vegetation on both sides of the railroad, and animals may use these linear structures as a pathway for movement [
90]. For such sections, for both sides of the railroad and roadway, physical separation, such as higher fences, can be added to prevent bobcats from straying into the area. And planting more native vegetation to increase cover to reduce the tendency of bobcats to cross railroads.
Section B is the area where roadkill incidents occur most frequently (
Figure 3). It bisects a region highly suitable for bobcats, separating the reserve from a water source on the other side. There is a direct positive correlation between the risk of road mortality for carnivores and the presence of water bodies [
97]. The proximity to water sources (Section 2-2 in
Figure 4) may lead bobcats to prefer these areas as crossing points, being both drinking spots and hunting grounds, and increasing potential conflicts with human activities. For such fragmented and divided habitats, in addition to establishing wildlife crossings, it is also advisable to set up wildlife monitoring systems near these critical water sources, intervening when necessary. Through spatial overlay analysis of factors affecting bobcat survival and habitat suitability, we recognize the importance of establishing wildlife corridors along highway edges near highly suitable habitats. Further assessments can utilize spatial capture–recapture models to estimate bobcat densities in urban environments [
74]. This will help to precisely identify bobcat activity hotspots. Moreover, studies like Reed et al. [
11] show that combining expert experience models with empirical models simulating landscape connectivity can effectively identify crucial ecological corridors for medium-sized carnivores like bobcats. Data collected on wildlife activity and roadkill statistics are crucial in determining optimal locations for road crossing structures. Research indicates that wildlife underpasses not built based on such data have significantly lower usage efficiency compared to those planned and constructed with relevant data [
98]. Section B is the area where roadkill occurs most frequently. Section B divides the area of high bobcat suitability and separates the reserve from the water source on the other side. The proximity to water sources may cause bobcats to prefer crossing at watering points and hunting areas. In addition, these areas may also be places where human activities are more concentrated, which increases the likelihood of bobcat conflicts with human. For such sections where suitable habitat is divided and fragmented, in addition to establishing animal passages, wildlife monitoring systems can be set up near these critical water sources and humans intervene when necessary.
Section C is characterized by its integration of rail and car lanes, adjacent to a nature reserve (Section 3-3 in
Figure 4). The fences here somewhat delineate the boundaries between wildlife and human activities. However, bobcats and other wildlife adapt their behaviors to avoid human activities and habitat fragmentation [
99]. The partial fencing along one side of Section C may not prevent bobcats from crossing to the other side of the road. Studies indicate a higher incidence of roadkill at fence ends than in fenced or unfenced sections, suggesting fences should be continuous or sufficiently long to encourage the use of crossing structures rather than movement around fence ends [
100]. Besides barriers, wildlife underpasses or highway crossings should be added to ensure bobcat safety. In addition to passages and fencing, the habitat variables related to forests, crucial for bobcats as emphasized by Woolf et al. [
73], should be considered. When designing and constructing such ecological infrastructure, efforts should be made to simulate and preserve key features of the bobcat’s natural habitat, like vegetation type and density, and prey availability. Overpasses and underpasses should be surrounded by ample vegetation resembling the adjacent habitat, avoiding elements that could startle or hinder wildlife [
101]. In Section C, considering the impact of large man-made structures like billboards and streetlights on bobcat use of crossing facilities should be included in the design of ecological infrastructure. Moreover, given the proximity to a nature reserve, competition and interactions with other feline species may affect bobcat distribution [
102]. In planning reserves and wildlife corridors, this competition should be accounted for by planning resources such as food, water, and shelter to minimize direct competition between species for the same resources. Section C is characterized by its blend of railway and motorway and its proximity to the nature reserve. Fencing here demarcates the boundary between wildlife and human activity to some extent; however, bobcats and other wildlife adjust their behaviors to avoid human activity and habitat fragmentation [
99]. Fencing around Section C that only partially encloses an area on one side may not prevent bobcats from abandoning their crossings to the other side of the road. Further measures, such as wildlife crossings or bridges, may be needed subsequently to ensure the safety of bobcats.
Section D, with its expansive meadow landscapes and significant topographical elevation differences (Section 4-4 in
Figure 4), provides bobcats with open vistas and abundant hunting grounds. This might also make them more prone to approaching human activity areas, such as nearby golf courses. While some studies suggest golf courses in urban landscapes can serve as refuges for wildlife, offering various habitats [
103,
104], associated factors like roads fragmenting habitats, extensive grasslands, and human activity presence may increase road mortality risks for species like bobcats. Previous research indicates that bobcat collision areas are characterized by smaller, fewer habitat patches, and larger, more isolated grassland patches [
105]. In a simulation study on Deer–Vehicle Collisions (DVCs), researchers found that reducing speed limits and roadside clearings are powerful mitigation tools to decrease DVC numbers [
106]. Although there is no similar simulation study on bobcat roadkill probability and vehicle speed, setting specific road signs and locally reducing speed on fast cross-city highways could be an effective and cost-efficient solution to mitigate roadkill incidents.
In conclusion, to reduce roadkill incidents, establishing crossing zones, such as wildlife underpasses or overpasses, along with barriers to prevent animal crossings, is a feasible approach [
107]. Furthermore, studies have shown that carnivores prefer large, open overpasses [
36,
108,
109]. Therefore, we recommend incorporating fences and wildlife passages into highway upgrade plans. Installing barriers along highways near reserves and other highly suitable habitats can reduce the likelihood of wildlife like bobcats entering roadways. Additionally, wildlife bridges can help bobcats cross, minimizing fragmentation of suitable habitats on either side of the highway.
In the practical implementation of these conservation measures, it is essential to tailor the design and placement of wildlife crossings and barriers based on the specific behavioral patterns and habitat requirements of bobcats. This entails conducting detailed studies of bobcat movement patterns, preferred habitats, and road-crossing behaviors. Such data can inform the strategic placement of wildlife overpasses and underpasses, ensuring these structures are located at key points where bobcats are most likely to cross roads. Additionally, the design of these crossings should mimic the natural environment to encourage usage by bobcats [
110] incorporating elements like native vegetation and ensuring an appropriate scale and layout. Barrier installations along roadways should be carefully planned to minimize habitat fragmentation while effectively deterring bobcats from entering high-risk road areas [
111]. Collaborative efforts with local authorities and communities are also vital in implementing traffic calming measures, such as speed limit reductions in areas with high wildlife activity, to further mitigate the risk of roadkill [
35]. In summary, a comprehensive and data-driven approach, considering the unique ecological characteristics of bobcats, is crucial to effectively implement these measures and enhance the safety and connectivity of bobcat habitats.