Introduction

Human car accidents due to collisions with animals and road mortality are main causes of human–nature conflicts and may lead to fewer green urban environments being planned1. In the context of traffic, human-wildlife conflicts can be defined as encounters between humans and wildlife, resulting in negative outcomes for both humans and their resources, and wildlife and their habitats2. Generally, roads and railways are recognised as linear landscape structures that negatively impact several species3. For example, tens of millions of birds are killed annually owing to collisions with automobiles in the US4, billions of pollinating insects are killed per annum across North America5, and hundreds of ungulates are subject to yearly railroad collisions6. However, it has been demonstrated that linear landscape structures, such as roads and railways, can positively benefit some species by providing foraging and nesting possibilities or pose migratory routes7,8, as well as benefit non-native species9. Associated linear landscape structures such as power lines or fences, can be used by insects as nesting spots10,11, by plants to climb up structures12, and by birds as perches for hunting activities, singing and displaying, or simply for resting13,14. Moreover, some bird species use railways to clean feathers during sand-bathing or collect grit as a source of calcium and as gastroliths13. However, the impact of transportation on the environment, especially biodiversity, is limited to roads and some aspects of railways15,16.

However, in urban areas of many regions of the world, trams are a very popular type of public transport, sometimes in historical contexts recognised as streetcars in the USA or modern light rail transit/light railway vehicles17. Tramways are intensively developed, and some aspects such as availability of stops points, design, speed, and low impact on the environment are especially important to passengers and inhabitants18. Tramway networks in cities have infrastructure similar to roads, such as paved surfaces, drainage, bridges, poles, or stops, and may provide similar pitfalls and opportunities for wildlife across different continents, countries, or cities19. Unfortunately, little is known about how these elements affect urban populations of animals and plants. Trams, a form of mass public transport, have been suggested environmentally friendly. However, surprisingly, tramways have not been studied in detail3,20, 21. Analysis of the potential influence of this type of public transport on wildlife seems to be important, especially in light of the resurrection and extension of tramway networks in many European cities in the last decade18 and significant new investments, such as NextGenerationEU (https://ec.europa.eu/info/strategy/recovery-plan-europe_en). On the other hand, since species may affect transport safety through collisions22, as well as by other means, for example, metal corrosion caused by excreta23, knowledge on how wildlife may use the transport infrastructure is important not only for biodiversity but also for ecosystem services, for example human safety purposes, cooling effects, and water retention. Thus, monetary methods for quantifying the non-market benefits from greening tramways and estimating increases in their ecosystem service values should be applied for sustainable decision-making in urban areas24,25.

The renaissance of trams originated in France, where a school for designing modern tram systems was developed. In addition to an innovative approach to urban issues (e.g. tram routes), the French School of Design is characterised by the widespread use of green tracks26. However, as historical photographs, films, and postcards show, green tracks have existed since the second decade of the twentieth century. The first city to turn green tracks into a symbol of a modern means of transport to protect the environment, and as a symbol of the city at all, was Freiburg im Breisgau. In 1978, a new line to Landwasser was opened. The new investment was characterised by several innovations, including green tracks27. Originally, green tracks were expected to improve aesthetics, reduce noise spread, and cool the urban heat island, in comparison to traditional tracks on ballast. With the popularity of green tracks, different types (grass track, sedum track, high- or low-level, etc.) and construction techniques have been distinguished28. Accordingly, green tracks are increasingly becoming a subject of local public policies, e.g. Urban Heat Island Strategy City of Vienna29; a subject of research projects, e.g. German-wide Grüngleisnetzwerk28; or a part of an Urbact project “RiConnect—Rethinking infrastructure” (https://urbact.eu/networks/riconnect).

It is likely that interest in public transport, after the temporary stagnation resulting from the COVID-19 lockdown30, will systematically increase. Therefore, it is important to prepare for a long-lasting debate on its importance, possibilities, limitations, and environmental impact in a changing world31. Importantly, management schemes based on policy-focused analysis should be ready for business and government administrations for world rebuilding after large-scale disturbances such as pandemics or climate change. Recently, ideas such as land-sharing and land-sparing have been woven into urban ecology, aiming to harmoniously blend green spaces with economic activities in cities32,33. Land-sharing advocates gently interspersing urban development with green elements, such as trees, grass, or small parks, nestled among structures. Green tramways may be a good example of land sharing policy in urban development using linear landscape structures, reducing human–nature conflict by combining active management and using the area for public transportation in urban areas, providing biodiversity, and benefiting human wellbeing34.

Therefore, there were two main aims of our study: (1) to summarise the state-of-the-art ways in which tramways and surrounding infrastructure affect biodiversity underlying commonness in urban landscapes worldwide, as well as the importance of trams for societies; and (2) to analyse potential gaps in the knowledge of the importance of trams for biodiversity, including wildlife. To realise the above purposes, in this study, we collected and classified available information on the main effects of trams and associated infrastructure on biodiversity. We hope to provide useful records for ecologists, road planners and other stakeholders engaged in conservation and urban planning.

Methods

Systematic review

A search of the relevant peer-reviewed literature was conducted using the Web of Science and Scopus databases on 26 January 2023. A set of keywords was used in the following search string: (tramway* OR trams OR “tram* track*” OR streetcar* OR “light rail transit” OR “light railway vehicles” OR lrt OR lrv) AND (*diversity OR wildlife OR vegetation* OR flora OR fauna OR richness OR disturbance OR birds OR mammals OR amphibians OR reptiles OR insects ) AND NOT (“Tram Chim”). The search was limited to the subject areas of Environmental Science, Agricultural and Biological Sciences in Scopus and was refined by the following Web of Science categories: Environmental Sciences, Ecology, Environmental Studies, Multidisciplinary Sciences, Plant Sciences, Evolutionary Biology, Biodiversity Conservation, Engineering Environmental, Biology, Horticulture, Ornithology, Zoology. Using the above search method, we identified 108 articles from Scopus and 82 from the Web of Science. After removing duplicates, 136 unique entries were considered for abstract screening. Based on the title, abstract, and keywords, we screened in Rayyan QCRI (https://rayyan.qcri.org/). We also included one additional record identified by a backward search of the previously included studies. We found only eight empirical studies published between 2013 and 2022 that investigated the impact of tramways on wildlife (Table 1). The remaining 128 papers subjected to screening were irrelevant regardless of the usage of specified keywords in the search strings.

Table 1 Studies included in this review.
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Global interest

We used Web of Science database (https://www.webofscience.com/wos/woscc/basic-search) to assess the changes in tramway studies published over time. In March 2023, a literature search was conducted using the Web of Science Core Collection for papers published between 1950 and 2022 that included the term “tramway” in their titles.

To assess public interest in trams as transport over time, we used the Google Trends database (https://trends.google.com/trends/). Google Trends is a public web facility provided by Google Inc. that measures how often a particular search item is entered into Google Search browsers relative to the total search volume. The trends provided by this tool estimate changes in searches for an item or phrase and are often used to examine temporal changes in socio-economic studies35. The search for the term “tram” was performed on 7.03.2023 and the region was set to World. To avoid biases, we set the “travels” filter, allowing searches to only find travel-related items when searching for trams as transport, thereby avoiding searches for other purposes.

Trams all over the World

The lengths of the tramways and light rail transit networks of cities, as well as the populations of cities, were obtained from Wikipedia. Wikipedia is a multilingual, free online encyclopaedia written and maintained by a community of volunteers (https://en.wikipedia.org/wiki/Wikipedia). Cities listed in the Wikipedia page “List of tram and light rail transit systems” (https://en.wikipedia.org/wiki/List_of_tram_and_light_rail_transit_systems) along with information about their tramway network (in English) were included in the database. In most cases, the length of a city tramway network is referred to as the length of the lines, routes, systems, or tracks. The data include networks which provide actual transit services (including heritage trams and streetcars), not those that are presently under construction or are qualified as metro networks. Networks in Russia and Turkey, including those in the European regions, are listed for convenience under Asia.

Data visualization

All visualizations were performed with R36, package ggplot237.

Results

Systematic review

Although tramways are frequent elements of many urban landscapes in the EU and other countries, their contribution to city biodiversity has not been thoroughly studied (Table 1). We found eight empirical studies published between 2013 and 2022 that investigated the impact of tramways on wildlife (Table 1). It is already recognized that tramways result in the development of a mosaic habitat covered by many plant species, including both spontaneous flora and cultivated plants38,39,40,41 (Table 1). The floral composition profile of infrastructure associated with tram communication is similar to that of the flora of industrial or urban habitats41. The recorded plant species were dominated by native species, but the tramways were also sources of alien and invasive taxa. However, the potential risk of plant invasion differs among the tramway infrastructure types. Rendeková et al.40 revealed that green tramways are habitats with fewer spontaneously growing alien taxa, and their frequency of occurrence is lower than that on conventional tracks. In the case of conventional tracks, alien species occurred directly in the rail yard more frequently than those growing at greater distances from the tracks39. Despite the abovementioned risks at these sites, tramways can be a valuable refuge for endemic and endangered species worthy of conservation action38,39. Moreover, city wildlife seem to use tramways as attractive habitats for food foraging, resting, or moving along. However, to the best of our knowledge, the value of tramways for animals has only been studied for birds and newts (Table 1). Szala et al.42. showed that the tramway infrastructure is used by 11 bird species, particularly corvids and pigeons. In winter, the abundance of hooded crows was positively influenced by tramway length. These habitats may constitute valuable foraging areas, especially during severe winter42. In contrast, some birds, such as silver gulls (Chroicocephalus novaehollandiae), avoid areas with high disturbance from traffic, trams, and overhead wires, despite the high availability of food at these sites43. Furthermore, tramways are important terrestrial habitats for smooth newts in late autumn and winter (Lissotriton vulgaris21). The rail aggregate provides a large number of shelters and cavities, thus reducing predation risk, and providing a prey-rich, humid habitat. Moreover, dense tramway networks may encourage more people to use tramways instead of cars, further reducing animal road mortality and pollution.

Global interest

Among the 335 studies containing the term “tramways” in their titles published between 1955 and 2022 (Fig. 1), the Web of Science Categories matched were: Transportation Science Technology (19%), Engineering Civil (13%) and Engineering Electrical Electronics (13%). The number of publications increased over time with a peak observed in the year 2017.

Figure 1
figure 1

The total number of published papers with the term “tramway” in a title. The literature search was conducted using Web of Science.

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Moreover, the trend of the Internet search for the term “tram” increased from the year 2005, but significantly decreased during the COVID-19 pandemic (Fig. 2).

Figure 2
figure 2

The blue line represents Annual Google Trend searches for the term “tram”. The red line marks the beginning of the COVID-19 pandemic, when public transport was restricted. The search was conducted across the entire world and the “travels” filter was set.

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Trams all over the World

The median value of the tramway network length was highest for European cities (33 km) and lowest for South American cities (12 km; Fig. 3). The tramway network lengths for Africa, Asia, North America and Oceania ranged between 23 and 20 km (Fig. 3).

Figure 3
figure 3

Length of tramway and light rail transit network of cities in six continents. Colour of points correspond with the referred length of a city’s tramway lines, routes, systems or tracks. Circles are arranged according to population size of a city (in thousands). The median length is presented as values in boxes. Jittering was added to aid visualization.

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Discussion

Significant efforts have been made to develop of protection plans to recover or sustain the current level of biodiversity and ecosystem services in urban areas44. Interventions in urban landscapes encouraging landowners to properly design gardens or create wildlife sanctuaries have been devised with the hope that wildlife will survive45,46. However, this approach to conserving species diversity faces many practical problems. The effectiveness of wildlife sanctuaries in an urban landscape depends on where they are implemented, the genus or order of the plants and animals being targeted, and the landscape structure31,44. Sanctuaries may be located in areas isolated from other semi-natural habitats and might play a minor role as a source habitat47. Many solutions for wildlife in urban areas are costly, and hence may be limited to the local scale or well-developed countries48.

A supplementary or alternative solution to the above-mentioned methods is to take advantage of the unrecognised benefits of artificial or novel habitats for wildlife49, according to the land-sharing concept33. Such novel habitats, usually associated with industrial or infrastructural development, may have high conservation value. For example, it has been shown that limestone quarries50, road verges51, former open-surface coal mines52, landfills53, sandpits54, gravel-pits55, gardens45, railway embankments56, levees57, or green roofs46 may be refuges for pollinator populations. Moreover, linear landscape structures, such as railways or levees, may act as corridors for insects that are highly affected by human landscape8. Thus, habitats created by human activities may significantly mitigate the negative effects of industry and agriculture58. Tramways are common landscape features worldwide (Fig. 3) which increase their potential value for biodiversity conservation and restoration of ecosystem services. Additionally, steadily increasing the interest of society and scientists in trams (Figs. 1,2), e.g. in an era of transportation rethinking, may lead to favourable conditions for implementing new ideas of bringing biodiversity back to cities59.

Having recognised the positive aspects of tramways for wildlife, one should also be aware of the possible threats to biodiversity brought about by tramways60. Tram traffic can cause animal mortality, and thus lower population abundance. However, there is no strong evidence suggesting that tram traffic kills many animals. Surprisingly, for both birds and newts, tramway infrastructure does not seem to be dangerous, and it is not an additional source of mortality, in contrast to roads21,61, perhaps because of the average speeds of trams and cars. Tram traffic, even in the busiest lines, is much lower than the traffic volume on roads, thus, it is reasonable to assume that mortality is probably lower than that on roads. Tramway transport can also be a serious source of different kind of pollutions62,63 which may negatively impact wildlife. Pollution also includes non-selective herbicides used to maintain tracks64 which, in turn, may negatively affect insect populations65, e.g. by lowering native flowering plant cover. Although tramway verges may act as functional biological corridors8, these may also pose a barrier for wildlife. Movements between habitat patches may also be diminished by tramways that “filter” individuals who are unwilling to move further when they encounter the tracks66. However, this indicates that the potential role of tramway infrastructure is even greater and that applying alternative methods of vegetation management may increase the positive role of this habitat (Fig. 4). Additionally, moving trams may also be a source of noise pollution; however, modern tracks and trams typically exhibit reduced noise emissions.

Figure 4
figure 4

Exemplary greening of tram infrastructure.

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Management recommendations

Tramway systems have evolved with technological advancements. Analysing public, political, and scientific debates, one of the key directions of technical development is the use of trams moving without overhead traction (Figs. 5, 6). This is primarily due to the protection of historic areas. There is also an interesting discussion in Munich regarding the use of catenary-free tramways to protect natural and landscape values in the planned northern tramway rings. “Tram-Nordtangente”, is planned to be a 2200 m long double-track line. Approximately 800 m will be located in the English Garden [German: Englischer Garten], one of the most famous parks in the city. This is the most controversial aspect of new investments. The primary consideration is to reduce the environmental impact of a new tramway67,68. The removal of overhead lines and equipment from cities was considered in two contexts. The first is the ground-level power supply, which, as mentioned above (Figs. 5,6), reduces the area of the biologically active surface, making it difficult to maintain because it divides the area between the rails into two narrow strips of greenery approximately 60 cm wide. The construction of l'alimentation électrique par le sol also requires protection against flooding. The second consideration is the development of battery or supercapacitor systems or the use of hydrogen fuel cells. Although the application of ground-level power supply or battery (supercapacitor) run trams is a common practice worldwide, hydrogen fuel cell trams remain a topic of research and development, e.g. the H2-Tram Project in Germany69,70.

Figure 5
figure 5

Space for greenery in tracks of guided buses, typical tramways and tramways supplied with electric power by the conductor rail built into the track (l'alimentation électrique par le sol).

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Figure 6
figure 6

Types of location of tracks in the urban space.

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It should be mentioned that there were many concerns about the development of green tracks at the turn of the 1980s and the 1990s, when low-floor (in various cities) or very low-floor (e.g. Vienna, Oradea) trams were introduced. However, this has not prevented the development of green tracks.

For sustainable tramway development, measurement of monetary losses of ecosystem services when a tram line is built, or ecosystem services gained after greening the existing lines should be utilized71. Methods of nonmarket valuation, such as the Biotope Valuation Method and Energy-Water-Vegetation Method can show the range of environmental values of nature. This includes assessing the societal costs of restoring landscape quality to its real ability to replace the core supporting and regulating services of ecosystems, such as climatising services, water-retention services, oxygen production, and habitats for biodiversity72. Thus, the estimated values for ecosystem services per unit length of green tram tracks should be incorporated into decision-making in urban landscapes.

Future studies

There has been a significant increase in the number of studies on tramway engineering (Fig. 1), social interest in trams as transport networks (Fig. 2) and urban management plans that consider greening tramways28. This is not surprising as tramway features are common in urban landscapes on all continents (Fig. 3). However, there is an urgent need to consider the effects, advantages, and disadvantages, of tram transport on biodiversity and ecosystem services in cities. Specifically, we need to understand the potential role of tramway infrastructure in:

- creating new habitats for biodiversity of rare/key group species,

- improving ecological processes, such as migration and primary production,

- increasing the economic valuation of ecosystem services (cooling—climate change, pollination, water retention, and aesthetic values),

- assessing and preventing the mortality of animals through engineering solutions,

- incorporating tramways into urban development strategies, that is, land sharing versus land sparing.