Urbanization: Impact on local fungal biodiversity

Urbanization represents one of the most significant anthropogenic transformations of the landscape, with profound and lasting effects on natural ecosystems. While public attention often focuses on the consequences for flora and larger fauna, fungal communities, essential components of terrestrial ecosystems, undergo equally important but less visible alterations. This article provides an in-depth exploration of how urbanization processes influence local fungal biodiversity, analyzing changes in species composition, the reduction of fungal richness, and the ecological implications of these transformations. Through scientific data, case studies, and statistical analyses, we will seek to understand the complex dynamics governing fungal communities in urban and peri-urban environments.

Fungi play fundamental ecological roles: as decomposers they recycle essential nutrients, as mycorrhizal symbionts they facilitate water and mineral absorption for plants, and as pathogens they regulate plant populations. Their sensitivity to environmental alterations makes them excellent bioindicators of ecosystem health. Understanding how urbanization modifies these communities is therefore not just a matter of mycological interest, but has broader implications for the management of urban ecosystems and biodiversity conservation in anthropized contexts.

In this study, we will examine the different components of urbanization that influence fungi: habitat fragmentation, soil and air pollution, microclimate alterations, the introduction of alien species, and the modification of substrate availability. We will also analyze how different types of urban green spaces (parks, private gardens, flower beds, cemeteries) can host distinct fungal communities, and discuss strategies to promote greater fungal diversity in urban environments.

Urbanization: Definition and Metrics 

Before delving into the impact on the fungal ecosystem, it is necessary to precisely define what we mean by urbanization and how it is measured. Urbanization is not simply the replacement of natural areas with buildings, but a complex process that includes population densification, infrastructure construction, soil sealing, and the modification of hydrological regimes. Different metrics are used to quantify urbanization, each of which can have distinct relationships with fungal biodiversity.

Quantitative Metrics of Urbanization

Quantifying the degree of urbanization is fundamental to studying its relationships with fungal biodiversity. Researchers use various metrics, including:

These metrics are not mutually exclusive and are often combined into composite indices to better capture the complexity of the urban phenomenon. For example, the urbanization index developed by McDonnell and Hahs (2008) combines different variables to classify areas along a continuum from rural to urban. It is important to note that different metrics can have different relationships with different fungal groups: while saprotrophic fungi might respond primarily to the availability of organic substrate, mycorrhizal fungi might be more sensitive to the presence and health of host plants.

Urban-Rural Gradients

A common approach in urban ecology studies is the concept of the urban-rural gradient, which considers the spatial transition from the city center to peri-urban areas and finally to rural areas. This approach allows studying how biological communities gradually change in response to urbanization, rather than making simple dichotomous comparisons between "urban" and "natural".

Along these gradients, complex patterns in fungal diversity are observed. Some studies have found a general reduction in species richness towards urban centers, while others have documented peaks of diversity in peri-urban areas, where ecotonal conditions can favor both species adapted to disturbed environments and species more typical of natural environments. The response of fungi to urbanization also depends heavily on the functional group considered: arbuscular mycorrhizal fungi tend to show more marked decreases in diversity compared to saprotrophic fungi, which can sometimes benefit from the greater availability of organic substrates of anthropogenic origin.

A crucial aspect is that urbanization does not simply create a "worse" environment for fungi, but rather a different environment, which selects for species with specific characteristics. Urban fungi tend to have more ruderal life strategies (rapid growth, abundant spore production, broad ecological tolerance) compared to their relatives in natural environments. This ecological differentiation has important implications for the functioning of urban ecosystems and the ecosystem services they can provide.

Impact of Urbanization on Mycorrhizal Fungi

Mycorrhizal fungi establish symbiotic relationships with the roots of most terrestrial plants, facilitating the absorption of nutrients and water in exchange for carbohydrates. These symbioses are fundamental for the health of plant ecosystems, but are particularly sensitive to environmental alterations associated with urbanization. In this section, we will examine in detail how different aspects of urbanization influence these important fungal communities.

Alterations of the Host Plant Community

Urbanization profoundly modifies the composition of plant communities, with cascading effects on associated mycorrhizal fungi. Urban areas are characterized by a strong dominance of non-native plant species, structural simplification of communities (fewer vegetation layers), and lower connectivity between plant populations. These alterations affect mycorrhizal fungi in various ways:

A particularly illuminating study conducted in Berlin demonstrated that the species richness of arbuscular mycorrhizal fungi (AMF) progressively decreased from peri-urban to city center, with a reduction of up to 40% in the most urbanized areas. This decline was closely correlated with the decrease in plant diversity and the increase in impervious soil cover. The loss of specialized mycorrhizal fungi is particularly concerning because these organisms perform ecological functions that cannot be easily replaced by generalist species.

The consequences of these alterations go beyond simple loss of diversity. The reduction in the abundance and diversity of mycorrhizal fungi can compromise the health of urban plants, making them more susceptible to water stress and nutritional deficiencies. This in turn can increase the need for anthropogenic interventions (irrigation, fertilization) to maintain urban vegetation, creating a vicious cycle that further degrades soil microbial communities.

Soil Pollution and Its Effects

Urban soils are subject to different forms of pollution that directly influence mycorrhizal fungi. Heavy metals (lead, zinc, copper), persistent organic compounds (polycyclic aromatic hydrocarbons, PAHs) and de-icing salt can accumulate in urban soils, reaching concentrations toxic to many soil organisms, including mycorrhizal fungi.

The response of mycorrhizal fungi to pollution is complex and species-specific. Some studies have documented a general reduction in abundance and diversity of mycorrhizal fungi in contaminated soils, while others have observed the emergence of tolerant strains, which in some cases can even facilitate the phytoremediation of contaminated sites. Tolerance to heavy metals, for example, varies considerably between different species and even between different strains of the same mycorrhizal fungal species.

In addition to chemical pollutants, urban soils are often characterized by physical alterations that affect mycorrhizal fungi. Soil compaction, resulting from trampling and the use of heavy machinery, reduces soil porosity and aeration, creating unfavorable conditions for the growth of fungal hyphae. At the same time, soil sealing drastically reduces the available habitat for fungi and disrupts the continuity of underground mycelial networks.

Saprotrophic Fungi in Urban Environments

Saprotrophic fungi play the essential role of decomposing dead organic matter, recycling nutrients in ecosystems. In urban environments, these fungi face conditions profoundly modified compared to natural ecosystems, with different substrates, microclimatic alterations and new sources of disturbance. In this section we will explore how urbanization affects these important decomposers.

Changes in Substrate Availability and Quality

One of the most evident impacts of urbanization on saprotrophic fungi is the alteration of the quantity, quality and diversity of substrates available for decomposition. While in natural ecosystems saprotrophic fungi evolve to decompose specific types of wood, leaves and other plant materials, in urban environments they must face a narrower but also more variable range of substrates, many of anthropogenic origin.

The management of urban organic waste has important implications for saprotrophic fungi. In many cities, the practice of quickly removing fallen leaves and dead wood deprives saprotrophic fungi of their natural substrates, favoring species capable of growing on alternative materials or completing their life cycle very quickly. On the other hand, the presence of compost and leaf piles in parks and gardens can create favorable microhabitats for some saprotrophic fungi, although these environments are often characterized by simplified fungal communities dominated by few generalist species.

An interesting aspect is the appearance of saprotrophic fungi on materials of anthropogenic origin. Species such as Schizophyllum commune and various members of the genus Trichoderma have been observed growing on treated wood, cardboard and even on some types of plastic, demonstrating remarkable metabolic plasticity. This ability to adapt to new substrates could be a key characteristic for the survival of fungi in rapidly evolving urban environments.

Urban Microclimate and Its Influence

The urban heat island effect creates distinctive microclimatic conditions that profoundly influence the ecology of saprotrophic fungi. The higher temperatures in urban areas, especially at night, can prolong the fungal growing season and accelerate decomposition rates. However, these warmer conditions can also select for thermotolerant species, potentially at the expense of species adapted to cooler temperatures.

In addition to temperatures, relative humidity is often reduced in urban environments due to lower evapotranspiration and the greater presence of impermeable surfaces that reduce water retention. This reduced humidity can limit the activity of many saprotrophic fungi, which require relatively high humidity conditions for hyphal growth and fruiting. Consequently, urban fungi often show adaptations to cope with water stress conditions, such as increased production of protective pigments or the ability to quickly enter quiescent states when conditions become unfavorable.

Urban microclimatic alterations interact with other factors in complex ways. For example, the irrigation of urban parks can create microhabitats with high humidity despite the generally drier urban context, allowing the survival of species with high humidity requirements. Similarly, the presence of buildings can create very pronounced microclimatic gradients, with significantly different conditions between the north and south side of the same building, offering distinct ecological niches for different fungal communities.

For further insights into urban fungal communities and their interactions with microclimate, we recommend consulting the website of the Italian Botanical Society, which contains numerous scientific publications on the subject.

Fungal Biodiversity in Urban Green Spaces

Parks, gardens and other urban green areas represent important refuges for biodiversity in anthropized contexts. These spaces not only provide crucial ecosystem services for human well-being, but can also support surprisingly diversified fungal communities. In this section we will examine how different types of urban green spaces influence fungal biodiversity and discuss strategies to maximize their value for fungal conservation.

Types of Urban Green Spaces and Their Fungal Communities

Not all urban green spaces are equal from the point of view of fungal biodiversity. Various factors, including the age of the park, the variety of plant species, management practices and the degree of isolation from other green areas, influence the composition and diversity of fungal communities. Research has shown that larger and older urban parks tend to host more diverse fungal communities, probably because they offer a wider range of microhabitats and have had more time to be colonized by less mobile species.

A particularly interesting aspect is the potential of urban cemeteries as reservoirs of fungal biodiversity. Several studies have documented a surprising richness of species in these environments, which often combine elements of moderate disturbance (favoring adapted species) with some stability over time (allowing the establishment of more specialized species). Furthermore, older cemeteries may contain mature trees that provide critical habitat for fungi associated with decaying wood, a microhabitat often scarce in highly managed urban environments.

The connectivity between different urban green areas is another crucial factor for fungal biodiversity. Fungi, despite their ability to produce wind-dispersed spores, often show distribution patterns that suggest dispersal limitations in urban environments. The presence of green corridors connecting different green areas can facilitate the exchange of fungal propagules between otherwise isolated populations, helping to maintain genetic diversity and allowing the recolonization of areas where local populations have gone extinct.

Strategies to Promote Urban Fungal Biodiversity

Given the ecological importance of fungi, it is desirable to incorporate mycological considerations into the design and management of urban green spaces. Various strategies can be implemented to promote more diverse and functionally effective fungal communities:

Maintenance of Dead Wood and Plant Debris: contrary to the common practice of quickly removing dead wood and fallen leaves, maintaining these materials in situ provides critical substrates for many saprotrophic fungi and creates microhabitats for other organisms. Even small piles of dead wood placed strategically can significantly increase fungal diversity in an urban park.

Diversification of Plant Species: since many fungi show host specificity, planting a variety of native plant species can support a wider range of mycorrhizal fungi. In particular, the inclusion of tree species that form ectotrophic mycorrhizal associations (such as oaks, beeches and pines) can significantly enrich urban fungal communities.

Reduction of Fungicide and Other Pesticide Use: many antifungal products used in urban settings have non-selective effects, suppressing both plant pathogens and beneficial fungi. The adoption of integrated management approaches that minimize the use of these products can favor more balanced and diverse fungal communities.

Creation of Diverse Microhabitats: designing green spaces that include a variety of light conditions, humidity and soil type can support a wider range of fungal species. Even small interventions, such as creating small depressions that collect rainwater or arranging stones that create humidity gradients, can increase the diversity of ecological niches available for fungi.

Case Studies and Recent Research

Research on the impact of urbanization on fungal biodiversity is a rapidly evolving field, with increasingly sophisticated studies revealing complex and sometimes counterintuitive patterns. In this section we will present some significant case studies that illustrate different dimensions of this relationship, as well as discuss the most promising directions for future research.

Case Study: The Urban-Rural Gradient of Milan

A particularly in-depth study conducted in the metropolitan area of Milan examined fungal communities along a gradient ranging from the city center to the Parco Agricolo Sud Milano. The researchers sampled fungi at 40 sites representative of different degrees of urbanization, using both traditional morphological methods and molecular metabarcoding techniques.

The results revealed a significant decline in the species richness of mycorrhizal fungi with increasing urbanization, with an average reduction of 35% in the city center compared to peri-urban areas. In contrast, the species richness of saprotrophic fungi showed a more complex pattern, with a peak in moderately urbanized areas, where the combination of natural and anthropogenic substrates created favorable conditions for a wide range of species.

A particularly interesting result emerged from the analysis of fungal community composition. While rural areas were dominated by specialized fungi with restricted geographical distributions, urban areas were characterized by more homogeneous communities composed mainly of generalist species with broad geographical distributions. This pattern of "biotic homogenization" has been observed in other organismal groups in response to urbanization, but this study was among the first to clearly document it for fungi.

The study also identified specific biological indicators of different degrees of urbanization. For example, the presence of Amanita rubescens and Boletus edulis was strongly associated with low urbanization areas, while species such as Coprinellus micaceus and Schizophyllum commune were more abundant in urban areas. These indicators could be useful for monitoring the health of urban ecosystems and assessing the effectiveness of conservation strategies.

Emerging Research: Urban Fungi as Bioindicators of Environmental Quality

A particularly promising line of research concerns the use of fungi as bioindicators of environmental quality in urban ecosystems. Several studies have demonstrated that fungal communities respond in a sensitive and predictable manner to different forms of environmental stress, including air pollution, soil contamination and alterations of the hydrological regime.

For example, research conducted in various European cities has documented significant correlations between the composition of fungal communities and the concentrations of specific air pollutants. In particular, the relative abundance of some fungal groups (such as corticoid fungi) seems to be a sensitive indicator of tropospheric ozone concentrations, a gaseous pollutant with harmful effects for both human health and ecosystems.

Similarly, the ability of some fungi to accumulate heavy metals and other contaminants makes them potential tools for biomonitoring soil contamination. Studies have shown that species such as Agaricus arvensis and Macrolepiota procera can accumulate significant concentrations of cadmium, lead and mercury in their fruiting bodies, providing an integrated indication of the bioavailability of these contaminants in the soil.

These applications of fungi as bioindicators are particularly interesting in urban environments, where conventional environmental quality monitoring can be expensive and logistically complex. The development of standardized protocols for sampling and analyzing urban fungal communities could therefore contribute significantly to the environmental management of cities.

Urbanization: Conservation Strategies and Future Perspectives

In light of scientific evidence demonstrating the significant impact of urbanization on fungal biodiversity, it becomes crucial to develop effective and integrated conservation strategies. In this final section, we will explore practical approaches to protect and promote fungal diversity in urban environments, as well as discuss future directions for research and management in this emerging field.

Integrated Approaches for Urban Fungal Conservation

The conservation of fungal biodiversity in urban environments requires a multidisciplinary approach that integrates knowledge of ecology, urban planning, soil science and, of course, mycology. Several guiding principles can inform the development of effective conservation strategies:

Incorporate Mycological Considerations into Urban Planning: fungi and their ecosystem services should be explicitly considered in urban planning and development processes. This includes protecting areas with particularly diverse fungal communities, designing ecological corridors that facilitate fungal dispersal, and integrating fungal habitats (such as dead wood piles) into the design of green spaces.

Promote Habitat Connectivity: habitat fragmentation is one of the main threats to urban fungal biodiversity. Strategies to increase connectivity include creating green corridors connecting parks and other green areas, incorporating green elements into grey infrastructure (e.g., green roofs, living walls), and designing ecological networks at the landscape scale.

Manage Urban Soils as Living Ecosystems: urban soils are often treated as inert substrates, but in reality they host complex microbial communities that include fungi essential for ecosystem functioning. Soil management practices that preserve their health include minimizing compaction, reducing the use of synthetic pesticides and fertilizers, and incorporating organic matter to support soil food webs.

Engage Citizens in Fungal Conservation: citizen science can play an important role in monitoring urban fungal biodiversity and promoting public awareness of the ecological importance of fungi. Programs that encourage citizens to document fungal observations in their urban environments can generate valuable data for research while building a base of public support for conservation.

Future Research Perspectives

Despite significant progress in recent years, many fundamental questions regarding urban fungal ecology remain poorly understood. Some of the most promising directions for future research include:

Long-term Studies on Urban Fungal Community Dynamics: most existing studies provide snapshots of fungal communities at a specific time, but few have examined how these communities change over time in response to factors such as climate change, the evolution of urban management practices and demographic fluctuations.

Investigations into the Ecological Functions of Urban Fungi: while we know that the composition of fungal communities changes with urbanization, we understand less how these alterations affect ecosystem processes such as decomposition, nutrient cycling and plant-soil relationships. Studies that explicitly link fungal diversity to ecosystem functionality are particularly needed.

Exploration of the Potential of Urban Fungi for Providing Ecosystem Services: beyond their traditional ecological roles, urban fungi could provide specific services for anthropized environments, such as bioremediation of contaminated soils, improvement of ornamental plant health, and perhaps even regulation of the urban microclimate through effects on evapotranspiration.

Applications of Emerging Technologies: advances in DNA sequencing, mass spectrometry and remote sensing offer unprecedented opportunities to study urban fungal communities at spatial and temporal resolutions previously impossible. Integrating these technological approaches with traditional ecological methods promises to revolutionize our understanding of urban fungal ecology.

In conclusion, while urbanization represents a significant challenge for the conservation of fungal biodiversity, it also offers unique opportunities to study fungal ecology in new and dynamic conditions. Understanding how fungi respond and adapt to urban environments not only enriches our fundamental knowledge of these fascinating organisms, but also provides scientific foundations for the design of more sustainable and resilient cities. The challenge for researchers, planners and citizens is to work together to create urban environments that support not only human well-being, but also the diversity and functioning of the fungal ecosystems on which we depend.