Aliens: otherworldly mushrooms found in Italy

Aliens: introduction to the concept of alien species in mycology

Before delving into the specific analysis of species, it is essential to understand what is meant by "alien species" in the mycological field. This concept, borrowed from ecology, refers to organisms introduced, voluntarily or accidentally, outside their historical range of distribution. Their presence in a new territory can represent a threat to local biodiversity, altering established ecological balances.

Definition and classification criteria for alien fungi

A fungus is classified as alien when its presence in a given territory is the result of a direct or indirect introduction by humans. The criteria for this classification include the absence of historical documentation of the species in the area, association with human introduction vectors (such as the trade of plants or the movement of soils), and the tendency to form self-sustaining populations.

The distinction between alien and native species is not always clear-cut, especially in a paleogeographic context where continents are constantly moving, but "aliens" generally refer to those species whose spread has been significantly accelerated by human activities in recent centuries.

Introduction pathways and modes of spread in Italy

The introduction pathways of alien fungi into Italy are multiple and often linked to human commercial and recreational activities. Among the main ones are:

• Import of ornamental and fruit plants: the global trade of plants, with their root systems and associated soil, represents one of the most significant vectors for the introduction of mycelium and spores of alien fungi.
• Trade of timber and wood products: untreated timber can host a wide range of wood-decaying fungi, some of which can establish themselves in new forests.
• Tourist and recreational activities: even hiking boots, camping equipment, or off-road vehicles can unintentionally transport spores from one region of the world to another.

Once introduced, these fungi spread through wind, water, insects, or the movement of animals, exploiting favorable environmental conditions to colonize new territories.

Morphological analysis of the main alien species in Italy

Identifying alien fungi requires an in-depth knowledge of their morphology. Macroscopic and microscopic characteristics are fundamental for distinguishing these species from native ones and for understanding their adaptation strategies.

Macroscopic characteristics: cap, stem, hymenium, and flesh

Macroscopic characteristics are the first step in recognizing a fungus. For alien species, it is possible to observe particular adaptations that have favored their success in new environments.

Let's take as an example Amanita phalloides var. alba, a variant that in some Italian regions shows characteristics slightly different from native populations. Its cap, initially hemispherical then convex-flattened, can present shades ranging from pure white to cream, a characteristic that distinguishes it from the more common green-olive form. The stem is slender, with a well-evident sac-like volva and a large, membranous ring. The hymenium consists of dense, free gills, white in color. The flesh, white and unchanging when cut, emits an initially sweetish odor that becomes unpleasant with aging. These characteristics, although similar to the type species, present significant variations that can mislead less experienced foragers.

Microscopic characteristics: spores, hyphae, and cystidia

Microscopic analysis is crucial for confirming the identity of an alien species and for studying its phylogenetic relationships. Spores, in particular, are diagnostic elements of primary importance.

To continue with the example of Amanita phalloides var. alba, its spores are spherical or sub-spherical in shape, hyaline (transparent) in water, and amyloid, meaning they stain blue-black with Melzer's reagent. Spore dimensions fall within a range of 8-11 µm in diameter. The hyphae of the cap and stem are of the interwoven type, with cells of variable sizes. Cystidia, if present, are rare and have no particular distinctive characteristics. This microscopic profile, when compared with that of North American or Asian populations of the same species, can reveal subtle genetic and adaptive differences, testifying to a separate evolutionary path.

Habitat and geographic distribution of alien fungi in Italy

The distribution of alien fungi in Italy is not uniform but tends to concentrate in those areas subject to greater human impact or with particularly favorable climatic conditions. Understanding their preferred habitats is essential for predicting their expansion and assessing their ecological risk.

Analysis of the most colonized ecosystems

Alien fungi show a marked preference for certain types of ecosystems, often those already modified by humans or particularly vulnerable.

• Urban parks and gardens: these environments, characterized by a high density of ornamental plants introduced from different parts of the world and by soils often compacted and rich in nutrients, represent fertile ground for the establishment of alien species. The lack of natural competitors and the stress conditions of the host plants favor their proliferation.
• Timber arboriculture plantations: monocultures of poplars, eucalyptus, or non-native conifers can host specific pathogenic fungi, introduced along with the plants themselves. These fungi can then make a "host jump" and attack related native plants.
• Ruderal areas and roadsides: these marginal environments, subject to continuous disturbances, offer opportunities for pioneer and opportunistic species, including many alien saprotrophic fungi that feed on organic debris.

Regional distribution maps and influential environmental factors

The distribution of alien fungi can be visualized through maps indicating their presence at a regional or provincial level. These maps often reveal a correlation with specific environmental factors.

For example, the presence of Clathrus archeri, a fungus of Australian origin known as the "octopus fungus" for its unmistakable appearance, was initially reported in the northern regions of Italy, particularly Piedmont, Lombardy, and Veneto, later expanding towards the center. Its spread is favored by mild and humid climates and the presence of humus rich in organic matter. The following table illustrates a possible regional distribution for some alien species:

Biological and ecological properties of alien fungi

The biological properties of alien fungi are the basis of their invasive success. Understanding their life cycle, trophic strategies, and interactions with other species is fundamental for informed management.

Life cycle and reproductive strategies

Alien fungi often possess flexible life cycles and highly efficient reproductive strategies that facilitate colonization. Many produce an enormous quantity of spores, which can remain viable in the soil for years, waiting for favorable conditions.

Others, like some pathogenic fungi, have developed the ability to reproduce both sexually and asexually, increasing the chances of success in new and variable environments. The rapid maturation of fruiting bodies and the resistance of spores to UV rays and dehydration are further adaptations that favor long-distance dispersal.

Interactions with native flora and fauna

The introduction of an alien fungus can profoundly alter existing food webs. Some species establish mycorrhizal symbiotic relationships with native plants, modifying the plants' nutrient absorption and drought resistance.

Others, like parasitic fungi, can decimate entire populations of plants or animals that have not developed effective defense mechanisms. An emblematic example is the pathogenic fungus Batrachochytrium dendrobatidis, responsible for the global decline of amphibians, whose spread has been facilitated by the international animal trade.

Environmental impact and considerations for conservation

The presence of alien fungi in Italy is not a phenomenon to be underestimated, as it can have far-reaching consequences for biodiversity and the functioning of ecosystems. Assessing their impact and defining management strategies is a priority for nature conservation.

Competition with native species and alteration of ecosystems

Alien fungi compete with native species for limited resources such as space, nutrients, and host plants. This competition can lead to the reduction of native populations or, in the most severe cases, to their local extinction. Furthermore, by altering the composition of soil microbial communities, alien fungi can influence decomposition processes and nutrient cycling, with cascading effects on the entire food chain. The simplification of ecosystems and the loss of ecological functionality are among the most significant risks associated with biological invasions of mycological origin.

Monitoring, containment, and management strategies

Controlling the spread of alien fungi requires an integrated approach, based on continuous monitoring, containment of existing populations, and prevention of new introductions.

• Monitoring: the creation of citizen science networks, involving enthusiasts and amateur mycologists in reporting suspicious species, can be an effective tool for tracking the expansion of alien species. The use of remote sensing technologies and environmental DNA (eDNA) analysis is also becoming increasingly important.
• Containment: once an outbreak is identified, it is possible to intervene with physical containment methods, such as the manual removal of fruiting bodies to reduce spore production, or the modification of environmental conditions to make them less favorable for the alien species.
• Prevention: the most effective measure remains prevention, which involves controlling introduction pathways, regulating the trade of plants and soils, and raising public awareness about the risks associated with the unintentional transport of spores.