1. INTRODUCTION
Anthropogenic disturbance on natural ecosystems is growing in frequency and magnitude worldwide, with consequent negative repercussions on biodiversity (Ceballos et al., 2015; Pereira et al., 2012; Pimm et al., 2014; Wagner et al., 2021). However, disentangling the underlying mechanisms that determine changes in biotic communities at the local scale is often hampered by habitat-dependent confounding effects (Cardinale et al., 2018; Gonzalez et al., 2016; McGill et al., 2015; Vellend et al., 2017). Notably, these limitations are evident in microbial communities due to rapid evolution (Niehus et al., 2015), high taxonomic richness (Shoemaker et al., 2017), functional redundancy (Curtis & Sloan, 2004), and dormancy (Locey et al., 2020). Insights into the main processesdetermine changes in local communities following external perturbations may be obtained by examining variation in species richness and composition in a metacommunity ecology perspective (Jurburg et al., 2021; Kinnunen et al., 2016). Metacommunity theory emphasises how species richness of a community can be explained by two coexisting limiting forces, summaried into two main paradigms, the island and the trait-environment paradigm. The former states that number of species in a community depends on dispersal limitation, imposed by both geographical barriers and species dispersal capacity (MacArthur & Wilson, 1967); the latter identifies niche assembly rules as the major force in selecting species, emphasising environmental filters based on species morphological, physiological and behavioural traits (Cody et al., 1975). Despite its potential capacity of disentangling the main selective mechanisms of biotic communities under human disturbance (Leibold & Chase, 2017), this framework has been rarely applied microbial communities (Jurburg et al., 2021).
Being characterised by highly predictable gradients in their environmental conditions, simplified trophic webs, climatic stability, and spatial confinement (Culver & Pipan, 2019; Poulson & White, 1969), subterranean ecosystems represent ideal ecological laboratories in this regard (Mammola, 2019). Among the main threats affecting subterranean ecosystems (Mammola et al., 2022), the ever-increasing conversion of natural caves into tourist attractions, i.e. the so-called ‘show caves’, imposes significant ecological pressure on the subterranean ecosystem (Cigna, 2016). Although this phenomenon provides both an economic and social opportunity to local development (Allan et al., 2015; Kim et al., 2008; Shavanddasht et al., 2017), the presence of visitors throughout the year significantly impacts the subterranean environment. Well-documented effects on abiotic component of the subterranean ecosystems include changes in microclimate (e.g. Šebela et al., 2015, 2019; Addesso et al., 2022a), carbon dioxide concentration (e.g. Lang et al., 2015, 2017; Addesso et al., 2022a), and rock composition (e.g. Addesso et al., 2019), with cascade effects on the subterranean fauna (e.g. Nicolosi et al., 2021; Pacheco et al., 2021) and energy fluxes (e.g. Addesso et al., 2022b; Fernandez-Cortes et al., 2011), also induced by the proliferation of alien photosynthetic microorganisms caused by the installation of artificial light at different intensities (e.g. Havlena et al., 2021; Piano et al., 2015) and duration (e.g. Borderie et al., 2014; Piano et al., 2021). In addition, evidence in literature indicates tourists as vehicles of microbial species alien to the cave, which represent a source of biological pollution for the cave air (Martin-Sanchez et al., 2014; Porca et al., 2011), water (Ando & Murakami, 2020; Moldovan et al., 2020), soil (Kukla et al., 2018; Mammola et al., 2017) and speleothems (Bercea et al., 2019). Also, their outbreaka can cause significant biochemical and biophysical degradation (Saiz-Jimenez et al., 2012) and perturbations of resident communities by potentially competing with, and even overriding, autochthonous species (Alonso et al., 2019; Griffin et al., 2014). However, the consequences of the touristic pressure on microbial communities in subterranean ecosystems have never been tested within a metacommunity framework .
With the aid of advanced molecular techniques, we examined changes in species richness and composition of three microbial components that naturally inhabit cave sediments, i.e. Fungi, Bacteria and Archaea, in four Italian show caves, facing anthropogenic disturbance. By adopting a replicated factorial design, we tested: i) to what extent species diversity and composition of microbial communities is determined by dispersal-driven mechanisms and/or environmental filtering; and ii) whether tourists’ disturbance influences local microbial communities. We hypothesised that: i) human-driven mechanisms represent the main selective force in microbial communities; and ii) propagules vehiculated by tourists would cause a replacement of the original species, thus emphasising the turnover process.