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.