The touristic use of caves causes multiple environmental alterations to the subterranean ecosystem, having potential effects on all components, from the atmosphere to lithosphere, hydrosphere, and biosphere. Setting a baseline on the current knowledge of the ecological status of world show caves is pivotal to implement monitoring and management programs aiming at their conservation. However, information on this topic is scattered throughout several publications, making it difficult to access data and ultimately delaying advances towards a sustainable touristic use of show caves. We provide a literature-based dataset relative to the knowledge on the ecological status of 265 show caves worldwide. Data were collated from 289 papers selected through a systematic literature survey of an initial set of more than 1,000 scientific papers. We made the compiled information available through two complementary datasets, reporting: (i) references of the selected papers and (ii) 44 fields relative to the main characteristics of show caves investigated in literature. These fields encompass information about geographic locations, cave general characteristics, natural heritage, and the specific environmental components—and related environmental parameters—investigated in each of the considered study. Such a dataset improves our accessibility to the basic information provided by literature on the ecological status of show caves, also pointing out some literature gaps that should be addressed by future research. By making these data freely available and re-usable, we hope to stimulate research in the field of cave tourism, cave conservation, and cave-based ecology.

Cave heritage, environmental components, environmental parameters, subterranean ecosystems, systematic search

Cave ecosystems are nutrient-deprived environments characterized by the absence of light, spatial confinement, climatic stability, and low biodiversity (Culver and Pipan 2019). Given such peculiar environmental conditions, caves are extremely susceptible to both natural and anthropogenic disturbance and subjected to multiple environmental pressures (Mammola et al. 2019). Among other threats, disturbance posed by cave tourism is gaining international attention in show cave managements (Cigna 2016) as well as remarkable traction in scientific literature (Mammola 2019). The use of caves by humans has long been central in history. If in the past we mostly used caves as shelters, places for rituals, or food storage (Mulec 2014), in later centuries several caves have been converted into touristic attractions—the so-called “show caves” defined as “any cavity where a fee is paid to gain access and visit it” (Cigna and Forti 2013). In show caves, paying visitors experience the natural beauty of caves via constructed trails, guided tours, artificial lighting systems, and regular opening hours (Cigna and Burri 2000). These infrastructural and environmental alterations, along with the presence of visitors themselves, threaten the integrity of the cave ecosystem across all its environmental components—i.e. atmosphere, lithosphere, hydrosphere, and biosphere.

For example, the presence of visitors may have direct impacts on the cave temperature (both in the air and in the water; Šebela and Turk 2014). Temperature increases cause “thermal waves” when caves are visited by high numbers of people which can persist even on subsequent days (Dominguez-Villar et al. 2010). These perturbations to the subterranean microclimate, together with the associated increase of CO₂ air concentration caused by tourists’ breath, may enhance carbonate dissolution, damaging geological formations, i.e. speleothems (Sanchez-Moral et al. 1999; Martin-García et al. 2010, 2011).

Moreover, tourists visiting the cave carry dust pollutants and propagules of microorganisms through their clothes and hands, which can be found on speleothems (e.g. Saiz-Jimenez et al. 2012; Iliev et al. 2018), in the water (e.g. Ando and Murakami 2020; Moldovan et al. 2020), in the air (e.g. Porca et al. 2011; Martin-Sanchez et al. 2014), and in the sediments on the ground (e.g. Mammola et al. 2017; Kukla et al. 2018). Another human-driven pressure that may enhance the growth of alien microorganisms in show caves is the presence of high input of organic matter in both the sediments (Marques et al. 2016, 2017) and in the water (Jiménez-Sánchez et al. 2008). In parallel, the presence of artificial lighting often drives massive growths of photosynthetic microorganisms, such as cyanobacteria, diatoms, and green algae (Falasco et al. 2015; Piano et al. 2015, 2021), but also bryophytes, ferns, and vascular plants (Castello 2014). All these organisms constitute the so-called ‘lampenflora’ (Mulec et al. 2008), causing aesthetic damage due to the formation of extensive patinas on speleothems and paleo-archaeological heritage (Saiz-Jimenez et al. 2012). Furthermore, the environmental instability generated by tourists (e.g. increasing temperature, high organic matter inputs, trampling) and artificial lights may negatively affect the subterranean invertebrate fauna (Isaia et al. 2011; Pellegrini and Ferreira 2012, 2016; Alonso et al. 2019; but see Faille et al. 2014; Nicolosi et al. 2021), but also bats reproducing or overwintering in caves, which may be disturbed by both visitors’ noise and artificial lighting (Mann et al. 2002; Cardiff et al. 2012; Ivanova 2017).

The sustainable touristic use of show caves represents an emerging topic in the frame of the conservation of subterranean ecosystems—also in light of the Sustainability Goals proposed by the United Nations (UN 2015). A publicly available systematic collection of the natural heritage of show caves worldwide and information about the status of the different environment components, i.e. atmosphere, lithosphere, hydrosphere and biosphere, in show caves is therefore necessary to set a base for the implementation of scientifically sound environmental monitoring programs. This could facilitate cave managers to measure the impact of tourism in show caves.

To mitigate this knowledge gap, we here provide a data paper based on systematic research in which we describe a database that builds on scientific and peer-reviewed literature. Our aims are: i) to quantify available information on the ecological status of show caves worldwide; and ii) to identify knowledge gaps in the study of ecological impacts in show caves globally. Our database includes a georeferenced set of show caves where researchers have evaluated variations of a certain environmental indicator used to monitor the impact of tourism and its related activities on the subterranean ecosystem. For each show cave, we provide information on the main cave’s characteristics, including its natural heritage, and the environmental components investigated therein. This database could, therefore, represent a basis to evaluate, support, and promote the study of the impacts caused by tourism on these fragile ecosystems in light of their conservation.

Our survey encompassed 265 show caves located in 39 countries. Most show caves are located in the Western Palearctic region (Fig. 2a), given the overrepresentation of European countries, namely Spain, Czech Republic, and France (Fig. 2b). The most studied show cave is Postojna cave in Slovenia, which is also recognized as the most important global subterranean biodiversity hotspot (Zagmajster et al. 2021), followed by two Spanish show caves, i.e. Nerja Cave and Castañar de Ibor Cave, and by Lascaux Cave in France (Fig. 2c). Although they are currently closed to tourism, Lascaux and Altamira caves are among the most studied show caves, pointing out that tourism-induced environmental changes may perdure much longer after eliminating the pressures exerted by tourism. Most studies were focused on a single study site, while only in a few cases, more than one show cave was investigated (Fig. 2d).

Figure 2.

A number of examined caves and relative percentages for the different biogeographic regions B list of countries where most of the studied show caves are located with their frequencies and relative percentages C list of the show caves most frequently studied in literature and their relative frequency and percentage in the considered papers D number of studied show caves and relative percentages within the examined papers. Percentages are calculated with respect to the total number of show caves in A, B, and with respect to the total number of papers in C, D.

Concerning environmental components (Fig. 3a) and their related parameters (Fig. 4a), most papers examined a single component and a single parameter. Similarly, in most of the caves, a single component and a single parameter were examined throughout literature. These results highlight the need to create collaborations among researchers of different disciplines to implement multidisciplinary research studies encompassing different environmental components and parameters. The atmosphere was the most studied component, followed by the biosphere and the lithosphere, while the hydrosphere was the least investigated (Fig. 3b). When considering the investigated parameters (Fig. 4b), an overwhelming interest towards factors potentially affecting human health in subterranean environments emerged. In this regard, the greatest part of the studies dedicated to the atmosphere focused on radon—mostly evaluated to quantify its potential negative effect on the human health and not as a proxy of the anthropogenic impact on show caves (e.g. Lario et al. 2006; Lu et al. 2009; Cevik et al. 2011)—, followed by the microclimate and CO₂ concentration; air particulate was the least investigated atmospheric parameter. Similarly, most of the available literature dealing with the biosphere focuses on microorganisms (fungi and prokaryotes), possibly due to their potential repercussions on human health, followed by the ‘lampenflora’ and fauna (invertebrates and bats). Regarding lithosphere, most of the papers focused on the crystalline structure of speleothems and, to a lesser extent, of paleo-archaeological findings, while the sediment composition was the less examined parameter. Papers dealing with the hydrosphere mostly focused on the water geochemistry, followed by water level and the water quality.

Figure 3.

A number of studied components and relative percentages within examined papers (left panel) and studied show caves (right panel) B frequency and relative percentage of papers (left panel) and show caves (right panel) examining the different environmental components.

Figure 4.

A number of studied parameters and relative percentages within examined papers (left panel) and studied show caves (right panel) B frequency and relative percentage of papers (left panel) and show caves (right panel) examining the different parameters.

According to the temporal trend of examined components (Fig. 5), studies on atmosphere and biosphere faced a huge increase in the last decade (from 2011 to 2022), probably thanks to the advent of advanced molecular techniques, e.g. Next-Generation Sequencing for the microbial component of the biosphere (e.g. Pfendler et al. 2018; Alonso et al. 2019), while the number of papers dealing with hydrosphere and lithosphere showed a less pronounced increase across decades.

Figure 5.

Temporal trend in the study of the different environmental components across considered publications.

Based on the data overview provided by our database, future research in the field of tourism sustainability in show caves should be dedicated to:

• the implementation of interdisciplinary studies encompassing multiple environmental components and parameters as well as multiple show caves;
• the development of our knowledge on show caves located outside the Western Palearctic Region;
• the increase of available information with reference to the lithosphere and especially of the hydrosphere, which are extremely underrepresented components in the current literature;
• the increase of our awareness on the possible impacts of tourism on the subterranean fauna and microorganisms and their interactions with other global impacts (e.g. climate change and land use change) in order to promote the conservation of subterranean biodiversity (Bontemps et al. 2021; Mammola et al. 2022).

In addition, information presented in this data paper could also be used to perform studies that:

• investigate which factors attract tourists in show caves in order to highlight which features may expose them to a touristic overexploitation;
• provide management guidelines that allow a sustainable touristic development of show caves not only from an environmental, but also from an economic and social point of view (Buonincontri et al. 2021).

Overall, this data paper could fill the lack of awareness towards the fragility of the natural heritage of show caves to favour a sustainable touristic use that would guarantee their preservation for future generations as well as the economic development of local communities (Chiarini et al. 2022).

This work was realized within the framework of the PRIN SHOWCAVE “A multidisciplinary research project to study, classify and mitigate the environmental impact in tourist caves” - code 2017HTXT2R (PI: Marco Isaia), funded by the Italian Ministry of Education, University and Research. SM is supported by the European Commission (program H2020-MSCA-IF-2019; grant award: 882221). The grant of EP is co-financed by the PON “Research and Innovation” Programme (Axis IV “Education and Research for recovery” – Action IV.6 “Research contracts on Green themes”).

EP, SM, GN and MI set the lines of enquiry and designed the study. AP, BB, EC, EP, GN, NM and VB collected the data. EP and GN summarized the data. EP led the writing of the paper. MI and SM critically reviewed the first draft of the paper. All authors provided important improvements to the original text.