An ecological survey of the invertebrate community at the epigean/hypogean interface

An ecological survey of the invertebrate community at the epigean/hypogean interface 27 Subterranean A peer-reviewed open-access journal Subterranean Biology 24: 27–52 (2017) doi: 10.3897/subtbiol.24.21585 RESEARCH ARTICLE http://subtbiol.pensoft.net Biology Published by The International Society for Subterranean Biology An ecological survey of the invertebrate community at the epigean/hypogean interface Stefano Mammola1, Elena Piano1, Pier Mauro Giachino2, Marco Isaia1 1 Department of Life Sciences and Systems Biology, University of Torino, Via Accademia Albertina, 13-10123 Torino, Italy 2 Settore Fitosanitario Regionale, Environment Park, Palazzina A2, Via Livorno, 60-10144 Torino, Italy Corresponding author: Stefano Mammola (); Marco Isaia () Academic editor: O. Moldovan | Received 11 October 2017 | Accepted 27 November 2017 | Published 11 December 2017 http://zoobank.org/899734E8-2028-4815-B3E7-316D251E47E9 Citation: Mammola S, Piano E, Giachino PM, Isaia M (2017) An ecological survey of the invertebrate community at the epigean/hypogean interface. Subterranean Biology 24: 27–52. https://doi.org/10.3897/subtbiol.24.21585 Abstract We studied the ecological continuum between caves and the associated network of fissures – Milieu Souterrain Superficiel (MSS) – in an hypogean site in the Graian Alps, Italy. Over one year, we surveyed the faunal assemblages by means of pitfall traps placed in the caves and specific subterranean sampling devices (SSD) buried in the MSS. We used generalized linear mixed models (GLMMs) and generalized additive mixed models (GAMMs) to compare the spatial and temporal dynamics of the subterranean invertebrates inhabiting the two environments. As expected, arthropod communities occurring near the surface were characterized by minor level of subterranean adaptations, and conversely, subterranean species were more abundant and diversified at higher depths, both in the caves and in the MSS. Diversity and abundance of external elements in the superficial layers were found to be highly seasonal dependent, with minor values in winter compared to the other seasons. We provided information about the faunal assemblages dwelling in the two hypogean compartments, and we characterized the microclimatic conditions therein. We discussed the existence of an ecological gradient of specialization extending from the surface to the deep hypogean layers, which can be interpreted in light of the microclimatic changes occurring at increasing depths and the parallel decrease in available organic matter. Keywords Mesovoid Shallow Substratum, Cave fauna, Superficial Subterranean Habitats, Subterranean biology, Subterranean Sampling Device, Ecological gradient, Troglobionts Copyright Stefano Mammola et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 28 Stefano Mammola et al. / Subterranean Biology 24: 27–52 (2017) Introduction According to the modern view of subterranean biology, subterranean organisms do not exclusively inhabit underground vacuums of wide dimensions (i.e. caves), but also naturally occupy the network of fissures the size of which is not commensurable to the human scale (Racovitza 1907, Jeannel 1926, 1942, 1943, Juberthie et al. 1980a, 1980b, 1981, Uéno 1987). Culver and Pipan (2009a, 2014) recently categorised the most superficial subterranean habitats colonized by a strictly subterranean fauna as “Shallow Subterranean Habitats (SSHs)”. Among the variety of SSHs listed by Culver and Pipan, the “Milieu Souterrain Superficiel (MSS)” is possibly one of the most inventively studied (Mammola et al. 2016). As a general definition, the MSS consists of a labyrinth of air-filled voids within rocky fragments that have accumulated for various morphogenetic reason on the bedrock, harbouring organisms showing adaptation to the subterranean conditions. In a hypothetical multilayer-structure, the MSS is generally found between the edaphic area – soil and rhizosphere – and the deep hypogean domain resulting, as a whole, in a “gradient from soil to cave” (Gers 1998). The climatic isolation of the MSS from the surface is usually achieved when the rocky layers are progressively covered by evolving soils with edaphic horizons (Giachino and Vailati 2010, Pipan et al. 2011), but other insulation mechanisms may be involved (Mammola et al. 2016). As a general rule, the external climatic variations are buffered in the MSS according to increasing depth (Nitzu et al. 2010, 2014), i.e. approaching a cave-like climate at higher distances from the surface (Badino 2010, Mammola et al. 2016). Since MSS is not accessible to men unless by indirect means (see, e.g., López and Oromí 2010), biological studies focusing on the subterranean fauna are usually set in caves or in more accessible SSHs, such as lava tubes (e.g., Howarth 1972, Aschmole and Ashmole 1997, Arnedo et al. 2007). As a matter of fact, the great majority of the published papers investigating the MSS dealt with single model taxa, and were usually strictly taxonomical (Mammola et al. 2016). The focus of the MSS studies was rarely set on the whole community and/or on ecological processes (but see, e.g., Gers 1998, Nitzu et al. 2010, Pipan et al. 2011, Rendoš et al. 2012, Ortuño et al. 2013, 2014, Langourov et al. 2014, Jimenéz-Valverde et al. 2015). As an example, all the studies conducted so far in Italy focused on taxonomic descriptions of new species, sampled in the MSS by means of buried pitfall traps or sight-collected after excavation (e.g. Monguzzi 1982, 2011, Casale and Rondolini 1983, Casale and Giachino 1988, Vailati 1988, Latella and Rampini 1994, Monguzzi and Regalin 2001, Giachino and Vailati 2008, Magrini et al. 2012, Monzini 2013). We conducted a one-year ecological study in an alpine hypogean site, aiming at investigating simultaneously the cave environment and the surrounding MSS. Our aims were to 1) compare the faunal assemblages characteristic of the two subterranean compartments; 2) investigate whether a temporal (seasonal) and/or a spatial gradient of specialization exists in the MSS – i.e. higher richness and abundance of specialized elements at increasing depth; 3) investigate whether the same gradients exist in the cave – i.e. variation in richness and abundance related to season and/or to vertical distance from the surface (subjacency) or to distance from the cave entrance. An ecological survey of the invertebrate community at the epigean/hypogean interface 29 Materials and methods Study area The study was set in the Pugnetto hypogean complex, in the nearby to the hamlet of Pugnetto, municipality of Mezzenile, Lanzo Valley, Graian Alps, Piedmont (NW-Italy). The site is protected under the European Habitat Directive 43/92 (S.C.I. IT 1110048) and hosts five natural caves classified (...truncated)