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

Vertical distribution of spiders (Araneae) in Central European shallow subterranean habitats

Shallow subterranean habitats are among the last habitats in Central Europe to be arachnologically researched. Using stratified pipe traps, we studied the vertical distribution of spiders in soil and interspaces in bedrock (shallow subterranean habitats). Specifically, we sampled fauna in different substrates, including limestone, sandy marlstone, sandy marl, claystone, loess, and artificial gravel accumulation. Employing stratified pipe traps allowed us to identify the depth at which particular species occurred. Across multiple years and sampling sites, we collected 76 spider species, 21 of which showed an affinity for subterranean microhabitats. Some of these species occurred in interspaces in soil and bedrock, whereas others have been previously found in subterranean ant nests and animal burrows. We collected five species (Iberina microphthalma, Centromerus cf. piccolo, Porrhomma cambridgei, P. microcavense, and P. microps) almost exclusively at depths over half a meter, suggesting the strong affinity of these species for a subterranean lifestyle. We provide diagrams of these species’ vertical distribution and photo-document eye reduction. Our study demonstrates that poorly studied shallow subterranean habitats harbor diverse subterranean spider fauna, including several previously considered rare species in Central Europe.

Species conservation profiles of the endemic spiders Troglohyphantes (Araneae, Linyphiidae) from the Alps and the north-western Dinarides

Background

The genus Troglohyphantes Joseph, 1882 (Araneae, Linyphiidae) includes 131 species, mainly distributed across the main European mountain ranges. The Alps and the north-western Dinarides account for 66 species, most of them showing narrow or even point-like distributions. The majority of Troglohyphantes spiders dwell in subterranean habitats including caves, mines, soil litter, rocky debris and other moist and shaded retreats. Despite being intensively studied from taxonomic, ecological and biogeographic standpoints, knowledge on the status of conservation and on the potential risk of extinction of these spiders is lagging. To date, only three species have been included in the global IUCN Red List, but their status has not been updated ever since their last assessment in 1996. The aim of this contribution is to assess the Alpine and north-western Dinaric species of the genus Troglohyphantes and to re-assess the species previously evaluated, according to the last version of the IUCN Red List Categories and Criteria.

New information

Amongst the 66 species here considered, 62 had sufficient data to allow the quantification of their Extent Of Occurrence (EOO) and Area Of Occupancy (AOO). Most of the species have a narrow distribution range, with an estimated EOO < 20,000 km² and AOO < 2,000 km², meeting the thresholds for the inclusion in the threatened categories. Five species have a more widespread distribution (EOO > 20,000 km²), extending across multiple countries. The quality of the data on distribution of four species was not sufficient to provide a reliable estimation of the distribution range.A continuing decline in EOO, AOO and habitat quality was inferred for 30 species. The majority of them were subterranean specialised species, with a reduced thermal tolerance and a low dispersal ability. Accordingly, changes in subterranean microclimatic conditions due to climate change represent a major threat for these species. Land-use change and habitat alteration were identified as additional relevant threats for several species.A considerable proportion of the species here assessed was found in protected areas and in sites of the Natura 2000 network. In addition, 14 species are formally protected by national and sub-national legislation. At present, 25 species are listed in the regional Red Lists.Long-term monitoring programmes, management plans for both the species and their habitats, expansion of the extant protected areas and designation of new ones, should be considered as the most effective approaches to species conservation.

Recent speciation and phenotypic plasticity within a parthenogenetic lineage of Levantine whip spiders (Chelicerata: Amblypygi: Charinidae)

Caves constitute ideal study systems for investigating adaptation and speciation, as the abiotic conditions shared by aphotic habitats exert a set of environmental filters on their communities. Arachnids constitute an important component of many cave ecosystems worldwide. We investigated the population genomics of two whip spider species: Sarax ioanniticus, a widely distributed parthenogenetic species found across the eastern Mediterranean; and S. israelensis, a recently described troglomorphic species that is endemic to caves in Israel. Here, we show that S. israelensis is completely genetically distinct from S. ioanniticus and most likely also constitutes a parthenogen. Counterintuitively, despite the lack of genetic variability within S. ioanniticus and S. israelensis, we discovered considerable variation in the degree of median eye reduction, particularly in the latter species. Natural history data from captive-bred specimens of S. israelensis validated the interpretation of parthenogenesis. Our results are most consistent with a scenario of a sexual ancestral species that underwent speciation, followed by independent transitions to apomictic parthenogenesis in each of the two daughter species. Moreover, the lack of genetic variability suggests that variation in eye morphology in S. israelensis is driven exclusively by epigenetic mechanisms.

Climatic stability, not average habitat temperature, determines thermal tolerance of subterranean beetles

The climatic variability hypothesis predicts the evolution of species with wide thermal tolerance ranges in environments with variable temperatures, and the evolution of thermal specialists in thermally stable environments. In caves, the extent of spatial and temporal thermal variability experienced by taxa decreases with their degree of specialization to deep subterranean habitats. We use Phylogenetic Generalized Least Squares to model the relationship between thermal tolerance (upper lethal limits), subterranean specialization (estimated using ecomorphological traits) and habitat temperature in sixteen beetle species of the tribe Leptodirini (Leiodidae). We found a significant, negative relationship between thermal tolerance and the degree of subterranean specialization. Conversely, habitat temperature had only a marginal effect on lethal limits. In agreement with the climatic variability hypothesis and under a climate change context, we show that the specialization process to live in deep subterranean habitats involves a reduction of upper lethal limits, but not an adjustment to habitat temperature. Thermal variability seems to exert a higher evolutionary pressure than mean habitat temperature to configure the thermal niche of subterranean species. Our results provide novel insights on thermal physiology of species with poor dispersal capabilities and on the evolutionary process of adaptation to subterranean environments. We further emphasize that the pathways determining vulnerability of subterranean species to climate change greatly depend on the degree of specialization to deep subterranean environments.

Don’t forget the vertical dimension: assessment of distributional dynamics of cave-dwelling invertebrates in both ground and parietal microhabitats

Biological studies on factors shaping underground communities are poor, especially those considering simultaneously organisms with different degrees of adaptation to cave life. In this study, we assessed the annual dynamics and use of both horizontal and vertical microhabitats of a whole community with the aim of understanding whether cave-dwelling organisms have a similar distribution among vertical and ground-level microhabitats and to find out which microhabitat features influence such distribution. We monthly assessed from 2017 to 2018, by direct observation combined with quadrat sampling method on the ground and transects on the walls, richness and abundance of 62 cave-dwelling species in a cave of Northern Italy. Environmental factors such as light intensity, temperature, relative humidity and mineralogical composition of the substrates were measured during each monitoring session, influencing the dynamics of the whole community and revealing significant differences between ground and wall microhabitats. A gradient of variation of the species assemblages occurred from the entrance toward inner areas, however, evidence that the dynamics of the walls are very different from those occurring at the ground independent from the distance from the surface are shown. Biodiversity indices highlighted sampling area diversity and a discrete total cave fauna biodiversity with the highest values found near the entrance and the lowest in the inner part of the cave.

A comparison of collecting methods in relation to the diversity of Collembola in scree habitats

We compared the species composition, relative abundances and life form structure of subterranean Collembola (Hexapoda) captured by two different methods along a depth gradient of five forested scree sites in the Western Carpathians, Slovakia: (1) high-gradient extraction of soil samples, and (2) collection using subterranean traps. Our results showed that the soil samples were more efficient in covering species richness at the majority of the sites. The body size of the captured animals depended remarkably on the sampling method. Extraction was more effective in collecting smaller, less active hemi- and euedaphic forms of Collembola, while collection by subterranean traps favoured both motile ground-dwelling as well as relatively large, active euedaphobionts. Additionally, different trends in the vertical stratification of Collembola life forms and their relative abundances were detected by the two methods. Atmobionts and epigeonts, forming the greater part of the communities in traps compared to soil samples, were distributed along the entire scree profiles, but their relative abundance and species numbers had a strongly decreasing trend with depth. Moreover, motile, large hemi- and euedaphic forms had high relative abundances in traps in the middle and deeper scree levels at three sites. In contrast, in soil samples the hemi- and euedaphobionts with small body size were abundant on the surface of the MSS sites. Thus, soil sampling applied before installation of subterranean traps may serve as an appropriate complementary technique to obtain a more complete pattern of Collembola diversity in forested scree habitats.

Subterranean biodiversity and depth distribution of myriapods in forested scree slopes of Central Europe

The shallow underground of rock debris is a unique animal refuge. Nevertheless, the research of this habitat lags far behind the study of caves and soil, due to technical and time-consuming demands. Data on Myriapoda in scree habitat from eleven localities in seven different geomorphological units of the Czech and Slovak Republics were processed. Based on previous studies, as well as knowledge of cave and soil fauna, it was hypothesised that the occurrence of a varied and peculiar fauna would show a pattern of depth distribution with variations due to local specificities. From 2005–2016 (at least one year on each site), macrofauna was collected via sets of three long-term exposed subterranean traps consisting of 110 cm long perforated tube, with ten cups located in a gradient at 5–95 cm below the soil surface. In total, 14 symphylans (not identified to species level), 271 centipedes (23 spp.) and 572 millipedes (32 spp.) were sampled. The overall depth distribution of centipedes and millipedes appeared to have relatively similar pattern, with both groups being found at all depth levels. Nevertheless, this pattern depends on locations. The depth distribution trend lines are mostly in the form of an asymmetric ‘U’, with decreased abundance until the middle of the gradient, followed by increase in the deepest levels. Epigeic species were sporadically distributed along the whole depth gradient, but concentrated at the soil surface, while some subterranean species, such as the centipede Lithobiuslucifugus and the millipedes Geoglomerissubterranea, Cibiniulusslovacus and Archiboreoiuluspallidus, were recorded in the deepest parts of the gradient. This characterises the debris community as a mixture of soil and subterranean species with an absence of species exclusively found in caves. The use of different fixation methods in traps had a significant and selective impact on samples; millipedes were either attracted by ethylene glycol or repelled by formaldehyde. Centipedes were also captured more frequently in ethylene glycol; however, the species composition varied in each of the fixatives. Depth distribution of myriapods was similar in both fixative solutions. Traps with these fixatives could be recommended for similar ecological studies.

Arthropod biodiversity patterns point to the Mesovoid Shallow Substratum (MSS) as a climate refugium

The determinants of biodiversity patterns in the subterranean habitat called Mesovoid Shallow Substratum (MSS) are not well-understood. In this study, thirty-three scree slopes at high altitudes were selected across the Sierra de Guadarrama National Park in central Spain to investigate the effect of ten environmental variables on the abundance and species diversity of the spider and springtail assemblages from the colluvial MSS. In each locality, a multiperforated PVC tube with a pitfall trap inside was buried up to 1 m deep, and generalized linear models and Mantel tests were used to analyze the effect of mainly climate- and habitat-related variables on the diversity patterns of both taxa. A total of 1143 individual spiders belonging to 54 species and 40 811 springtail individuals belonging to 62 species were collected. The analyses indicated that cold temperatures and the presence of forest cover on the surface significantly enhance richness and abundance in the two taxa. Environmental similarity also had a small positive effect on faunistic similarity. However, the effects of temperature and habitat detected on spider richness and abundance were stronger than on springtails, whereas the reverse was found regarding faunistic similarity. These results indicate that subterranean dwellers respond differently to the same environmental factors, which in turn, points to a different degree of affinity for the MSS. The MSS plays an important role in the survival of high mountain arthropod species, acting as a climate refuge, so the protection of this habitat should be prioritized.

Type of fixative solution in pitfall traps as a decisive factor affecting community parameters of Collembola (Hexapoda) inhabiting superficial subterranean habitats

Vertical distribution of collembolan communities along a depth gradient was studied at three forested karst scree slopes in the Western Carpathians, differing in topography, microclimate, soil-chemical parameters, and forest associations. Two different fixative solutions were used in subterranean pitfall traps. The activity and species richness of Collembola in the vertical scree profiles were significantly higher in traps filled with ethylene glycol than in those filled with formaldehyde at all three sites. Using traps filled with formaldehyde, both Collembola species numbers and activity positively correlated with the soil moisture and carbon content, while for ethylene glycol traps this relation was the same only for species numbers and carbon content. Ecological groups of Collembola showed a different response, distinguished by the level of association to subterranean habitats, to environmental factors and to the fixation liquid used in the subterranean traps. Eutroglophiles, the forms relatively well-adapted to subterranean habitats, were associated with depth exclusively when using ethylene glycol traps. This implies that ethylene glycol is a more suitable preservative in terms of species richness and activity of Collembola in deeper scree layers compared to traditionally used formaldehyde. Thus, the effect of environmental factors on diversity and vertical distribution of Collembola in scree habitats may be substantially superimposed by the type of fixation used in subterranean traps.

Ortuño V. The study of hidden habitats sheds light on poorly known taxa: spiders of the Mesovoid Shallow Substratum

The scarce and biased knowledge about the diversity and distribution of Araneae species in the Iberian Peninsula is accentuated in poorly known habitats such as the Mesovoid Shallow Substratum (MSS). The aim of this study was to characterize the spiders inventory of the colluvial MSS of the Sierra de Guadarrama National Park, and to assess the importance of this habitat for the conservation of the taxon. Thirty-three localities were selected across the high peaks of the Guadarrama mountain range and they were sampled for a year using subterranean traps specially designed to capture arthropods in the MSS. Species accumulation curves were built both for the observed species richness and for the non-parametric richness estimators. The literature was reviewed in order to update the distributional maps of the rarest species. Forty-two species were collected, of which four were species new to science. More than half were represented by one or two individuals which caused the accumulation curves to rise slowly and to end without reaching an asymptote. Almost half of the species showed significant increases in their Iberian distribution ranges. Two species were recorded for the first time in the Iberian Peninsula and 32 species were new additions to the spider checklist of the Sierra de Guadarrama National Park.

Day–night and seasonal variations of a subterranean invertebrate community in the twilight zone

Being characterized by the absence of light and a reduced environmental cyclicity, the subterranean domain is generally regarded as temporally stable. Yet, in the proximity of cave entrances (twilight zones), patterns of sunlight and darkness can be detected within the 24-hour day–night cycle. In parallel, changes in the abiotic and biotic conditions are expected; however, these patterns have been rarely explored in animal communities dwelling in the twilight zone. We performed a biological investigation in a small abandoned mine in the Western Alps, monitoring it once per season, both during the day and at night. At each survey, we collected data on the spatial distribution of the resident species, their activity patterns, and the main microclimatic parameters. We observed significant daily variations in the environmental conditions during winter and spring, namely higher temperature, relative humidity and availability of trophic resources at night. In conjunction with these disparate nocturnal conditions, the abundance of troglophile species was also higher, as well as the activity patterns of one of the most frequent species inhabiting the entrance area – the orb-weaver spiderMetamenardi. We further documented temporal changes in the composition of the parietal community, due to species using the mine as a diurnal, nocturnal or overwintering shelter. Overall, our results suggest that the communities of the twilight zone are not temporally stable and we highlight the importance of taking into account not only their seasonal, but also their daily variations.