Ecology and life history of Meta bourneti (Araneae: Tetragnathidae) from Monte Albo (Sardinia, Italy)

Abstract

The orb-weaver spider Meta bourneti Simon 1922 (Araneae: Tetragnathidae) is one of the most common cave predators occurring in the Mediterranean basin. Although the congeneric M. menardi represented the model species in several studies, our knowledge of M. bourneti is only founded on observations performed on a handful of populations. In this study M. bourneti spiders were studied in caves of Monte Albo (Sardinia, Italy) over a year. Generalized Linear Mixed Models were used to analyze spider occupancy inside cave environments, as well as spider abundance. Analyses on M. bourneti occupancy and abundance were also repeated for adults and juveniles separately. Generalized Linear Models, were used to weight species absence based on its detection probability. Linear Mixed Models were used to detect possible divergences in subterranean spatial use between adult and juvenile spiders. Although widespread on the mountain, M. bourneti generally showed low density and low detection probability. Most of the individuals observed were juveniles. The spiders generally occupied cave sectors with high ceilings that were deep enough to show particular microclimatic features. Adults tended to occupy less illuminated areas than juveniles, while the latter were more frequently found in sectors showing high humidity. The abundance of M. bourneti was strongly related to high humidity and the presence of two troglophile species, Hydromantes flavus Wake, Salvador & Alonso-Zarazaga, 2005 (Amphibia: Caudata) and Oxychilus oppressus (Shuttleworth, 1877) (Gastropoda: Panpulmonata). The abundance of juveniles was related to sector temperature and humidity, the presence of H. flavus and O. oppressus and to morphological sector features. However, when only adults were considered, no significant relationships were found. Adult and juvenile spiders did not differ in their spatial distribution inside the caves studied, but a seasonal distribution of the species along cave walls was observed. Microclimate was one of the most important features affecting both the presence and abundance of M. bourneti in subterranean environments. Individuals tended to occupy lower heights during hot seasons.

Are the Neglected Tipuloidea Crane Flies (Diptera) an Important Component for Subterranean Environments?

“…A smooth wall may facilitate the attachment of individuals, so they probably choose this feature to facilitate their resting. On the other hand, a wall with pronounced irregularities may shield their presence from surveyor view, and reduce the overall species detection [53]. However, this hypothesis was not confirmed by the analysis of detection probability, as we did not find relationships between crane-fly detectability and wall heterogeneity.…”

“…Diversity 2020, 12, x FOR PEER REVIEW 7 of 11 detection [53]. However, this hypothesis was not confirmed by the analysis of detection probability, as we did not find relationships between crane-fly detectability and wall heterogeneity.…”

“…For further details on data recording refer to Lunghi, Corti, Mulargia, Zhao, Manenti, Ficetola and Veith [49]. We surveyed each sector to assess the presence of crane flies through Visual Encounter Survey (VES) [52]; sectors were surveyed with similar effort to limit bias due to imperfect species detection [53]. Adopting the same methodology, we also recorded the presence and the abundance of five predators (traditionally considered to be troglophiles or trogloxenes) usually occurring on cave walls: the cave salamander Hydromantes italicus (Dunn, 1923), and four spider species, Meta menardi (Latreille, 1804), Metellina merianae (Scopoli, 1763), spiders of the family Agelenidae C. L. Koch, 1837, and Amaurobius ferox (Walckenaer, 1830) [10,48,54].…”

“…We first estimated the detection probability of crane flies; the detectability of small cave species usually is <1 [53,55]. We calculated the species detection probability on the base of 35 pairs of survey performed within 14 days; this ensures to meet the assumption of population closure [55].…”

Distributional dynamics of a specialized subterranean community oppose the classical understanding of the preferred subterranean habitats

“…Significant vertical migration between MSS and caves, corresponding to seasonal fluctuations in temperature, has also been recorded in other subterranean species (Chapman, ; Mammola et al, ; Polak, ). In addition to variation in climatic conditions, trophic availability was found to be an important factor affecting the spatial distribution of several spider species inhabiting caves (e.g., Lunghi, ; Mammola & Isaia, , ; Manenti et al, ; Novak et al, ). Furthermore, the spatial distribution of particular subterranean species is also affected by the structural complexity of cave walls (e.g., Bourne, , ; Mammola & Isaia, , ).…”