Abstract
Caves possess a continuum of ecological zones that differ in their microhabitat conditions, resulting in a gradient of nutrients, climate, and illumination. These conditions engender relatively rapid speciation and diverse assemblages of highly specialised spider fauna. It is unclear, however, how zonation of these caves affects spider assemblage composition and structure. Surveys of 35 Levantine caves were conducted to compare the assemblages of spiders between their different ecological zones. The diverse spider assemblages of these caves differed between the entrance, twilight, and dark zones, with troglophiles and accidental species occupying the cave entrance, endemic troglobites occupying the dark zones, and hybrid assemblages existing in the twilight zones. The progression of assemblage composition and divergence throughout cave zones is suggestive of processes of ecological specialisation, speciation, and adaptation of cave-endemic troglobites in the deepest zones of caves, while cave entrance assemblages are composed of relatively common species that can also be found in epigean habitats. Moreover, the cave entrance zone assemblages in our study were similar in the different caves, while the cave dark zone assemblages were relatively distinct between caves. Cave entrance assemblages are a subset of the regional species pool filtered by the cave conditions, while dark zone assemblages are likely a result of adaptations leading to local speciation events.
Highlights
Subterranean habitats such as caves are home to species with adaptations and preadaptations to darkness and nutrient limitation
While it is suggested that species richness of troglophiles and accidentals is explained by local ecological factors and seasonality, species richness of troglobites may be better explained by historical biogeography [8,11]
We have previously demonstrated that southern Levantine caves harbour diverse assemblages of troglophile and troglobite arachnids, with 62 observed spider species in 35 caves in Israel and Palestine, including 32 troglobite and troglophile spiders [19]
Summary
Subterranean habitats such as caves are home to species with adaptations and preadaptations to darkness and nutrient limitation Given their unique environments and reduced connectivity, caves resemble islands for obligate troglobite (obligated to life in caves) inhabitants, with little gene flow occurring between populations [1]. These conditions lead to a formation of unique assemblages of highly specialised invertebrates. The cave structure— the characteristics of cave openings—has a direct effect on the abiotic conditions within the cave, such as light intensity, climatic and air conditions, and energy and nutrient influx These abiotic conditions establish several ecological zones—entrance, twilight, and dark [2,3,4,5]—that are primarily characterised by decreasing illumination with progression into the cave. While it is suggested that species richness of troglophiles (species that have source populations in both hypogean and epigean habitats [10,11]) and accidentals (occasional visitors in caves) is explained by local ecological factors and seasonality, species richness of troglobites may be better explained by historical biogeography [8,11]
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