Field study links distinct invertebrate communities to light gradients in Western Caucasus cave entrances

A field study of 15 cave watercourses in Abkhazia found that macroinvertebrate assemblages in partially illuminated cave entrances had a taxonomic composition and dominance structure distinct from those both inside the caves and on the surface. The entrance communities nevertheless occupied an intermediate position between the two environments, while illuminance was associated with differences in genus- and family-level assemblage structure.

The researchers investigated streams in the Duripshski, Gumistinsko-Panavskiy and Dzhalski speleo districts of the Western Caucasus. The study covered 14 caves and an abandoned railway tunnel near the mining village of Akarmara. Sampling took place in February 2018, February 2020 and March 2021, all during the early-spring period for the region. This was an observational sampling and statistical analysis, not an experiment, so the results demonstrate associations rather than establishing causal effects.

Where access allowed, investigators established sampling stations in complete darkness inside caves, in partial sunlight at entrances and in fully illuminated surface reaches downstream. Entrance stations were available in 12 caves; streams at three caves could not be sampled there because rubble concealed the watercourse. Four surface samples were also omitted because of hidden flow or research-time constraints. Each mixed quantitative sample covered a total area of 0.5 square metres and included organisms from three locations spaced 1–3 metres apart, representing deeper areas and water edges. Invertebrates were collected with an 11-centimetre hemisphere sampler-sieve with 0.5-millimetre mesh and preserved in 90% ethanol.

Identified specimens were assigned to three ecological groups under the Schiner–Racovitza system: stygobionts restricted to subterranean habitats, stygophiles capable of completing their life cycle in caves, and stygoxenes, treated in the study as surface organisms. The investigators measured selected hydrological and chemical variables and analysed assemblage structure using distance-based linear models, distance-based redundancy analysis, ANOSIM and SIMPER. The samples contained 84 aquatic invertebrate species, including 23 stygobionts, 17 stygophiles and 44 stygoxenes.

Species richness was highest in external reaches, where 58 species were recorded, compared with 51 in entrance zones and 45 inside caves. The richness of stygoxenes declined from surface to internal stations, whereas stygobiont richness followed the opposite pattern. Stygophile richness was greatest in the entrance ecotone. Insect species richness decreased toward the cave interior, while crustacean richness increased. Macroinvertebrate abundance was also greater outside the caves: mean abundance was 480 ± 542 individuals per square metre at external stations, compared with 83 ± 37 in entrance zones and 68 ± 49 at internal stations. The maximum at an external station was 1,842 individuals per square metre.

Stygoxenes dominated the external assemblages, while stygobionts predominated inside the caves. In the entrance zone, the contributions of stygoxenes, stygobionts and stygophiles to abundance were similar. Mean insect abundance was higher outside the caves, at 92 ± 102 individuals per square metre, than in the entrance zone, at 27 ± 25 individuals per square metre. Families associated with the ecotone included Dugesiidae, Erpobdellidae, Hydrobiidae, Leuctridae, Elmidae, Hydraenidae and Scirtidae.

The six environmental variables considered together explained 32–35% of variation in assemblage structure, leaving a substantial proportion unexplained. At the species, genus and family levels, cave location, illuminance and pH were significant predictors. Their relative contributions changed with taxonomic resolution: illuminance accounted for 7.9% of species-level variation, 11.9% of genus-level variation and 16.2% of family-level variation. Cave location explained 15.1% of species-level variation but 4.4% at family level. ANOSIM detected significant differences among light zones at genus and family levels, with R-statistics of 0.132 and 0.218, respectively, but not at species level, where R was 0.06. The largest pairwise differences occurred between surface assemblages and either entrance-zone or internal assemblages.

Species-level composition was associated with proximity among caves, consistent with restricted dispersal among isolated subterranean habitats, whereas family-level composition tracked the light gradient more strongly. The entrance zone showed ecological specificity rather than simply resembling one adjacent habitat. The entrance fauna included 62.5% of the species recorded at surface stations and 60.0% of those recorded inside caves. In the opposite direction, 76.1% of entrance-zone species also occurred in surface habitats, while 58.7% occurred in the cave interior. The expectation that richness and abundance would consistently peak in ecotones was not clearly supported: most streams had more organisms outside the cave entrance, although Adzaba, Kveda Shakurani and Gupskaya showed entrance-zone richness exceeding that of neighbouring zones.

These findings have implications for interpreting the roles of light and geography, as well as for assessing anthropogenic disturbance. Abrskili Cave has artificial lighting extending 400 metres from its entrance, and surface stonefly larvae were found on rotting wood more than 100 metres inside. The authors note that artificial lighting may contribute to ecological change as surface insects are attracted into caves and deposit eggs along upstream watercourses, but the present study did not experimentally test those effects. Its conclusions are bounded by one-time, early-spring sampling, unequal station availability, incomplete measurement of some environmental variables, substantial unexplained variation and the observational design.

Subject of Research: Associations between light zones, geography and macroinvertebrate assemblages in Western Caucasus cave streams

Article Title: At the edge of darkness and light: macrozoobenthic assemblages of streams in the entrance zone of caves in the Western Caucasus

Article References: Borisov, R. R., Chertoprud, E. S., Palatov, D. M., Novichkova, A. A., Vorobjeva, L. V., & Chertoprud, E. M. (2025). At the edge of darkness and light: macrozoobenthic assemblages of streams in the entrance zone of caves in the Western Caucasus. Subterranean Biology, 53, 73-101. https://doi.org/10.3897/subtbiol.53.152803

Image Credits: AI Generated

DOI: 10.3897/subtbiol.53.152803

Keywords: cave streams, macroinvertebrates, stygobionts, stygophiles, stygoxenes, ecotones, illuminance, Western Caucasus, Abkhazia, field study, edge, darkness