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 spider Meta menardi. 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.